Beyond iron: interface-coupled non-ferrous photo-Fenton-like catalysts for boosted H₂O₂ activation in environmental remediation

Abstract

The persistent presence of emerging pollutants, including antibiotics, dyes, and pharmaceutical residues in water environments, poses substantial environmental and public health risks. Conventional treatment approaches, such as Fe-based Fenton processes, are restricted by narrow pH requirements, excessive sludge formation, and poor utilization of visible light. In recent years, non-ferrous transition-metal-based catalysts, particularly those incorporating Cu, Co, Mn, Bi, and Ce, have emerged as promising alternatives due to their flexible redox chemistry, broader pH adaptability, and enhanced photo-assisted H₂O₂ activation. More importantly, interface engineering has been established as a pivotal strategy to amplify photo-Fenton-like performance by promoting interfacial charge separation, accelerating reactive oxygen species (ROS) generation, and stabilizing metal redox cycling. Approaches involving semiconductor heterojunctions, carbonaceous frameworks, layered double hydroxides, defect-rich architectures, and oxygen-vacancy modulation are systematically discussed. This review critically summarizes recent advances in the design, mechanistic understanding, and performance of interface-engineered non-ferrous photo-Fenton catalysts for environmental remediation. We focus on structure–activity relationships, synergistic effects, and pollutant-specific degradation pathways, while also addressing challenges and opportunities toward sustainable water remediation. Current challenges and future perspectives toward scalable, sustainable, and efficient Fenton-like systems are also outlined, providing a comprehensive roadmap for the rational development of next-generation Fenton-like systems beyond iron.

---

1. Introduction

Since the beginning of the 21st century, water pollution has become a serious problem that exerts a significant impact on the ecological environment and human survival.^1,2 Organic dyes,^3,4 benzene-based organics,^5,6 pesticides,⁷ and antibiotics^8,9 are the main pollutants of concern in wastewater due to their high toxicity and hardly degradable characteristics. Recent comprehensive analyses of Fenton and Fenton-like advanced oxidation processes (AOPs) have highlighted their effectiveness in degrading pharmaceutical and pesticide contaminants, while also emphasizing the potential formation of transformation by-products and associated environmental risks, underscoring the need for more controlled and sustainable oxidation strategies.¹⁰ Conventional biological and physicochemical processes, though widely implemented, often prove inadequate for the degradation of emerging contaminants (ECs) due to their complex structures, low biodegradability, and persistent environmental behaviour. To overcome these drawbacks, powerful treatment technologies exploiting reactive oxygen species (ROS), notably hydroxyl radicals (·OH), achieve rapid, non-selective degradation and mineralization of refractory organic pollutants.^11–13 Among these, the homogeneous Fenton reaction, which produces highly reactive hydroxyl radicals (·OH) through the Fe²⁺/H₂O₂ system, has been extensively applied owing to its high efficiency, low cost, and operational simplicity.^14,15 The catalytic performance can be further enhanced under solar or ultraviolet irradiation in the photo-Fenton process, where light-assisted electron transfer accelerates metal redox cycling and promotes sustained H₂O₂ activation, leading to the continuous generation of ROS such as ·OH, ¹O₂, and ·O₂⁻.^16,17 In a typical heterogeneous photo-Fenton process, metal ions immobilized on a solid support react with H₂O₂ under UV/visible light, creating electron–hole pairs and ·OH radicals, which decompose organic contaminants.¹⁸ Despite these advantages, classical homogeneous Fenton processes suffer from intrinsic drawbacks, including the requirement for strongly acidic conditions (pH 2–3), generation of iron-containing sludge, and loss of the catalyst in the effluents, which causes secondary pollution and catalyst loss.^19,20 To address these challenges, heterogeneous Fenton catalysts, such as iron-containing solids,^21–23 have been developed as alternatives to the homogeneous Fenton process. These catalysts avoid the need for large amounts of reducing metal ions, which has been an active research area in this field. Consequently, a variety of iron-based heterogeneous photo-Fenton catalysts, such as Fe@MIL-100 (Fe), Fe/Fe₃O₄ composites, Fe₃O₄@void@TiO₂, Fe–Co PBAs, and α-Fe₂O₃@g-C₃N₄ have been developed to improve catalyst recovery and stability.^24–28 However, most of these systems remain effective only under acidic conditions, exhibiting significantly diminished catalytic performance at neutral or alkaline pH, thereby limiting their practical applicability. Moreover, iron-based catalysts often suffer from sluggish charge-transfer kinetics, insufficient active-site availability, and poor visible-light utilization.^29,30

To overcome these limitations, recent research has shifted toward the development of non-iron photo-Fenton-like catalysts capable of operating over a broader pH range with improved efficiency and stability. Alternative transition and post-transition metal systems, including Cu, Co, Mn, Bi, and Ce, have attracted increasing attention due to their favourable redox properties, more flexible operating conditions, and enhanced visible-light absorption. This paradigm shift mirrors the development of non-TiO₂ based photoanodes, where the focus has moved toward optimizing visible-light utilization and establishing clear synthesis–structure–mechanism–activity relationships to enhance practical wastewater treatment performance. In particular, their ability to function efficiently from near-neutral to alkaline pH conditions provides a distinct advantage over Fe-based catalysts.³¹ For example, Cu-based catalysts activate H₂O₂ at near-neutral pH through the Cu⁺/Cu²⁺ redox cycle,^32–34 while Co- and Mn-based systems rely on multivalent redox and oxygen-vacancy-mediated pathways that are enhanced under visible light;^35–37 Bi- and Ce-based catalysts, in contrast, facilitate H₂O₂ activation primarily via photogenerated charge carriers and oxygen-vacancy-assisted interfacial electron transfer.^38,39

However, the standalone catalytic performance of these metals is often limited by inefficient separation of photoinduced charge carriers, poor light-harvesting capability, or leaching of active species. To overcome these challenges, interface engineering strategies have gained significant attention. By constructing heterojunctions with suitable semiconductors (e.g., MoS₂, BiOBr), introducing carbonaceous supports (e.g., rGO, CNTs, biochar), or integrating porous scaffolds (e.g., MOFs, LDHs), researchers have been able to modulate the electronic structure, improve charge separation, and stabilize active species. The interfacial design is not merely structural; it fundamentally reshapes the energy band alignment, oxygen vacancies, and surface functionalities that dictate ROS generation. Importantly, interface engineering establishes built-in electric fields, suppresses charge recombination, and minimizes metal leaching, advantages not achievable through simple blending or doping.

Recent studies have demonstrated that Z-scheme and S-scheme heterojunctions integrating non-ferrous Fenton-like active components can markedly enhance photo-Fenton degradation by promoting spatial separation of photogenerated electrons and holes while preserving strong redox potentials for H₂O₂ activation. For instance, CuO/CDs/g-C₃N₄ nanocomposites exhibit superior visible-light-driven activity due to accelerated interfacial charge transfer and rapid Cu⁺/Cu²⁺ cycling, which sustains continuous H₂O₂ activation.⁴⁰ Similarly, Bi₂WO₆/CoAl-LDHs S-scheme heterojunctions exhibit outstanding photo-Fenton-like activity by leveraging an interfacial electric field to drive directional charge separation while maintaining strong redox potentials, thereby enabling efficient H₂O₂ activation and cobalt-mediated ·OH generation for rapid antibiotic degradation.⁴¹

In parallel, the incorporation of carbonaceous matrices such as reduced graphene oxide (rGO), graphitic carbon nitride (g-C₃N₄), and biomass-derived biochar offers multifaceted advantages for non-ferrous photo-Fenton-like catalysis. These conductive frameworks promote rapid electron transport, provide abundant surface functionalities for H₂O₂ adsorption, and stabilize metal active sites, thereby facilitating efficient redox cycling. When coupled with Mn or Co-based oxides, the resulting composites demonstrate enhanced Fenton-like reactivity, often under neutral pH conditions and with low metal leaching. Moreover, the interfacial synergy between the metal oxide and the carbon support serves to suppress radical scavenging pathways and extend ROS lifetimes. Carbon-based interfaces also improve mechanical stability, surface area, and pollutant adsorption, further supporting improved photocatalytic performance. For instance, in CuO_x·Ag₂O/RGO systems, the highly conductive and defect-rich carbon support acts as an efficient electron mediator and anchoring scaffold, promoting rapid charge transport, stabilizing metal active sites, and sustaining metal redox cycling for efficient H₂O₂ activation.⁴²

Bismuth-based materials, particularly layered BiOX (X = Cl, Br, I) compounds, represent an emerging yet highly promising class of non-ferrous photo-Fenton-like catalysts.^36,43 Their unique internal electric fields (IEF) facilitate rapid charge separation, while their tunable band structures allow for visible-light absorption and high ROS generation. Interface engineering with materials like MOFs or g-C₃N₄ further augments their catalytic properties, enabling efficient removal of diverse organic contaminants under simulated or natural sunlight with minimal secondary pollution. Consequently, Bi-based heterostructures have emerged as powerful candidates for sunlight-driven advanced oxidation processes, particularly when combined with strategies to modulate defects and interfaces.

Despite these advances, a unified understanding of the underlying reaction pathways, advantages, and challenges of non-ferrous photo-Fenton catalysts remains fragmented. Existing reviews often focus on individual metal systems or broadly discuss Fenton-like processes without systematically addressing the synergistic role of interface engineering, such as heterojunction construction, carbon coupling, and defect modulation. Furthermore, few reviews provide a comparative perspective across different non-ferrous systems under similar conditions, nor do they sufficiently address key practical issues such as catalyst stability, reusability, pollutant selectivity, or scalability. Thus, a unified and comparative evaluation of interface-engineered non-ferrous catalysts is urgently needed to guide future material development.

This review aims to bridge these gaps by presenting a systematic and comparative analysis of non-ferrous (Cu, Co, Mn, Ce, and Bi) photo-Fenton catalysts, with a special emphasis on interface engineering approaches that have enabled performance breakthroughs. It will explore:

1. The fundamentals of redox cycling and ROS generation in non-ferrous systems.

2. Recent advances in nanocomposite design with structural and electronic interface modulation.

3. Reported degradation performance of key pollutants (antibiotics, dyes, PPCPs).

4. Mechanistic insights into charge dynamics and active species generation.

5. Current limitations, environmental considerations, and future research outlook.

In doing so, this review highlights not only the evolution of photo-Fenton chemistry “beyond iron”, but also the emerging paradigm of interfacial nanoarchitectures as the foundation for next-generation water treatment technologies. An overview of this review is illustrated in Fig. 1. We aim to provide a blueprint for researchers and engineers working at the nexus of materials chemistry, environmental catalysis, and sustainable water remediation, with a critical eye toward the practical hurdles of selectivity, long-term stability, and scalable deployment that must be overcome for real-world application.

Fig. 1 Overview of interface-engineered non-ferrous photo-Fenton-like catalysis for pollutant degradation.

2. Fundamentals of non-ferrous photo-Fenton catalysis

The photo-Fenton process is a synergistic system that integrates semiconductor photocatalysis with Fenton-like redox chemistry. Its enhanced efficacy relies on multiple, often simultaneous, pathways for generating reactive oxygen species (ROS). To understand the advantages of non-ferrous catalysts, it is essential to first define the baseline mechanisms and limitations of the classic iron-based system.

2.1 Mechanism overview of photo-Fenton and Fenton-like reactions

The traditional Fenton mechanism centres on Fe²⁺ activating H₂O₂ to generate hydroxyl radicals (·OH), as represented by:⁴⁴

Fenton reaction (dark).

Fe²⁺ + H₂O₂ → Fe³⁺ + OH⁻ + ·OH (k = 40–80 M⁻¹ s⁻¹)

Fe³⁺ + H₂O₂ → Fe²⁺ + H⁺ + HO₂· (k = 0.001–0.01 M⁻¹ s⁻¹)

The primary limitation of the dark Fenton process is the extremely slow regeneration of Fe²⁺ from Fe³⁺, which is the rate-determining step and requires acidic conditions to keep Fe³⁺ in solution.

Photo-Fenton enhancement. In photo-Fenton systems, light exposure accelerates Fe³⁺ reduction and promotes additional radical formation:

[Fe(OH)]²⁺ + hν → Fe²⁺ + ·OH

This photoreduction step is essential for maintaining continuous Fe²⁺ regeneration, but it is inefficient under visible light and becomes unstable at higher pH.

Non-ferrous metal-based photo-Fenton-like redox cycles. When non-ferrous metals are introduced, similar redox cycles are invoked:

• Cu⁺ + H₂O₂ → Cu²⁺ + ·OH + OH⁻

• Cu²⁺ + H₂O₂ → Cu⁺ + ·OOH + H⁺

• Co²⁺ + H₂O₂ → Co³⁺ + ·OH + OH⁻

• Ce³⁺ + H₂O₂ → Ce⁴⁺ + ·OH + H⁻

• Mn²⁺ + H₂O₂ → Mn³⁺ + ·OH + OH⁻

• Ce³⁺/Ce⁴⁺ cycles facilitate electron shuttling and oxygen-vacancy creation, although Ce-based reactions often rely more on interface-assisted H₂O₂ activation than direct Fenton reactions.

• Bi-based systems typically activate H₂O₂ indirectly through oxygen vacancies and photogenerated charge carriers rather than classical redox cycling.

• Cu⁺/Cu²⁺, Co²⁺/Co³⁺, and Mn²⁺/Mn³⁺ efficiently mediate ROS generation in the presence of H₂O₂ and photons.

These metals often show consistent activity and consistently maintain activity over a broader pH range, and avoid sludge formation. Their faster electron-transfer kinetics also facilitate the rapid generation of ROS under solar or visible-light illumination.

Photo-induced charge carrier participation. In photo-Fenton and photo-Fenton-like systems, the light-driven excitation of electrons (e⁻) and holes (h⁺) in semiconductors or composites further enhances radical formation:

h⁺ + H₂O → ·OH + H⁺

h⁺ + OH⁻ → ·OH

e⁻ + O₂ → ·O₂⁻

·O₂⁻ + H⁺ → HO₂· → (secondary ROS)

These pathways indicate that both redox cycling and semiconductor photoexcitation contribute simultaneously, giving rise to multiple ROS species (·OH, ·O₂⁻, ·OOH, ¹O₂, and even H₂O₂ regeneration in some cases).

Non-radical oxidation pathways and interface effects. In addition to conventional radical pathways dominated by ·OH, increasing evidence suggests that non-radical oxidation mechanisms play a crucial role in non-ferrous photo-Fenton systems, particularly in interface-engineered catalysts. These pathways primarily involve singlet oxygen (¹O₂) generation and direct interfacial electron transfer, which exhibit higher selectivity and resistance to matrix interference.

Interface features such as oxygen vacancies (OVs), defect-rich surfaces, and specific exposed crystal facets significantly influence this transition. Oxygen vacancies act as electron-rich centres that facilitate the adsorption and activation of dissolved O₂ or H₂O₂, promoting the formation of surface-bound superoxide intermediates (·O₂⁻), which can subsequently convert into ¹O₂ via energy-transfer pathways. Unlike free ·OH radicals, these processes occur on the catalyst surface, minimizing non-selective oxidation.

Moreover, facets engineering (e.g., BiOBr (010) or CeO₂ (111)) alters the local electronic structure and adsorption configuration, enabling selective electron transfer to oxygen species rather than homolytic H₂O₂ cleavage. In addition, interfacial electric fields in S-scheme and Z-scheme heterojunctions drive directional charge migration, enriching electrons at specific active sites that favour O₂ activation and ¹O₂ generation.

Such non-radical pathways are particularly prominent in Cu–N₄ coordinated systems and oxygen-vacancy-rich Bi- and Ce-based catalysts, where strong metal–support interactions and defect-mediated charge redistribution suppress excessive ·OH formation and promote surface-mediated oxidation routes. These findings highlight that rational interface design not only enhances catalytic efficiency but also fundamentally alters the dominant reaction mechanism.

Additional synergistic pathways. Synergistic mechanisms typically include:

• Direct oxidation of pollutants by h⁺ (valence band holes).

• Reduction of H₂O₂ by photo-generated e⁻ to yield ·OH or ·O₂⁻:

H₂O₂ + e⁻ → ·OH + OH⁻

H₂O₂ + e⁻ → ·O₂⁻ + 2H⁺

• Activation of H₂O₂ at oxygen vacancies (OVs), common in CeO₂, MnO_x, and BiOX:

Ov + H₂O₂ → ·OH + OH⁻

• Energy-band alignment in heterojunctions, enabling vectorial charge migration and suppressing carrier recombination.

Thus, non-ferrous photo-Fenton catalysis often involves multi-pathway ROS generation driven by both metal-centred reactions and photocatalytic charge transfer. This hybrid mechanism differentiates it from classical Fe-based systems and underpins the superior efficiency observed in interface-engineered composites (Fig. 2). Consistent with these mechanistic advantages, most non-ferrous photo-Fenton-like catalysts exhibit significantly higher degradation rates in H₂O₂-assisted systems than in H₂O₂-free photocatalytic processes, as summarized in Table 1.

Fig. 2 Comparison of classical Fenton, photo-Fenton, and non-iron photo-Fenton-like catalytic processes.

Table 1 Photocatalytic performance of non-ferrous photo-Fenton-like catalysts with and without H₂O₂

Catalyst	Pollutant	Light source	k (min^−1)	Degradation (%)	Main ROS	Activity without H2O2 (%)	Activity with H2O2	Ref.
VO–Cu2O@Cu/CNx	Tetracycline (TC, 20 mg L^−1)	Visible LED (λ > 420 nm)	0.2	100	^1O2	Moderate (54.4)	Strong enhancement due to Cu^0 and oxygen vacancies, which accelerates the Cu redox cycle	45
Cu-Bi2O2S	Tetracycline hydrochloride (TCH, 50 mg L^−1)	Visible LED lamp	0.0243	77.76	·OH	Low (18.4)	Strong due to Cu-cocatalysts, which enhance charge carrier mobility and separation	46
BiWO6/CoAl LDH	Oxytetracycline (OTC, 10 mg L^−1)	300 W xenon lamp (λ > 420 nm)	—	98.47	·O2^−, ·OH	Moderate (41.37)	Stronger S-scheme heterojunctions facilitate cobalt-mediated H2O2 activation	41
Cu2−xS/g-C3N4	Rhodamine B (RhB, 30 mg L^−1)	300 W xenon lamp	0.172	99.6	·O2^−	Very low (2.1)	High due to nitrogen vacancies and internal electric fields, which promote Cu^+/Cu^2+ cycling and suppress recombination	47
BiOBr/Co3O4	Methylene blue (MB, 10 mg L^−1)	500 W xenon lamp	—	96	·O2^−, ·OH	Moderate (27)	Significantly higher due to hierarchical nanowires and BiOBr/Co3O4 heterojunctions, which drive visible-light, Co-mediated H2O2 activation	36
Prussian blue/Mn3O4	Levofloxacin (LVF)	30 W visible LED	0.076	93.9	^1O2	Moderate (74.2)	High due to Z-scheme p–n heterojunctions, which enable Fe/Mn valence cycling and continuous non-radical ^1O2 generation	48
Mn doped cobalt silicate@diatomite	Methyl Orange (MO, 50 mg L^−1)	UV lamp	0.0226	95	·OH	Low (20)	Stronger due to due to manganese doping, which increases surface area and creates a Co–Mn bimetallic synergy that lowers the H2O2 activation energy	35
g-C3N4/ZnCo2O4	Rhodamine B (RhB, 10 mg L^−1)	40 W visible LED	0.03	94	·OH	Moderate (39)	High due to due to a Z-scheme heterojunction enhancing separation and enables the effective activation of H2O2 at neutral pH	49

---

2.2 Unique roles of Cu, Co, Mn, Bi, and Ce in Fenton-like Photocatalysis

Copper (Cu). Copper-based photocatalysts, including CuO, Cu₂O, and Cu-modified g-C₃N₄ exhibit efficient redox cycling between Cu⁺ and Cu²⁺, enabling rapid activation of H₂O₂ through:

Cu⁺ + H₂O₂ → Cu²⁺ + ·OH + OH⁻

This redox versatility promotes continuous ROS production, especially under visible light, where photoexcited electrons readily reduce Cu²⁺ back to Cu⁺. Cu–O bonds and oxygen-vacancy-rich CuO_x surfaces facilitate interfacial electron transfer, suppressing charge recombination and extending radical lifetimes.

Due to their suitable band gaps (1.2–2.1 eV), Cu-based catalysts exhibit strong visible-light absorption and maintain high activity near neutral pH, overcoming the acidic limitations of Fe-based Fenton systems.⁵⁰ Their performance further improves in heterojunctions with g-C₃N₄, TiO₂, or BiVO₄, where Z-scheme or p–n architectures enable directional charge migration.^40,51,52 Nanostructuring (e.g., hollow spheres, nanowires) and hybridization with MOFs or carbon dots enhance surface reactivity, adsorption behaviour, and long-term stability.^40,53 Collectively, Cu-based photocatalysts represent one of the most efficient and tunable non-ferrous systems for photo-Fenton water purification.

Cobalt (Co). Cobalt-based catalysts, including Co²⁺ salts, Co₃O₄, CoAl₂O₄, and Co-LDHs, exhibit strong Fenton-like behaviour due to the Co²⁺/Co³⁺ redox couple.^35,41 The primary H₂O₂ activation step occurs via:

Co²⁺ + H₂O₂ → Co³⁺ + ·OH + OH⁻

Under illumination, photoexcited electrons reduce Co³⁺ to Co²⁺, sustaining continuous ROS generation. Cobalt oxides are typically p-type semiconductors, enabling strong hole-mediated oxidation pathways, especially under visible light. When integrated with carbon materials, MOFs, or semiconductors (BiVO₄, g-C₃N₄), Co-based composites exhibit enhanced charge transfer, broadened light absorption, and improved recyclability.⁵⁴ Their stability, tunable structure, and robust catalytic kinetics make cobalt systems excellent candidates for solar-driven Fenton-like processes.

Manganese (Mn). Manganese-based catalysts, such as MnO₂, Mn₃O₄, MnOOH, and Mn-doped semiconductors benefit from multi-valence cycling among Mn²⁺/Mn³⁺/Mn⁴⁺, enabling activation of H₂O₂ and even O₂ under illumination:^35,55

Mn²⁺ + H₂O₂ → Mn³⁺ + ·OH + OH⁻

Defect-rich MnO₂ contains abundant surface hydroxyls and oxygen vacancies, which facilitate H₂O₂ adsorption, pollutant binding, and radical production. Although MnO₂ alone shows limited light absorption, its performance improves significantly when coupled with g-C₃N₄, BiVO₄, carbon supports, or LDHs, which enhance charge separation and visible-light utilization.^56,57 Mn-based perovskites (e.g., LaMnO₃) offer strong structural stability and flexible electronic configurations.⁵⁸ Due to their low toxicity, favourable redox characteristics, and cost-effectiveness, Mn-based catalysts are well-suited for the degradation of antibiotics, dyes, and phenolic pollutants under mild conditions.

Bismuth (Bi). Bismuth-based photocatalysts BiOBr, Bi₂O₃, Bi₄MoO₉, and BiVO₄ are uniquely characterized by their layered structures and internal electric fields (IEFs), which strongly promote charge separation under visible light.^36,43,59 While Bi is not classically redox-active with H₂O₂, Bi-based materials can indirectly activate H₂O₂ through photogenerated h⁺/e⁻ pairs, oxygen vacancies, and interfacial charge transfer within heterostructures.

BiOBr and BiVO₄ readily form type-II, S-scheme, or Z-scheme junctions with g-C₃N₄, TiO₂, or MoS₂, enabling enhanced ROS formation (·OH, ·O₂⁻, ¹O₂).^60,61 Their high visible-light response, low toxicity, and tunable band structures make Bi-based systems particularly effective for pharmaceuticals, dyes, and endocrine-disrupting chemicals under ambient or solar irradiation.

Cerium (Ce). Cerium-based catalysts, particularly CeO₂ and Ce-doped composites, are distinguished by the Ce³⁺/Ce⁴⁺ redox couple and high concentration of oxygen vacancies (OVs). Unlike Cu or Co, Ce does not strongly activate H₂O₂ via classical homogeneous Fenton reactions. Instead, Ce-based catalysis relies on:

• OV-assisted H₂O₂ adsorption and activation,

• Ce³⁺/Ce⁴⁺ electron shuttling,

• Surface hydroxyl-mediated ROS formation, and

• Improved charge separation in heterojunctions.

CeO₂ readily forms composites with g-C₃N₄, BiOBr, or rGO, where its OVs enhance interfacial electron transfer and promote the generation of ·OH and ·O₂⁻.^62–64 Ce-based photocatalysts also maintain structural stability over a wide pH range, making them attractive for practical wastewater treatment applications. These structure–activity relationships, including oxygen-vacancy-mediated H₂O₂ activation, redox shuttling behaviour, and associated challenges, are comparatively summarized in Table 2.

Table 2 Key features, H₂O₂ activation mechanisms, and challenges of non-ferrous (Cu, Co, Mn, Bi, Ce) photo-Fenton-like catalysts

Catalyst system	Key unique features & advantages	Primary H2O2 activation mechanism	Challenge & interface strategy
Copper (Cu)	• Strong visible-light absorption	Direct (Fenton-like): Cu^+/Cu^2+ redox couple	Leaching: addressed by carbon supports and MOF encapsulation
	• High activity at neutral pH
Cobalt (Co)	• High intrinsic catalytic activity	Direct (Fenton-like): Co^2+/Co^3+ redox couple	Toxicity/cost: mitigated by doping (Ni/Mn) and core–shell structures
	• Magnetic recoverability (spinels)
Manganese (Mn)	• Low toxicity	Direct (Fenton-like): Mn^2+/Mn^3+/Mn^4+ redox couple	Low solar response: improved via Z-scheme S-scheme coupling with g-C3N4 or TiO2
	• Earth-abundant
	• Multi-valence states
Bismuth (Bi)	• Strong internal electric fields (IEF)	Indirect (Photocatalytic): via h^+/e^−, oxygen vacancies (OVs), and IEFs	Weak activation: enhanced by oxygen vacancy & construction of advanced heterojunctions
	• Excellent charge separation
	• Low toxicity
Cerium (Ce)	• High oxygen-vacancy concentration	Indirect (Photocatalytic): via OVs & Ce^3+/Ce^4+ shuttle	Defect-dependence & low visible-light absorption: increasing OV density and forming a heterojunction with BiOBr or g-C3N4
	• Wide pH stability
	• OV-mediated activation pathway

---

2.3 Key parameters influencing the activity of non-ferrous photo-Fenton catalysts

The catalytic performance of non-ferrous photo-Fenton-like systems is governed by a complex interplay of intrinsic electronic properties, surface chemistry, and interfacial architecture. Optimizing these parameters is essential for efficient H₂O₂ activation, ROS generation, and visible-light-driven pollutant degradation. This section discusses the fundamental physicochemical factors redox potential, band structure, surface area and porosity, defect chemistry, and interface modulation with a specific emphasis on cerium (Ce), alongside Cu, Co, Mn, and Bi.

a. Redox potential and valence flexibility. The ability of a metal centre to undergo reversible redox transformations directly influences its capacity to activate H₂O₂ and sustain ROS production. Non-ferrous metals such as Cu, Co, Mn, and Ce possess diverse valence states that enable electron exchange under light illumination. Their standard redox potentials (E⁰) determine the thermodynamic feasibility of the ROS generation process:^44,65

• Cu⁺/Cu²⁺: +0.17 V vs. NHE

• Co²⁺/Co³⁺: +1.81 V vs. NHE

• Mn²⁺/Mn³⁺: +1.51 V vs. NHE

• Fe²⁺/Fe³⁺: +0.77 V vs. NHE

• Ce³⁺/Ce⁴⁺: +1.72 V vs. NHE

Copper's relatively low redox potential favours rapid Cu²⁺/Cu⁺ cycling even at near-neutral pH,⁵⁰ while Co and Mn serve as stronger oxidants capable of promoting efficient ·OH generation. The Ce³⁺/Ce⁴⁺ couple provides an effective electron-shuttling mechanism; photoexcited electrons readily reduce Ce⁴⁺ to Ce³⁺, which enhances catalytic cycling and simultaneously promotes oxygen-vacancy formation. This valence flexibility plays a pivotal role in pollutant mineralization, particularly when these metals are incorporated into composite or heterojunction frameworks.³⁸

b. Band gap and light absorption. The catalyst's band gap governs its ability to absorb sunlight and generate electron–hole pairs. Representative band-gap values for key non-ferrous photocatalysts include:^66–69

CuO: ∼1.3 eV

MnO_x (e.g., alpha-MnO₂): ∼1.5 eV

BiOBr: ∼2.7 eV

Co₃O₄: ∼1.6 eV

CeO₂: ∼3.0–3.2 eV (can be narrowed by doping/defects)

While pristine CeO₂ absorbs mainly UV light due to its wide band gap, defect engineering (oxygen vacancies),⁷⁰ aliovalent doping,⁷¹ or hybridization with visible-light-responsive semiconductors⁷² (e.g., g-C₃N₄, BiOBr, WO₃) induces mid-gap or defect states. The presence of these states enables visible-light utilization and accelerates charge-carrier separation, which collectively improves ROS generation efficiency. For instance, Liu et al. developed mesoporous ceria nanoparticles (MCNs) and showed that band-gap narrowing and enhanced light absorption are the dominant factors governing their photo-Fenton activity. The optimized MCNs-240, featuring a larger pore diameter and reduced band gap, achieved >90% Acid Orange 7 dye degradation within 3 min across a wide pH (2.5–8.5) and H₂O₂ range, far outperforming conventional CeO₂. Mechanistic analysis revealed that the narrower band gap improved visible-light harvesting, while the mesoporous structure accelerated adsorption equilibrium, enabling ultrafast ROS generation. This work highlights band-gap engineering as a key strategy for boosting ceria-based photo-Fenton catalysts.⁷³

c. Surface area and porosity. Photo-Fenton reactions occur predominantly on catalyst surfaces; therefore, increasing specific surface area and accessible porosity enhances pollutant adsorption, mass transfer, and active-site density. Nanostructuring strategies such as hollow architectures, nanosheets, nanorods, or MOF-derived frameworks considerably improve catalytic interactions.

CeO₂ nanosheets, MnO_x nanoflowers, and Co-based layered double hydroxides (LDHs) exhibit high pollutant affinity and rapid charge migration due to their accessible surfaces and layered structure. Notably, the tunable interlayer chemistry and high surface-to-volume ratio of LDH-based materials make them exceptionally effective for the advanced photocatalytic oxidation of recalcitrant pharmaceuticals, where the porous architecture facilitates the diffusion of large medicinal molecules into active interfacial zones.⁷⁴ Ce-based MOFs, aerogels, and ordered mesoporous oxides also offer tunable porosity and high defect concentration, enabling simultaneous adsorption–oxidation pathways for improved degradation performance. For instance, it was reported that porosity and surface area are critical contributors to the superior photo-Fenton performance of supramolecularly assembled CA-Cu/TCN catalysts. The hydrogen-bond-induced self-assembly strategy generates a porous tubular g-C₃N₄ framework with high mesopore density (H4 hysteresis), enlarged pore volume, and increased specific surface area (29.3 m² g⁻¹), markedly exceeding that of pristine CN. This hierarchical porous architecture enhances mass transport, pollutant diffusion, and accessibility of ultra-dispersed CuN_x active sites and nitrogen vacancies, while suppressing Cu aggregation. The increased interfacial contact between H₂O₂, pollutants, and active sites accelerates redox reactions and ROS generation, enabling rapid ciprofloxacin degradation (95.9%, k = 0.0948 min⁻¹). These results underscore that engineered porosity and surface accessibility are key structural parameters for maximizing active-site utilization and photo-Fenton efficiency.⁷⁵

d. Oxygen vacancies and lattice defects. Oxygen vacancies and lattice defects critically influence both catalytic activity and charge dynamics in non-ferrous systems. These defects trap photogenerated electrons, reducing recombination, enhancing surface polarity and H₂O₂ adsorption, altering electronic structure and promoting sub-bandgap transitions, and introducing active sites for ROS production.

CeO₂ is particularly noted for its CeO_2−x structure, where abundant OVs create Ce³⁺-rich surface regions that facilitate H₂O₂ activation through both radical and non-radical pathways. For instance, Yang et al. reported a cerium-rich CeO_2−x/Bi₂MoO₆ heterojunction, non-stoichiometric CeO_2−x introduces abundant surface oxygen vacancies that enrich Ce³⁺ sites and promote rapid Ce³⁺/Ce⁴⁺ self-circulation.³⁸ These vacancies create defect states that enhance visible-light absorption, act as electron-trapping centres to suppress recombination, and facilitate interfacial electron migration. Consequently, oxygen-vacancy-assisted Ce³⁺ efficiently activates H₂O₂ to generate ·OH, while photogenerated electrons rapidly regenerate Ce³⁺ from Ce⁴⁺, sustaining continuous ROS production. This self-circulating, OV-driven mechanism enables markedly enhanced antibiotic degradation under low-power visible light, demonstrating oxygen vacancy engineering as a key lever for high-performance non-ferrous photo-Fenton systems.

In another study, it was reported that Oxygen vacancies act as key enablers in Cu–MgO photo-Fenton systems by coupling defect-mediated charge transfer with multivalent Cu redox cycling.⁷⁶ UV photodeposition of Cu on MgO creates rich surface oxygen vacancies, which extend visible-light response, suppress electron–hole recombination, and enhance interfacial charge separation through Schottky junction formation. These vacancies work synergistically with coexisting Cu⁰/Cu⁺/Cu²⁺/Cu³⁺ species to facilitate efficient electron shuttling and sustained H₂O₂ activation, leading to enhanced ·O₂⁻ generation. As a result, 3% Cu–MgO achieves high photo-Fenton efficiency (92.8% RhB removal, k = 0.0436 min⁻¹) over a wide pH range (3.6–10), highlighting oxygen-vacancy engineering as a decisive factor for stabilizing multivalent Cu states and boosting non-ferrous photo-Fenton activity.

e. Interface modulation and heterojunction engineering. Interface engineering is one of the most effective design strategies for improving photo-Fenton catalysis. Well-constructed interfaces accelerate charge separation, extend light absorption, and provide directional transport pathways for electrons and holes. Typical heterojunction architectures include:

• Type-II junctions for stepwise electron–hole migration.

• Z-Scheme systems that retain strong redox potentials during charge transfer.

• S-Scheme heterojunctions providing spatial charge separation via built-in electric fields.

• p–n junctions that generate internal electric fields to accelerate electron extraction.

Carbonaceous matrices such as rGO, CNTs, and biochar contribute additional conductivity and adsorption capacity, while MOFs provide ordered porous structures and abundant active sites. The synergy between redox flexibility of Ce and interfacial charge-transfer pathways allows advanced multi-component composites to achieve high degradation efficiencies even at neutral pH and under natural sunlight, surpassing the limitations of traditional Fe-based systems.

3. Interface engineering strategies for enhanced photo-Fenton activity

Interface engineering plays a pivotal role in enhancing the photocatalytic performance of non-ferrous photo-Fenton catalysts. By rationally tailoring interfacial architectures, charge-carrier dynamics can be optimized, and redox activity can be modulated, leading to more efficient ROS generation. Furthermore, engineered interfaces improve pollutant adsorption, H₂O₂ activation, and structural stability. This section presents a comprehensive overview of the dominant interfacial strategies, beginning with heterojunction construction, followed by integration with carbonaceous supports, defect engineering, and the use of MOF/LDH-derived scaffolds. Different types of Interface Engineering Strategies and their applications are summarized in Table 3.

Table 3 Representative photo-Fenton-like degradation performance of non-ferrous catalysts with different interface-engineering strategies

Interface engineering strategy	Catalyst	Pollutant	Light source	H2O2	Stability & leaching assessment	Efficiency/time	Ref.
Heterojunctions (type II)	Cu2(OH)3F/BiOBr	TC-HCl	300 W Xe lamp 400 ≥ λ	40 mM	5 cycles/89.8% retention/structurally stable	98.8%/60 min	43
	CuCo2S4/Bi2WO6	TC-HCl	300 W metal halide lamp	100 μL, 30 wt%	4 cycles/high activity maintained/crystalline structure preserved (XRD)	80%/60 min	78
	Cu2ZnSnS4/Bi2WO6	RhB	300 W metal halide lamp	—	4 cycles/high activity/crystalline stability (XRD)	94%/50 min	79
	CeO2−x/Bi2MoO6	OFX	5 W visible LED	1.5 mM	5 cycles/nearly unchanged/structurally stable (XRD/XPS); stable redox cycle with low leaching	97.64%/120 min	38
Heterojunctions (Z-scheme)	CPDs/CuBi2O4	CIP	Visible LED lamp 420 ≥ λ	0.5 mL	5 cycles/>79.3% removal/high structural stability (XRD/XPS/TEM/FT-IR confirmed); Z-scheme durability	92.3%/30 min	51
	MnO2/CNK-OH-Mn	TC	300 W Xe lamp	10 mM	—	96.7%/120 min	81
	CDs/CuO/g-C3N4	MB	VIS lamp	5 mM	6 cycles/high retention/excellent universality	100%/40 min	40
	g-C3N4/ZnCo2O4	RhB	40 W visible LED	1 mL (30 wt%)	5 cycles/79% retention/structurally stable (XRD constant)	94%/80 min	49
Heterojunctions (S-scheme)	CuInS2/WO3	TCH	300 W Xe lamp	50 μL	5 cycles/stable activity/structural and compositional integrity (XRD confirmed)	42.7%/6 h	85
	Bi2WO6/CoAl LDH	OTC	300 W Xe lamp 420 ≥ λ	50 mM	4 cycles/88.74% retention/structurally stable (XRD); Co^2+ leaching <1.0 mg L^−1	98.47%/60 min	41
	C3N4x/AgOy@Co1−xBi1−yO7	OTC	100 W tungsten bulb	—	4 cycles/75–78% retention	93%/160 min	83
	MgO/g-C3N4	RhB	30 W xenon lamp	2 mL	—	80%/60 min	84
Cocatalyst & electron mediators	Cu–Bi2O2S	TCH	Visible lamp	30 μL	5 cycles/>50% retention/moderate stability; decline due to Cu nanoparticle peeling and Cu^0 surface oxidation (XPS/SEM confirmed)	77.76%/60 min	46
	CNNS/Ag/Ni heterostructure	Metronidazole	Visible lamp	—	5 cycles/91.7% retention/high structural stability (XRD/SEM/FT-IR confirmed)	92.9%/60 min	94
	Cu2(OH)3F/CQDs-BiVO4	CIP	300 W xenon lamp	0.2 mL	5 cycles/∼90% retention/high structural stability (XRD/XPS)	98.1%/60 min	97
	CuO/g-C3N4	Dimethyl phenol (2,4-DMP)	35 W visible LED	0.2 mM	High stability/leaching significantly suppressed by H2O2 (<0.085 mg L^−1); stabilization via CuO2 intermediate formation	99%/120 min	99
Defect & oxygen vacancy (OV)	CeO2–Ba4TaO8Cl	OFX	5 W LED lamp	4.5 mM	5 cycles/93.69% retention/high structural stability (XRD confirmed)	98.78%/40 min	102
	Cu^0@CuZ	Phenol	Visible lamp	—	High reusability/wide pH tolerance/excellent stability in actual complex wastewater	100%/15 min	104
	Cu–MgO	Rhodamine B (RhB)	Xe lamp 400 ≥ λ	20 mM		92.8%/60 min	76
Porous scaffolds (BC/MOFs/LDHs)	CuOx/MnOy@BC	Metronidazole (MNZ)	300 W Xe lamp	20 mM	5 cycles/85.7% retention/high stability	99.7%/120 min	56
	Bi/Ce-BMOFs	TC	5 W visible LED	10 mM	4 cycles/high retention/structurally and chemically stable (XRD/SEM/XPS confirmed)	83%/60 min	39
	CuNi2Al-LDH	Phenol	30 W xenon lamp	6 mM	4 cycles/high retention/high stability; Cu leaching (0.73 ppm)	100%/30 min	111
Atomically dispersed sites	Cu–C3N4	CIP	30 W xenon lamp	—	5 cycles/∼100% retention/exceptional stability; universal degradation	99%/30 min	112
	CuCN	TC	300 W xenon lamp	80 mM	5 cycles/83.3% retention/high stability; Cu leaching <30 μg L^−1 (ICP confirmed)	93.6%/60 min	114
	Cu-SA/CNH/PF	TC	Visible Xe lamp 420 ≥ λ	160 mM	5 cycles/80.3% retention/excellent stability; Cu leaching <25 μg L^−1 (ICP-MS)	98.1%/30 min	52
Bimetallic/multicomponent	Mn-doped cobalt silicate@diamoite	MO	UV lamp	25 mM	5 cycles/>90% efficiency	95%/90 min	35
	AlMnCuZnBi alloy	MO	White LED lamp	0.5 mM	5 cycles/∼94% retention/high stability; high-entropy effect prevents significant active site leaching	100%/170 min	116
	Co9Se8/Ni3Se4/Cu2Se	RhB	LED lamp	—	—	97.14%/120 min	117

---

3.1 Heterojunction engineering

Heterojunction engineering represents one of the most potent design strategies for advancing non-ferrous photo-Fenton and Fenton-like catalysis. By coupling two semiconductors with complementary band structures, heterojunctions facilitate the efficient spatial separation of photogenerated charge carriers, promote directional electron–hole migration, and minimize recombination losses. This improved charge dynamics directly enhances the redox cycling of active metal centres (Cu⁺/Cu²⁺, Co²⁺/Co³⁺, Mn²⁺/Mn³⁺, Ce³⁺/Ce⁴⁺) and accelerates the generation of reactive oxygen species (ROS), particularly ·OH, ·O₂⁻, and ¹O₂ during H₂O₂ activation. Consequently, heterojunction construction has become indispensable in interface-engineered non-ferrous photo-Fenton systems.

A representative example of this strategy is the design of a Ce₄O₇/Bi₄MoO₉ heterojunction synthesized via molten salt-assisted synthesis strategy.⁵⁹ Transmission electron microscopy (TEM) analysis of this system demonstrated that the Ce₄O₇ nanoparticles were intimately loaded on the surface of Bi₄MoO₉ nanoblocks, creating a compact interface that reduces resistance to charge migration. Photoluminescence (PL) quenching studies revealed a drastic suppression of electron–hole recombination, while electrochemical impedance spectroscopy (EIS) confirmed enhanced interfacial conductivity. These improvements allowed a higher density of electrons to participate in the Ce⁴⁺/Ce³⁺ and Mo⁶⁺/Mo⁵⁺ dual redox cycle, thereby accelerating H₂O₂ reduction. It is important to note that distinguishing between different heterojunction mechanisms, particularly S-scheme and Z-scheme systems, requires rigorous experimental validation. While band structure alignment provides a preliminary indication, definitive identification should be supported by complementary techniques such as in situ or operando X-ray photoelectron spectroscopy (XPS) to probe interfacial charge transfer, electron paramagnetic resonance (EPR) spin-trapping to identify active species, Mott–Schottky analysis for band structure determination, and Kelvin probe or work function measurements to verify Fermi level equilibration and internal electric field formation.

In S-scheme heterojunctions, charge transfer is governed by Fermi level differences and internal electric fields, resulting in selective recombination of low-energy electrons and holes while preserving high redox potential carriers. In contrast, Z-scheme systems involve recombination between photogenerated electrons and holes across the interface, typically mediated by direct contact or electron mediators, while retaining strong oxidative and reductive capabilities. Therefore, in this review, mechanistic assignments are made based on the available experimental evidence, and in cases where such validation is limited, the systems are described more generally as heterojunctions with enhanced charge separation.

Generally, heterojunctions used in these catalytic systems are classified into type-II, Z-scheme, S-scheme, p–n junctions, and Schottky junctions (Fig. 3), each offering distinct charge transfer pathways.

Fig. 3 Schematic diagram representing the transfer of photogenerated electrons and holes for different types of heterojunctions. (a) Type II heterojunction, (b) Z-scheme, (c) S-scheme, (d) p–n heterojunction, and (e) Schottky junction.

a. Type-II heterojunctions. As exhibited in Fig. 3a, type-II heterojunctions form when two semiconductors with staggered band structures are coupled such that the conduction band (CB) and valence band (VB) edges of one material lie at lower energies than those of the other. This configuration directs photogenerated electrons toward the semiconductor with a more negative conduction band, while holes migrate to the component with a more positive valence band. This physical separation across the interface strongly suppresses recombination and increases the lifetime of charge carriers involved in Fenton-like reactions.⁷⁷

Guo et al.⁴³ constructed a type-II Cu₂(OH)₃F/BiOBr heterojunction with highly exposed BiOBr (010) facets, enabling accelerated charge transfer and efficient Cu⁺/Cu²⁺ cycling in the photo-Fenton process. The (010) facet provided fast electron transport channels, allowing photogenerated electrons from BiOBr to migrate to Cu₂(OH)₃F (Fig. 4a), where they rapidly reduced Cu²⁺ and boosted ·OH generation from H₂O₂. Simultaneously, holes accumulated on the BiOBr oxidized surface, OH⁻/H₂O to additional ·OH, while electrons on Cu₂(OH)₃F also reduced O₂ to ·O₂⁻. This dual-pathway ROS generation markedly enhanced the synergistic degradation of TC-HCl, achieving 98.8% removal, 4.2 and 2.5 times higher than photocatalysis and Fenton-like oxidation alone (Fig. 4b). Stable performance over five cycles (Fig. 4c) further confirmed that type-II band alignment and facet-directed charge separation effectively overcame Cu-based recombination limitations, establishing Cu₂(OH)₃F/BiOBr as a robust non-ferrous photo-Fenton catalyst.⁴³ In another report, Wang et al.⁷⁸ constructed a 2D/2D type-II CuCo₂S₄/Bi₂WO₆ heterojunction through an in situ hydrothermal–solvothermal route, enabling intimate interfacial contact between both nanosheet components. The CuCo₂S₄ cocatalyst broadened visible-light absorption and accelerated electron–hole separation, while also mediating Cu²⁺/Cu⁺¹ and Co³⁺/Co²⁺ redox cycling for efficient H₂O₂ activation. As a result, the heterojunction generated abundant ·OH and ·O₂⁻ radicals and delivered markedly enhanced photo-Fenton degradation of tetracycline, with a rate constant nearly 1.8-fold higher than pristine Bi₂WO₆. The catalyst also retained high activity over multiple cycles, confirming the stability of the interfacial charge-transfer pathway.⁷⁸

Fig. 4 (a) A type II heterojunction for charge transfer between Cu₂(OH)₃F and BiOBr in 25% CFO/BOR catalyst, (b) catalyst activity under different situations over 25% CFO/BOR, and (c) stability performance over 5 cycles. Reproduced from ref. 43 with permission from Elsevier, Guo, Y.; Liu, Y.; Hao, S.; Zhang, X.; Yu, Y., J. Water Process Eng., 2023, 55, 104140. Copyright 2023. (d) Schematic diagram for photo-Fenton-like catalytic mechanism of S-scheme heterojunction of Bi₂WO₆/CoAl-LDHs, (e) photocurrent, and (f) EIS study of Bi₂WO₆/CoAl-LDHs-20 nanohybrid. Reproduced from ref. 41 with permission from Elsevier, Shao, B.; Liu, Z.; Tang, L.; Liang, Q.; He, Q.; Wu, T.; Pan, Y.; Cheng, M.; Liu, Y.; Tan, X., Chemosphere, 2022, 291, 133001. Copyright 2021.

Similarly, Guo et al.⁷⁹ synthesized a series of 2D/2D Cu₂ZnSnS₄/Bi₂WO₆ (CZTS/BWO) type-II heterojunctions through a secondary solvothermal growth of ultrathin CZTS nanosheets onto Bi₂WO₆, forming intimate interfacial contact that was confirmed by XPS and HRTEM analysis. The optimized 2 wt% CZTS/BWO composite exhibited markedly enhanced visible-light photo-Fenton activity toward dyes and antibiotics, outperforming pristine BWO due to broadened light absorption, abundant surface active sites, and accelerated charge separation across the type-II interface. The presence of H₂O₂ further boosted degradation efficiency by promoting rapid Cu²⁺/Cu¹⁺ cycling, enabling efficient ROS generation. ESR and radical-trapping studies verified ·O₂⁻ and ·OH as the dominant reactive species. The heterojunction maintained high catalytic stability over four cycles, demonstrating that coupling CZTS with BWO effectively strengthens charge mobility and redox cycling in non-ferrous photo-Fenton systems.⁷⁹

In another study, it was reported that the superior photo-Fenton activity of cerium-rich CeO_2−x/Bi₂MoO₆ originates from the formation of an efficient type-II heterojunction, which promotes directional charge separation and accelerates interfacial redox reactions.³⁸ Due to favourable band alignment, photogenerated electrons migrate from the conduction band of Bi₂MoO₆ to CeO_2−x, while holes transfer from the valence band of CeO_2−x to Bi₂MoO₆, resulting in efficient spatial separation of charge carriers. This directional charge transfer suppresses electron–hole recombination and enables sustained accumulation of electrons on CeO_2−x for rapid Ce³⁺/Ce⁴⁺ self-circulation and H₂O₂ activation, while holes retained on Bi₂MoO₆ drive direct oxidation and ·OH generation. The type-II heterointerface thus couples enhanced photocatalytic charge separation with accelerated Fenton redox cycling, leading to markedly improved antibiotic degradation efficiency under low-power visible light.³⁸

Overall, type-II heterojunctions offer a robust platform for enhancing photo-Fenton-like catalysis by facilitating interfacial electron transfer and suppressing charge recombination, making them particularly attractive for non-ferrous catalytic systems. Their performance can be further strengthened when coupled with auxiliary strategies such as oxygen-vacancy engineering or LDH-based hybridization. Nevertheless, intrinsic limitations remain. The staggered band alignment in type-II heterojunctions inevitably lowers the redox potential of photogenerated electrons and holes, weakening the oxidative and reductive driving forces required for efficient H₂O₂ activation. Moreover, the accumulation of holes on one semiconductor surface can hinder cross-interface charge migration, restricting full utilization of photogenerated carriers. These drawbacks highlight the need for more advanced charge-regulation architectures beyond conventional type-II configurations.

b. Z-Scheme systems. The Z-scheme heterojunction concept originates from the charge-transfer principles observed in photosynthetic systems and is regarded as an advanced alternative to conventional type-II architectures for reactions demanding strong redox capability. Unlike type-II heterojunctions, where charge transfer results in a net loss of redox ability, Z-scheme systems enable the spatial separation of photogenerated electron–hole pairs while preserving the strong reducing and oxidizing capabilities of the respective components. In a typical Z-scheme configuration (specifically the direct Z-scheme, which eliminates liquid redox mediators), two semiconductors are physically coupled as shown in Fig. 3b. When exposed to light, electrons excited in the CB of the oxidation photocatalyst migrate to the interface and recombine with VB holes of the reduction photocatalyst, driven by the band potential difference.⁸⁰

Lu et al.⁵¹ developed a ball-milled CPDs/CuBi₂O₄ Z-scheme heterojunction as an efficient photo-Fenton catalyst for antibiotic wastewater treatment. In this system, carbonized polymer dots (CPDs) served as interfacial mediators to construct a stable Z-scheme junction with CuBi₂O₄, promoting extended light absorption and rapid charge-carrier separation. The optimized 20 wt% CPDs/CuBi₂O₄ composite showed markedly improved performance, achieving 92.3% ciprofloxacin degradation, 36% higher than pristine CuBi₂O₄. ESR and scavenger experiments revealed that ¹O₂, ·OH, h⁺, and ·O₂⁻ were the dominant reactive species generated through the Z-scheme pathway. Overall, this work demonstrates a scalable ball-milling strategy to engineer CPD-modified CuBi₂O₄ heterojunctions with enhanced redox capability for practical photo-Fenton degradation of antibiotics.⁵¹

Zhang et al.⁸¹ constructed a porous Z-scheme MnO₂/Mn-modified alkalinized g-C₃N₄ (MnO₂/CNK-OH-Mn) heterojunction via a calcination–impregnation route. The optimized MnO₂/CNK-OH-Mn-15% delivered markedly higher Fenton-like photocatalytic activity, achieving 96.7% tetracycline degradation and notable TOC removal (74.9%), attributed to synergistic ROS generation. ESR and trapping studies verified ·OH and ·O₂⁻ as dominant species, derived from Z-scheme charge transfer and accelerated Mn⁴⁺/Mn³⁺/Mn²⁺ redox cycling for efficient H₂O₂ activation. The catalyst also maintained high stability over multiple cycles and enabled effective treatment of pharmaceutical wastewater, underscoring the role of Mn-based Z-scheme heterojunctions in enhancing Fenton-like pathways.⁸¹

In another study, the formation of a Z-scheme heterojunction was identified as a key factor enabling the synergistic coupling of photocatalytic processes and Fenton-like redox cycling in CuO/CDs/g-C₃N₄ systems.⁴⁰ In this ternary architecture, carbon dots act as interfacial electron-transfer bridges that reinforce the Z-scheme charge migration, enabling electrons in the conduction band of CuO to recombine with holes in the valence band of g-C₃N₄ while preserving strong redox potentials. This configuration retains highly oxidative holes on CuO for ·OH generation and pollutant oxidation, while electrons accumulated on g-C₃N₄ efficiently reduce Cu²⁺ to Cu⁺, sustaining the Fenton-like cycle. The dual Z-scheme-assisted charge transfer pathways effectively suppress carrier recombination and accelerate Cu²⁺/Cu⁺ conversion, resulting in synergistically enhanced ·OH and ·O₂⁻ production under visible light. These findings highlight Z-scheme interface engineering as a powerful strategy to maximize redox efficiency and catalytic durability in non-ferrous photo-Fenton systems.⁴⁰

Researchers constructed a g-C₃N₄/ZnCo₂O₄ Z-scheme heterojunction, which plays a crucial role in regulating charge separation and redox balance in photo-Fenton-like systems.⁴⁹ Upon visible-light illumination, electrons excited to the conduction band of g-C₃N₄ selectively recombine with valence-band holes of ZnCo₂O₄ via a Z-scheme charge-transfer route, preserving strongly reducing electrons on ZnCo₂O₄ and oxidative holes on g-C₃N₄. This charge-transfer configuration suppresses carrier recombination while enabling efficient Co³⁺/Co²⁺ cycling, which drives the activation of both H₂O₂ and persulfate to generate ·OH and SO₄·⁻ radicals. As a result, the Z-scheme architecture ensures sustained ROS production and enhanced dye degradation under visible light, highlighting its effectiveness in coupling photocatalytic charge dynamics with Fenton-like oxidant activation.⁴⁹

Jia et al.⁵⁵ constructed a Z-scheme MnO_x/g-C₃N₄ photo-electro-Fenton-like system in which polyvalent Mn species (Mn²⁺/Mn³⁺/Mn⁴⁺) served as the core Fenton-like redox centres, fully replacing Fe in H₂O₂ activation. The MnO_x domains were uniformly anchored on g-C₃N₄ nanosheets, forming a solid Z-scheme interface that accelerated electron–hole separation and promoted the simultaneous generation of ·OH, ·O₂⁻, and h⁺ under visible light. The rapid Mn⁴⁺/Mn³⁺/Mn²⁺ conversion cycle, strengthened by abundant oxygen vacancies generated from low-valence Mn, enabled efficient H₂O₂ decomposition and boosted electrocatalytic activity under applied bias. Benefitting from these features, the MnO_x/g-C₃N₄ catalyst achieved 96.2% CIP removal in 120 min with widened pH applicability and excellent durability. This study highlights the indispensable role of Mn multivalence and OV-induced electronic modulation in enabling Fe-free Z-scheme PEF systems with high oxidative efficiency and rapid Mn valence transitions, enabling efficient H₂O₂ activation even under visible light.⁵⁵

Yang et al.⁴² constructed a CuO_x·Ag₂O/RGO multiphase heterostructure via electrochemical post-treatment of a bimetallic ICP/RGO precursor, in which the intimate CuO_x–Ag₂O coupling on conductive RGO established a direct Z-scheme charge transfer pathway. In this configuration, photogenerated electrons with strong reducing power and holes with high oxidation potential are spatially retained on their respective components, avoiding the redox loss typical of type-II junctions. This Z-scheme-driven charge migration not only accelerates electron–hole separation but also sustains efficient Cu-based redox cycling and H₂O₂ activation, leading to robust ·OH generation during photo-Fenton degradation. The study highlights that Z-scheme architectures are highly advantageous for integrating bimetallic oxides with carbon mediators, enabling stable, recyclable, and high-efficiency photo-Fenton catalysis under visible light.

It was reported that the Z-scheme p–n heterojunction is critical for maintaining redox capability while enhancing charge separation in Prussian blue (PB)/Mn₃O₄ photo-Fenton systems.⁴⁸ In the p-Mn₃O₄/n-PB heterostructure, the built-in electric field drives Z-scheme charge migration, where photogenerated electrons in the conduction band of PB recombine with holes in the valence band of Mn₃O₄. This process suppresses carrier recombination while retaining highly oxidative holes on PB and strongly reductive electrons on Mn₃O₄, enabling simultaneous ·OH and ·O₂⁻ generation. Such charge regulation also facilitates efficient Fe³⁺/Fe²⁺ and Mn⁴⁺/Mn³⁺/Mn²⁺ redox cycling, sustaining H₂O₂ activation and favouring a non-radical pathway dominated by ¹O₂. Consequently, the Z-scheme architecture enhances photo-Fenton efficiency, stability, and resistance to environmental interference, underscoring its importance in advanced non-ferrous photo-Fenton catalyst design.

c. S scheme systems. While the direct Z-scheme model effectively describes the vectoral charge transfer pathway, it often lacks a rigorous physical explanation regarding the driving forces at the atomic level. To address this, the S-scheme (step-scheme) mechanism was recently proposed to clarify the charge transfer dynamics based on Fermi level differences and work functions. This design preserves the high redox potential of the photogenerated carriers while simultaneously achieving spatial separation, making it exceptionally effective for driving the demanding redox cycles in non-ferrous photo-Fenton catalysis. Structurally, an S-scheme heterojunction is composed of a reduction photocatalyst (RP) with a high conduction band (CB) position and an oxidation photocatalyst (OP) with a deep valence band (VB) as shown in Fig. 3c. When these materials come into contact, electrons flow from the material with the higher Fermi level to the one with the lower Fermi level until equilibrium is reached. This creates an electron-depletion layer and an electron-accumulation layer, resulting in a built-in internal electric field (IEF) at the interface. Under irradiation, the IEF and band bending drive the “useless” electrons (those in the lower CB of the OP) to recombine with the “useless” holes (those in the higher VB of the RP). Consequently, the photogenerated electrons with the highest reducing power are retained in the CB of the RP, and the holes with the highest oxidizing power are retained in the VB of the OP.⁸²

For non-ferrous photo-Fenton systems, this mechanism is vital. For instance, Panda et al.⁵³ synthesized AgAu–C/N–CeO₂@C/N–CuO S-scheme nanohybrids from a Ce/Cu-MOF precursor via sequential hydrothermal growth, calcination, and photoreduction. The MOF-derived 1D/2D C/N–CeO₂/C/N–CuO heterostructure provided abundant surface defects and intimate interfacial contact, while plasmonic AgAu nanoparticles were uniformly anchored onto the composite. Comprehensive XRD, XPS, TEM, ESR, and trapping experiments confirmed the formation of a compact S-scheme interface that preserved strong redox potentials and enhanced exciton separation. Owing to the synergistic contributions of LSPR-induced hot-electron injection, defect-mediated charge trapping, and accelerated S-scheme charge transfer, the catalyst delivered a remarkable H₂O₂ production rate (3289.3 μmol g⁻¹ h⁻¹) and efficient in situ photo-Fenton degradation of ofloxacin (91% in 1 h), maintaining excellent photostability across multiple cycles.⁵³

In another work, Shao et al.⁴¹ constructed hortensia-like Bi₂WO₆/CoAl-LDHs nanohybrids via a hydrothermal route to engineer an S-scheme heterojunction for strengthened photo-Fenton-like activity. The intimate Bi₂WO₆/CoAl-LDH interface established a robust internal electric field that accelerated directional charge migration while preserving strong oxidizing holes on Bi₂WO₆ and reducing electrons on CoAl-LDHs (Fig. 4d). The photocurrent and EIS results (Fig. 4e and f) suggested that the existence of heterojunction of Bi₂WO₆/CoAl-LDHs-20 nanohybrid, which was benefit for the electron transfer process, thus effectively decreased the recombination of photoexcited e⁻–h⁺ pairs and promoted the H₂O₂ activation, enabling rapid ·OH formation. Benefiting from this S-scheme charge-transfer pathway and cobalt-mediated Fenton-like reaction, the composite achieved 98.47% oxytetracycline removal under visible light and H₂O₂, significantly outperforming the individual components. DFT calculations confirmed improved H₂O₂ adsorption and optimized band alignment as key contributors to its superior photo-Fenton-like performance.

Kumar et al.⁸³ synthesized a g-C₃N_4x/AgO_y@Co_1−xBi_0.1−yO₇ composite, in which the S-scheme heterojunction plays an important role in preserving redox potential while suppressing charge recombination in photo-Fenton catalysis. Unlike conventional type-II or Z-scheme pathways that compromise reduction or oxidation capability, the S-scheme configuration enables selective recombination of low-energy charge carriers while retaining highly oxidative holes on g-C₃N₄ and strongly reductive electrons on the cobalt–bismuth oxide component. This charge-selective migration maintains a continuous supply of high-energy electrons and holes under visible light, promoting efficient O₂ reduction to ·O₂⁻ and subsequent H₂O₂/·OH generation. As a result, the S-scheme heterointerface synergistically enhances ROS production and Fenton-assisted oxidation, leading to high degradation efficiencies of methylene blue and oxytetracycline under neutral pH with improved stability and reusability.

Researchers reported that the formation of an S-scheme heterojunction is essential for maintaining strong redox capability and promoting efficient charge migration in 2D/2D MgO/g-C₃N₄ photo-Fenton catalysts.⁸⁴ Owing to the large work-function difference between MgO and g-C₃N₄, an internal electric field is established at the tightly bonded MgO/g-C₃N₄ interface, which promotes the recombination of energetically unfavourable charge carriers while preserving highly reductive electrons on MgO and strongly oxidative holes on g-C₃N₄. This S-scheme charge regulation effectively suppresses electron–hole recombination and sustains high redox potential for ROS generation. Notably, the formation of interfacial Mg–N bonds strengthen the S-scheme contact, accelerates interfacial charge transfer, and simultaneously induces oxygen vacancies in MgO, which further promote H₂O₂ activation and ·OH formation. The synergistic coupling of S-scheme charge migration and oxygen-vacancy-enhanced Fenton-like reactions enables markedly improved pollutant degradation under visible light.

Wang et al.⁸⁵ synthesized S-scheme CuInS₂/WO₃ hybrid heterostructures, in which the S-scheme heterojunction plays a key role in maintaining strong redox capability while enabling efficient charge separation during photo-Fenton degradation. Owing to the Fermi-level difference between p-type CuInS₂ and n-type WO₃, an internal electric field is established at the interface, driving selective recombination of low-energy electrons in WO₃ with holes in CuInS₂. This S-scheme charge migration preserves highly reductive electrons on CuInS₂ and strongly oxidative holes on WO₃, which are essential for generating ·O₂⁻ and ·OH radicals, respectively. Consequently, the S-scheme architecture suppresses charge recombination while sustaining efficient photo-Fenton redox reactions, leading to enhanced tetracycline degradation under visible light.

S-scheme and Z-scheme heterojunctions, although often used interchangeably in the literature, are fundamentally distinct in their charge transfer mechanisms. In S-scheme systems, charge migration is governed by Fermi level equilibration and the formation of an internal electric field, which drives the recombination of low-energy electrons and holes while preserving high-energy charge carriers with strong redox potential. In contrast, Z-scheme heterojunctions involve the recombination of photogenerated electrons and holes across the interface, either through direct contact or via an electron mediator, while maintaining strong oxidative and reductive capabilities on the respective semiconductors. Therefore, careful differentiation between these mechanisms is necessary when interpreting photocatalytic behaviour.

d. p–n junctions. The construction of p–n junctions represents a distinct heterojunction strategy that exploits the contrasting electronic characteristics of p-type (hole-rich) and n-type (electron-rich) semiconductors. Unlike conventional heterojunctions, where charge separation is driven primarily by band-edge offsets, p–n junctions operate through the formation of a strong internal electric field (IEF) created by carrier diffusion. When a p-type material contacts an n-type counterpart, majority carriers, holes from the p-side and electrons from the n-side, diffuse across the interface until their Fermi levels align (Fig. 3d). This diffusion produces a depletion layer and a built-in electric field directed from the n-region toward the p-region.⁸⁶ Under illumination, this internal field acts as an efficient driving force for rapid electron–hole separation: electrons drift toward the n-type conduction band, while holes migrate toward the p-type valence band. Such field-assisted separation markedly increases carrier lifetimes and accelerates interfacial charge transport. This behaviour is particularly advantageous in non-ferrous photo-Fenton and Fenton-like systems, where prolonged carrier lifetimes are crucial for sustaining multistep H₂O₂ activation and ROS generation.

Wang et al.⁸⁷ synthesised a Cu₂O/Bi₂MoO₆ (CBM) p–n heterojunction, in which the tightly coupled interface generated a strong built-in electric field that directed photogenerated electrons from p-Cu₂O to n-Bi₂MoO₆. This interfacial field greatly enhanced charge separation and created an efficient pathway for electron-driven H₂O₂ activation, producing abundant ·OH through both photoinduced reduction and accelerated Cu²⁺/Cu⁺ cycling. The optimized CBM-25% catalyst leveraged this engineered p–n interface to deliver nearly complete ciprofloxacin degradation within 120 min, far outperforming single-component catalysts, and retained >92% efficiency over multiple cycles, demonstrating the critical role of p-n junction interface design in boosting non-iron photo-Fenton-like reactivity.

Shi et al.⁸⁸ constructed a Bi₄Ti₃O₁₂/CuBi₂O₄ p–n heterojunction piezo-photocatalyst that markedly enhanced photo-Fenton degradation of dyes and antibiotics. The intimate n-Bi₄Ti₃O₁₂/p-CuBi₂O₄ interface generated a strong built-in electric field, while piezoelectric polarisation further intensified carrier drift under ultrasonic excitation, jointly suppressing recombination. This interfacial driving force enabled rapid electron migration to Bi₄Ti₃O₁₂ and hole accumulation in CuBi₂O₄, thereby accelerating Cu²⁺/Cu¹⁺ cycling and H₂O₂ activation. As a result, the composite achieved superior degradation of MB, LEV, and OTC, outperforming single-component catalysts and maintaining high stability. This work highlights how robust p–n junctions coupled with piezoelectric polarization can dramatically boost charge separation and Fenton-like reactivity.⁸⁸

Researchers developed a Cu₂(OH)₃F/Cu₂O p–n heterojunction that plays a critical role in accelerating charge separation and facilitating the Cu²⁺/Cu⁺ redox cycle in photo-Fenton-like systems.⁸⁹ In this n-type Cu₂(OH)₃F and p-type Cu₂O composite, the built-in electric field at the p–n interface drives directional migration of photogenerated electrons toward Cu₂(OH)₃F and holes toward Cu₂O, effectively suppressing electron–hole recombination. This charge redistribution enriches Cu⁺ species and facilitates rapid Cu²⁺/Cu⁺ interconversion, thereby enhancing H₂O₂ activation and continuous ·OH and ·O₂⁻ generation. Consequently, the p–n heterojunction synergistically couples efficient photocatalytic charge transport with Fenton-like redox kinetics, leading to near-complete tetracycline degradation and improved stability over a wide pH range.

Researchers reported that the Cu_2−xS/g-C₃N₄ system forms a p–n heterojunction in which the internal electric field promotes efficient charge separation, while redox preservation plays a secondary role.⁴⁷ Mott–Schottky analysis confirms the formation of a p–n junction between p-type Cu_2−xS and n-type g-C₃N₄, which promotes directional migration of photogenerated electrons and holes and effectively suppresses charge-carrier recombination. This interfacial electric field stabilizes Cu²⁺/Cu⁺ cycling and enables complementary enhancement between photocatalysis and Fenton-like reactions, even though the band structure of Cu_2−xS alone is not highly favourable for ROS generation. Consequently, the p–n heterojunction mainly functions as a charge-regulation and redox-stabilization interface, supporting efficient ·O₂⁻-dominated degradation and high TOC removal under photo-Fenton conditions.

Guo et al.⁹⁰ synthesized a 2D in-plane CuS/Bi₂WO₆ p–n heterojunction, which plays a key role in enhancing charge separation and stabilizing Cu-based photo-Fenton activity. The intimate coupling of p-type CuS with n-type Bi₂WO₆ establishes an internal electric field that promotes directional migration of photogenerated electrons and holes, thereby suppressing carrier recombination under visible light. This charge redistribution enables efficient electron transfer from Bi₂WO₆ to CuS, facilitating H₂O₂ activation and sustaining Cu²⁺/Cu⁺ cycling, while holes retained on Bi₂WO₆ directly participate in pollutant oxidation. Consequently, the p–n heterointerface synergistically couples photocatalytic charge regulation with heterogeneous Fenton reactions, resulting in rapid degradation, improved stability, and excellent reusability.

e. Schottky junctions. As exhibited in Fig. 3e, Schottky junctions arise at metal–semiconductor interfaces due to differences in Fermi levels and work functions, leading to the formation of a Schottky barrier and an interfacial space-charge region. In such systems, metallic cocatalysts serve both as electron sinks and as active reaction sites. Under light irradiation, photogenerated electrons in the semiconductor conduction band are preferentially transferred to the cocatalyst with a lower Fermi level, while holes remain in the semiconductor, resulting in efficient charge separation and suppressed recombination. This directional electron migration is particularly advantageous in non-ferrous photo-Fenton-like catalysis, as the accumulated electrons facilitate continuous redox cycling of active metal species and promote H₂O₂ activation. Consequently, Schottky interface engineering provides an effective strategy for enhancing charge-transfer kinetics and overall photocatalytic efficiency.⁹¹

Zhong et al.⁹² reported in situ formed Ti³⁺-self-doped TiO₂/Ti₃C₂ MXene Schottky heterojunctions exhibiting highly efficient photo-Fenton-like degradation of cationic dyes over a wide pH range (3–10). The Schottky barrier arising from the work-function mismatch between TiO₂ and metallic Ti₃C₂ promoted directional charge separation, enabling effective utilization of photogenerated electrons and holes for H₂O₂ activation. Negligible degradation was observed without light or H₂O₂, whereas nearly complete dye removal (∼99%) occurred under visible light in the Ti₃C₂/H₂O₂ system, confirming a synergistic photo-Fenton pathway. Radical quenching, EPR, and DFT analyses revealed that the Schottky interface not only enhanced carrier separation but also weakened –OH adsorption, facilitating H₂O₂ decomposition and boosting ·OH and ·O₂⁻ generation. This work underscores the dual role of Schottky junctions in regulating both charge-transfer dynamics and surface reaction kinetics in non-ferrous photo-Fenton-like systems.

In another study, it was reported that the metal semiconductor Schottky junction is central to efficient charge separation and accelerated H₂O₂ activation, leading to enhanced photo-Fenton activity in Ag/g-C₃N₄–H₂O₂ systems.⁹³ Deposited Ag nanoparticles act as electron sinks due to their lower Fermi level, establishing a Schottky barrier at the Ag/g-C₃N₄ interface that promotes directional transfer of photogenerated electrons from g-C₃N₄ to Ag. This interfacial electron trapping effectively inhibits charge recombination and extends carrier lifetimes, allowing a greater fraction of electrons to participate in H₂O₂ and O₂ reduction to generate ·OH and ·O₂⁻ radicals. Simultaneously, the retained holes on g-C₃N₄ contribute to direct oxidation pathways. As a result, the Schottky junction synergistically couples photocatalytic charge regulation with oxidant activation, leading to accelerated ROS production, enhanced degradation kinetics, and improved stability in photo-Fenton-like dye removal.

3.2 Cocatalyst and electron mediator incorporation

Cocatalyst incorporation is an effective strategy for accelerating charge transfer, suppressing recombination, and promoting H₂O₂ activation in non-ferrous photo-Fenton systems. By integrating conductive nanoparticles or carbonaceous electron mediators, the interfacial electron mobility is significantly enhanced, enabling more efficient ROS generation during visible-light irradiation.

a. Noble and transition metal nanoparticles. Metallic nanoparticles such as Cu⁰, Ag⁰, Au⁰, and Pd⁰ are frequently employed as cocatalysts due to their high electrical conductivity and strong electron-capturing capability. When deposited onto semiconductor surfaces, they form Schottky junctions that facilitate directional electron trapping. This process serves two major functions:

• Suppressing photogenerated electron–hole recombination, thereby extending charge-carrier lifetimes.

• Enhancing H₂O₂ activation through accelerated electron transfer to catalytic sites.

For instance, Xiong et al.⁹⁴ developed a CNNS/Ni–Ag heterostructure in which dual-metal cocatalysts were interfacially anchored to regulate charge flow and enable sacrificial-agent-free H₂O₂ production and self-photo-Fenton degradation. The bimetallic interface generated a strong built-in electric field that directed photogenerated electrons from CNNS and LSPR-derived hot electrons from Ag toward Ni centres, markedly suppressing recombination. Ni acted as the primary electron mediator, optimizing O₂ adsorption and the two-electron ORR pathway, while Ag broadened visible-light absorption. This engineered interface delivered a high H₂O₂ production rate (1541.3 μmol g⁻¹ h⁻¹ with IPA) and enabled efficient in situ Fenton degradation of metronidazole (92.9% in 60 min), demonstrating the power of dual-metal synergy and interfacial electric-field tuning in boosting non-ferrous Fenton-like catalysis.⁹⁴

Dihingia et al.⁹⁵ green-synthesized a bimetallic (Cu⁰ + Ag⁰)@bentonite heterojunction using Psidium guajava leaf extract to anchor zero-valent Cu and Ag nanoparticles onto bentonite. The resulting Cu–Ag interface lowered the band gap (2.32 eV), enhanced visible-light absorption, and promoted efficient ·OH- and peroxy-radical generation under photo-Fenton-like conditions. The catalyst achieved 84% AMX and 74% SMZ degradation (UV-A), with significant mineralisation and minimal Cu²⁺ leaching, and retained stability over repeated cycles. The study demonstrates that a simple bio-assisted route can yield a robust Cu–Ag heterojunction with strong antibiotic-removal capability.⁹⁵

Guo et al.⁴⁶ reported a Cu nanoparticle-modified Bi₂O₂S nanosheet, where surface-anchored Cu nanoparticles act as electron sinks, rapidly extracting photogenerated electrons from the semiconductor and thereby suppressing electron–hole recombination. This interfacial charge separation facilitates efficient reduction of Cu²⁺ to Cu⁺, enabling continuous Cu²⁺/Cu⁺ redox cycling that is essential for sustained H₂O₂ activation. Meanwhile, Cu⁺ directly catalyses H₂O₂ decomposition to generate abundant ·OH radicals, while excess electrons reduce dissolved O₂ to ·O₂⁻, jointly accelerating pollutant mineralization. Importantly, the noble-metal-free nature of Cu nanoparticles offers a cost-effective alternative to Au or Ag cocatalysts while retaining key functionalities such as enhanced charge mobility, increased surface adsorption, and durable redox mediation. This dual role electron mediator and Fenton active centre-highlights metal nanoparticles as pivotal components for boosting both efficiency and stability in advanced photo-Fenton processes.

In the Cu-decorated C@Bi/Bi₂MoO₆ heterojunction reported by Hu et al.,⁹⁶ metallic Cu and Bi nanoparticles act as plasmonic and redox-active centres that enhance visible-light absorption through surface plasmon resonance (SPR) while simultaneously promoting interfacial charge separation. The SPR-excited electrons generated on metallic Bi nanoparticles are efficiently transferred to dissolved O₂, producing reactive intermediates (·O₂H and ¹O₂), whereas Cu species facilitate H₂O₂ activation to yield abundant ·OH radicals. Meanwhile, the carbon-anchored metal nanoparticles serve as electron mediators, suppressing electron–hole recombination and sustaining rapid metal redox cycling during the Fenton process. This dual functionality—plasmon-induced charge generation and metal-assisted oxidant activation-highlights how noble and transition metal nanoparticles synergistically bridge photocatalysis and Fenton chemistry, enabling ultrafast pollutant degradation under mild conditions.

b. Carbon-based mediators. Carbonaceous materials, including reduced graphene oxide (rGO), carbon nanotubes (CNTs), carbon quantum dots (CQDs), and biochar, serve as highly conductive electron mediators due to their delocalized Π electron systems and large surface areas. Their advantages include:

• Rapid electron transport through conjugated networks.

• Improved H₂O₂ reduction kinetics.

• Enhanced pollutant adsorption through Π–Π interactions.

• Stabilization of metal species through strong interfacial bonding.

Zhang et al.⁹⁷ engineered a Cu₂(OH)₃F/CQDs–BiVO₄ photo-Fenton-like catalyst in which 0D carbon quantum dots acted as an electron mediator between BiVO₄ and Cu₂(OH)₃F, enabling rapid and directional charge shuttling (Fig. 5a). As shown in Fig. 5(b–e), SEM analyses reveal decahedral BiVO₄ crystals uniformly decorated with Cu₂(OH)₃F nanosheets confirming successful construction of a hierarchical heterointerface. Crystal-facet tuning of BiVO₄ (highly exposed (010) facet) enhanced its electron-donor capability, while CQDs bridged electron transfer and regenerated Cu⁺, thereby strengthening H₂O₂ activation. Benefiting from accelerated carrier separation and intensified ROS generation, the CQDs-bridged heterojunction achieved 98.1% ciprofloxacin removal within 1 h, maintained stable activity over pH 3–11, as shown in Fig. 5f, and exhibited excellent structural stability across five cycles. This work highlights how electron mediators and cocatalyst interfaces synergistically overcome recombination limits and significantly boost non-ferrous photo-Fenton efficiency.

Fig. 5 (a) A schematic illustration showing carbon quantum dots acting as an electron mediator between BiVO₄ and Cu₂(OH)₃F in CFO/CQDs (3)-BVO, SEM images of Cu₂(OH)₃F (b), BiVO₄ (c), CQDs (3)-BVO (d), and CFO/CQDs (3)-BVO (e), and (f) stability performance over wide pH of 3–11. Reproduced from ref. 97 with permission from Elsevier, Zhang, X.; Liu, Y.; Zhai, Y.; Yu, Y.; Guo, Y.; Hao, S., Environ. Res., 2023, 222, 115347. Copyright 2023. (g) Proposed mechanism for CR and DY degradation in Cu-N/C-500/H₂O₂ system, (h) effect of different reactive species scavengers on the degradation efficiencies of DY, and (i) degradation efficiencies of DY under different catalytic conditions. Reproduced from ref. 98 with permission from Elsevier, Tan, Z.; Wang, R.; Yang, S.; Shi, Z.; Wang, D., J. Mol. Struct., 2025, 1337, 142248. Copyright 2025.

N-doped carbon matrices derived from MOF precursors function as active carbon mediators rather than inert supports in photo-Fenton systems by regulating electron transfer, stabilizing metal redox sites, and enhancing H₂O₂ activation. In the Cu–Cu₂O/N-doped carbon catalyst reported by Tan et al.,⁹⁸ the graphitized N-doped carbon framework serves as an efficient electron-transfer highway, as shown in Fig. 5g, facilitating rapid migration of photogenerated electrons from Cu/Cu₂O heterojunctions to adsorbed H₂O₂ molecules. Pyridinic- and graphitic-N sites introduce additional electron-rich centres that lower the activation barrier for H₂O₂ decomposition, thereby accelerating ·OH generation. Consistently, the addition of tert-butanol (TBA) suppressed DY degradation by 73.5%, confirming ·OH as the dominant reactive species (Fig. 5h). Meanwhile, the carbon matrix spatially confines Cu/Cu₂O domains, suppressing metal aggregation and leaching while maintaining stable Cu²⁺/Cu⁺ redox cycling under visible light. As a result, the optimized Cu–N–C-500 catalyst achieved 93.3% DY removal within 30 min under visible light, with complete degradation observed by 45 min, as shown in Fig. 5i, showing the best performance in the photo-Fenton catalytic system. This work highlights that carbon mediators play a decisive role in coupling photocatalysis with Fenton chemistry by synchronizing charge transport, redox stability, and oxidant activation.

Metal-free platforms can also act as electron mediators; Taha et al.¹⁰⁰ synthesized N-doped porous carbon nanofibers with abundant pyridinic-N/graphitic-N sites that efficiently activated H₂O₂ and mediated multi-electron transfer pathways. Their high conductivity and porous 1D architecture enabled a solar-Fenton-like reaction active across pH 3–11, without metal leaching.¹⁰⁰

Carbon-based semiconductors such as graphitic carbon nitride (g-C₃N₄) play a dual mediator role in Cu-based photo-Fenton systems by simultaneously facilitating charge transport and stabilizing metal redox cycling. In the CuO/g-C₃N₄/H₂O₂ system reported by Sharma et al., g-C₃N₄ acts as an efficient electron reservoir under visible-light irradiation, promoting rapid transfer of photogenerated electrons to CuO and thereby accelerating the Cu²⁺/Cu⁺ redox cycle essential for H₂O₂ activation. This interfacial electron mediation suppresses charge recombination, enhances ·OH generation, and significantly improves pollutant degradation efficiency. Moreover, the strong CuO–g-C₃N₄ interaction stabilizes surface Cu species, effectively minimizing Cu²⁺ leaching during repeated photo-Fenton cycles. These findings highlight that carbon mediators do not merely serve as photocatalytic supports but actively regulate electron flow, oxidant activation, and catalyst durability in non-ferrous photo-Fenton-like systems.

Carbon-based mediators such as reduced graphene oxide (RGO) play a pivotal role in photo-Fenton systems by acting as conductive charge bridges and structural stabilizers at multicomponent interfaces. Yang et al.⁴² fabricated an ultra-fine Z-scheme CuO_x·Ag₂O/RGO heterostructure via electrochemical post-treatment of a bimetallic ICP/RGO precursor, in which RGO provided a highly conductive platform for rapid electron transport and interfacial charge redistribution. The exposed oxygenated functional groups and defect sites on RGO facilitated intimate anchoring of CuO_x and Ag₂O nanoparticles while suppressing aggregation, thereby enhancing catalyst stability and recyclability. More importantly, rGO functioned as an efficient electron shuttle, facilitating Z-scheme charge separation while preserving the redox reactivity of photogenerated carriers, thereby sustaining Cu-based redox cycling and effective H₂O₂ activation during the photo-Fenton process. This study highlights that integrating metal oxides with carbon mediators such as RGO is an effective strategy to couple efficient charge transport, interfacial stability, and high photo-Fenton activity under visible light.

Carbon-based architectures such as active carbon fibres (ACFs) play a multifunctional mediator role in advanced photo-Fenton-like systems by simultaneously regulating electron transfer, reactant enrichment, and catalyst dispersion. In the Mn₃O₄@ZnO/ACFs micromotor system reported by Cui et al.,¹⁰¹ the 3D hierarchical ACF framework acts as an efficient electron-transfer bridge, promoting rapid migration of photogenerated electrons from the semiconductor heterojunction to Mn-based Fenton-active sites, thereby accelerating H₂O₂ activation and ·OH generation. In parallel, the high surface area and porous nature of ACFs enhance pollutant adsorption, increasing local reactant concentration near reactive oxygen species. Importantly, ACFs stabilize and uniformly disperse Mn₃O₄@ZnO heterostructures, maximizing accessible active sites and light utilization. These functions collectively demonstrate that carbon mediators extend beyond passive supports, actively governing charge flow, ROS formation, and catalytic efficiency in non-ferrous photo-Fenton systems. These carbon-based mediators not only enhance charge-separation efficiency but also increase the structural durability of non-ferrous catalysts, enabling stable operation under continuous photo-Fenton cycles.

3.3 Oxygen vacancy and defect engineering at interfaces

Oxygen vacancies (OVs) and defect-rich surfaces are powerful tools for enhancing interfacial charge dynamics and catalytic reactivity in non-ferrous photo-Fenton systems. OVs, in particular, function as powerful catalytic hotspots by modulating the local electronic and coordination environment.

Their primary functions at the interface are to:

• Act as electron-rich centres that can trap photogenerated electrons, suppressing e⁻ and h⁺ recombination.

• Modulate the electronic structure by creating mid-gap states, which often extends light absorption into the visible region.

• Facilitate H₂O₂ adsorption and dissociation by altering surface polarity and providing active binding sites.

• Modulate the Fermi level at the interface, lowering the activation energy for subsequent redox reactions.

Guo et al.¹⁰² constructed a CeO₂/Bi₄TaO₈Cl (CE-BTC) heterojunction via a molten-salt-mediated defect-engineering route, yielding highly dispersed CeO₂ nanodots and abundant oxygen vacancies (OVs) without damaging the perovskite morphology. The coexistence of OVs and the Ce⁴⁺/Ce³⁺ redox pair markedly strengthened charge separation and H₂O₂ activation, enabling a 7.78-fold higher photo-Fenton degradation rate of ofloxacin compared to pristine Bi₄TaO₈Cl. OVs served as crucial electron-trapping and activation centres, boosting ·OH and ·O₂⁻ generation while facilitating rapid Ce³⁺/Ce⁴⁺ cycling. The CE-BTC catalyst also degraded multiple antibiotics with excellent stability, demonstrating OV-driven enhancement as the key factor governing its superior photo-Fenton performance.¹⁰²

Zhang et al.¹⁰³ developed an aeration-free in situ Fenton-like system by integrating TiO_2−x/W₁₈O₄₉ heterophase oxygen-vacancy structures with floating rGO-modified polyurethane foam, enabling continuous solar-driven H₂O₂ production (4.2 ppm h⁻¹) and >90% antibiotic removal. In this catalyst, interfacial oxygen vacancies act as powerful electron mediators, tuning the d-band centre to selectively adsorb and activate O₂, weakening the O–O bond, and accelerate the hydrogenation of ·O₂⁻/·OOH intermediates. These vacancies also enhance spontaneous H₂O₂ adsorption and ROS formation, thereby eliminating the need for external aeration. The work highlights oxygen-vacancy engineering as a critical cocatalytic pathway for enabling self-sustained Fenton-like reactions with drastically reduced energy inputs.¹⁰³

Gao et al.⁸⁴ constructed a 2D/2D MgO/g-C₃N₄ S-scheme heterojunction in which MgO nanosheets were in situ anchored onto g-C₃N₄ through strong Mg–N interfacial bonds. These chemical bonds not only created a tightly coupled S-scheme interface that accelerated directional charge migration but also induced abundant oxygen vacancies within MgO, boosting H₂O₂ activation and ROS generation (Fig. 6a). UV–vis DRS revealed enhanced visible-light absorption (250–450 nm) (Fig. 6b), while suppressed PL intensity and reduced EIS charge-transfer resistance confirmed efficient carrier separation and interfacial electron migration driven by Mg–N bonding, collectively accounting for the superior photo-Fenton degradation performance (Fig. 6c and d). Consequently, the optimized heterojunction achieved markedly higher photo-Fenton degradation efficiency (80% RhB removal) compared to pristine g-C₃N₄ (28%) and MgO (56%), demonstrating how vacancy engineering combined with interfacial S-scheme charge transfer significantly enhances photocatalytic performance.⁸⁴

Fig. 6 (a) A schematic illustration showing oxygen vacancy mediated photo-Fenton degradation of Rhodamine by the Mg–CN, (b) UV–visible diffuse-reflectance spectra, (c) photoluminescence spectra, (d) electrochemical impedance spectra of MgO, g-C₃N₄, Mg–CN, and PM Mg–CN. Reproduced from ref. 84 with permission from Elsevier, Gao, M.; Li, Z.; Su, X.; Zhang, X.; Chang, J.; Geng, D.; Lu, Y.; Zhang, H.; Wei, T.; Feng, J., Chemosphere, 2023, 343, 140285. Copyright 2023. (e) High resolution XPS spectra of Ce 3d of 10Bi–CeO₂ photocatalyst dark and light reaction, (f) PL spectra of CeO₂ and Bi–CeO₂, (g) transient photocurrent response of CeO₂ and 10Bi–CeO₂. (h) EIS Nyquist plots of CeO₂ and 10Bi–CeO₂. Reproduced from ref. 39 with permission from Elsevier, Zhang, X.; Zhang, L.; Ye, H.; Zeng, Z.; Yang, J.; Han, C.; Liang, Y., J. Environ. Chem. Eng., 2025, 117489. Copyright 2025.

In the Cu⁰@Cu-zeolite system reported by Zhang et al.,¹⁰⁴ V_O synergistically cooperate with plasmonic Cu⁰ nanoparticles to enhance visible-light harvesting and accelerate charge utilization. The vacancy sites preferentially trap photogenerated electrons and facilitate their directional transfer to Cu²⁺ species, thereby expediting the Cu²⁺/Cu⁺ redox cycle that governs Fenton-like activity. Simultaneously, oxygen vacancies lower the activation energy for H₂O₂ adsorption and, if recovery and dissociation, promote sustained ·OH generation under visible light. This dual-channel electron-transfer pathway, mediated by V_O and Cu⁰, ensures rapid redox cycling, suppresses charge recombination, and underpins the exceptionally high degradation kinetics observed. These findings highlight oxygen vacancy engineering as a powerful and generalizable strategy for boosting non-ferrous photo-Fenton efficiency beyond conventional metal-centre control.

In Co/Ni co-doped ZnO nanorods, aliovalent substitution of Zn²⁺ by Co²⁺ and Ni²⁺ induces lattice distortion and defect levels that stabilize surface oxygen vacancies (V_O) rather than allowing their rapid annihilation during photocatalysis.¹⁰⁵ These vacancies act as electron-trapping centres that suppress electron–hole recombination and facilitate directional transfer of photogenerated electrons toward surface-adsorbed H₂O₂, thereby accelerating ·OH radical generation. Simultaneously, V_O introduce sub-bandgap states that extend visible-light absorption and sustain charge excitation under mild irradiation. The stabilized defect environment enables continuous ROS production and preserves catalytic integrity over repeated cycles, demonstrating that defect-engineered ZnO systems can overcome the intrinsic instability of vacancy-rich oxides in photo-Fenton reactions. This work highlights oxygen-vacancy stabilization, rather than mere vacancy creation, as a key design principle for long-term, neutral-pH photo-Fenton catalysis.

Oxygen vacancies play a decisive role in amplifying Ce-based photo-Fenton activity by coupling redox cycling with charge-carrier regulation. In Bi-doped CeO₂ nanorods derived from bimetallic-organic frameworks, aliovalent Bi³⁺ incorporation induces a high density of adaptive oxygen vacancies without phase segregation, even at high doping levels.³⁹ These vacancies act as electron-trapping centres that suppress electron–hole recombination and promote directional electron transfer toward surface-adsorbed H₂O₂, thereby accelerating ·OH and ·O₂⁻ generation. In situ Ce 3d XPS fitting (Fig. 6e) revealed a markedly higher light-induced Ce³⁺ fraction in 10Bi–CeO₂ (5.90%) than in pristine CeO₂ (3.02%), indicating that oxygen-vacancy-enhanced visible-light excitation promotes efficient Ce⁴⁺ → Ce³⁺ reduction and sustains continuous Ce³⁺/Ce⁴⁺ redox cycling for photo-Fenton activation. PL spectra reveal that optimal Bi doping (10Bi–CeO₂) suppresses charge recombination relative to CeO₂, while excessive Bi loading (13Bi–CeO₂) enhances recombination and weakens photo-Fenton activity (Fig. 6f). Photocurrent and EIS analyses as shown in Fig. 6g and h reveal that 10Bi–CeO₂ exhibits a higher photocurrent density and smaller charge-transfer resistance than pristine CeO₂, confirming oxygen-vacancy-assisted Bi doping enhances charge separation, accelerates interfacial electron transport, and thereby improves photo-Fenton degradation efficiency. The synergistic interaction between vacancy-mediated charge separation and cerium redox shuttling endows the Bi–CeO₂ system with exceptional photo-Fenton efficiency and durability, underscoring oxygen-vacancy engineering as a central design principle for advanced non-iron Fenton catalysts.

Oxygen vacancies play a central mechanistic role in enabling efficient photo-Fenton activity in Cu-modified MgO systems by coupling charge separation with Cu redox cycling.⁷⁶ In Cu-deposited MgO nanosheets, UV photodeposition induces abundant surface oxygen vacancies that act as electron-trapping centres, suppressing electron–hole recombination and extending visible-light absorption. These vacancies stabilize high-valence Cu species (Cu³⁺/Cu²⁺) and facilitate rapid interconversion among Cu⁰/Cu⁺/Cu²⁺/Cu³⁺, thereby sustaining continuous H₂O₂ activation under illumination. Moreover, oxygen-vacancy-rich sites promote the accumulation and directional transfer of photogenerated electrons toward Cu centres, accelerating the Cu²⁺/Cu⁺ redox cycle and enhancing ·O₂⁻ and ·OH generation. The synergistic interaction between oxygen vacancies and multivalent Cu species enables efficient photo-Fenton degradation across a broad pH range, highlighting oxygen-vacancy engineering as a powerful strategy for activating wide-bandgap oxides in non-iron Fenton-like systems. Overall, oxygen vacancy and defect engineering at interfaces play a decisive role in expanding light absorption, steering charge transfer, and increasing surface reactive sites. This strategy is especially crucial for Ce- and Bi-based systems, where these OV-mediated pathways rather than classical Fenton chemistry, dominate H₂O₂ activation and govern the efficiency of visible-light-assisted reactions.

3.4 Integration with porous scaffolds (MOFs, LDHs, biochar)

Coupling non-ferrous photo-Fenton catalysts with porous supports such as metal–organic frameworks (MOFs), layered double hydroxides (LDHs), mesoporous carbons, and biochar has emerged as an effective strategy to enhance surface reactivity, increase active-site density, and improve H₂O₂ utilization. These porous architectures provide high surface areas, abundant anchoring sites, and tunable pore networks that facilitate mass transport, pollutant adsorption, and dispersion of catalytic species. The resulting interfacial synergy significantly accelerates visible-light-driven Fenton-like processes.

Porous carbonaceous materials such as biochar, activated carbon, and mesoporous carbon not only offer large accessible surface areas but also contain oxygen- and nitrogen-functional groups that anchor transition-metal centres. Biochar-derived porous scaffolds play a multifunctional role in photo-Fenton catalysis, serving simultaneously as catalyst supports, electron mediators, and adsorption platforms. Mansoori et al.⁵⁶ reported a CuO_x–MnO_y@biochar catalyst in which FESEM images (Fig. 7a) show spherical CuO_x–MnO_y nanoparticles (10–100 nm) well dispersed on a rough, mesoporous biochar surface derived from rice straw. This carbon framework ensures high dispersion and stabilization of Cu and Mn oxide nanodomains while providing abundant surface sites for pollutant enrichment, and its conductive network with persistent free radicals facilitates interfacial electron transfer under visible light, thereby promoting rapid Cu²⁺/Cu⁺ and Mn³⁺/Mn²⁺ redox cycling and efficient H₂O₂ activation. ESR analysis (Fig. 7b) confirmed persistent free radical–mediated ·OH generation in the biochar/H₂O₂ system, and this synergistic coupling between the porous biochar scaffold and Cu–Mn bimetallic active centres enabled sustained ROS production over a wide pH range, leading to near-complete metronidazole degradation with appreciable mineralization. These results highlight biochar as an effective porous scaffold for integrating adsorption, charge mediation, and Fenton-like redox chemistry in sustainable photo-Fenton systems.

Fig. 7 (a) FESEM image of CuO_x–MnO_y@BC, and (b) ESR spectra of DMPO–OH for BC. Reproduced from ref. 56 Mansoori, S.; Ozumchelouei, E. J.; Davarnejad, R.; Zahrani, A. A., Catal. Commun., 2022, 171, 106517. Under the terms of the Creative Commons CC-BY license. (c) TEM of CuO@C, (d) HR-TEM of CuO@C, (e) role of various processes for PCM photodegradation, and (f) effect of radical scavengers on PCM photodegradation by CuO@C. Reproduced from ref. 108 with permission from Elsevier, Abdelhaleem, A.; Abdelhamid, H. N.; Ibrahim, M. G.; Chu, W., J. Cleaner Prod., 2022, 379, 134571. Copyright 2022.

In another study, Wang et al.¹⁰⁶ prepared CuO/Cu₂O/Cu–N–codoped biochar (Cu–N–C-700) by pyrolyzing a chitosan–Schiff-base–Cu precursor, enabling the in situ formation of CuO, Cu₂O, metallic Cu, and N-doped carbon within a conductive biochar matrix. The resulting mesoporous biochar (21 nm average pore size) provided high surface area, strong adsorption affinity, and efficient electron transport, which collectively enhanced the interaction between pollutants and catalytic sites. Benefiting from multiple Cu redox centres and the adsorption–activation synergy of the biochar scaffold, the Cu–N–C-700/H₂O₂ system achieved 98.5% tetracycline degradation within 15 min and maintained stable performance over five cycles. The study highlights how embedding multi-valent copper species into N-doped biochar frameworks enables robust, pH-tolerant photo-Fenton catalysis.¹⁰⁶

In the Cu/HAP-sBC system reported by Wang et al.,⁵⁰ straw-derived biochar provides a hierarchically porous carbon matrix that anchors hydroxyapatite-stabilized Cu species, ensuring uniform metal dispersion while suppressing Cu leaching and acid etching. The large surface area of the biochar scaffold enhances pollutant enrichment near active sites, while the conductive carbon framework promotes interfacial electron transfer, facilitating continuous Cu²⁺/Cu⁺ redox cycling and effective H₂O₂ activation under near-neutral conditions. This adsorption–catalysis synergy enables sustained ·OH generation and rapid p-nitrophenol mineralization, demonstrating that biochar scaffolds are not passive supports but active structural components that stabilize metal centres, broaden operational pH windows, and enhance the durability of heterogeneous photo-Fenton systems.

MOF-based supports (e.g., ZIF-67, MIL-88, UiO-66) also provide well-defined pore channels and uniform metal sites that assist in dispersing non-ferrous catalytic centres. Upon partial pyrolysis, MOFs can generate highly porous carbon–metal composites that combine conductive frameworks with abundant active sites, resulting in enhanced photo-Fenton activity.

Wang et al.¹⁰⁷ developed self-supported 2D TCPP-MOF arrays vertically grown on conductive copper foam via a hydrothermal route, forming an integrated heterointerface that significantly boosts photo-Fenton-like catalysis. The honeycomb MOF architecture provided a large accessible surface area and rapid mass transfer, while the uniformly dispersed M–N₄ active sites enabled efficient visible-light harvesting and charge separation. Incorporating bimetallic centres further strengthened interfacial redox activity, enhancing electron–hole utilization for ROS generation. Benefiting from the intimate MOF–substrate contact and fast interfacial carrier transport, the catalyst achieved robust mineralization of diverse organic pollutants and exhibited excellent recyclability due to its mechanically stable, self-supported structure. This work highlights the power of MOF interface engineering in designing high-efficiency, recoverable photo-Fenton-like catalysts.¹⁰⁷

Abdelhaleem et al.¹⁰⁸ synthesized a Cu-based MOF-derived CuO@C catalyst via direct pyrolysis, forming a tightly integrated CuO–carbon interface that enhanced H₂O₂ activation under visible LED irradiation. TEM and HR-TEM analyses (Fig. 7c and d) confirm that CuO nanoparticles are uniformly embedded within the MOF-derived carbon matrix with exposed CuO (111) facets, which ensures homogeneous dispersion, suppresses Cu leaching via in situ redeposition, and facilitates rapid interfacial electron transfer during the photo-Fenton-like process. Control studies (Fig. 7e) demonstrate that the CuO@C/H₂O₂/visible LED system achieves 95% paracetamol degradation within 60 min, far surpassing CuO@C/H₂O₂ (47%) and H₂O₂/LED (26%), thereby underscoring the dominance of synergistic heterogeneous photo-Fenton catalysis enabled by the robust interfacial architecture. TBA quenching (Fig. 7f) confirmed ·OH dominated degradation, underscoring that MOF-derived CuO@C provides a conductive and stable scaffold for efficient H₂O₂ activation and sustained photo-Fenton catalysis.¹⁰⁸

In the POM-incorporated MOF Cu(I)W–DPNDI reported by Si et al.,¹⁰⁹ the ordered porous architecture aligns Cu(I) Fenton-active centers, polyoxometalate (POM) oxidation sites, and photoactive DPNDI ligands at molecular proximity, facilitating efficient interfacial electron transfer and reactive oxygen species generation. The confinement effect of the MOF pores suppresses radical over-diffusion and promotes controlled ROS utilization, which is critical for achieving high selectivity in oxidation reactions. Meanwhile, the rigid MOF scaffold stabilizes Cu(I) species and POM polyanions, enabling sustained redox cycling under light irradiation while preventing catalyst deactivation. This work demonstrates that MOF porosity is not merely structural but functionally decisive, allowing the integration of light harvesting, charge transport, and Fenton-like redox chemistry into a single, highly selective photo-Fenton catalytic platform.

It was reported that porous self-supported scaffolds play a decisive role in enhancing photo-Fenton-like catalysis by simultaneously governing mass transfer, active-site accessibility, and charge transport. In vertically aligned 2D porphyrinic MOF arrays grown on conductive copper foam, the open honeycomb architecture provides interconnected meso–macroporous channels that enrich organic pollutants and H₂O₂ at catalytic interfaces while minimizing diffusion resistance.¹⁰⁷ The rigid, porous scaffold enables uniform exposure of M–N₄ active sites, promoting efficient light harvesting and rapid separation of photogenerated charge carriers. Moreover, the conductive and self-standing framework facilitates directional electron transport and suppresses catalyst aggregation or leaching during redox cycling. This integrated porous-scaffold design demonstrates that architectural engineering, beyond chemical composition, is critical for sustaining high ROS flux, long-term stability, and efficient mineralization in advanced photo-Fenton systems.

In multivariate Cu-ZIF-9-ica frameworks reported by Rodríguez-Sánchez et al.,⁵⁴ the intrinsic microporous ZIF scaffold concentrates organic pollutants and H₂O₂ within confined pores, enhancing local reactant availability and accelerating interfacial ROS generation. The open framework ensures uniform dispersion of Cu redox centres and mixed-ligand environments, shortening charge-migration pathways and suppressing recombination of photogenerated carriers. Moreover, the rigid porous architecture stabilizes the catalytic sites against leaching during repeated Cu²⁺/Cu⁺ cycling, enabling sustained photo-Fenton activity under neutral pH and low oxidant dosage. These findings highlight that porous scaffold engineering in MOFs is not merely structural but mechanistically integral to achieving high efficiency, stability, and practical applicability in visible-light-driven photo-Fenton systems.

Layered double hydroxides (LDHs) offer well-ordered two-dimensional galleries, tunable metal compositions, and abundant surface hydroxyl groups that enable uniform dispersion of non-ferrous catalytic centres and facilitate H₂O₂ adsorption and activation. Upon thermal transformation or partial reduction, LDHs can evolve into porous mixed-metal oxides or metal/oxide–carbon hybrids with enlarged surface area, enhanced electrical conductivity, and stabilized multivalent redox sites, collectively leading to accelerated charge transfer and improved photo-Fenton efficiency under visible light.

Ultrathin LDH nanosheets provide an effective porous scaffold for photo-Fenton catalysis by combining high surface area, short carrier diffusion lengths, and uniformly distributed multivalent metal centres. For example, He et al.¹¹⁰ reported ultrathin ternary CuZnCr-LDH nanosheets in which the layered porous architecture facilitates rapid charge migration and exposes abundant redox-active Cu and Cr sites. The LDH scaffold promotes efficient Cu⁺/Cu²⁺ and Cr³⁺/Cr⁶⁺ cycling under light irradiation, thereby accelerating H₂O₂ activation and sustaining continuous ·OH and ·O₂⁻ generation. Benefiting from the synergistic coupling of photocatalysis and Fenton chemistry enabled by the ultrathin LDH framework, the catalyst achieved high degradation and mineralization efficiencies for ciprofloxacin and methylene blue, highlighting the critical role of LDH porous scaffolds in stabilizing metal redox cycles and enhancing photo-Fenton performance.

Zhu et al.¹¹¹ reported an ultrathin engineered CuNi₂Al-LDH nanosheet as a porous layered scaffold to accelerate electron transport and enrich redox-active sites. The brucite-like LDH framework enabled intimate spatial proximity between Cu and Ni ions, where Ni acted as an internal electron mediator to continuously shuttle electrons to Cu centres, generating a high proportion of Cu(I) required for efficient H₂O₂ activation. The porous LDH nanosheet architecture simultaneously enhanced visible-light harvesting through Cu/Ni d–d transitions and provided abundant exposed active sites. This synergistic scaffold-induced electron transfer dramatically boosted Cu²⁺/Cu⁺ cycling and promoted rapid ROS generation, enabling complete degradation of phenol, tetracycline, and dyes within 30 min. Their work highlights how integrating bimetallic centres within a high-surface-area LDH scaffold offers an effective pathway to strengthen charge separation and Fenton-like redox dynamics in non-ferrous systems.

Overall, the integration of non-ferrous catalysts with porous supports strengthens adsorption activation processes, improves charge mobility, and offers structural stability, ultimately leading to faster reaction kinetics and higher pollutant removal efficiencies in visible-light-assisted Fenton-like systems.

3.5 Atomically dispersed metal sites on interfaces

Atomically dispersed metal sites represent a cutting-edge strategy in the design of highly efficient non-ferrous photo-Fenton catalysts. By distributing metal atoms as isolated single-site centres (e.g., M–N_x, M–O_x coordination environments), the maximum atomic utilization is achieved while preventing metal agglomeration and leaching, two major challenges in conventional heterogeneous Fenton systems. These atomic-scale active centres provide well-defined coordination environments that promote uniform H₂O₂ activation, enhanced charge transfer, and improved catalytic selectivity.

Dong et al.¹¹² constructed atomically dispersed Cu–N₄ sites on g-C₃N₄, with SEM and TEM confirming the preservation of the wrinkled nanosheet architecture (Fig. 8a and b). These isolated Cu–N₄ centres enable efficient H₂O₂ activation and accelerated charge separation, thereby overcoming sluggish redox kinetics in non-ferrous photo-Fenton systems. The optimized Cu–C₃N₄ achieved nearly 99% ciprofloxacin removal within 30 min, far outperforming pristine g-C₃N₄ and all control systems due to efficient H₂O₂ activation at Cu–N₄ single-atom sites, with the degradation kinetics following a pseudo-first-order model as shown in Fig. 8c. Spectroscopic analyses (EPR and XAFS) revealed that Cu–N₄ sites mediate a nonradical H₂O₂ activation pathway via O=Cu–N₄ and O=Cu–N₄=O intermediates, consistent with the weak ·OH signals observed by Scavenger study (Fig. 8d). This atomic-scale dispersion maximized the catalytic utilization of Cu, enabled fast electron mobility across the interface, and established a robust platform for next-generation non-ferrous photo-Fenton catalysts.¹¹²

Fig. 8 (a) SEM, (b) TEM (inset SAED) results of Cu–C₃N₄, (c) pseudo-first-order kinetic plots for CIP degradation in Cu–C₃N₄/Vis, Cu–C₃N₄/H₂O₂, and Cu–C₃N₄/vis/H₂O₂ systems, and (d) the relative contributions of different scavengers over the Cu–C₃N₄/vis/H₂O₂ system. Reproduced from ref. 112 with permission from American Chemical Society, S. Dong, X. Chen, L. Su, Y. Wen, Y. Wang, Q. Yang, L. Yi, W. Xu, Q. Yang, P. He, ACS ES&T Engineering, 2022, 3, 150–164. Copyright 2022. (e) UV-vis diffuse reflectance spectra of CuBTC-300, Bi₂MoO₆ and BC300-30, (f) corresponding (αhν)² or (αhν)^1/2 plots versus photon energy (hν), (g) instantaneous time-response current of Bi₂MoO₆, CuBTC-300 and BC300-30, and (h) EIS Nyquist plots of Bi₂MoO₆, CuBTC-300 and BC300-30. Reproduced from ref. 96 with permission from Elsevier, Hu, Y.; Ke, J.; Yan, Z.; Zhao, L.; Liu, J., Colloids Surf., A, 2025, 137409. Copyright 2025.

Guo et al.⁵² developed hydrangea-like carbon nitride decorated with atomically dispersed Cu–N₄ sites, forming a strong metal–support interaction that markedly enhanced photo-Fenton efficiency. The Cu–N₄ coordination created intermediate electronic states that accelerated the Cu²⁺/Cu⁺ redox cycle and increased the binding affinity of Cu d orbitals (especially d_xz) toward H₂O₂ by nearly ten-fold, enabling faster peroxide activation. Benefiting from the maximized atomic utilization and rapid carrier transfer, the Cu-SA/CNH catalyst achieved 98.1% tetracycline degradation and complete E. coli inactivation within 30 min, outperforming pristine CN. This work highlights the pivotal role of single-atom dispersion in strengthening redox cycling and H₂O₂ activation pathways during photo-Fenton treatment.⁵²

Zichao Lian et al.¹¹³ engineered atomically dispersed Ag–Co dual sites on g-C₃N₄ (AgCo–CN), forming densely distributed bimetallic coordination centres that simultaneously promote charge separation and create self-Fenton redox pairs. The isolated Ag and Co atoms act cooperatively, Ag accelerates photoelectron extraction while Co functions as the primary H₂O₂-activating site, enabling efficient in situ generation of ·OH and ¹O₂ under visible light. Benefiting from this highly dispersed dual-metal interface, AgCo–CN achieved near-complete mineralization of phenol even under harsh conditions and maintained long-term activity in a custom photo-filter reactor. This work highlights how atomic-scale bimetallic dispersion can unlock self-driven photo-Fenton pathways for degrading recalcitrant pollutants.¹¹³

In the CuN_x–carbon nitride catalyst reported by Zhang et al.,¹¹⁴ isolated CuN_x moieties embedded within a porous C–N matrix function as well-defined single-atom-like active centres for H₂O₂ activation. Such high-density atomic dispersion prevents Cu aggregation and leaching while enabling efficient Cu²⁺/Cu⁺ redox cycling under visible light. The strong metal–nitrogen coordination modulates the local electronic structure of Cu, lowers the activation barrier for H₂O₂ dissociation, and promotes selective ·OH generation, as corroborated by DFT calculations. Simultaneously, the conductive carbon nitride framework facilitates charge separation and transport, allowing photogenerated electrons to be efficiently funnelled toward CuN_x sites. This work highlights that atomic-scale metal dispersion is not merely a stabilization strategy but a decisive design principle for synchronizing charge transport, oxidant activation, and redox durability in high-performance photo-Fenton catalysts.

This concept extends across a diverse spectrum of the non-ferrous family. Single-atom Ce sites, for example, leverage the Ce³⁺/Ce⁴⁺ redox couple at the atomic level to enhance oxygen vacancy-mediated turnover, while atomic Bi sites create unique, highly localized charge centres to promote specific photocatalytic pathways.¹¹⁵ These single-atom sites effectively modulate the local electronic structure of the host material, creating localized charge-density regions that accelerate light-driven redox reactions. Overall, atomic dispersion of non-ferrous metals provides a powerful avenue for designing highly active, durable, and structurally uniform photo-Fenton catalysts. By combining biomimetic site architecture with strong interfacial bonding, single-atom catalysts (SACs) offer enhanced catalytic precision and significantly improved pollutant degradation efficiency under visible-light-assisted conditions.

3.6 Bimetallic and multicomponent interfaces

Bimetallic and multicomponent interface engineering has emerged as a powerful strategy for significantly improving the catalytic efficiency of non-ferrous photo-Fenton systems. By integrating two or more metal species with complementary redox behaviours, it is possible to create synergistic reaction pathways, broaden light absorption, and optimize H₂O₂ activation. Such interfaces fine-tune electronic structures, promote multivalent cycling, and offer multiple active sites for simultaneous radical generation. Dai et al.³⁵ synthesized Mn-doped cobalt silicate@diatomite (Co–MnSi@DE) composites to enhance Fenton-like oxidation via synergistic bimetallic interactions. The introduction of Mn into the Co-silicate framework enlarged surface area, narrowed the band gap, and created a more redox-active Co–Mn interface that accelerated H₂O₂ activation. DFT analysis confirmed that Mn incorporation lowered the reaction energy barrier for peroxide cleavage, while ESR revealed that the Co/Mn dual sites enabled the concurrent formation of ·OH and ·O₂⁻ under UV light. Benefiting from these cooperative interfacial effects, the optimized Co_0.25MnSi@DE achieved >95% degradation of methyl orange and maintained stability over repeated cycles, demonstrating the strong performance advantages of bimetallic interface engineering in non-ferrous photo-Fenton catalysis.³⁵

Xiong et al.⁹⁴ constructed a CNNS/Ni–Ag heterostructure in which bimetallic Ni/Ag sites and an interfacial built-in electric field cooperatively boosted photo-self-Fenton reactivity. The LSPR-induced hot electrons from Ag and photogenerated electrons from CNNS were directionally driven toward Ni centres, where Ni acted as the primary electron mediator to accelerate O₂ adsorption and the two-electron ORR pathway. This dual-metal synergy enabled high in situ H₂O₂ generation (1541.3 μmol g⁻¹ h⁻¹ with IPA) and rapid metronidazole removal (92.9% in 60 min), outperforming single-metal or external-H₂O₂ systems. The study highlights how bimetallic interfaces can simultaneously enhance light harvesting, carrier separation, and surface redox kinetics to realize efficient non-ferrous self-Fenton catalysis.

Hu et al.⁹⁶ constructed a Cu-decorated carbon@Bi/Bi₂MoO₆ plasmonic heterojunction derived from a MOF precursor, in which metallic Cu, in situ generated Bi, and Bi₂MoO₆ formed an integrated dual-metal interface. UV–vis DRS and Tauc analyses (Fig. 8e and f) show that the Cu/Bi bimetallic coupling with Bi₂MoO₆ introduces SPR-enhanced visible-light absorption (∼580 nm) and narrows the bandgap (2.58 → 1.48 eV), thereby promoting charge generation and synergistic photo-Fenton activity. Transient photocurrent and EIS analyses (Fig. 8g and h) reveal that the Cu/Bi–carbon coupled BC300-30 heterojunction exhibits the highest photocurrent density and the smallest charge-transfer resistance confirming that bimetallic interfaces, carbon skeletons, and oxygen vacancies synergistically enhance charge separation and migration, thereby boosting photo-Fenton activity. This work highlights that rational engineering of bimetallic and carbon-coupled interfaces is a powerful strategy to amplify interfacial charge dynamics and ROS productivity in non-ferrous photo-Fenton catalysts.

In the rhombohedral AlMnCuZnBi HEA reported by Anuraag et al.,¹¹⁶ the homogeneous atomic-level mixing of Mn, Cu, Zn, Al, and Bi generates a complex electronic landscape with diverse local coordination environments that collectively facilitate H₂O₂ activation and charge transfer. The coexistence of multiple transition and post-transition metals enables parallel redox pathways, enhancing electron availability for Fenton-like reactions while suppressing metal leaching commonly observed in single-metal systems. Such multicomponent interfaces promote synergistic interactions among different metal centres, improving electron delocalization and sustaining catalytic activity under photo-assisted conditions. This study highlights HEAs as an emerging class of photo-Fenton catalysts, where compositional complexity and interfacial synergy replace conventional heterojunction design to achieve robust and durable pollutant degradation.

In the ternary selenide nanocomposite Co₉Se₈/Ni₃Se₄/Cu₂Se reported by Mohapatra et al.,¹¹⁷ the intimate interfacial coupling among Co, Ni, and Cu-based selenides establishes multiple electron-transfer pathways under visible light, promoting efficient separation and migration of photogenerated charge carriers. These heterogeneous interfaces facilitate cooperative redox cycling (Cu²⁺/Cu⁺, Co³⁺/Co²⁺, Ni³⁺/Ni²⁺), which accelerates H₂O₂ activation and sustains continuous ROS generation during photo-Fenton reactions. Moreover, the multinary junctions broaden light absorption and provide abundant interfacial active sites, enabling high degradation efficiencies for both cationic and anionic dyes. This study highlights that interface-driven electronic synergy in multicomponent catalysts is a powerful strategy for overcoming kinetic limitations in conventional single-metal photo-Fenton systems.

These hybrid structures often operate through dual or cooperative pathways, including simultaneous radical and non-radical oxidation, broadening the scope of degradable pollutants.

Overall integration of interface strategies. Interface engineering has evolved into a foundational design principle for next-generation non-ferrous photo-Fenton catalysts. Through the rational construction of semiconductor junctions, cocatalyst decoration, oxygen-vacancy engineering, atomic dispersion of metal centres, and multicomponent design, catalytic efficiency has been dramatically improved. These interfacial strategies synergistically enhance charge separation, strengthen ROS generation, increase pollutant affinity, and improve structural durability. Consequently, multicomponent and interface-engineered catalysts represent the most promising direction for developing advanced, sustainable oxidation systems for water purification.

4. Mechanistic insights into reactive oxygen species (ROS) generation in non-iron photo-Fenton catalysts

The performance of non-ferrous photo-Fenton catalysts is governed by the generation and management of reactive oxygen species (ROS), including hydroxyl radicals (·OH), superoxide anions (·O₂⁻), hydroperoxyl radicals (·OOH), and singlet oxygen (¹O₂). In contrast to classical Fe-based systems that require acidic pH, the combination of visible-light-responsive semiconductors with redox-active metals such as Cu, Co, Mn, Bi, and Ce enables efficient H₂O₂ activation under near-neutral conditions. These processes are strongly influenced by metal valence transitions, semiconductor band alignment, interfacial charge separation, and the presence of oxygen vacancies or other defect sites. Rational interface engineering via heterojunctions, cocatalyst deposition, or atomic dispersion, therefore plays a central role in steering electron flow, stabilizing intermediates, and enhancing ROS productivity in non-ferrous systems.

4.1 Copper-based catalysts: redox cycling and enhanced charge transfer

Copper-based catalysts exhibit some of the most efficient non-ferrous photo-Fenton activity due to the favourable redox potential of the Cu⁺/Cu²⁺ pair (+0.17 V), which enables rapid and reversible electron transfer. The core mechanism consists of two interconnected steps:

(1) Cu⁺-driven H₂O₂ decomposition

Cu⁺ + H₂O₂ → Cu²⁺ + OH + OH⁻

This reaction is thermodynamically favored and proceeds quickly, generating ·OH radicals as the primary oxidant.

(2) Photoreduction of Cu²⁺ to regenerate Cu⁺

Cu²⁺ + e⁻_(photo) → Cu⁺

The integration of Cu species with semiconductors significantly enhances this redox cycle by providing a continuous supply of photogenerated electrons. Zhang et al.¹¹⁴ developed a Cu-doped carbon nitride catalyst featuring highly dispersed CuN_x single sites that markedly strengthened H₂O₂ activation in a photo-Fenton system. Embedding CuN_x centres into the porous C₃N₄ framework enhanced visible-light absorption and accelerated charge transport, enabling rapid generation of ·OH, ·O₂⁻, h⁺, and ¹O₂. The optimized CuCN catalyst achieved 93.6% tetracycline degradation within 60 min, with strong performance even in real water matrices. DFT calculations confirmed that CuN_x moieties lower the energy barrier for H₂O₂ adsorption and cleavage, thereby boosting the Cu²⁺/Cu⁺ redox cycle and sustaining active radical formation. This work highlights the pivotal role of atomically dispersed Cu centres in accelerating charge transfer and maximizing non-ferrous photo-Fenton reactivity.¹¹⁴

Rozmyślak et al.¹¹⁸ prepared Cu₃(PO₄)₂ by a simple precipitation route and demonstrated its strong potential as an iron-free Fenton-like catalyst for ciprofloxacin degradation. Owing to the redox-active Cu²⁺/Cu⁺ couple and favourable surface adsorption, Cu(II) phosphate delivered a ∼7-fold higher Fenton-like degradation rate than commercial CuO and markedly higher activity than Fe₂O₃, CeO₂, and other metal phosphates. Under visible-light assistance, the catalyst further accelerated CIP oxidation (k = 0.00445 min⁻¹), confirming enhanced charge excitation and faster Cu²⁺ → Cu⁺ cycling. Radical-quenching and EPR analyses identified ·OH as the dominant reactive species, underscoring the key role of copper sites in H₂O₂ activation. Collectively, the strong redox flexibility of Cu species, together with efficient photo-activation, make Cu(II) phosphate a promising copper-based platform for photo-Fenton degradation of antibiotics.

Almahri et al.¹¹⁹ synthesized hexahydroxy copper–stannate (CuSn(OH)₆) and demonstrated its strong sunlight-driven Fenton-like degradation of Rh6G and tetracycline, highlighting the crucial role of Cu-based redox centres in ROS generation. The well-defined cubic CuHS particles (150–300 nm) possessed abundant surface Cu²⁺/Cu⁺ sites and oxygen-defect-rich domains, which significantly enhanced H₂O₂ activation and visible-light electron transfer, enabling rapid pollutant removal with maximum activity at alkaline pH. Mechanistic analyses confirmed that Cu sites facilitated efficient charge separation and promoted the production of ·OH and ·O₂⁻, driving chromophore cleavage in Rh6G and multistep hydroxylation and ring fragmentation in TC. Overall, the study establishes CuSn(OH)₆ as an efficient Cu-driven Fenton-like photocatalyst, where copper redox cycling and defect-assisted ROS generation govern its high degradation performance.¹¹⁹

Zhang et al.¹⁰⁴ developed a Cu-doped zeolite catalyst (Cu⁰@CuZ) featuring synergistic metallic Cu⁰ nanoparticles and abundant oxygen vacancies, which together enabled exceptional photo-Fenton-like performance. The dual-channel electron-transfer pathway, where oxygen vacancies trap photogenerated electrons and plasmonic Cu⁰ rapidly donates them to Cu(II), greatly accelerated the Cu²⁺/Cu⁺ redox cycle and boosted H₂O₂ activation. Benefiting from enhanced visible-light harvesting and rapid charge transport, Cu⁰@CuZ completely degraded phenol (20 mg L⁻¹) within 15 min, with rate constants 40–65 times higher than Cu₂O, CuO, and Cu⁰ alone. This work demonstrates how coupling Cu⁰ with vacancy-rich zeolite frameworks provides a powerful strategy to maximize Cu-based photo-Fenton reactivity.¹⁰⁴

In the plasmon-driven V_O-Cu₂O@Cu/CN_x heterojunction reported by Wang et al.,⁴⁵ mixed-valence Cu⁰/Cu⁺/Cu²⁺ sites act as dynamic redox mediators that couple visible-light excitation with sustained Cu⁺/Cu²⁺ cycling. Metallic Cu⁰ and Cu₂O serve as electron-rich reservoirs that harvest plasmon-induced hot electrons, while interfacial Cu⁺ sites directly participate in H₂O₂ activation. Notably, the deprotonated H₂O₂ species (HO₂⁻) accelerates reversible Cu⁺ disproportionation, stabilizing the catalytic cycle and suppressing metal deactivation. This copper-centred dual-channel mechanism enables both radical-assisted (·OH, ·O₂⁻) and non-radical (¹O₂, electron-transfer) oxidation pathways, imparting high reaction robustness over a wide pH range. These findings underscore that precise engineering of copper oxidation states and interfacial Cu environments is pivotal for achieving efficient, stable, and interference-resistant photo-Fenton catalysis beyond conventional Fe-based systems.

Copper nanoparticles act as highly effective noble-metal-free cocatalysts in photo-Fenton-like systems by simultaneously enhancing charge transfer and accelerating Cu-mediated H₂O₂ activation. In the Cu-modified Bi₂O₂S nanosheets reported by Guo et al.,⁴⁶ surface-anchored Cu nanoparticles function as electron sinks, facilitating rapid extraction of photogenerated electrons from Bi₂O₂S and suppressing electron–hole recombination. The accumulated electrons promote the reduction of Cu²⁺ to Cu⁺, which serves as the key active species for catalytic H₂O₂ decomposition into highly oxidative ·OH radicals. This Cu-assisted redox cycling not only increases reactive oxygen species generation but also stabilizes the photo-Fenton process under visible light. Meanwhile, the intimate Cu–Bi₂O₂S interface improves charge carrier mobility and adsorption affinity toward pollutants, collectively leading to markedly enhanced tetracycline degradation kinetics. These results highlight that judicious incorporation of metallic Cu nanoparticles provides a cost-effective strategy to boost photo-Fenton efficiency by coupling semiconductor photoexcitation with sustained Cu⁺/Cu²⁺ redox catalysis.

Copper species act as the pivotal redox mediators in piezo-photo-Fenton systems by enabling efficient Cu²⁺/Cu⁺ cycling and sustained H₂O₂ activation. In the Bi₄Ti₃O₁₂–CuBi₂O₄ p–n heterojunction, interfacial electric fields and piezoelectric polarization continuously drive photogenerated electrons toward Cu²⁺ sites, accelerating their reduction to Cu⁺.⁸⁸ This rapid Cu redox turnover enhances H₂O₂ decomposition into reactive oxygen species, thereby coupling mechanical–photonic charge separation with Cu-centred Fenton chemistry. The results highlight that copper not only serves as the active Fenton centre but also dynamically links interfacial charge transport with oxidant activation, leading to markedly improved degradation kinetics.

In the Cu@MoS₂/PAAm/CA double-network hydrogel, Cu sites immobilized within the 3D polymer matrix catalyze ·OH generation, while MoS₂ and the hydrogel scaffold promote charge transfer and pollutant enrichment.¹²⁰ The spatial confinement of Cu species suppresses leaching and enables stable Cu-mediated Fenton reactions even under dark conditions, highlighting copper's decisive role in coupling adsorption-assisted enrichment with durable photo-Fenton degradation. Copper centres act as the decisive photo-Fenton active sites in ligand-engineered Cu-MOFs by enabling efficient Cu²⁺/Cu⁺ redox cycling and H₂O₂ activation under visible light. In secondary-ligand-modulated Cu-MOFs, tetranuclear Cu cluster configurations provide abundant coordinatively unsaturated Cu sites that promote photoinduced charge separation and accelerate interfacial electron transfer to H₂O₂, thereby boosting ·OH and ·O₂⁻ generation. This work highlights that precise ligand control over Cu coordination environments directly governs photo-Fenton efficiency, stability, and pH tolerance in Cu-MOF catalysts.¹⁸

Overall, copper-based catalysts benefit from rapid Cu⁺/Cu²⁺ cycling, strong visible-light absorption when coupled with semiconductors, and accelerated interfacial charge transfer in engineered heterojunctions. These features enable Cu-based systems to outperform classical Fe-based catalysts, especially under environmentally relevant pH conditions.

4.2 Cobalt-based catalysts: multivalent redox mediation

Cobalt-based photo-Fenton catalysts derive their activity from the reversible Co²⁺/Co³⁺ redox couple (E° ≈ +1.81 V vs. NHE), which enables rapid H₂O₂ activation and efficient ·OH generation under mild conditions. The Fenton-like reaction proceeds through the oxidation of Co²⁺ by H₂O₂, followed by the reduction of Co³⁺ to regenerate catalytically active Co²⁺:

Co²⁺ + H₂O₂ → Co³⁺ + ·OH + OH⁻

Co³⁺ + e⁻_(photo) → Co²⁺

This redox cycling can be significantly accelerated under visible-light irradiation, especially when cobalt is integrated into semiconductor interfaces that promote directional electron transfer. Tang et al.¹²¹ developed a cobalt-based ZIF-67/defective TiO_2−x heterostructure (B–TiO_2−x@ZIF-67) to enhance visible-light-driven photo-Fenton-like degradation of bisphenol A. The intimate coupling between oxygen-vacancy-rich TiO_2−x and Co-containing ZIF-67 constructed an efficient charge-transfer interface, where TiO_2−x vacancies trapped electrons to accelerate carrier separation, while Co(II) sites served as active Fenton centres for rapid H₂O₂ decomposition to ·OH. Benefitting from the synergistic roles of vacancy-mediated charge migration and Co-induced ROS generation, the B–TiO_2−x@ZIF-67/H₂O₂/vis system achieved ∼95% BPA removal and maintained stable activity over repeated cycles. This work highlights the critical role of Co-based centres integrated within MOF–oxide interfaces in promoting peroxidative pathways for efficient photo-Fenton-like oxidation.¹²¹

Li et al.¹²² constructed a Co-POM/N-TiO₂ heterostructure in which cobalt polyoxometalate clusters acted as an efficient cocatalyst to mediate electron transfer during the photo-Fenton-like reaction. Decorating Co-POM onto N-doped TiO₂ not only broadened visible-light absorption but also established a favourable interfacial pathway that accelerated electron migration from TiO₂ to the Co-POM sites, where Co(II) species activated H₂O₂ to generate ·OH. Benefiting from this cobalt-driven interfacial charge-transfer mechanism, the hybrid catalyst achieved rapid RhB degradation (∼97% within 40 min at neutral pH), exhibiting a rate constant nearly nine times higher than N–TiO₂ alone. The strong Co–TiO₂ electronic coupling and efficient Co²⁺/Co³⁺ redox cycling underpin the markedly enhanced photo-Fenton-like activity.¹²²

Cobalt nodes in ZIF-9 frameworks act as the primary photo-Fenton catalytic centres by facilitating visible-light-driven charge separation and promoting ROS generation under neutral conditions. In multivariate Co-based MOFs, electronic modulation via mixed ligands lowers the band gap and enhances carrier mobility, enabling efficient interaction between photogenerated electrons and H₂O₂ to produce ·OH and ·O₂⁻.⁵⁴ This study demonstrates that cobalt-centred MOF scaffolds provide structurally stable, pH-flexible platforms for heterogeneous photo-Fenton reactions, where ligand engineering further amplifies Co-mediated redox activity without metal leaching.⁵⁴

Zhou et al.¹²³ reported a Pt/CoAl₂O₄ heterostructured catalyst derived from spent Pt/γ-Al₂O₃ exhibited highly efficient visible-light-driven photo-Fenton-like degradation of malathion. In this system, CoAl₂O₄ acts as the primary Fenton-active phase, where continuous Co²⁺/Co³⁺ redox cycling governs H₂O₂ activation and ·OH generation, while epitaxially grown Pt nanocrystals enhance light absorption and suppress electron–hole recombination. The interfacial charge transfer from Pt to CoAl₂O₄ accelerates cobalt redox turnover, thereby sustaining ROS production under mild conditions. This work highlights that cobalt-based spinel oxides provide robust and recyclable photo-Fenton centres, with noble metals mainly serving as photonic and electronic promoters rather than active Fenton sites.

Zhu et al.¹²⁴ reported that an amorphous–crystalline CoS_x@TiO₂ heterointerface bridged by S–O bonds markedly accelerates photo-Fenton degradation of ciprofloxacin. In this system, CoS_x serves as the cobalt-based photo-Fenton centre, enabling efficient Co²⁺/Co³⁺-mediated H₂O₂ activation and abundant ·OH generation, while S–O interfacial bridges promote directional electron transfer and in situ H₂O₂ formation under light irradiation. Consistently, EPR spectroscopy (Fig. 9a and b) detected strong ·OH and ·O₂⁻ signals after light irradiation, while radical quenching experiments (Fig. 9c) showed that isopropanol markedly suppressed ciprofloxacin degradation (from 100% to 47%), collectively confirming ·OH as the dominant oxidative species with minor contributions from electrons, holes, and ·O₂⁻. These results demonstrate that cobalt sulphide heterointerfaces effectively couple photocatalytic charge transport with cobalt redox cycling to drive ·OH-dominated photo-Fenton oxidation without the need for noble metals.

Fig. 9 (a) CIP removal efficiency over CoS_x@TiO₂ in the presence of radical quenchers and (b) the ESR of ·O₂⁻, and (c) ·OH. Reproduced from ref. 124 with permission from Elsevier, Zhu, Y.; Li, J.; Lai, Y.; Cao, Y.; Li, J.; Wei, Z.; Yang, L.; Chen, Z.; Zou, J., Environ. Res., 2025, 279, 121785. Copyright 2025. (d) Schematic diagram of the possible degradation mechanism of dye wastewater degradation in the Co_0.25MnSi@DE/H₂O₂-UV system, (e) the effects of quenching of different radicals on MO degradation, and (f) the corresponding reaction constant. Reproduced from ref. 35 with permission from Elsevier, Dai, N.; Yang, L.; Liu, X.; Gao, L.; Zheng, J.; Zhang, K.; Song, D.; Sun, T.; Luo, S.; Liu, X., J. Colloid Interface Sci., 2023, 652, 1812–1824. Copyright 2023.

Overall, cobalt-based catalysts offer high intrinsic oxidizing power through Co²⁺/Co³⁺ transitions, efficient H₂O₂ activation at near-neutral pH, strong compatibility with Z-scheme and defect-engineered systems, and improved ROS generation through oxygen-vacancy-enabled pathways. These characteristics make Co an exceptionally potent non-ferrous metal for advanced photo-Fenton applications.

4.3 Manganese-based catalysts: valence versatility and defect engineering

Manganese-based catalysts leverage the rich redox chemistry of Mn²⁺/Mn³⁺/Mn⁴⁺ to activate H₂O₂ and dissolved oxygen for ROS production. The multivalent nature of Mn enables both radical and non-radical oxidation pathways, making manganese oxides particularly attractive for visible-light-assisted Fenton-like reactions, especially under neutral or slightly alkaline conditions.

The primary Fenton-like step involves the oxidation of Mn²⁺ by H₂O₂ to produce ·OH, followed by the photoreduction of Mn³⁺ to sustain the cycle:

Mn²⁺ + H₂O₂ → Mn³⁺ + ·OH + OH⁻

Mn³⁺ + e⁻_(photo) → Mn²⁺

Meanwhile, higher-valent Mn⁴⁺ species commonly present in MnO₂ can participate in surface redox reactions that facilitate the production of ·OOH and ·O₂⁻, especially when oxygen vacancies (OVs) are present. These OVs promote electron trapping, enhance Mn valence cycling, and improve adsorption of H₂O₂ and O₂.

Mansoori et al.⁵⁶ reported a biochar-supported CuO_x-MnO_y nanocatalyst, highlighting the key role of manganese redox chemistry in visible-light-driven photo-Fenton degradation of metronidazole. In this system, Mn²⁺ acts as an efficient H₂O₂ activator, promoting sustained ·OH generation under near-neutral pH, while Cu species and biochar-derived persistent free radicals synergistically facilitate electron transfer and Mn redox regeneration. The Mn-centred Fenton-like pathway dominates ROS production, enabling almost complete MNZ removal with high stability and recyclability, underscoring manganese oxides as robust, non-iron active sites for practical photo-Fenton applications.

Dai et al.³⁵ synthesized Mn-doped cobalt silicate@diatomite (Co-MnSi@DE) composites via a hydrothermal route, where Mn incorporation enhances synergistic Co/Mn redox cycling; as shown in Fig. 9d, MO degradation proceeds via UV-driven Co/Mn Fenton-like pathways that continuously generate ·OH radicals following initial pollutant adsorption. The introduction of Mn increased surface area, narrowed the band gap, and strengthened H₂O₂ adsorption/activation, which DFT confirmed by showing a much lower H₂O₂ adsorption energy on Mn-doped surfaces. Benefiting from this bimetallic synergy, the optimized Co_0.25MnSi@DE achieved >95% methyl orange degradation under UV/H₂O₂, significantly outperforming Mn-free counterparts. Radical quenching experiments (Fig. 9e and f) revealed that both ·OH and ·OOH contribute to methyl orange degradation in the Co_0.25MnSi@DE/H₂O₂-UV system, with a much stronger inhibition observed upon ·OH scavenging (k = 0.0033) than ·OOH scavenging (k = 0.008), indicating that hydroxyl radicals play the dominant role in the catalytic process. The catalyst remained reusable and structurally stable, underscoring Mn doping as a potent strategy to boost cobalt-based photo-Fenton activity.

Zhang et al.⁸¹ reported that a porous Z-scheme MnO₂/Mn-modified alkalinized g-C₃N₄ heterojunction exhibits outstanding Mn-driven photo-Fenton performance for pharmaceutical degradation. In this system, multivalent Mn species (Mn⁴⁺/Mn³⁺/Mn²⁺) act as the core Fenton-like active centres, efficiently activating H₂O₂ to generate abundant ·OH, while the Z-scheme charge transfer preserves strong redox ability and suppresses charge recombination. The synergistic coupling of Mn redox cycling, surface hydroxyl groups, and Z-scheme photocatalysis enables deep mineralization of tetracycline and high TOC/COD removal, highlighting manganese oxides as robust, iron-free photo-Fenton catalysts.

Singh et al.⁵⁷ reported that a Cu–Mn codoped BiVO₄ demonstrates efficient Mn-driven photo-Fenton degradation of ciprofloxacin by exploiting the multivalent Mn²⁺/Mn³⁺/Mn⁴⁺ redox cycle. In this system, Mn acts as the primary Fenton-like centre, facilitating rapid H₂O₂ activation to generate ·OH radicals, while Cu serves as an electron mediator that regenerates lower-valence Mn species and stabilizes the redox cycle. The coexistence of Mn multivalence with oxygen vacancies and favourable band alignment prolongs charge-carrier lifetimes, enabling sustained ·OH production and high photocatalytic stability, underscoring the critical role of manganese redox chemistry in non-iron photo-Fenton systems.

Overall, manganese-based photo-Fenton catalysts benefit from multi-valence redox flexibility (Mn²⁺/Mn³⁺/Mn⁴⁺), Oxygen-vacancy-enhanced H₂O₂ and O₂ activation, Strong synergy with carbonaceous and semiconductor supports, and robust performance over a wider pH window than classical Fe-based systems. These advantages position Mn-based catalysts as promising candidates for energy-efficient and environmentally compatible advanced oxidation processes.

4.4 Bismuth-based catalysts: surface coordination and interfacial activation

Bismuth-based materials represent a distinct class of non-ferrous photo-Fenton catalysts, primarily due to their layered structures, strong internal electric fields (IEFs), and intrinsic ability to form oxygen vacancies (OVs). Unlike transition metals such as Cu, Co, or Mn, bismuth does not undergo classical multivalent Fenton cycling. Instead, its catalytic activity arises from photocatalytic activation of H₂O₂ and interfacial charge modulation, both of which are strongly influenced by crystal anisotropy and defect engineering.

Layered bismuth oxyhalides (BiOX, X = Cl, Br, I) and oxides (Bi₂O₃, BiVO₄) possess unique (001) or (010) facets that exhibit strong IEFs, which promote spatial charge separation and directional electron migration.⁶⁴ Upon visible-light excitation, photogenerated electrons can reduce H₂O₂ to ·O₂⁻, while valence-band holes oxidize surface hydroxyls or H₂O to yield ·OH. This dual-pathway ROS formation compensates for the absence of classical Bi redox cycling. The presence of oxygen vacancies significantly enhances these processes. OVs act as electron-trapping sites, prolong photocarrier lifetime, and facilitate the adsorption and activation of H₂O₂ and O₂.

Guo et al.⁴⁶ constructed a noble-metal-free Cu-modified Bi₂O₂S nanosheet photocatalyst (Cu–Bi₂O₂S), where Cu nanoparticles act as an efficient cocatalyst to accelerate charge transfer and H₂O₂ activation. The optimized 13 wt% Cu–Bi₂O₂S showed a 3.95-fold increase in the TCH photo-Fenton-like degradation rate compared to pristine Bi₂O₂S, attributed to enhanced electron mobility, higher ROS generation, and stronger pollutant adsorption. Importantly, Bi₂O₂S served as the high-responsivity visible-light absorber, while Cu introduction markedly boosted interfacial electron extraction and surface redox activity. Beyond pollutant degradation, the Cu–Bi₂O₂S catalyst also achieved significantly higher CO₂ photoreduction performance (CH₄ formation rate: 2.27 μmol g⁻¹ h⁻¹, 7× higher than Bi₂O₂S), demonstrating the strong synergy between Bi-based layered structure and Cu cocatalyst. This work highlights Bi-based hosts as versatile platforms where cocatalyst engineering can simultaneously enhance photo-Fenton activity and solar-fuel conversion.⁴⁶

Hu et al.⁹⁶ constructed a dual-metal plasmonic heterojunction (Cu-decorated C@Bi/Bi₂MoO₆) by carbonizing CuBTC and subsequently introducing Bi₂MoO₆ with in situ reduced metallic Bi. The Bi/Bi₂MoO₆ interface played a central role in boosting the photo-assisted Fenton process by providing plasmon-active Bi sites and forming an efficient charge-transfer heterojunction that accelerated electron–hole separation. Coupled with Cu-derived SPR enhancement, the catalyst achieved rapid tetracycline degradation (96.1% in 5 min) with excellent pH tolerance and recyclability. Mechanistic studies confirmed ·OH as the dominant ROS, with metallic Bi enabling fast O₂ reduction and ROS generation, thereby driving the superior Fenton activity.

Zhang et al.³⁹ fabricated ultra-high Bi-doped CeO₂ nanorods by pyrolyzing Bi/Ce-BMOFs, achieving uniform Bi incorporation (up to 20 mol%) without phase segregation. The Bi doping, together with abundant oxygen vacancies, markedly enhanced visible-light absorption and accelerated the Ce³⁺/Ce⁴⁺ redox cycling, enabling fast H₂O₂ activation. The resulting Bi–CeO₂ catalyst delivered outstanding photo-Fenton activity for tetracycline removal across a broad pH range (2–9). Mechanistic investigations confirmed ·O₂⁻ and ·OH as dominant species, with OVs serving as electron-trapping sites to promote charge separation. Overall, the study highlights how Bi doping effectively modulates the electronic structure of CeO₂, strengthens redox cycling, and boosts photo-Fenton efficiency, offering a robust strategy for MOF-derived Bi-modified catalysts.

Reported by Wang et al.,⁷⁸ the 2D/2D CuCo₂S₄/Bi₂WO₆ heterojunction underscores the active role of the Bi₂WO₆ host in photo-Fenton catalysis. The layered Bi–O–W framework of Bi₂WO₆ serves as a robust visible-light absorber and hole-rich oxidation platform, while its suitable band structure enables directional charge transfer in the type-II heterojunction. Coupling with CuCo₂S₄ primarily accelerates electron extraction and Fenton redox cycling, whereas Bi₂WO₆ preserves strong oxidative holes and stabilizes interfacial charge separation, promoting sustained ·OH and ·O₂⁻ generation. This study highlights that bismuth-based oxides are not passive supports but key photoredox backbones that govern light harvesting, hole-driven oxidation, and interfacial stability in non-ferrous photo-Fenton systems.

Overall, the superior performance of Bi-based photo-Fenton catalysts arises from strong internal electric fields that promote charge separation, oxygen-vacancy-mediated H₂O₂ and O₂ activation, broad visible-light absorption across layered BiOX structures, and enhanced interfacial charge migration in Z/S-scheme heterojunctions. These attributes allow Bi-based materials to generate ROS effectively without relying on classical redox cycling, making them highly promising for visible-light-driven pollutant degradation in aqueous environments.

4.5 Cerium-based catalysts: redox buffering and oxygen vacancy-mediated ROS generation

Cerium-based catalysts occupy a unique position among non-ferrous photo-Fenton systems due to the exceptional redox flexibility of the Ce³⁺/Ce⁴⁺ couple and the strong tendency of CeO₂ to form oxygen-deficient structures (CeO_2−x). Unlike classical Fenton catalysts, Ce does not rely primarily on direct homogeneous H₂O₂ activation; instead, its catalytic behaviour is governed by oxygen-vacancy (OV)-mediated electron transfer, surface Ce³⁺ enrichment, and interfacial photoreduction pathways.

Oxygen vacancies in CeO₂ create localized Ce³⁺ sites that serve as active centres for H₂O₂ adsorption and electron transfer. The Ce³⁺/Ce⁴⁺ shuttle facilitates redox cycling under illumination:

Ce³⁺ + H₂O₂ → Ce⁴⁺ + ·OH + OH⁻

Ce⁴⁺ + e⁻_(photo) → Ce³⁺

Although this reaction proceeds more slowly than Cu or Co-based Fenton pathways, the high OV concentration is significantly compensated for by enhancing H₂O₂ activation efficiency. In addition to ·OH formation, CeO_2−x surfaces promote ·O₂⁻ and ¹O₂ generation through electron transfer to dissolved O₂, enabling both radical and non-radical oxidation routes.

Several interfacial engineering strategies have been reported to substantially enhance Ce-based photo-Fenton performance. Yang et al.³⁸ constructed a cerium-rich CeO_2−x/Bi₂MoO₆ heterojunction in which abundant oxygen-deficient CeO_2−x nanodots were uniformly anchored onto Bi₂MoO₆ nanosheets. The introduction of non-stoichiometric CeO_2−x proved crucial: oxygen vacancies accelerated the Ce⁴⁺/Ce³⁺ self-circulating redox cycle, markedly enhancing H₂O₂ activation and preventing metal sludge formation typically seen in stoichiometric CeO₂. Benefiting from this rapid Ce-centred redox cycling and the intimate heterojunction interface, the optimized CEO/BIM-3 catalyst achieved >97% degradation of ofloxacin under only 5 W visible LED irradiation, far surpassing pristine CeO_2−x and Bi₂MoO₆. The catalyst also efficiently removed norfloxacin and ciprofloxacin with similarly high efficiencies. Mechanistic studies confirmed that the accelerated Ce³⁺ regeneration boosted ·OH production, while the Bi₂MoO₆ component enhanced visible-light absorption and charge separation. Overall, the work highlights cerium's pivotal role as a self-renewing Fenton cocatalyst, enabling highly stable, efficient, and sludge-free photo-Fenton degradation of fluoroquinolone antibiotics.³⁸

Peng et al.¹²⁵ engineered Ce-induced lattice distortion and oxygen vacancies to generate surface FLP sites (Ce⁴⁺–Ov–O²⁻), which strengthened Lewis acidity/basicity and promoted interfacial adsorption/activation of both H₂O₂ and escaped O₂. This FLP-mediated O₂/·O₂⁻/¹O₂/H₂O₂ redox cycling enabled highly efficient ROS generation, giving Ce-BMO a 2.15-fold activity boost over pristine BMO for tetracycline degradation. EPR and quenching tests confirmed ¹O₂ and ·O₂⁻ as dominant species. Overall, Ce doping establishes an FLP-driven catalytic environment that maximizes H₂O₂ utilization and accelerates photo-Fenton pathways.¹²⁵

An iron-free Ce–Ag nano-Fenton catalyst (ZY@(CeAg)) was green-synthesized by anchoring Ce and Ag nanoparticles onto a zeolite-Y matrix using papaya-leaf phytochemicals.¹²⁶ The introduction of Ce, supported by XPS and XRD evidence of reduced Ce species, significantly narrowed the band gap of zeolite Y (4.2 → 3.0 eV) and created abundant redox-active Ce³⁺/Ce⁴⁺ sites that accelerated H₂O₂ activation and ROS generation. The resulting heterojunction exhibited strong photo-Fenton-like degradation of tetracycline and ciprofloxacin (≈87% removal under UV-A), with ·OH radicals identified as the dominant oxidants. Benefiting from Ce-driven redox cycling, stable surface dispersion, and minimal Ce leaching, ZY@(CeAg) maintained high activity over multiple cycles and displayed consistent performance in natural waters, highlighting cerium's pivotal role in enabling robust, iron-free Fenton catalysis.¹²⁶

Hu et al.¹²⁷ constructed ZIF-67-derived Co₃O₄-CeO₂ nanocage micromotors, where CeO₂ nanoparticles uniformly anchored on Co₃O₄ created abundant Ce³⁺/Ce⁴⁺ redox sites that enhanced peroxide-like activity and accelerated ROS generation. The Ce-modified nanocages enabled rapid self-propulsion in H₂O₂, providing efficient mass transfer and boosted photo-Fenton-like degradation, achieving 98.2% hydroquinone removal. The CeO₂ contribution, strong redox cycling, oxygen-vacancy-mediated activation, and reinforced catalytic centres, was pivotal to both sensitive colorimetric detection and high-efficiency pollutant oxidation.

Overall, cerium-based catalysts benefit from OV-rich structures, enabling efficient H₂O₂ and O₂ activation, Ce³⁺/Ce⁴⁺ redox shuttling under visible light, compatibility with Z-scheme and S-scheme heterojunctions, and broad pH stability and high structural durability. These properties allow CeO₂-based systems to operate as robust, versatile photo-Fenton catalysts capable of engaging both radical and non-radical oxidation pathways, making them particularly valuable for the treatment of persistent pollutants in realistic water conditions.

4.6 Emerging paradigms: non-radical pathways and surface-activated complexes

In addition to conventional radical pathways dominated by ·OH, increasing evidence indicates that non-radical oxidation mechanisms play a pivotal role in non-ferrous photo-Fenton systems, particularly in interface-engineered catalysts. Importantly, these processes should not be interpreted within a binary framework of radical presence or absence; rather, they involve a dynamic interplay between radical and non-radical pathways. Non-radical routes include singlet oxygen (¹O₂) generation and surface-activated complex-mediated electron transfer, both of which exhibit higher selectivity and enhanced resistance to matrix interference.

Interface features such as oxygen vacancies (OVs), defect-rich surfaces, and specific exposed crystal facets significantly facilitate these pathways. Oxygen vacancies act as electron-rich centres that promote the adsorption and activation of dissolved O₂ or H₂O₂, leading to the formation of surface-bound superoxide intermediates (·O₂⁻), which can subsequently convert into ¹O₂ via energy-transfer processes. Unlike free ·OH radicals, these reactions predominantly occur at the catalyst surface, thereby minimizing non-selective oxidation and reducing oxidant loss.

Furthermore, facet engineering (e.g., BiOBr (010) or CeO₂ (111)) modulates local electronic structure and adsorption configurations, enabling selective electron transfer to oxygen species rather than homolytic H₂O₂ cleavage. In parallel, interfacial electric fields in S-scheme and Z-scheme heterojunctions drive directional charge migration, enriching electrons at specific active sites that favour O₂ activation and ¹O₂ formation.

Notably, these non-radical pathways are especially prominent in Cu–N₄ coordinated systems and oxygen-vacancy-rich Bi and Ce-based catalysts. In such systems, the pollutant and H₂O₂ can form a metastable surface-activated ternary complex with the catalytic site, enabling direct and localized electron transfer without the involvement of freely diffusing radicals. This surface-complex-mediated mechanism provides enhanced selectivity, improved resistance to scavenging effects, and superior catalytic stability, thereby underpinning the high performance observed in interface-engineered composites.

4.7 Delineation of oxidative pathways and practical implications

In photo-Fenton systems, it is essential to distinguish between different oxidation pathways, as they have significant implications for catalytic selectivity and performance in real water matrices. Free ·OH radicals generated in solution are highly reactive and non-selective; however, they are readily scavenged by co-existing ions and natural organic matter, which can significantly reduce their effectiveness in practical applications. In contrast, surface-bound ·OH species are localized at the catalyst interface and participate in more controlled oxidation processes with reduced susceptibility to scavenging.

Furthermore, direct interfacial electron transfer represents a distinct non-radical pathway, wherein pollutants are oxidized through interaction with catalyst-bound activated species without the involvement of freely diffusing radicals. This mechanism, commonly observed in defect-rich and single-atom catalytic systems, offers enhanced selectivity, improved resistance to matrix interference, and greater stability under realistic conditions. Therefore, understanding and differentiating these pathways is crucial for the rational design of efficient and application-oriented photo-Fenton catalysts (Table 4).

Table 4 Recent non-ferrous photo-Fenton-like catalytic systems and their degradation performance toward organic pollutants

Catalyst	Target pollutant	Light source	Operational conditions	K (min^−1)	Degradation efficiency (%)	Main ROS	Ref.
Cu(OH)3F/Cu2O	Tetracycline (TCH)	300 W Xe lamp 400 ≥ λ	Catalyst dosage: 0.6 g L^−1, C0: 20 mg L^−1, [H2O2]: 40 mM, pH: 7, 30 min	0.212	99.93	·OH	89
Cu–MgO	Rhodamine B (RhB)	Xe lamp 400 ≥ λ	Catalyst dosage: 0.05 g L^−1, C0: 50 mg L^−1, [H2O2]: 20 mM, 60 min	0.0435	92.8	·O2^−	76
VO-Cu2O@Cu/CNx	Tetracycline (TC)	Visible LED lamp	Catalyst dosage: 0.2 g L^−1, C0: 20 mg L^−1, [H2O2]: 20 mM, pH: 7, 30 min	0.2	100	^1O2	45
Ag-doped CuS	Rhodamine (RhB)	UV lamp	Catalyst dosage: 15 mg, C0: 30 mg L^−1, pH: 5	—	94.9	^1O2, ·OH	128
Mesoporous CuO nanoplate	Sulfathiazole (STZ)	Visible LED lamp	[H2O2]: 20 mM, 90 min	0.0547	100	·OH	129
Cu nanoparticles modified Bi2O2S	TC HCL	Visible LED lamp	Catalyst dosage: 0.75 g L^−1, C0: 50 mg L^−1, [H2O2]: 30 μL, pH: 4, 60 min	0.243	77.76	·OH	46
Cu2−xS/g-C3N4	RhB	300 W Xe lamp	Catalyst dosage: 0.4 g L^−1, C0: 30 mg L^−1, [H2O2]: 0.5 mL, 30 min	0.172	99.6	O2·^−	47
Cu@MoS2/PAAm/CA NCDN	TC	300 W Xe lamp	Catalyst dosage: 1 g L^−1, C0: 200 mg L^−1, [H2O2]: 1.5 mM, pH: 5	—	90	·O2^−	120
CuSn(OH)6	Rh6G	500 W Xe lamp	Catalyst dosage: 30 mg, C0: 20 mg L^−1, [H2O2]: 20 mM, pH: 11, 50 min	0.084	99	·OH	119
(Cu2O/CFYO)	MO	Visible Xe lamp	[H2O2]: 1 mM, 60 min	0.0474	97	·OH, ·O2^−	130
BiOBr/Co3O4	MB	500 W Xe lamp	C0: 10 mg L^−1, [H2O2]: 50 μL, 60 min	—	96	·OH, ·O2^−	131
Pt-Cu BTC	RhB	Near IR light	Catalyst dosage: 1 g L^−1, pH: 7, 120 min	—	99	·OH	132
CPDS/CuBiO4	CIP	Visible LED lamp 420 ≥ λ	Catalyst dosage: 0.5 g L^−1, C0: 10 mg L^−1, [H2O2]: 0.5 mL, 30 min	—	92.3	·OH, O2·^−, ^1O2	51
CuO/g-C3N4	TC	400 W Na lamp 400 ≥ λ	Catalyst dosage: 0.2 g L^−1, C0: 10 mg L^−1, [H2O2]: 300 μL L^−1, 15 min	0.1254	99	·OH, O2·^−	133
Basalt powder	Atrazine (ATZ)	UV C lamp	Catalyst dosage: 0.5 g L^−1, C0: 5 mg L^−1, [H2O2]: 1.6 mM, pH: 6, 180 min	—	96	·OH	134
Cu2(OH)3F/BiOBr	TC-HCl	300 W Xe lamp 400 ≥ λ	Catalyst dosage: 0.3 g L^−1, C0: 20 mg L^−1, [H2O2]: 40 mM, pH: 5, 60 min	0.0635	98.8	·OH, O2·^−	43
Cu-decorated C@Bi/Bi2MoO6	TC	VIS lamp 400 ≥ λ	Catalyst dosage: 0.16 g L^−1, C0: 50 mg L^−1, [H2O2]: 10 μL (20 wt%), 5 min	—	96.12	·OH	96
HOFs/Cu/g-CN	TC	500 W Xe lamp 420 ≥ λ	Catalyst dosage: 0.3 g L^−1, C0: 20 mg L^−1, [H2O2]: 150 μL (30 wt%), 20 min	—	96	·OH	135
Cu mesh/Cu7S4	MB	VIS lamp	C0: 10 mg L^−1, [H2O2]: 5 mL (30 wt%), 140 min	—	98.4	·OH	136
CuLPMo12	RhB	300 W Hg lamp	Catalyst dosage: 0.136 g L^−1, C0: 4.8 mg L^−1, [H2O2]: 10 mM, pH: 7	—	99.1	·OH, ^1O2	137
Cu-ZIF-9-ica	MB	24 W LED lamp	Catalyst dosage: 0.5 g L^−1, C0: 5 mg L^−1, [H2O2]: 35 μL, 45 min	0.0475	94	·OH	54
Posnjakite (CP)	MB	Vis Xe lamp	Catalyst dosage: 30 mg, C0: 20 mg L^−1, [H2O2]: 0.2 M, 20 min	0.638	97	·OH	138
Mn-doped cobalt silicate@diamoite	MO	UV lamp	Catalyst dosage: 0.2 g L^−1, C0: 20 mg L^−1, [H2O2]: 25 mM, pH: 2.88, 90 min	0.0226	95	·OH	35
CDs/CuO/g-C3N4	MB	VIS lamp	Catalyst dosage: 0.6 g L^−1, C0: 20 mg L^−1, [H2O2]: 5 mM, 40 min	0.198	100%	·OH, O2·^−	40
Cu–N/C-500	CR	300 W xenon lamp	Catalyst dosage: 0.8 g L^−1, C0: 10 mg L^−1, 60 min	0.041	91.8	·OH	98
Cu2(OH)3F/CQDs-BiVO4	CIP	300 W xenon lamp	Catalyst dosage: 0.2 g L^−1, C0: 20 mg L^−1, [H2O2]: 0.2 mL, pH: 7, 60 min	0.0642	98.1	·OH	97
Cu-SA/CNH/PF	TC	Visible Xe lamp 420 ≥ λ	Catalyst dosage: 0.2 g L^−1, C0: 10 mg L^−1, [H2O2]: 160 mM, 30 min	0.121	98.1	·OH, ^1O2	52
CeO2−x/Bi2MoO6	OFX	5 W visible LED	Catalyst dosage: 0.2 g L^−1, C0: 20 mg L^−1, [H2O2]: 1.5 mM, 120 min	0.026	97.64	·OH, ^1O2	38
Bi/Ce-BMOFs	TC	5 W visible LED	Catalyst dosage: 0.6 g L^−1, C0: 20 mg L^−1, [H2O2]: 10 mM, 60 min	—	83	·OH, O2·^−	39
Co3O4–CeO2	Hydroquinone (HQ)	500 W Xe lamp	Catalyst dosage: 30 mg, C0: 33 mg L^−1, [H2O2]: 3 mL (30 wt%)	—	98.21	·OH, O2·^−	127
Ce-BMO	TC	Visible LED lamp	[H2O2]: 9.5 mM, 40 min	0.0487	90.7	O2·, ^1O2	38
CeO2–Ba4TaO8Cl	OFX	5 W LED lamp	Catalyst dosage: 0.2 g L^−1, C0: 20 mg L^−1, [H2O2]: 4.5 mM	0.096	98.78	·OH, O2·^−	102
Mesoporous ceria nanoparticles	AO7	LED lamp	Catalyst dosage: 1 g L^−1, C0: 70 mg L^−1, 3 min	—	90	·OH	73
Ce4O7/Bi4MoO9	TC	5 W visible LED lamp	Catalyst dosage: 0.4 g L^−1, C0: 10 mg L^−1, [H2O2]: 3 mM, 120 min	0.036	99.8	·OH, O2·^−	59
Cu–Bi2O2S	TCH	Visible lamp	Catalyst dosage: 0.75 g L^−1, C0: 50 mg L^−1, [H2O2]: 30 μL, pH: 4, 60 min	0.0243	77.76	·OH	46
Pt/CoAl2O4	Malathion	300 W Xe lamp	Catalyst dosage: 1 g L^−1, C0: 10 mg L^−1, [H2O2]: 4 mM, pH: 8	—	94.61	·OH, O2·^−	123
ZnCo2O4–ZnO Li(Mn0.75Ni0.25)2O4 mixed oxide	MB	20 W UV lamp	Catalyst dosage: 0.4 g L^−1, C0: 20 mg L^−1, [H2O2]: 19 mM, 90 min	—	93	·OH	139
B-TiO2@ZIF-67	Bisphenyl-A	Visible lamp	Catalyst dosage: 0.5 g L^−1, C0: 50 mg L^−1, [H2O2]: 16 mM, 6 min	0.045	95.30	O2·^−	121
CoSx@TiO2	CIP	300 W Xe lamp	C0: 10 mg L^−1, pH: 5, 100 min	0.0356	100	·OH, O2·^−	124
Cu-ZBP	MO	72 W UV A lamp	C0: 3 mg L^−1, [H2O2]: 22 mM, pH: 6.12, 60 min	—	90	O2·^−	140
Bi2WO6/CoAl LDH	OTC	300 W Xe lamp 420 ≥ λ	Catalyst dosage: 1 g L^−1, C0: 10 mg L^−1, [H2O2]: 50 mM, 60 min	—	98.47	·OH, O2·^−	41
Co–POM/N–TiO2	RhB	300 W Xe lamp	Catalyst dosage: 0.8 g L^−1, [H2O2]: 2 μL (30 wt%), pH: 7, 40 min	0.092	97.47	·OH, O2·^−	83
BiOBr/Co3O4	MB	500 W xenon lamp	C0: 10 mg L^−1, [H2O2]: 50 μL, 60 min	—	96	·OH, O2·^−	131
Zn0.96Co0.03Ni0.01O nanorods	Chloramphenicol (CAP)	Visible lamp	Catalyst dosage: 0.05 g L^−1, [H2O2]: 100 μL, 500 min	0.0052	94.2	·OH	105
g-C3N4/ZnCo2O4	RhB	40 W visible LED	Catalyst dosage: 01 g L^−1, C0: 10 mg L^−1, [H2O2]: 1 mL (30 wt%), pH: 7, 80 min	0.03	94%	·OH	49
C3N4x/AgOy@Co1−xBi1−yO7	OTC	100 W tungsten bulb	C0: 25 mM, pH: 7, 160 min	—	93	·OH, O2·^−	83
Co-g-C3N4/DE	4-Chlorophenol (4CP)	500 W Xe lamp	Catalyst dosage: 0.3 g L^−1, C0: 10 mg L^−1, [H2O2]: 2 mM, pH: 3, 100 min	—	85	·OH, O2·^−	37
Ti/TiO2–CoWO4	MO	100 W UV lamp	C0: 10 mg L^−1, [H2O2]: 10 mM, pH: 6.8, 180 min	—	85	·OH	141
Co-SMA	MO	Visible lamp	Catalyst dosage: 0.029 g L^−1, C0: 30 mg L^−1, [H2O2]: 38 mM, pH: 4.31	—	94.79	·OH	142
ZnAl2O4/BiPO4	MB	100 W UV Hg lamp	Catalyst dosage: 0.8 g L^−1, C0: 30 mg L^−1, 180 min	—	92.55	·OH	143
Prussian blue/Mn3O4	Levofloxacin (LVF)	30 W LED lamp	[H2O2]: 20 mM, 60 min	0.076	93.9	^1O2	48
AlMnCuZnBi alloy	MO	White LED lamp	Catalyst dosage: 1 g L^−1, C0: 10 mg L^−1, [H2O2]: 0.5 M, 170 min	—	100%	·OH	116
Mn3O4/ZnO on ACFs	MB	500 W visible Xe lamp	Catalyst dosage: 0.6 g L^−1, C0: 5 mg L^−1, 120 min	0.0328	100	·OH, O2·^−	101
CuOx/MnOy@BC	Metronidazole (MNZ)	300 W Xe lamp	Catalyst dosage: 0.75 g L^−1, C0: 20 mg L^−1, [H2O2]: 20 mM, pH: 7	0.0216	99.7	·OH	56
Cu(II) quinoline complex	MB	30 W UV light	Catalyst dosage: 0.2 g L^−1, C0: 5 mg L^−1, 150 min	—	95	·OH, O2·^−	144
Al/Cu-PILC	Paracetamol	8 W UV C Hg lamp	Catalyst dosage: 0.5 g L^−1, C0: 100 mg L^−1, [H2O2]: 145 μL, pH: 5.8, 120 min	0.231	100	·OH	145
CuSe(Cu2Se)/g-C3N4	MB	Visible Xe lamp	Catalyst dosage: 0.4 g L^−1, C0: 30 mg L^−1, [H2O2]: 0.1 mL, 60 min	0.083	98.3	·OH	146
Ag/SnO2	Naproxen (NPS)	18 W visible LED	Catalyst dosage: 0.2 g L^−1, C0: 40 mg L^−1, [H2O2]: 0.2 mL, pH: 6, 40 min	0.0946	96.85	·OH	147
Cu–Cu2O/g-C3N4	RhB	Visible lamp	[H2O2]: 3 mM, 50 min	0.041	99	·OH	148
Cu^0@CuZ	Phenol	Visible lamp	C0: 20 mg L^−1, 15 min	—	100	·OH	104
Phosphate doped ZnO	MB	72 W visible LED lamp	Catalyst dosage: 0.3 g L^−1, C0: 10 mg L^−1, [H2O2]: 1.5 mL (30 wt%), pH: 3, 120 min	0.024	92.35	·OH	149
LMSN	RhB	Sunlight	0.22 g L^−1, C0: 1 mg L^−1, [H2O2]: 0.8 mL,105 min	—	97	·OH, ·O2^−	150
Copper(II) phosphate	CIP	200 W visible XE lamp	C0: 15 mg L^−1, [H2O2]: 50 μL (30 wt%), pH: 6.5, 60 min	0.004	95	·OH	118
Gd2−xLaxZr2O7	Crystal violet (CV)	36 W UV lamp	C0: 10 mg L^−1, [H2O2]: 1 mL (33 wt%), 60 min	—	90	·OH, ·O2^−	151
CeO2–MoO3	RhB	Sunlight	C0: 1 mg L^−1, [H2O2]: 10 mM, pH: 7, 120 min	—	95.36	·OH, ·O2^−	152
CuN/C-500	CR	300 W Xe lamp	Catalyst dosage: 0.8 g L^−1, C0: 10 mg L^−1, 60 min	0.0418	91.8	·OH, ·O2^−	98
CuxMn0.03−xBi1–xV0.97+xO4	CIP	Visible LED	Catalyst dosage: 0.2 g L^−1, C0: 10 mg L^−1, [H2O2]: 1 M, pH: 3, 100 min	—	100	·OH	57
CuO/g-C3N4	Dimethyl phenol (2,4-DMP)	35 W visible LED	Catalyst dosage: 0.5 g L^−1, [H2O2]: 0.2 mM, 120 min	0.0006	99	·OH	99
CNNS/Ag/Ni heterostructure	Metronidazole	Visible lamp	Catalyst dosage: 1 g L^−1, C0: 20 mg L^−1, 60 min	—	92.9	·OH	94
CuCo2S4/BiWO4	TC-HCl	300 W metal halide lamp	Catalyst dosage: 1 g L^−1, C0: 40 mg L^−1, [H2O2]: 100 μL, pH: 5	—	80	·OH, ·O2^−	78
MnO2/CNK-OH-Mn	Tc	300 W Xe lamp	Catalyst dosage: 0.5 g L^−1, C0: 10 mg L^−1, [H2O2]: 10 mM	—	96.7	·OH, ·O2^−	81
C/N-CCO@AgAu	Ofloxacin (OFX)	Hg lamp	Catalyst dosage: 1 g L^−1, C0: 20 mg L^−1, 60 min	—	91	·OH, ·O2^−	53
Cu2O/Bi2MoO6	CIP	300 W Xe lamp	Catalyst dosage: 0.4 g L^−1, C0: 50 mg L^−1, pH: 3, 120 min	—	100	·OH, ·O2^−	87
Ba4Ti3O12–CuBi2O4	OTC	300 W Xe lamp	Catalyst dosage: 0.4 g L^−1, C0: 10 mg L^−1, [H2O2]: 0.7 mL (30 wt%), pH: 7, 50 min	—	96	·OH, ·O2^−	88
Cu^0 + Ag^0@bentonite	AMX	9 W UV A light	C0: 2 mg L^−1, [H2O2]: 0.2 mL L^−1, pH: 3, 120 min	0.0095	84	·OH, ·O2^−	95
MgO/g-C3N4	RhB	30 W xenon lamp	Catalyst dosage: 1 g L^−1, C0: 100 mg L^−1, [H2O2]: 2 mL	0.0263	80	·OH, ·O2^−	84
Cu–N–C-700	TC	30 W xenon lamp	Catalyst dosage: 0.6 g L^−1, C0: 10 mg L^−1, [H2O2]: 0.2 mL, 15 min	—	98.5	·OH	106
CuNi2Al-LDH	Phenol	30 W xenon lamp	Catalyst dosage: 0.4 g L^−1, C0: 20 mg L^−1, [H2O2]: 6 mM, pH: 3, 30 min	—	100	·OH, ·O2^−	111
CuO@C	Paracetamol	Visible LED lamp	Catalyst dosage: 1 g L^−1, C0: 0.008 mM, [H2O2]: 5 mM, 60 min	—	95	·OH	108
Cu–C3N4	CIP	30 W xenon lamp	Catalyst dosage: 0.4 g L^−1, C0: 10 mg L^−1, pH: 7, 30 min	0.0978	99	^1O2	112
Ag–Co-CN	Phenol	Visible lamp	Catalyst dosage: 20 mg, C0: 10 mg L^−1, 180 min	0.0135	94	·OH, ^1O2	113

---

5. Challenges and outlook

Despite the impressive progress in the development of non-ferrous photo-Fenton catalysts, particularly Cu, Co, Mn, Bi, and Ce-based systems, several scientific and technological barriers must still be overcome to transition from lab-scale studies to real-world environmental remediation applications. A critical assessment of these challenges is essential to chart a path forward for the design of more efficient, stable, and sustainable systems.

5.1 Catalyst stability and leaching: a quantitative comparison

A persistent issue across non-iron photo-Fenton systems is the instability of active metal sites under prolonged photocatalytic conditions. For instance, Cu⁺-based catalysts are prone to oxidation into Cu²⁺, leading to structural collapse or leaching of active Cu ions into solution.¹⁵³ Similarly, Co²⁺ and Mn²⁺ ions can migrate out of the catalyst lattice during cyclic operations, raising concerns about secondary contamination and limiting reusability. Even cerium-based materials, despite the robust Ce⁴⁺/Ce³⁺ redox pair, may suffer from partial surface dissolution under highly acidic or oxidative environments. Therefore, designing stable anchoring frameworks, such as mesoporous supports or MOFs with strong metal–ligand bonding, is crucial for minimizing leaching while retaining catalytic activity.

In addition to qualitative stability assessment, recent studies increasingly report quantitative metal leaching data to evaluate catalyst durability. A critical factor in the practical deployment of non-ferrous photo-Fenton systems is the stability of active metal sites and the mitigation of secondary pollution via leaching. A quantitative comparison of modern interface engineering strategies reveals significant variations in metal retention and structural durability after repeated reaction cycles.

• Atomically dispersed and single-atom sites: this paradigm offers the highest degree of metal immobilization. For instance, the CuCN system, featuring atomically dispersed sites, demonstrated high stability with 83.3% retention after 5 cycles and Cu leaching levels confirmed via ICP at less than 30 μg L⁻¹. Similarly, the Cu-SA/CNH/PF architecture maintained excellent stability over 5 cycles (80.3% retention), with extremely low Cu leaching (25 μg L⁻¹) as verified by ICP-MS.

• Bimetallic and high-entropy effects: multicomponent systems like the AlMnCuZnBi alloy utilize the “high-entropy effect” to prevent significant leaching of active sites. This strategy yielded high structural stability and approximately 94% retention after 5 cycles.

• Cocatalysts and intermediate stabilization: the use of cocatalysts and electron mediators, such as in the CuO/g-C₃N₄ composite, provides a chemical pathway for stabilization. In this system, leaching is significantly suppressed to less than 0.085 mg L⁻¹ by the presence of H₂O₂, which facilitates the formation of a protective CuO₂ intermediate.

• Porous scaffolds and heterojunction engineering: frameworks that provide lattice anchoring or directional charge dynamics also show promising results. The Bi₂WO₆/CoAl-LDH S-scheme heterojunction remained structurally stable (verified by XRD) after 4 cycles with 88.74% retention, keeping Co²⁺ leaching below 1.0 mg L⁻¹. Porous scaffolds such as CuNi₂Al-LDH exhibited high retention over 4 cycles with a reported Cu leaching level of 0.73 ppm.

As summarized in Table 3, the shift toward atomic-level dispersion and high-entropy alloying represents the most effective technical approach for ensuring the long-term environmental safety of non-ferrous catalysts. These quantitative findings provide a robust roadmap for designing interfaces that minimize leaching while maintaining high catalytic flux in complex wastewater matrices.

5.2 Matrix effects and practical adaptability

In practical wastewater treatment, the performance of non-ferrous photo-Fenton catalysts is significantly influenced by the presence of co-existing ions and natural organic matter, collectively referred to as matrix effects. Unlike model laboratory conditions, real water systems contain a complex mixture of inorganic anions (e.g., Cl⁻, HCO₃⁻, PO₄³⁻, SO₄²⁻) and dissolved organic substances such as humic and fulvic acids, which can interfere with catalytic processes.

Among these, chloride ions (Cl⁻) can react with hydroxyl radicals (·OH) to form less reactive chlorine-based radicals (Cl·, Cl₂·⁻), thereby reducing oxidation efficiency. Similarly, bicarbonate (HCO₃⁻) and carbonate (CO₃²⁻) ions act as scavengers of ·OH, forming carbonate radicals (CO₃·⁻) with lower oxidation potential. Phosphate ions (PO₄³⁻), on the other hand, can strongly adsorb onto catalyst surfaces, blocking active sites and inhibiting H₂O₂ activation. In addition, natural organic matter such as humic acids can compete with target pollutants for adsorption sites, alter surface charge properties, and even quench reactive oxygen species.

Importantly, the extent of matrix interference depends on the dominant reaction pathway. Radical-based systems relying on free ·OH are highly susceptible to scavenging by co-existing species, whereas non-radical pathways, such as singlet oxygen (¹O₂) generation and surface-mediated electron transfer, exhibit greater selectivity and resistance to interference. Interface engineering plays a crucial role in mitigating matrix effects by promoting selective adsorption, enhancing charge separation, and favouring localized reaction pathways. For example, oxygen-vacancy-rich surfaces and S-scheme heterojunctions can facilitate controlled ROS generation and reduce undesired side reactions with background ions.

Overall, understanding and addressing matrix effects is essential for translating laboratory-scale catalytic performance to real-world wastewater treatment applications.

5.3 Quantitative benchmarking and performance metrics

A significant challenge in evaluating the performance of non-ferrous photo-Fenton systems is the lack of standardized experimental protocols across the literature. Direct comparison of pseudo-first-order rate constants (k, min⁻¹) is often misleading, as k is highly sensitive to parameters such as catalyst dosage (C_cat), initial pollutant concentration (C₀), light intensity, and reactor configuration.

To enable more meaningful comparison, normalized performance descriptors can be considered. One such parameter is the mass-normalized rate constant (k_m), defined as:

where k_m is expressed in L g⁻¹ min⁻¹. This metric accounts for variations in catalyst loading and provides a more representative indication of intrinsic catalytic activity.

However, it is important to note that even normalized parameters may be influenced by additional experimental factors, including light intensity and solution chemistry. Therefore, such metrics should be interpreted cautiously and used primarily to identify general performance trends rather than for strict quantitative comparison across different studies.

Overall, the development of standardized testing protocols and reporting guidelines will be essential for enabling reliable benchmarking and accelerating the rational design of high-performance photo-Fenton catalysts.

5.4 H₂O₂ consumption and economic viability

While degradation efficiency is the most frequently reported metric in photo-Fenton studies, the utilization efficiency of H₂O₂ is also a critical factor in determining the economic and environmental feasibility of the process. In many systems, a significant fraction of H₂O₂ may be lost through non-productive pathways, including self-decomposition and scavenging reactions with excess oxidant. From a practical perspective, inefficient H₂O₂ utilization leads to increased operational costs and reduced process sustainability. Therefore, it is essential to evaluate how effectively H₂O₂ is converted into reactive oxygen species (ROS) for pollutant degradation.

Interface engineering provides an effective strategy to improve H₂O₂ utilization efficiency. By modulating surface electronic structure and adsorption behaviour, engineered interfaces can promote selective activation of H₂O₂ toward ROS generation while suppressing unproductive decomposition pathways. For example, defect-rich catalysts, heterojunction systems, and atomically dispersed metal sites can enhance controlled H₂O₂ activation and improve oxidant efficiency.

In this context, metrics such as H₂O₂ consumption efficiency (η) can be considered for a more realistic assessment of catalytic performance. These parameters are particularly important for bridging the gap between laboratory-scale studies and practical wastewater treatment applications.

5.5 Interface engineering complexity

The creation of efficient heterojunctions or atomically precise interfaces remains synthetically challenging, particularly for large-scale production. Techniques such as atomic layer deposition (ALD), electrospinning, or solvothermal synthesis often require stringent control over temperature, pH, or precursor ratios to maintain desired morphology and interface composition. Moreover, ensuring reproducibility and uniform dispersion of active sites is still difficult in batch-to-batch synthesis. Scalable and green fabrication methods like mechanochemical synthesis or bio-templating need to be developed to make such advanced materials industrially viable.

5.6 Narrow light absorption range and photocorrosion

Although visible-light-responsive materials like g-C₃N₄, BiOBr, and Co₃O₄ have expanded the working wavelength of photo-Fenton catalysts, many systems still suffer from limited light absorption or rapid electron–hole recombination. Additionally, photocorrosion of materials such as Bi-based catalysts under high-flux light can compromise long-term durability. Incorporating plasmonic nanoparticles (e.g., Ag, Au) or constructing broadband absorbers via doped semiconductors offers a potential path to widen spectral absorption while suppressing photocorrosion.

5.7 ROS selectivity and byproduct toxicity

Current systems often rely on nonspecific ROS such as ·OH and ·O₂⁻, which indiscriminately attack organic pollutants. However, this lack of selectivity may lead to incomplete mineralization or the formation of toxic intermediates, particularly in pharmaceutical degradation. Advanced characterization techniques and LC-MS studies have shown accumulation of quinolone or aromatic byproducts even at high degradation percentages. Future systems should aim to steer ROS pathways using ligand-functionalized surfaces or microenvironment engineering (e.g., acidic/basic zones) to maximize pollutant selectivity and safety.

5.8 Real-world wastewater complexity

While the specific effects of co-existing ions and natural organic matter on catalytic activity have been discussed in section 5.2, additional challenges arise when translating non-ferrous photo-Fenton systems from controlled laboratory conditions to real wastewater environments. Most reported studies rely on model pollutants (e.g., rhodamine B, tetracycline) in deionized or simplified aqueous systems, which do not adequately represent the complexity of real wastewater. In practical scenarios, wastewater streams typically contain diverse pollutant mixtures, including pharmaceuticals, dyes, and industrial organics, along with fluctuating pH, variable ionic strength, and suspended solids. These factors can significantly alter catalyst surface properties, light penetration, and mass transfer dynamics.

Furthermore, the presence of turbidity and coloured dissolved organic matter can reduce photon utilization efficiency by attenuating light intensity, thereby limiting photocatalytic activation. In addition, competitive adsorption among multiple pollutants may reduce the effective availability of active sites, leading to decreased degradation efficiency compared to single-component systems. From an operational perspective, real-world implementation also requires consideration of catalyst recovery, long-term stability under continuous flow conditions, and scalability of synthesis methods. The transition from batch laboratory experiments to pilot-scale or continuous-flow reactors introduces additional constraints related to hydrodynamics, catalyst dispersion, and reactor design.

Importantly, the impact of real wastewater matrices varies significantly depending on the nature of the metal system and its dominant reaction pathway. Cu and Co-based catalysts, which often rely on radical-driven mechanisms, are more susceptible to inhibition by common anions such as Cl⁻ and HCO₃⁻ due to scavenging of ·OH radicals. In contrast, Bi and Ce-based systems, particularly those with abundant oxygen vacancies, tend to operate via non-radical pathways (e.g., ¹O₂ or surface-mediated electron transfer), which exhibit greater resistance to such interference. Mn-based catalysts typically display intermediate behaviour, where both radical and non-radical pathways may coexist, leading to variable sensitivity depending on surface structure and oxidation state. Additionally, natural organic matter (e.g., humic substances) can compete for active sites and modify surface charge, further influencing catalyst performance in a system-dependent manner.

Therefore, future research should prioritize testing under realistic wastewater matrices, integrating continuous-flow systems, and conducting pilot-scale studies to bridge the gap between laboratory-scale performance and practical environmental applications.

5.9 The environmental trade-off: secondary toxicity and regulatory compliance

A significant hurdle for the large-scale transition from iron-based Fenton processes to non-ferrous systems is the inherent toxicity of the metals themselves. While iron is an environmentally benign baseline, transition metals such as Cu, Co, and Mn possess complex toxicity profiles that can lead to secondary contamination if leaching is not strictly controlled. This creates a “dual challenge” where the catalyst must maintain high interfacial activity while ensuring metal dissolution remains below stringent parts-per-billion (ppb) levels.

• Regulatory thresholds and metal-specific risks: the environmental impact varies significantly across different metal systems. For instance, cobalt (Co) is a high-priority concern due to its potential bioaccumulation and carcinogenicity, with industrial discharge often capped at 1.0 mg L⁻¹.¹⁵⁴ Copper (Cu), while less toxic to humans, exhibits high aquatic toxicity to invertebrates and fish, with WHO and EPA limits set between 1.3 and 2.0 mg L⁻¹.¹⁵⁵ Excessive manganese (Mn) is associated with neurotoxicity. Health-based limits are generally set around 0.4 mg L⁻¹.¹⁵⁶

• The “green” alternative gap: bismuth (Bi) and cerium (Ce) are frequently proposed as “greener” alternatives due to their low systemic toxicity. However, their catalytic turnover frequencies often lag behind Cu or Co-based systems, requiring more intensive interface engineering to reach competitive degradation rates.

• Long-term operational stability: a major challenge remains in assessing “real-world” leaching over hundreds of hours. Most current studies report stability over only 5 to 10 cycles, which is insufficient to guarantee that metal–N_x sites or high-entropy alloys will remain leaching-free under the continuous oxidative stress of industrial-scale H₂O₂ activation.

To overcome this, future research must prioritize quantitative mass-balance studies and standardized leaching protocols that test catalysts in complex wastewater matrices where competing anions (like Cl⁻ or PO₄³⁻) might accelerate metal dissolution. Ensuring that treated water meets both organic pollutant limits and heavy metal safety standards is the final bridge that non-ferrous photo-Fenton technology must cross.

Outlook.
(1) Atomic-level engineering of active sites. Although atomically dispersed catalysts offer maximum atom efficiency and tunable electronic properties, their controlled synthesis and long-term stability under reaction conditions remain challenging. Most current methods for anchoring single or dual metal atoms (e.g., Cu, Co, Mn) on supports such as g-C₃N₄ or graphene suffer from aggregation and metal leaching. Future strategies should focus on stabilizing metal–N/O coordination environments using defect-rich or heteroatom-doped carbon frameworks, as well as in situ/operando spectroscopy to track coordination dynamics during catalysis.
(2) Design of multimodal interface architectures. While individual interfaces like type-II or Z-scheme heterojunctions have shown promise, hybrid systems integrating multiple interfacial types (e.g., type-II with atomically dispersed metals or embedded in porous frameworks) remain underexplored. Developing composite catalysts with synergistic interfaces can enhance directional charge transfer and suppress ROS recombination. Rational design of such architectures requires integrated synthetic computational approaches to optimize spatial configurations and electron pathways at the nano–micro scale.
(3) Surface functionalization and charge regulation via metal nanoparticles. Noble and transition metal nanoparticles (e.g., Ag, Cu, Pt) can act as electron sinks or plasmonic sensitizers, but their catalytic role in non-ferrous photo-Fenton systems often lacks mechanistic clarity. Future studies should explore size-dependent effects, support–metal interactions, and plasmon-induced hot carrier dynamics through advanced characterization and theoretical modelling. Tailoring interfacial charge densities and local electric fields will be key to enhancing H₂O₂ activation and ROS selectivity.
(4) Scaffolded interfaces with porous frameworks. Despite the success of MOFs, COFs, and porous carbon materials in pollutant capture and dispersion of active species, their integration with photo-Fenton centres is still in its infancy. The development of hierarchically structured supports with controlled pore size distribution, hydrophilicity, and functional anchoring groups is crucial. Additionally, structure–activity correlations between support chemistry and catalytic efficiency should be systematically studied using DFT and experimental mapping techniques.
(5) Mechanistic exploration of ROS pathways. The true identity and interplay of ROS (·OH, ·O₂⁻, ¹O₂) in Cu, Co, Mn, Bi, and Ce-based systems remain ambiguous due to overlapping generation mechanisms. Although scavenger tests and EPR studies are widely used, they often provide indirect or incomplete evidence. Time-resolved spectroscopy, isotope labelling, and operando techniques (e.g., XAS, TRPL) are necessary to delineate the ROS evolution kinetics, charge transfer routes, and interfacial energy band modulations during light-assisted H₂O₂ activation.
(6) AI/ML-driven catalyst discovery and design. The integration of artificial intelligence and machine learning (AI/ML) with materials chemistry offers a powerful strategy to bypass the traditional “trial-and-error” bottleneck in non-ferrous photo-Fenton catalyst design. ML models can be trained on Density Functional Theory (DFT)-derived datasets to predict key physicochemical descriptors that govern the catalytic cycle at the interface.

A primary challenge in photo-Fenton-like systems is optimizing the adsorption energy of H₂O₂. If the binding is too weak, the activation is inefficient; if too strong, the active sites become poisoned by intermediates. ML algorithms can rapidly screen metal-support combinations and defect structures to identify the “volcano plot” peak for H₂O₂ binding, promoting efficient reactive oxygen species (ROS) generation. Similarly, band alignment is a crucial descriptor for heterojunction engineering. ML models trained on electronic structure data can predict the Fermi levels and band edge positions of new semiconductor pairs, enabling the rational construction of S-scheme or Z-scheme systems with maximized redox potential.

Furthermore, ML can predict defect formation energies, particularly for oxygen vacancies, which are vital for tuning charge density. By combining these computational descriptors with experimental datasets through Active Learning loops, researchers can establish quantitative structure–activity relationships (QSAR). This data-driven approach facilitates the high-throughput screening of interfacial architectures, ultimately accelerating the discovery of optimized catalysts with enhanced H₂O₂ activation efficiency and long-term operational stability.

(7) Bifunctional systems for in situ H₂O₂ generation. A major limitation in the practical deployment of photo-Fenton systems is the reliance on externally added H₂O₂. A promising high-risk research direction involves the development of bifunctional catalytic interfaces capable of driving the two-electron oxygen reduction reaction (2e⁻ ORR) for in situ H₂O₂ generation.

It is hypothesized that coupling plasmonic nanoparticles (e.g., Ag, Au) with Cu–N_x coordinated sites or oxygen-vacancy-rich Bi-based surfaces can enable simultaneous generation and activation of H₂O₂ at the same interface. Such a synergistic system could facilitate localized oxidant production from dissolved O₂, thereby minimizing mass transport limitations and improving overall process efficiency. This self-sustaining catalytic cycle has the potential to significantly reduce chemical input requirements and enhance the economic feasibility of photo-Fenton processes.

(8) Pulsed-light operation and time-resolved catalytic dynamics. Most reported photo-Fenton systems operate under continuous irradiation, which can lead to inefficient charge utilization due to the mismatch between photogenerated charge carrier flux and the intrinsic kinetics of surface redox cycles (e.g., Co²⁺/Co³⁺).

An emerging hypothesis is that pulsed-light irradiation, employing microsecond (μs) or millisecond (ms) timescales, can be used to better synchronize charge carrier generation with catalytic turnover processes. By temporally modulating light input, this approach may reduce charge recombination and allow active sites to undergo complete redox cycling between pulses. Such dynamic operation could enhance quantum efficiency and provide new insights into time-resolved catalytic mechanisms in non-ferrous photo-Fenton systems.

6. Conclusion

The development of non-ferrous photo-Fenton catalysts has unlocked new possibilities in sustainable water remediation by expanding the catalytic toolbox beyond traditional iron-based systems. Transition metals such as Cu, Co, Mn, Bi, and Ce have demonstrated remarkable redox activity under visible light in the presence of H₂O₂, facilitating the generation of reactive oxygen species (ROS) essential for efficient pollutant degradation. Recent advances in interface engineering, ranging from type-II and Z-scheme heterojunctions, p–n junctions, to atomically dispersed metal sites and bimetallic interfaces, have significantly enhanced charge separation, optimized band alignment, and improved H₂O₂ activation kinetics. These tailored architectures have also shown superior selectivity, faster degradation rates, and better adaptability to diverse water matrices. Nonetheless, challenges persist, particularly regarding catalyst leaching, limited reusability, and incomplete understanding of ROS formation mechanisms across various interfacial configurations. Furthermore, real-world water treatment often involves complex mixtures, variable pH, and interfering species, which are underexplored in current lab-scale studies. Future efforts should focus on (i) integrating machine learning and theoretical modelling to guide rational interface design, (ii) advancing operando characterization tools to capture transient species during photocatalysis, and (iii) scaling up these systems through green synthesis, stable supports, and reactor design optimization. By addressing these aspects, non-iron photo-Fenton-like systems with engineered interfaces can transition from promising academic discoveries to practical solutions for global water pollution challenges.

Conflicts of interest

The authors declare that they have no conflict of interest.

List of abbreviations

AI/ML	Artificial intelligence/machine learning
AOPs	Advanced oxidation processes
BiOX	Bismuth oxyhalides (X = Cl, Br, I)
CB	Conduction band
g-C3N4	Graphitic carbon nitride
DIET	Direct interfacial electron transfer
DFT	Density functional theory
ECs	Emerging contaminants
EPR	Electron paramagnetic resonance
H2O2	Hydrogen peroxide
CNT	Carbon nanotubes
rGO	Reduced graphene oxide
IEFs	Internal electric fields
LDHs	Layered double hydroxides
LMCT	Ligand-to-metal charge transfer
MOFs	Metal–organic frameworks
NHE	Normal hydrogen electrode
NOM	Natural organic matter
OVs	Oxygen vacancies
PPCPs	Pharmaceuticals and personal care products
QSAR	Quantitative structure–activity relationships
ROS	Reactive oxygen species
SACs	Single-atom catalysts
VB	Valence band
XPS	X-ray photoelectron spectroscopy

Data availability

This article is based on an analysis of previously published data. All relevant data supporting the findings of this study are included within the article. Original sources of the data are cited throughout the manuscript.

References

1. M. A. Montgomery and M. Elimelech, Water and Sanitation in Developing Countries: Including Health in the Equation, Environ. Sci. Technol., 2007, 41(1), 17–24.
2. M. A. Shannon, P. W. Bohn, M. Elimelech, J. G. Georgiadis, B. J. Mariñas and A. M. Mayes, Science and Technology for Water Purification in the Coming Decades, Nature, 2008, 452(7185), 301–310.
3. G. K. Ramesha, A. V. Kumara, H. B. Muralidhara and S. Sampath, Graphene and Graphene Oxide as Effective Adsorbents toward Anionic and Cationic Dyes, J. Colloid Interface Sci., 2011, 361(1), 270–277.
4. C. Pan and Y. Zhu, New Type of BiPO₄ Oxy-Acid Salt Photocatalyst with High Photocatalytic Activity on Degradation of Dye, Environ. Sci. Technol., 2010, 44(14), 5570–5574.
5. J. Xu, L. Wang and Y. Zhu, Decontamination of Bisphenol A from Aqueous Solution by Graphene Adsorption, Langmuir, 2012, 28(22), 8418–8425.
6. F. Liang and Y. Zhu, Enhancement of Mineralization Ability for Phenol via Synergetic Effect of Photoelectrocatalysis of G-C₃N₄ Film, Appl. Catal., B, 2016, 180, 324–329.
7. P. Ghorbani, A. Hassani, P. Eghbali and A. Abbasi, Activated carbon-based photocatalysts for the removal of pesticides, in Pesticide Removal Methods from Wastewater: Proactive Approaches and Future Trends, ed. A. Hassani, CRC Press, 2025, pp. 203 –236.
8. J. Wang and R. Zhuan, Degradation of Antibiotics by Advanced Oxidation Processes: An Overview, Sci. Total Environ., 2020, 701, 135023.
9. M. J. F. Calvete, G. Piccirillo, C. S. Vinagreiro and M. M. Pereira, Hybrid Materials for Heterogeneous Photocatalytic Degradation of Antibiotics, Coord. Chem. Rev., 2019, 395, 63–85.
10. A. Hassani, N. Pourshirband, Z. Sayyar and P. Eghbali, Fenton and Fenton-like-based advanced oxidation processes, in Innovative and Hybrid Advanced Oxidation Processes for Water Treatment, ed. O. Hamdaoui, Elsevier, 2025, pp. 171 –203.
11. X. Bai, C. Sun, D. Liu, X. Luo, D. Li, J. Wang, N. Wang, X. Chang, R. Zong and Y. Zhu, Photocatalytic Degradation of Deoxynivalenol Using Graphene/ZnO Hybrids in Aqueous Suspension, Appl. Catal., B, 2017, 204, 11–20.
12. Y. Deng and C. M. Ezyske, Sulfate Radical-Advanced Oxidation Process (SR-AOP) for Simultaneous Removal of Refractory Organic Contaminants and Ammonia in Landfill Leachate, Water Res., 2011, 45(18), 6189–6194.
13. Y. Deng and R. Zhao, Advanced Oxidation Processes (AOPs) in Wastewater Treatment, Curr. Pollut. Rep., 2015, 1(3), 167–176.
14. J. P. Ribeiro, C. C. Marques, I. Portugal and M. I. Nunes, Fenton Processes for AOX Removal from a Kraft Pulp Bleaching Industrial Wastewater: Optimisation of Operating Conditions and Cost Assessment, J. Environ. Chem. Eng., 2020, 8(4), 104032.
15. D. B. Miklos, C. Remy, M. Jekel, K. G. Linden, J. E. Drewes and U. Hübner, Evaluation of Advanced Oxidation Processes for Water and Wastewater Treatment–A Critical Review, Water Res., 2018, 139, 118–131.
16. G. Peng, F. Gao, J. Zou, X. Wang, Y. Gao, H. Zhou, S. Liu, M. Li and L. Lu, One-Step Electrochemical Synthesis of Tremella-like Co-MOFs/Carbon Nanohorns Films for Enhanced Electrochemical Sensing of Carbendazim in Vegetable and Fruit Samples, J. Electroanal. Chem., 2022, 918, 116462.
17. Y. Zhou, L. Zhou, Y. Zhou, M. Xing and J. Zhang, Z-Scheme Photo-Fenton System for Efficiency Synchronous Oxidation of Organic Contaminants and Reduction of Metal Ions, Appl. Catal., B, 2020, 279, 119365.
18. W.-L. Duan, Y.-X. Li, W.-Z. Li and J. Luan, Controllable Synthesis of Copper-Organic Frameworks via Ligand Adjustment for Enhanced Photo-Fenton-like Catalysis, J. Colloid Interface Sci., 2023, 646, 107–117.
19. M. Zhang, H. Dong, L. Zhao, D. Wang and D. Meng, A Review on Fenton Process for Organic Wastewater Treatment Based on Optimization Perspective, Sci. Total Environ., 2019, 670, 110–121.
20. J. P. Ribeiro and M. I. Nunes, Recent Trends and Developments in Fenton Processes for Industrial Wastewater Treatment–A Critical Review, Environ. Res., 2021, 197, 110957.
21. L. Li, Z. Yin, M. Cheng, L. Qin, S. Liu, H. Yi, M. Zhang, Y. Fu, X. Yang and X. Zhou, Insights into Reactive Species Generation and Organics Selective Degradation in Fe-Based Heterogeneous Fenton-like Systems: A Critical Review, Chem. Eng. J., 2023, 454, 140126.
22. V. Cleveland, J.-P. Bingham and E. Kan, Heterogeneous Fenton Degradation of Bisphenol A by Carbon Nanotube-Supported Fe₃O₄, Sep. Purif. Technol., 2014, 133, 388–395.
23. H. Lv, H. Zhao, T. Cao, L. Qian, Y. Wang and G. Zhao, Efficient Degradation of High Concentration Azo-Dye Wastewater by Heterogeneous Fenton Process with Iron-Based Metal-Organic Framework, J. Mol. Catal. A: Chem., 2015, 400, 81–89.
24. M. Cheng, C. Lai, Y. Liu, G. Zeng, D. Huang, C. Zhang, L. Qin, L. Hu, C. Zhou and W. Xiong, Metal-Organic Frameworks for Highly Efficient Heterogeneous Fenton-like Catalysis, Coord. Chem. Rev., 2018, 368, 80–92.
25. A. P. F. M. De Urzedo, C. C. Nascentes and R. Augusti, Degradation of the Insecticides Thiamethoxam and Imidacloprid in Aqueous Solution as Promoted by an Innovative Fe⁰/Fe₃O₄ Composite, J. Braz. Chem. Soc., 2009, 20, 51–56.
26. D. Du, W. Shi, L. Wang and J. Zhang, Yolk-Shell Structured Fe₃O₄@ Void@TiO₂ as a Photo-Fenton-like Catalyst for the Extremely Efficient Elimination of Tetracycline, Appl. Catal., B, 2017, 200, 484–492.
27. X. Li, J. Liu, A. I. Rykov, H. Han, C. Jin, X. Liu and J. Wang, Excellent Photo-Fenton Catalysts of Fe–Co Prussian Blue Analogues and Their Reaction Mechanism Study, Appl. Catal., B, 2015, 179, 196–205.
28. T. Guo, K. Wang, G. Zhang and X. Wu, A Novel α-Fe₂O₃@ g-C₃N₄ Catalyst: Synthesis Derived from Fe-Based MOF and Its Superior Photo-Fenton Performance, Appl. Surf. Sci., 2019, 469, 331–339.
29. B. Ma, Y. Zha, H. Shi, Y. Qin, M. Zhao, J. Li, S. Wang, B. Yan, B. Zhao and Y. Ma, Enhanced Photocatalysis-Fenton Degradation of Levofloxacin by Fe Doped BiOCl Microspheres with Rich Surface Oxygen Vacancies: The Accelerated Redox Cycle Of≡ Fe (III)/≡ Fe (II), Sep. Purif. Technol., 2025, 354, 129086.
30. X. Wang, L. Wu, J. Wang, Y. Zhou, Y. Wang, W. D. Wu, W. Li and Z. Wu, Oxygen Vacancies and Interfacial Iron Sites in Hierarchical BiOCl Nanosheet Microflowers Cooperatively Promoting Photo-Fenton, Chemosphere, 2022, 307, 135967.
31. J. Liu, H. Wu, J. Sun, S. Li, A. Hassani and M. Zhou, Non-TiO₂-based photoanodes for photoelectrocatalytic wastewater treatment: electrode synthesis, evaluation, and characterization, EES Catal., 2025, 3(5), 921–942.
32. Y. Tian, N. Jia, H. Ma, G. Liu, Z. Xiao, Y. Wu, L. Zhou, J. Lei, L. Wang and Y. Liu, 0D/3D Coupling of g-C₃N₄ QDs/Hierarchical Macro-Mesoporous CuO-SiO₂ for High-Efficiency Norfloxacin Removal in Photo-Fenton-like Processes, J. Hazard. Mater., 2021, 419, 126359.
33. J. Li, A. N. Pham, R. Dai, Z. Wang and T. D. Waite, Recent Advances in Cu-Fenton Systems for the Treatment of Industrial Wastewaters: Role of Cu Complexes and Cu Composites, J. Hazard. Mater., 2020, 392, 122261.
34. H. Tian, M. Zhang, G. Jin, Y. Jiang and Y. Luan, Cu-MOF Chemodynamic Nanoplatform via Modulating Glutathione and H₂O₂ in Tumour Microenvironment for Amplified Cancer Therapy, J. Colloid Interface Sci., 2021, 587, 358–366.
35. N. Dai, L. Yang, X. Liu, L. Gao, J. Zheng, K. Zhang, D. Song, T. Sun, S. Luo and X. Liu, Enhanced Photo-Fenton-like Performance of Biotemplated Manganese-Doped Cobalt Silicate Catalysts, J. Colloid Interface Sci., 2023, 652, 1812–1824.
36. T. Mokoba, Z. Li, T. C. Zhang and S. Yuan, Superwetting Sea Urchin-like BiOBr@Co₃O₄ Nanowire Clusters-Coated Copper Mesh with Efficient Emulsion Separation and Photo-Fenton-like Degradation of Soluble Dye, Appl. Surf. Sci., 2022, 594, 153497.
37. L.-M. Zhang, X.-X. Lv, A.-L. Shu, X.-Z. Lu and X. Chen, Insight on the Degradation of P-Chlorophenol Based on the Co-g-C₃N₄/Diatomite Composite Photo-Fenton Process, J. Ind. Eng. Chem., 2024, 136, 305–316.
38. G. Yang, Y. Liang, H. Zheng, J. Yang, S. Guo and H. Yu, A Self-Circulating Cerium-Rich CeO_2-x/Bi₂MoO₆ Heterojunction Catalyst for Boosting Photo-Fenton Degradation of Fluoroquinolone Antibiotics, Sep. Purif. Technol., 2023, 309, 123084.
39. X. Zhang, L. Zhang, H. Ye, Z. Zeng, J. Yang, C. Han and Y. Liang, Bimetallic-Organic Frameworks Derived Bi-Doped CeO₂ Nanorods with Oxygen Vacancies for Boosted Photo-Fenton Activity, J. Environ. Chem. Eng., 2025, 117489.
40. X. Wu, Q. Zhao, J. Zhang, S. Li, H. Liu, K. Liu, Y. Li, D. Kong, H. Sun and M. Wu, 0D Carbon Dots Intercalated Z-Scheme CuO/g-C₃N₄ Heterojunction with Dual Charge Transfer Pathways for Synergetic Visible-Light-Driven Photo-Fenton-like Catalysis, J. Colloid Interface Sci., 2023, 634, 972–982.
41. B. Shao, Z. Liu, L. Tang, Q. Liang, Q. He, T. Wu, Y. Pan, M. Cheng, Y. Liu and X. Tan, Construction of Bi₂WO₆/CoAl-LDHs S-Scheme Heterojunction with Efficient Photo-Fenton-like Catalytic Performance: Experimental and Theoretical Studies, Chemosphere, 2022, 291, 133001.
42. B. Yang, J. Li, Y. Li, M. Zhang, J. Zhu, T. Zhou and J. Deng, Electrochemical Post-Treatment of Bimetallic-ICP/RGO Precursor for Z-Scheme CuO_x·Ag₂O/RGO Hetero-Structure with Catalytic Activity Enhancement for Visible-Light-Driven Photo-Fenton Degradation of Tetracycline, Sep. Purif. Technol., 2022, 299, 121741.
43. Y. Guo, Y. Liu, S. Hao, X. Zhang and Y. Yu, In-Situ Decoration of Cu₂(OH)₃F on BiOBr with Exposed (010) Facets: Heterojunctions and Photo-Fenton-like Synergistic Promote Cu⁺/Cu²⁺ Circulation, J. Water Process Eng., 2023, 55, 104140.
44. A. D. Bokare and W. Choi, Review of Iron-Free Fenton-like Systems for Activating H₂O₂ in Advanced Oxidation Processes, J. Hazard. Mater., 2014, 275, 121–135.
45. Y. Wang, C. Hu, Y. Gan, Z. Sun, Y. Deng, M. Xie, Y. Wang, J. Xie, W. Zhang and L. Huang, Plasmon-Drive Multi-Channel Hot Electron Transfer in VO-Cu₂O@ Cu/CN_x Heterojunction for Enhanced Photo-Fenton-Like Reaction: Critical Roles of Copper Species and H₂O₂, Appl. Catal., B, 2025, 125500.
46. Y. Guo, T. Han, Y. Luo, J. Xing and Y. Jia, Noble Metal-Free Cu Nanoparticles Modified Bi₂O₂S Nanosheets for Enhanced Photo-Fenton-Like Degradation and CO₂ Photoreduction. Available at SSRN 5229724.
47. Z. Kang, X. Ren, Y. Wang, J. Li, Y. Ma, Z. Feng and T. Sun, In-Situ Stripping Synthesis of Cu_2-XS/g-C₃N₄ Heterojunction with Feeble Nitrogen Vacancy Sites for Constructing Complementary Enhanced Photo-Fenton-like Systems, J. Water Process Eng., 2025, 69, 106876.
48. F. Xi, M. Yue, J. Zhao, K. Xie, J. Ma, X. Song, L. Liang and A. Hou, Photo-Fenton Enhanced Self-Floating Z-Scheme Prussian Blue/Mn₃O₄ Heterojunction for Efficient Levofloxacin Degradation: Mechanism, Pathway and DFT Analysis, Colloids Surf., A, 2025, 138653.
49. B. Palanivel, M. S. Hossain, I. N. Reddy, A. M. Al-Enizi, M. Ubaidullah, R. R. Macadangdang Jr. and J. Shim, Chemical Oxidants (H₂O₂ and Persulfate) Activated Photo-Fenton like Degradation Reaction Using Sol-Gel Derived g-C₃N₄/ZnCo₂O₄ Nanocomposite, Diamond Relat. Mater., 2022, 130, 109413.
50. Z. Wang, D. Ren and R. Chi, Synthesis of Copper-Based Fenton-like Catalyst (Cu/HAP-SBC) for Oxidative Degradation of p-Nitrophenol in Aqueous Solution, J. Mol. Struct., 2025, 1339, 142383.
51. S. Lu, X. Zhang, M. Wang, Y. Huang, S. Yin, B. Wang, J. Xia and H. Li, Ball-Milling Synthesis of CPDs/CuBi₂O₄ Z-Scheme Heterojunction Photocatalysis for Photofenton Antibiotic Wastewater Treatment, J. Environ. Chem. Eng., 2025, 118820.
52. Z. Guo, W. Wei, Y. Li, X. Niu, F. Hou, J. Li, X. Zhang, X. Zhang and A. Wei, Cu Single Atoms Anchored on Hydrangea-like Carbon Nitride for Facilitating Photo-Fenton: Role of Cu²⁺/Cu⁺ Cycle, Sep. Purif. Technol., 2024, 344, 127290.
53. J. Panda, N. Priyadarshini, S. Mansingh and K. Parida, Ce/Cu-MOF-Derived AgAu–C/N–CeO₂@ C/N–CuO S-Scheme Nanohybrids for Photocatalytic H₂O₂ Production and Photo-Fenton Antibiotic Degradation, Langmuir, 2025, 41(46), 31456–31470.
54. N. Rodríguez-Sánchez, B. Bhattacharya, F. Emmerling, C. Prinz, P. Prieto-Laria, A. R. Ruiz-Salvador and M. Ballesteros, Engineering a Multivariate Cobalt Metal–Organic Framework for High Photocatalytic Activity: The Impact of Mixed Ligands and Metal Incorporation in a Visible Light-Driven Heterogeneous Photo-Fenton Reaction for Water Treatment, Nanoscale Adv., 2025, 7(8), 2255–2265.
55. X. Jia, L. Xie, Z. Li, Y. Li, R. Ming, Q. Zhang, X. Mi and S. Zhan, Photo-Electro-Fenton-like Process for Rapid Ciprofloxacin Removal: The Indispensable Role of Polyvalent Manganese in Fe-Free System, Sci. Total Environ., 2021, 768, 144368.
56. S. Mansoori, E. J. Ozumchelouei, R. Davarnejad and A. A. Zahrani, CuO_x-MnO_y@ Biochar Nanocatalyst Synthesis for Heterogeneous Visible-Light-Driven Fenton-like: A Resistant Antibiotic Degradation, Catal. Commun., 2022, 171, 106517.
57. N. Singh and I. Sinha, Development of Cu, Mn Codoped BiVO₄ (Cu_xMn_0.03-XBi_1–xV_0.97+x O₄) Photocatalyst for Photo-Fenton Degradation of Ciprofloxacin, Langmuir, 2025, 41(23), 14935–14945.
58. E. Campagnoli, A. Tavares, L. Fabbrini, I. Rossetti, Y. A. Dubitsky, A. Zaopo and L. Forni, Effect of Preparation Method on Activity and Stability of LaMnO₃ and LaCoO₃ Catalysts for the Flameless Combustion of Methane, Appl. Catal., B, 2005, 55(2), 133–139.
59. G. Yang, Y. Liang, Z. Xiong, J. Yang, K. Wang and Z. Zeng, Molten Salt-Assisted Synthesis of Ce₄O₇/Bi₄MoO₉ Heterojunction Photocatalysts for Photo-Fenton Degradation of Tetracycline: Enhanced Mechanism, Degradation Pathway and Products Toxicity Assessment, Chem. Eng. J., 2021, 425, 130689.
60. B. Gao, Y. Pan and H. Yang, Enhanced Photo-Fenton Degradation of Fluoroquinolones in Water Assisted by a 3D Composite Sponge Complexed with a S-Scheme MoS₂/Bi₂S₃/BiVO₄ Ternary Photocatalyst, Appl. Catal., B, 2022, 315, 121580.
61. R. Liu, R. Li, L. Yang, R.-X. Wang, H.-Y. Chen, N. Wang, Z. Li, Y.-Q. Chen, S. Ramakrishna and Y.-Z. Long, FeOOH Quantum Dot-Coupled Z-Scheme BiVO₄/g-C₃N₄ Heterojunction for Enhancing Photo-Fenton Reactions, ACS Appl. Nano Mater., 2023, 6(18), 17009–17020.
62. Q. Zhou, A. Xing, J. Li, D. Zhao, K. Zhao and M. Lei, Synergistic Enhancement in Photoelectrocatalytic Degradation of Bisphenol A by CeO₂ and Reduced Graphene Oxide Co-Modified TiO₂ Nanotube Arrays in Combination with Fenton Oxidation, Electrochim. Acta, 2016, 209, 379–388.
63. K. V. Suryaa, A. Balakrishnan, M. Chinthala, K. B. Devi, H. Tripathy, A. Kumar, T. M. Aminabhavi and S. Rtimi, Photocatalytic Self-Fenton Degradation of Tetracycline over Z-Scheme Functionalized g-C₃N₄/CeO₂/Bi₂S₃ Hydrogel Beads: Dynamics, Mechanism, Degradation Pathways and Toxicity Analysis, Chem. Eng. J., 2025, 505, 159470.
64. L. Liang, S. Gao, J. Zhu, L. Wang, Y. Xiong, X. Xia and L. Yang, The Enhanced Photocatalytic Performance toward Carbamazepine by Nitrogen-Doped Carbon Dots Decorated on BiOBr/CeO₂: Mechanism Insight and Degradation Pathways, Chem. Eng. J., 2020, 391, 123599.
65. Y. Liu and J. Wang, Multivalent Metal Catalysts in Fenton/Fenton-like Oxidation System: A Critical Review, Chem. Eng. J., 2023, 466, 143147.
66. J. Prajapati, N. Maharana, B. Bhoi, A. Badola, S. Chaturvedi and V. Chandra, Solar-Powered Degradation of Malachite Green Dye via α-MnO₂ Nanorods with CTAB-Mediated Electron Trapping, Next Mater., 2026, 10, 101511.
67. T. V. Anitha, K. G. Menon, K. Venugopal and T. V. Vimalkumar, Investigating the Role of Film Thickness on the Physical Properties of Sol-Gel Coated CuO Thin Films: Discussing Its Potentiality in Optoelectronic Applications, Mater. Sci. Eng., B, 2024, 299, 116960.
68. S. Aktas, I. S. Hasanli, A. Demiroglu and M. Caglar, Band Gap Tunability and Optical Properties of Sol-Gel Derived Fe-Doped CeO₂ Films, Phys. B, 2024, 675, 415621.
69. F. Li, W. Wu, G. Liu, X. Liu, L. Dong, D. Wang, Y. Huang, J. Zhang, Z. Yang and W. Yang, Synergistical Promotion of Carrier Separation of Z-Scheme BiOBr/Bi₂O₃ Heterojunctions with Band-Gap Design and Oxygen Defects for Co-Catalysis of CIP and Cr (VI), J. Alloys Compd., 2025, 181810.
70. M. Ahamed, S. M. Ali, R. Lateef, G. L. Dotto and H. A. Alhadlaq, Oxygen-Vacancy Engineering and Junction Design in CeO₂ Nanomaterials for Photocatalysis and Antibacterial Action: A Review, Results Eng., 2025, 108076.
71. A. Wang, Z. Zheng, H. Wang, Y. Chen, C. Luo, D. Liang, B. Hu, R. Qiu and K. Yan, 3D Hierarchical H2-Reduced Mn-Doped CeO₂ Microflowers Assembled from Nanotubes as a High-Performance Fenton-like Photocatalyst for Tetracycline Antibiotics Degradation, Appl. Catal., B, 2020, 277, 119171.
72. S. R. A. Kumar, D. V. Mary, G. A. S. Josephine and A. Sivasamy, Hydrothermally Synthesized WO₃: CeO₂ Supported GC₃N₄ Nanolayers for Rapid Photocatalytic Degradation of Azo Dye under Natural Sunlight, Inorg. Chem. Commun., 2024, 164, 112366.
73. Q. Liu, J. He, W. Yang, Q. Wu, L. Zou, Y. Wu, L. Yang, G. Shi and X. Yang, Mesoporous Ceria Nanoparticles for Ultra-Fast and Highly Flexible Photo-Fenton Catalytic Reaction, J. Photochem. Photobiol., A, 2023, 435, 114309.
74. A. Hassani, P. Eghbali and F. Ghanbari, Layered double hydroxide (LDH)-based materials applied in advanced photocatalytic oxidation of pharmaceuticals, in Advanced materials for pharmaceutical wastewater treatment, ed. A. Hassani, CRC Press, 2024, pp. 147–171.
75. R. Huang, D. Liang, W. Zhang, T. Gan, H. Hu, Z. Huang and Y. Zhang, Hydrogen Bond-Induced Supramolecular Self-Assembly Strategy to Fabricate Ultra-Dispersed Cu-Loaded Porous Tubular Graphitic Carbon Nitride with Rich Nitrogen Vacancies and CuN_x Sites for Efficient Photo-Fenton Catalysis, J. Colloid Interface Sci., 2025, 678, 987–1000.
76. W. Zhou, X. Ding, W. Li, L. Zhao, Y. Lu, M. Zhao, F. Ma, Z. Wen, H. Zhang and J. Feng, Copper Deposition on MgO Surfaces: Synergistic Effect of Multivalent Cu and Oxygen Vacancies Efficiently Activates H₂O₂ and Enhances Photo-Fenton Efficiency, Colloids Surf., A, 2025, 139074.
77. Y. Wang, Q. Wang, X. Zhan, F. Wang, M. Safdar and J. He, Visible Light Driven Type II Heterostructures and Their Enhanced Photocatalysis Properties: A Review, Nanoscale, 2013, 5(18), 8326–8339.
78. Y. Wang, C. Yang, K. Zhang, L. Guo, R. Li, A. Zaheer, F. Fu, B. Xu and D. Wang, In-Situ Construction of 2D/2D CuCo₂S₄/Bi₂WO₆ Contact Heterojunction as a Visible-Light-Driven Fenton-like Catalyst with Highly Efficient Charge Transfer for Highly Efficient Degradation of Tetracycline Hydrochloride, Colloids Surf., A, 2022, 634, 127965.
79. L. Guo, K. Zhang, X. Han, Q. Zhao, Y. Zhang, M. Qi, D. Wang and F. Fu, 2D/2D Type-II Cu₂ZnSnS₄/Bi₂WO₆ Heterojunctions to Promote Visible-Light-Driven Photo-Fenton Catalytic Activity, Chin. J. Catal., 2020, 41(3), 503–513.
80. J. Low, C. Jiang, B. Cheng, S. Wageh, A. A. Al-Ghamdi and J. Yu, A Review of Direct Z-scheme Photocatalysts, Small Methods, 2017, 1(5), 1700080.
81. Q. Zhang, Y. Peng, F. Deng, M. Wang and D. Chen, Porous Z-Scheme MnO₂/Mn-Modified Alkalinized g-C₃N₄ Heterojunction with Excellent Fenton-like Photocatalytic Activity for Efficient Degradation of Pharmaceutical Pollutants, Sep. Purif. Technol., 2020, 246, 116890.
82. X. Wang, M. Sayed, O. Ruzimuradov, J. Zhang, Y. Fan, X. Li, X. Bai and J. Low, A Review of Step-Scheme Photocatalysts, Appl. Mater. Today, 2022, 29, 101609.
83. O. P. Kumar, K. Shahzad, M. A. Nazir, N. Farooq, M. Malik, S. S. A. Shah and A. ur Rehman, Photo-Fenton Activated C₃N_4x/AgO_y@Co_1-XBi_{0. 1-Y}O₇ Dual S-Scheme Heterojunction towards Degradation of Organic Pollutants, Opt. Mater., 2022, 126, 112199.
84. M. Gao, Z. Li, X. Su, X. Zhang, J. Chang, D. Geng, Y. Lu, H. Zhang, T. Wei and J. Feng, 2D/2D MgO/g-C₃N₄ S-Scheme Heterogeneous Tight with Mg–N Bonds for Efficient Photo-Fenton Degradation: Enhancing Both Oxygen Vacancy and Charge Migration, Chemosphere, 2023, 343, 140285.
85. J.-W. Wang, W. Chen, D.-M. Han and J.-R. Chen, S-Scheme CuInS₂/WO₃ Hybrid Heterostructures as Catalysts for Visible-Light-Driven Photo-Fenton Degradation of Tetracycline Hydrochloride, Opt. Mater., 2023, 138, 113635.
86. A. Sreedhar, Q. T. H. Ta and J.-S. Noh, Role of P-n Junction Initiated Mixed-Dimensional 0D/2D, 1D/2D, and 2D/2D BiOX (X= Cl, Br, and I)/TiO₂ Nanocomposite Interfaces for Environmental Remediation Applications: A Review, Chemosphere, 2022, 305, 135478.
87. J. Wang, X. Jia, D. Shang, L. Xie, Y. Li, H. Zhang, S. Zhan and W. Hu, Constructing Cu₂O/Bi₂MoO₆ p–n Heterojunction towards Boosted Photo-Assisted-Electro-Fenton-like Synergy Degradation of Ciprofloxacin, Environ. Sci.:Nano, 2021, 8(12), 3629–3642.
88. X. Shi, Q. Chen, X. Qin, X. Rao, S. Li, G. Liu, J. Wang, X. Dong, F. Chen and D. Luo, Robust Bi₄Ti₃O₁₂-CuBi₂O₄ p-n Heterojunction with Enhanced Piezo-Photocatalytic Activity Boosted by Cu (II)-Catalysed Fenton-like Reaction, J. Alloys Compd., 2025, 1020, 179472.
89. K. Liu, L. Shi, Y. Guo, S. Hao, Y. Zhai and Y. Liu, Construction of a Novel Cu₂(OH)₃F/Cu₂O Np Heterojunction by in-Situ Reduction: Photo-Fenton-like Synergistic Promotion of Efficient Cu²⁺/Cu⁺ Cycling, J. Catal., 2024, 435, 115544.
90. L. Guo, K. Zhang, X. Han, Q. Zhao, D. Wang and F. Fu, 2D In-Plane CuS/Bi₂WO₆ P-n Heterostructures with Promoted Visible-Light-Driven Photo-Fenton Degradation Performance, Nanomaterials, 2019, 9(8), 1151.
91. P. Kumari, N. Bahadur, L. Kong, L. A. O’Dell, A. Merenda and L. F. Dumée, Engineering Schottky-like and Heterojunction Materials for Enhanced Photocatalysis Performance–a Review, Mater. Adv., 2022, 3(5), 2309–2323.
92. Q. Zhong, Y. Li, J. Liu, J. Li and G. Zhang, In Situ Construction of Ti³⁺ Self-Doped TiO₂/Ti₃C₂ Schottky Heterojunctions for Highly Selective Photo-Fenton-like Degradation of Organic Pollutants: Surface/Interface Effect and Mechanism Insight, Appl. Surf. Sci., 2024, 667, 160376.
93. Y. Wang, W. Yang and K. Ding, Synergistic Ag/g–C₃N₄ H₂O₂ System for Photocatalytic Degradation of Azo Dyes, Molecules, 2024, 29(16), 3871.
94. L. Xiong, R. Li, K. Ba, L. Chen, Y. Shi, Y. Yu and M. Yang, Dual-Metal Synergy and Interfacial Electric Field Engineering in CNNS/Ni-Ag Heterostructure for Sacrificial Agent-Free in-Situ H₂O₂-Enabled Self-Photo-Driven Fenton Degradation, Appl. Catal., B, 2025, 126032.
95. H. Dihingia and D. Tiwari, Green and Facile Synthesis of Heterojunction Nanocatalyst (Cu0+ Ag0@ Supported Bentonite): Insights into the Removal of Potential Antibiotics. Available at SSRN 4193816.
96. Y. Hu, J. Ke, Z. Yan, L. Zhao and J. Liu, Facile Construction of Dual-Metal Modified Bi₂MoO₆ Plasmonic Heterojunction for Enhanced Photo-Assisted Fenton Performances, Colloids Surf., A, 2025, 137409.
97. X. Zhang, Y. Liu, Y. Zhai, Y. Yu, Y. Guo and S. Hao, An Optimization Strategy for Photo-Fenton-like Catalysts: Based on Crystal Plane Engineering of BiVO₄ and Electron Transfer Properties of 0D CQDs, Environ. Res., 2023, 222, 115347.
98. Z. Tan, R. Wang, S. Yang, Z. Shi and D. Wang, Fabrication of Cu-Cu₂ON Doped Carbon with High Photo-Fenton Catalytic Activity Using a Cu-Nicotinic Acid Framework: Insight into Structural Transformation, J. Mol. Struct., 2025, 1337, 142248.
99. K. Sharma, P. Raizada, A. Hosseini-Bandegharaei, P. Thakur, R. Kumar, V. K. Thakur, V.-H. Nguyen and S. Pardeep, Fabrication of Efficient CuO/Graphitic Carbon Nitride Based Heterogeneous Photo-Fenton like Catalyst for Degradation of 2, 4 Dimethyl Phenol, Process Saf. Environ. Prot., 2020, 142, 63–75.
100. A. A. Taha, L. Huang, H. Ma, S. Ramakrishna and Y. Liu, Metal-Free Nitrogen-Doped Porous Carbon Nanofiber Catalyst for Solar-Fenton-like System: Efficient, Reusable and Active Catalyst over a Wide Range of pH, Colloids Surf., A, 2023, 663, 131021.
101. X. Cui, J. Li, D. H. L. Ng, J. Liu, Y. Liu and W. Yang, 3D Hierarchical ACFs-Based Micromotors as Efficient Photo-Fenton-like Catalysts, Carbon, 2020, 158, 738–748.
102. Z. Guo, G. Yang, Y. Wu, J. Long, H. He, X. Ren, B. Huang, X. Pan and Y. Liang, Molten Salt Defect Engineering Regulation of Bi₄TaO₈Cl Perovskite Structure for Boosting Photo-Fenton Degradation of Antibiotics, Chem. Eng. J., 2024, 496, 153923.
103. S. Zhang, R. Wang, K. Wang, M. Wang, Z. He, H. Chen and S.-H. Ho, Aeration-Free in Situ Fenton-like Reaction: Specific Adsorption and Activation of Oxygen on Heterophase Oxygen Vacancies, Environ. Sci. Technol., 2024, 58(4), 1921–1933.
104. W. Zhang, L. Wang, C. Hou, Z. Zhu, E. Lichtfouse, C. Trapalis and C. Wang, Combined Effect of Cu⁰ and Oxygen Vacancies in Cu-Based Zeolites Enables Highly Efficient Photo-Fenton-like Performance for Water Purification, Environ. Sci.:Nano, 2024, 11(6), 2481–2493.
105. B. Brindha, A. Syed, R. Balakrishnaraja, A. M. Elgorban, I. Abid, L. S. Wong and S. S. Khan, Stabilize the Oxygen Vacancies in ZnO via Altering Local Electronic Structure by Co and Ni Doping: A Novel Strategy for Achieving Durable Visible Light Driven Photo-Fenton Degradation of Chloramphenicol, Colloids Surf., A, 2025, 707, 135796.
106. D. Wang, H. Wang, Z. Tan, A. S. A. Almalki, A. M. Fallatah, S. Yang and Z. Shi, In-Situ Preparation of CuO/Cu₂O/Cu/N-Codoped Biochar from Chitosan Derivative: Adsorption Property and Photo-Fenton Catalytic Performance in Removal of Antibiotic, Adv. Compos. Hybrid Mater., 2025, 8(1), 2.
107. S. Wang, Y. Wang, L. Cao, J. Zhang, Y. Liu, B. Wang and M. Li, Boosting Photoelectricity Based on Self-Supported Flexible Metal–Organic Framework Arrays for Photo-Fenton-like Degradation of Organic Pollutants, J. Mater. Chem. C, 2025, 13(11), 5526–5535.
108. A. Abdelhaleem, H. N. Abdelhamid, M. G. Ibrahim and W. Chu, Photocatalytic Degradation of Paracetamol Using Photo-Fenton-like Metal-Organic Framework-Derived CuO@C under Visible LED, J. Cleaner Prod., 2022, 379, 134571.
109. C. Si, X. Liu, T. Zhang, J. Xu, J. Li, J. Fu and Q. Han, Constructing a Photocatalyst for Selective Oxidation of Benzyl Alcohol to Benzaldehyde by Photo-Fenton-like Catalysis, Inorg. Chem., 2023, 62(10), 4210–4219.
110. Z. He, T. Shi, D. Chen, Y. Wang, Y. Feng, F. Zhou and Y. Li, Ultrathin CuZnCr-LDH Nanosheets for Photo-Fenton Synergistic Degradation of Ciprofloxacin and Methylene Blue, J. Alloys Compd., 2025, 1010, 177302.
111. Z. Zhu, L. Wang, W. Zhang, C. Hou, C. Wang and J. Zhao, Ultrathin CuNi₂Al-LDH Nanosheets with Enhanced Electron Transfer for Visible-Light-Driven Photo-Fenton-like Water Decontamination, Chem. Eng. J., 2024, 481, 148313.
112. S. Dong, X. Chen, L. Su, Y. Wen, Y. Wang, Q. Yang, L. Yi, W. Xu, Q. Yang and P. He, Integration of Atomically Dispersed Cu–N₄ Sites with C₃N₄ for Enhanced Photo-Fenton Degradation over a Nonradical Mechanism, ACS ES&T Eng., 2022, 3(2), 150–164.
113. Z. Lian, F. Gao, H. Xiao, D. Luo, M. Li, D. Fang, Y. Yang, J. Zi and H. Li, Photo-self-Fenton Reaction Mediated by Atomically Dispersed Ag− Co Photocatalysts toward Efficient Degradation of Organic Pollutants, Angew. Chem., Int. Ed., 2024, 63(8), e202318927.
114. X. Zhang, B. Xu, S. Wang, X. Li, B. Liu, Y. Xu, P. Yu and Y. Sun, High-Density Dispersion of CuN_x Sites for H₂O₂ Activation toward Enhanced Photo-Fenton Performance in Antibiotic Contaminant Degradation, J. Hazard. Mater., 2022, 423, 127039.
115. D. Li, Y. Zhang, H. Zhu, R. Zhu, X. Bai, J. Dang, F. Song and J. Zhou, Investigation on Photo-Assisted Fenton-like Mechanism of Bismuth Single-Atom Charge Transfer Bridge in Bi@g-C₃N₄/CoNi-LDH Step-Scheme Heterojunction, J. Environ. Chem. Eng., 2025, 119483.
116. N. S. Anuraag, U. Kumar, S. Pradhan, S. K. Shaw, A. Sharma, I. Sinha and N. K. Prasad, Rhombohedral Phase High-Entropy Alloy of AlMnCuZnBi as a Photo-Fenton Catalyst for Methyl Orange Degradation, J. Alloys Compd., 2025, 1010, 177142.
117. S. Mohapatra, H. T. Das, B. C. Tripathy and N. Das, Unleashing the Electrochemical/Photocatalytic Activity of Co₉Se₈/Ni₃Se₄/Cu₂Se Ternary Nanocomposites for Sustainable Energy Storage and Photo-Fenton Based Pollutants Degradation, Adv. Sustainable Syst., 2024, 8(12), 2400405.
118. M. Rozmyślak, A. Walkowiak, M. Frankowski and L. Wolski, Copper (II) Phosphate as a Promising Catalyst for the Degradation of Ciprofloxacin via Photo-Assisted Fenton-like Process, Sci. Rep., 2024, 14(1), 7007.
119. A. Almahri, Fenton-like Photocatalytic Degradation of Rhodamine 6G and Tetracycline under Sunlight Irradiation Using Hexahydroxy Copper-Stannate CuSn(OH)₆, Inorg. Chem. Commun., 2025, 177, 114340.
120. G. Qin, X. Song, Q. Chen, W. He, J. Yang, Y. Li, Y. Zhang, J. Wang and D. D. Dionysiou, Novel Durable and Recyclable Cu@MoS₂/Polyacrylamide/Copper Alginate Hydrogel Photo-Fenton-like Catalyst with Enhanced and Self-Regenerable Adsorption and Degradation of High Concentration Tetracycline, Appl. Catal., B, 2024, 344, 123640.
121. Y. Tang, X. Li, H. Zhang, T. Ouyang, Y. Jiang, M. Mu and X. Yin, Cobalt-Based ZIF Coordinated Hybrids with Defective TiO_2-x for Boosting Visible Light-Driven Photo-Fenton-like Degradation of Bisphenol A, Chemosphere, 2020, 259, 127431.
122. S. Li, J. Zhang, Y. Cao, Y. Yang, T. Xie and Y. Lin, Visible Light Assisted Heterogeneous Photo-Fenton-like Degradation of Rhodamine B Based on the Co-POM/N-TiO₂ Composites: Catalyst Properties, Photogenerated Carrier Transfer and Degradation Mechanism, Colloids Surf., A, 2022, 648, 129248.
123. Q. Zhou, J. Zhang, Q. Shu, F. Yu and Y. Guo, Construction of Platinum/Cobalt Aluminate Heterostructured Catalyst via Spent Catalyst Recycling for Photo-Fenton-like Degradation of Malathion, J. Water Process Eng., 2026, 81, 109253.
124. Y. Zhu, J. Li, Y. Lai, Y. Cao, J. Li, Z. Wei, L. Yang, Z. Chen and J. Zou, Accelerated Photo-Fenton Degradation of Ciprofloxacin on CoS_x@ TiO₂ Amorphous-Crystalline Interface with SO Bond Bridging, Environ. Res., 2025, 279, 121785.
125. G. Peng, Y. Xie, H. Chen, J. Zou, L. Li, C. Luo, L. Lu and G. Mao, Frustrated Lewis Pairs Created by Ce-Doped Bi₂MoO₆: A Universal Strategy to Promote Efficient Utilization of H₂O₂ for Fenton-like Photodegradation, Chemosphere, 2024, 356, 141952.
126. L. Ralte, S. Goswami, D. Tiwari and J. Jung, Novel Iron Free Nano-Fenton-like Catalyst for Sustainable Treatment of Antibiotics and Cyanobacteria, J. Cleaner Prod., 2025, 496, 144982.
127. K. Hu, J. Li, Y. Han, D. H. L. Ng, N. Xing and Y. Lyu, A Colorimetric Detection Strategy and Micromotor-Assisted Photo-Fenton-like Degradation for Hydroquinone Based on the Peroxidase-like Activity of Co₃O₄–CeO₂ Nanocages, Catal. Sci. Technol., 2022, 12(23), 7161–7170.
128. C. Han, Q. Zhou, Q. Tao, X. Li, B. Zhao and Q. Jing, Photochemical Fabrication of CuS-Based Compounds Driven by UV Irradiation and Their Photo-Fenton-like Catalytic Activity, J. Alloys Compd., 2025, 184749.
129. G. Tzvetkov, M. Tsvetkov and T. Spassov, Self-Sacrificing Template Fabrication of Mesoporous CuO Micro/Nanoplates for Photo-Fenton-like Oxidation of Sulfathiazole, Mater. Lett., 2025, 378, 137582.
130. Q. Liu, C. Zhou, X. Li, Y. He, Y. Fan, Y. Meng and M. Xu, Dual-Functional Photocatalytic Composites Featuring Direct Z-Scheme Heterojunctions and Photo-Fenton Reactions, J. Photochem. Photobiol., A, 2025, 116937.
131. T. Mokoba, Z. Li, T. C. Zhang and S. Yuan, Superwetting Sea Urchin-like BiOBr@Co₃O₄ Nanowire Clusters-Coated Copper Mesh with Efficient Emulsion Separation and Photo-Fenton-like Degradation of Soluble Dye, Appl. Surf. Sci., 2022, 594, 153497.
132. J. Li, T. Ma, P. Guo, C. Zhao, P. Xu and D. Luo, Octahedral Alloyed PtCu-BTC for Photothermal Synergistic Enhanced Photo-Fenton Degradation of Organic Dye Contaminant, Environ. Res., 2025, 122863.
133. M. Imran, A. Z. Abdullah, M. E. Khan, Y.-M. Kim and F. Khan, Excellent Successive Photo-Induced Degradation of Tetracycline Using CuO/g-C₃N₄ Nanocomposites: Synergistic Effects of CuO Integration and H₂O₂ in a Photo-Fenton System, J. Taiwan Inst. Chem. Eng., 2025, 171, 106068.
134. A. C. da Rocha, Á. de Dantas, P. A. Vieira and V. L. Cardoso, Evaluation of Basalt Powder as a Natural Heterogeneous Catalyst in Photo-Fenton-like Treatment of Atrazine, J. Photochem. Photobiol., A, 2024, 446, 115149.
135. Y. Liu, W. Liu, M. Du, W. Li, H. Masuda and S. Kang, HOFs Structured CN Template Enables the Dual Regulation of Cu–O and Cu–N Active Sites in Porous Cu-g-CN for Synergistic Photo-Fenton Process, J. Environ. Chem. Eng., 2024, 12(2), 112291.
136. R.-M. He, Y.-L. Yang, H.-J. Chen, J.-J. Liu, Y.-M. Sun, W.-N. Guo, D.-H. Li, X.-J. Hou, G.-Q. Suo and X.-H. Ye, In Situ Controllable Growth of Cu₇S₄ Nanosheets on Copper Mesh for Catalysis: The Synergistic Effect of Photocatalytic Fenton-like Process, Colloids Surf., A, 2022, 642, 128651.
137. J.-J. Ji, Z.-W. Cui, Q.-Q. Liu, S. Shi, J.-Y. Long, X.-L. Wang and B.-L. Fei, Synthesis, Characterization and Heterogeneous Photo-Fenton-like Catalyst Activity of a Novel Inorganic-Organic 2D Framework Based on Reduced α-Keggin Phosphomolybdate, Mater. Chem. Phys., 2023, 302, 127692.
138. T. Wen, Y. Zhao, X. Jiao, G. Yang, Z. Zhang, W. Wang, T. Zhang, Q. Zhang and S. Song, Use of Posnjakite Containing Sludge as Catalyst for Decoloring Dye via Photo-Fenton-like Process, J. Cleaner Prod., 2021, 293, 126184.
139. F. S. Betim, A. A. L. Marins, E. L. D. Coelho, M. F. F. Lelis and M. Freitas, Evaluation of Photocatalytic Properties of Zinc and Cobalt Mixed Oxide Recycled from Spent Li-Ion and Zn–MnO₂ Batteries in Photo-Fenton-like Process, Mater. Res. Bull., 2023, 162, 112179.
140. P. Kaewdee, C. Tupberg, S. Tandorn, W. Boontakam, K. Saenkam, A. Limpichaipanit, G. Rujijanagul, P. Thavornyutikarn and C. Randorn, Redox-Active Shapeable Phosphate-Based Glass Containing CuO for Long-Life Photo-Fenton Catalytic Applications, Chem. Eng. J. Adv., 2025, 100833.
141. M. S. Vasilyeva, I. V. Lukiyanchuk, A. A. Sergeev, K. A. Sergeeva, A. Y. Ustinov, V. V. Tkachev and O. D. Arefieva, Plasma Electrolytic Synthesis and Characterization of Oxide Coatings with MWO₄ (M= Co, Ni, Cu) as Photo-Fenton Heterogeneous Catalysts, Surf. Coat. Technol., 2021, 424, 127640.
142. P. Mahamallik and A. Pal, Photo-Fenton Process in Co (II)-Adsorbed Admicellar Soft-Template on Alumina Support for Methyl Orange Degradation, Catal. Today, 2020, 348, 212–222.
143. Q. Tian, M. Ran, G. Fang, L. Ding, A. Pan, K. Shen and Y. Deng, ZnAl₂O₄/BiPO₄ Composites as a Heterogeneous Catalyst for Photo-Fenton Treatment of Textile and Pulping Wastewater, Sep. Purif. Technol., 2020, 239, 116574.
144. T. Khudkham, D. Channei, B. Pinchaipat and R. Chotima, Degradation of Methylene Blue with a Cu(II)–Quinoline Complex Immobilized on a Silica Support as a Photo-Fenton-like Catalyst, ACS Omega, 2022, 7(37), 33258–33265.
145. L. Hurtado, O. Avilés, S. Brewer, K. K. Donkor, R. Romero, R. M. Gómez-Espinosa, O. Alvarado and R. Natividad, Al/Cu-PILC as a Photo-Fenton Catalyst: Paracetamol Mineralization, ACS Omega, 2022, 7(27), 23821–23832.
146. C. Liu and S. Xue, Efficient Photo-Assisted Fenton-like Reaction of Yolk–Shell CuSe (Cu₂Se)/GC₃N₄ Heterojunctions for Methylene Blue Degradation, RSC Adv., 2023, 13(13), 8464–8475.
147. B. Hazarika, B. J. Hazarika and M. J. K. Ahmed, Visible-Light Induced Photo-Fenton-Like Degradation of Pharmaceuticals Via Advanced Oxidation Process Using Ag@SnO₂ Nanocomposite Fabricated Via a Green in-Situ Strategy, J. Cluster Sci., 2025, 36(6), 233.
148. G. Zhou, F. Yang, X. Zhu, W. Feng, D. Chen and J. Fang, Copper-Copper Oxide Heterostructural Nanocrystals Anchored on g-C₃N₄ Nanosheets for Efficient Visible-Light-Driven Photo-Fenton-like Catalysis, Molecules, 2025, 30(1), 144.
149. S. Nehhal, M. Ben Ali, Y. Abrouki, K. Ofqir, Y. Elkahoui, N. Labjar, H. Nasrellah and S. El Hajjaji, Photocatalytic and Photo-Fenton-like Degradation of Methylene Blue Using Green-Synthesized Phosphate-Doped ZnO Under Visible LED Light, Reactions, 2025, 6(4), 64.
150. R. Muthusami, V. Ramachandran, M. Palaniappan, S. Arumugam, K. Palanisamy, K. Irena and R. Rangappan, Cu (II) Schiff Base Complex Functionalized Mesoporous Silica Nanoparticles as an Efficient Catalyst for the Synthesis of Questiomycin A and Photo-Fenton-like Rhodamine B Degradation, J. Solid State Chem., 2021, 302, 122429.
151. M. Abdelbaky, A. M. Abdelghany, A. H. Oraby, E. M. Abdelrazek and M. M. Rashad, Efficacious Elimination of Crystal Violet Pollutant via Photo-Fenton Process Based on Gd_(2−x)La_(x)Zr₂O₇ Nanoparticles, Sci. Rep., 2023, 13(1), 7723.
152. A. Dhadage, A. Nagare, P. Ghare, A. Athare, P. Gogoi and M. Navgire, Ultrasonic Wave-Assisted Synthesis of CeO₂-MoO₃ Nanocomposite for Photo-Fenton Degradation of Rhodamine-B Dye, Chem. Pap., 2024, 78(10), 5833–5850.
153. A. Santos, P. Yustos, A. Quintanilla, G. Ruiz and F. Garcia-Ochoa, Study of the Copper Leaching in the Wet Oxidation of Phenol with CuO-Based Catalysts: Causes and Effects, Appl. Catal., B, 2005, 61(3–4), 323–333.
154. L. S. Mustapha, O. V. Obayomi and K. S. Obayomi, A comprehensive review on potential heavy metals in the environment: Persistence, bioaccumulation, ecotoxicology, and agricultural impacts, Ecol. Front., 2026, 46(2), 434–449.
155. S. Bang, J. W. Choi, K. Cho, C. Chung, H. Kang and S. W. Hong, Simultaneous reduction of copper and toxicity in semiconductor wastewater using protonated alginate beads, Chem. Eng. J., 2016, 288, 525–531.
156. D. S. Patil, S. M. Chavan and J. U. K. Oubagaranadin, A review of technologies for manganese removal from wastewaters, J. Environ. Chem. Eng., 2016, 4, 468–487.

---