Advancement in photocatalytic degradation of ciprofloxacin: mechanisms, materials and environmental remediation

Abstract

Ciprofloxacin (CIP), a widely used fluoroquinolone antibiotic, poses significant ecological risks due to its persistence in water systems and contribution to antibiotic resistance. Traditional wastewater treatment methods are ineffective in completely degrading CIP, necessitating more advanced solutions. This review explores the potential of photocatalytic degradation as a promising approach for CIP removal, focusing on the generation of reactive oxygen species (ROS) using various photocatalysts. The influence of operational parameters such as catalyst type, CIP dosage, pH, temperature, and light source on degradation efficiency is critically evaluated. Recent advancements in visible-light-responsive photocatalysts and hybrid treatment systems are discussed, along with their performance metrics. The paper concludes with future directions for developing novel materials and integrated systems to enhance environmental remediation, offering a comprehensive evaluation of the photocatalytic degradation process of CIPs.

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1. Introduction

1.1. Background and environmental concerns

Ciprofloxacin (Fig. 1) is a popular second-generation fluoroquinolone antibiotic.¹ Considering the carboxylic acid group and the basic-N-moiety, the acid disintegration constant pK_a readings of CIP species are 5.9 ± 0.15 and 8.89 ± 0.11, respectively. Because of the charge equilibrium of the two aforementioned groups (Fig. 2), the zwitterionic form is the dominant type between pH values of 5.9 and 8.89.^2,3 The piperazine moiety and fluorinated quinolone backbone that make up CIP's structural makeup give it potent antibacterial activity, high chemical stability, and the ability to resist biodegradation. Because of its clinical effectiveness and comparatively low toxicity, fluoroquinolone use has grown dramatically over the last 20 years on a global scale.^4,5 While CIP is highly effective in treating bacterial infections in humans and animals, a large portion of the administered dose is excreted unmetabolized in urine and feces, making its way into wastewater systems.^6–8 Unfortunately, conventional wastewater treatment methods are ineffective in completely removing CIP, resulting in its accumulation in rivers, lakes, and groundwater. The environmental persistence of CIP is attributed to its high chemical stability, low biodegradability, and photostability, which make it resistant to natural degradation processes.^9,10 Furthermore, CIP is considered a pseudo-persistent pollutant, meaning that despite its moderate individual half-life, it continuously enters ecosystems due to prolonged human and veterinary use.^11,12 The presence of CIP in water bodies is particularly concerning because it can disrupt aquatic microbial communities, affect nutrient cycling, and contribute to the development of antibiotic-resistant bacteria. These factors make CIP a priority compound for environmental remediation.^13,14 Given these environmental and public health risks, this study focuses on ciprofloxacin as a model contaminant for exploring advanced degradation methods, specifically photocatalytic degradation, as a potential solution to mitigate its environmental impact.^15–19 Some key advantages of photocatalysis over other treatments are summarized in Fig. 3.

Fig. 1 Chemical structure of ciprofloxacin, possible modification in its structure with some possible derivatives.

Fig. 2 Molecular structure and ionic forms of CIP as a function of pH, and pK_a values.

Fig. 3 Show some advantages of photocatalysis.

1.2. Limitations of existing wastewater treatment approaches

Traditional wastewater treatment plants, which are primarily engineered to eliminate suspended solids, organic load, and pathogens, are not specifically equipped to remove pharmaceutical pollutants such as ciprofloxacin (CIP).^20,21 Physical separation techniques such as sedimentation, filtration, and adsorption onto activated carbon can temporarily reduce CIP concentrations; however, they merely transfer the pollutant to another phase, resulting in concentrated secondary waste that still requires proper disposal.^22,23 Activated sludge systems and other biological treatment techniques are generally ineffective against fluoroquinolones due to their low biodegradability and high resistance to microbial enzymatic attack.²⁴ Chemical oxidation methods like ozonation or chlorination can partially degrade CIP, but they may also result in dangerous transformation products like halogenated byproducts, which could pose additional risks to human health and the environment.^25,26 Despite having high removal efficiencies, membrane-based technologies like reverse osmosis and nanofiltration suffer from membrane fouling, high operating costs, and frequent replacement.^27,28 Furthermore, none of these conventional techniques can guarantee that CIP is completely broken down into nontoxic end products primarily CO₂ and H₂O so residual active compounds or metabolites may still enter natural water bodies. These disadvantages highlight the urgent need for innovative, eco-friendly, and mineralization-capable technologies that can handle both the parent compound and its dangerous intermediates, such as photocatalytic degradation.^29,30 Some of the common approaches that are used for the removal of CIPs from wastewater are illustrated in Fig. 4A–E.

Fig. 4 Some common approaches for the removal of CIPs (A) electro-Fenton degradation pathway reproduced with permission from ref. 31 Copyright 2024, Elsevier. (B) Adsorption adopted with permission from ref. 32 Copyright 2025, Elsevier. (C) Hybrid Bio-electric degradation mechanism & (D) biodegradation of CIP in sulfate-reducing bacteria (SRB) mechanism adopted with permission from ref. 33 Copyright 2018, Elsevier. (E) Biodegradation mechanisms and pathways of ciprofloxacin retracted with permission³⁴ Copyright 2023, Elsevier.

2. Scope and objectives of the review

This paper will provide a complete summary of the photocatalytic degradation of CIP with an emphasis on degradation pathways, operational parameters, catalyst design strategies, and degradation mechanisms. Special attention is paid to recent advancements in nanostructured photocatalysts and their application in solar or visible light irradiation. This review also critically examines the constraints, challenges of large-scale implementation, and potential directions for future research to support sustainable pharmaceutical wastewater treatment.

2.1. Ciprofloxacin as an emerging contaminant

The term “emerging contaminant” refers to recently found toxins in the environment that, while their effects are unknown, offer a significant risk of harm³⁵ as some are illustrated in Fig. 5. Pharmaceuticals are increasingly being studied as potential pollutants in water and wastewater. Pharmaceuticals enter water sources through a variety of routes, including home wastewater, hospital effluents, and agricultural runoff, damaging freshwater systems.³⁶ Antibiotics are a type of drug that is widely used globally to treat bacterial infections in humans, animals, and plants. They are used to treat bacterial and fungal infections in both humans and animals.³⁷ Ciprofloxacin (CIP) is a common second-generation fluoroquinolone antibiotic used to treat a wide range of bacterial infections in both humans and animals.³⁸ It is a synthetic antibiotic that operates by blocking the bacterial DNA gyrase and topoisomerase IV enzymes, which are required for DNA replication and bacterial cell division.³⁹ Ciprofloxacin is increasingly being categorized as a Contaminant of Emerging Concern (CEC) despite its high efficacy due to its prolonged use and perseverance, incomplete metabolism in humans and animals, and especially low biodegradability in wastewater treatment systems.⁴⁰ Significant amounts of administered ciprofloxacin are either eliminated unchanged or as active metabolites, and they find their way into the aquatic environment through municipal sewage, livestock runoff, hospital discharges, and pharmaceutical manufacturing effluents.^41,42 These pathways make CIP a pseudo-persistent compound in natural ecosystems, despite its intermittent input. According to the European Environment Agency (EEA) and the United States Geological Survey (USGS), pharmaceuticals such as ciprofloxacin are commonly found in sediments, groundwater, and surface water because they are not completely removed by traditional wastewater treatment plants (WWTPs).⁴³ Both human health and a variety of other organisms are at risk from ciprofloxacin residues in aquatic environments. The presence of ciprofloxacin can induce physicochemical changes that reduce the ability of high-surface-area soil components—such as clay minerals and metal oxides—to control the mobility and bioavailability of nutrients and other contaminants, thereby negatively affecting water quality.⁴⁴ Furthermore, native bacterial populations may experience chromosomal mutations because of environmental exposure to ciprofloxacin, which could lead to the emergence of antibiotic-resistant strains. Higher dosages of ciprofloxacin may therefore be needed to treat infections, and in extreme circumstances, treatment may become ineffective, raising serious public health concerns.⁴⁵ Fig. 6A1 and A2, summarized the sources and health effects of CIPs.⁶

Fig. 5 Summary of some emerging organic pollutants.

Fig. 6 (A1) Sources of CIPs. (A2) Health effects of CIPs.

2.1.1. Advanced oxidation processes (AOPs). An advanced oxidation processes (AOPs), processes depend on the production of extremely reactive free radicals that can oxidize and degrade organic pollutants, such as hydroxyl and singlet oxygen species. Fig. 7 represents the categories of AOPs in terms of their main activation method. AOPs like ozonation (O₃), UV/H₂O₂, Fenton process (Fe²⁺/H₂O₂), and photo-Fenton process (Fe²⁺/H₂O₂/UV) are most common.⁴⁶ These techniques rely on the production of extremely reactive hydroxyl radicals, which can break down recently discovered pollutants with low biodegradability and high chemical stability. When it comes to encouraging the full mineralization of pollutants into inorganic compounds, water, and CO₂, AOPs are highly effective.^47,48 The variety of AOPs includes photocatalysis (photo-Fenton reactions, UV/TiO₂), chemical oxidation processes (Fe²⁺/H₂O₂, H₂O₂/O₃, O₃), and photochemical processes (H₂O₂/UV, O₃/UV), all of which generate OH˙ radicals.⁴⁹ The degradation pathway of CIP entails numerous key reactions, together with alterations of the piperazinyl ring, oxidation of the quinolone structure that leads to defluorination and hydroxyl substitution, and oxidative assault at the cyclopropyl group that in the end reasons ring cleavage.^50,51 Although advanced oxidation techniques by myself may additionally display confined efficiency, their performance may be drastically more desirable via the incorporation of chemical components along with persulfate, nanoparticles, and catalytic materials.^52,53 Nevertheless, AOPs normally yield high CIP degradation efficiencies through producing surprisingly reactive oxygen species, especially hydroxyl radicals. Advanced oxidation and discount approaches employ hydroxyl radicals (˙OH) as strong oxidants and hydrated electrons or hydrogen atoms (H˙) as decreasing agents to eliminate CIP from consuming water and wastewater.^54,55 Fig. 8 summarized general pathways of some AOPs for CIPs degradation.

Fig. 7 Generation of oxidative radicals by AOPs in wastewater treatment.

Fig. 8 Represent general pathways of some AOPs for CIPs degradation.

AOPs are effective techniques for breaking down CIP because they use reactive species, particularly hydroxyl radicals, to convert pollutants into non-toxic byproducts. Ozonation, UV/H₂O₂, Fenton, and photo-Fenton are common methods that are frequently improved with catalysts such as layered double hydroxides or nanoparticles.^23,56 Despite their high efficiency, these processes usually require acidic conditions, which raises the cost and complexity of operations. Furthermore, certain pathways may produce hazardous byproducts, and their scalability and economic feasibility may be constrained by their high energy requirements and requirement for precise pH control.

2.2. Photocatalysis

Photocatalysis is an adjunct to conventional techniques such as membrane filtration, adsorption processes, and precipitation. This approach may be carried out as a very last step in water treatment and is in particular effective while CIP concentrations are low.⁵⁷ Through the motion of reactive species such as OH˙ and/or , the method targets to absolutely mineralize organic pollution into CO₂, H₂O, and inorganic byproducts.⁵⁸ Fig. 9A, represented the detail mechanism of photocatalysis, that how it works while Fig. 9B demonstrated the generalized mechanism of mechanism for creation of ROS and pollutants degradation. Antibiotics like ciprofloxacin (CIP) and other organic pollutants can be transformed by these reactions into innocuous byproducts like CO₂, H₂O, and inorganic ions.⁵⁹ The system is initiated when mild energy is same to or greater than the bandgap of a photocatalyst—which includes metal oxides absorbed. Fig. 10, summarized some metal oxide recently used for CIPs degradation⁶⁰ while Fig. 11 illustrated various pathway for obtaining these oxides.⁶¹ This strength excites an electron (e⁻) from the valence band (VB) to the conduction band (CB), leaving in the back of a hollow (h⁺) inside the VB.⁶² The formation of these electron–hole pairs is essential for triggering redox reactions at the catalyst surface. Once they migrate to the surface, the photogenerated electrons and holes participate in chemical reactions with adsorbed species: the holes oxidize water or hydroxide ions to generate hydroxyl radicals (˙OH), even as the electrons lessen molecular oxygen to form reactive oxygen species, along with superoxide anions (O₂⁻). Depending on the use, these extremely reactive species can destroy microorganisms, split water to create hydrogen, or break down organic pollutants.^63,64 However, its effectiveness is constrained at better antibiotic concentrations, wherein excessive pollutant stages can hinder light penetration and decrease touch with the catalyst surface, thereby slowing the reaction.⁶⁵ There are two categories of photocatalysis: heterogeneous and homogeneous as they are presented in Fig. 12. Heterogeneous photocatalysis relies on surface-mediated redox reactions on solid semiconductors, whereas homogeneous photocatalysis proceeds via dissolved photoactive species that generate reactive oxygen species directly in the reaction medium.⁶⁶ Heterogeneous photocatalysis is more frequently researched and used for photodegradation of CIPs. Various photocatalyst heterojunction interface are represented in Fig. 13.⁶⁷ Rapid electron–hole pair recombination limited visible light absorption (since many conventional photocatalysts only react to UV light), and stability problems like photo corrosion are some of the obstacles that photocatalysis must overcome despite its potential.⁶⁸

Fig. 9 (A) Detailed mechanism of photocatalysis. (B) Pictorial presentation of photodegradation of pollutants.

Fig. 10 Various metal oxides for photocatalysis of CIPs reprinted with permission from ref. 60 Copyright 2023, mdpi.

Fig. 11 Methods for synthesizing these catalysts modified with permission from ref. 61 Copyright 2022, Elsevier.

Fig. 12 Difference b/w hetero- and homogenous photocatalysis.

Fig. 13 Various photocatalyst heterojunction interfaces: (a) type-II heterojunction, (b) P–N heterojunction, (c) direct Z-scheme heterojunction, (d) all-solid-state Z-scheme heterojunction, and (e) S-scheme heterojunction reproduced with permission from ref. 67 Copyright 2024, Elsevier.

Researchers are looking at a number of approaches to overcome these constraints, such as creating heterojunctions to improve charge separation, doping with metal or non-metal elements to narrow the band gap, and creating plasmonic photocatalysts that improve light absorption through surface plasmon resonance.^69,70 All things considered, photocatalysis is still developing and presents viable options for sustainable energy generation and environmental cleanup. To overcome present constraints and make it possible for photocatalytic technologies to be used on a large scale, advances in material science and nanotechnology are essential.

2.2.1. Emerging trends in photocatalytic materials. Photocatalytic materials have developed quickly in recent year to improve light absorption and charge separation efficiency, and surface reactivity for the degradation of persistent pollutants such as ciprofloxacin (CIP).⁷¹ Traditional photocatalysts like ZnO and TiO₂ are still being studied in great detail due to their stability and lack of toxicity,.⁷² Fig. 14A shows general photodegradation mechanism followed by ZnO photocatalyst.⁷³ Similarly, Fig. 14B illustrates a schematic of the degradation of contaminants by forming photo-induced charge carrier electrons/holes (e⁻/h⁺) on the TiO₂ surface. Also, Fig. 14C shows how photodegradation occur on TiO₂ photocatalyst.⁷⁴ But their use in visible light is limited by their wide band gaps. Some serious modification strategies are implemented to overcome these limitations, as some are listed in Fig. 14D.⁷⁵ One modification strategy that has been developed to overcome these limitations is doping with metal or non-metal ions (e.g., Fe, Ag, N, or C) to tune band gap energy and extend photo-response into the visible region^76,77 as some are shown in Fig. 15A.⁷⁸ It has also been shown that heterojunction engineering, such as coupling TiO₂ with g-C₃N₄, Cds, or reduced graphene oxide (rGO), can improve charge carrier separation and reduce recombination rates.⁷⁹ 2D nanostructures (like graphitic carbon nitride nanosheets and MXenes) have further increased catalytic activity by providing large surface areas and a large number of reactive sites.⁸⁰ Additionally, plasmonic photocatalysts that use noble metals (Au, Ag) utilize localized surface plasmon resonance (LSPR) effects to improve visible light harvesting, generalized mechanism are pictorially represented in Fig. 15B.⁸¹ Recent studies have also focused on Z-scheme systems and MOF-derived photocatalysts, which combine a high redox potential with a strong charge separation ability. Fig. 16A depicted application of MOF as photocatalyst⁸² while Fig. 16B demonstrating CIP photodegradation via MOF based photocatalyst (BiOX/GaMOF).⁶³ These advancements demonstrate a clear trend toward multi-component, visible, light-active, environmentally safe photocatalysts that are intended to effectively degrade antibiotics.

Fig. 14 (A) Illustration of photocatalytic mechanism followed by ZnO photocatalyst adopted with permission from ref. 73 Copyright 2025, mdpi. (B) Creation of photo-induced charge carrier e⁻/h⁺ on the TiO₂ surface & (C) illustration of photodegradation on TiO₂ surface adopted with permission from ref. 74 Copyright 2024, mdpi. (D) Techniques for improving photocatalytic efficiency reproduced with permission from ref. 75 Copyright 2025, Taylor & Francis.

Fig. 15 (A) Illustration of the photocatalytic mechanism of (a) pure-TiO₂, (b) metal-doped, and (c) nonmetal-doped TiO₂ reproduced with permission from ref. 78 Copyright 2025, Viley-VcH. (B) Mechanism diagram for plasmonic metal–semiconductor photocatalysis reproduced with permission from ref. 81 Copyright 2024, Royal Society of Chemistry.

Fig. 16 (A) MOF as photocatalyst reprinted with permission from ref. 82 Copyright 2024, Heliyon. (B) LC-MS of photodegradation of CIP using BiOX/GaMOF photocatalyst adopted with permission from ref. 63 Copyright 2025, Elsevier.

3. Photocatalyst for ciprofloxacin degradation

A common fluoroquinolone antibiotic, ciprofloxacin (CIP), has been successfully and sustainably removed from aquatic environments by photocatalytic degradation. Over the past decade, numerous semiconducting substances were advanced and modified to decorate the photocatalytic degradation performance of CIP below both visible and UV light. These materials' structure, composition, light-absorbing ability, and charge carrier dynamics all affect how well they work in photocatalytic applications.⁸³ Some common categories of catalysts are presented in Fig. 17.

Fig. 17 Some common categories of photocatalysts.

3.1. TiO₂-based photocatalysts

Titanium dioxide (TiO₂) remains the most considerably studied photocatalyst for CIP degradation due to its strong oxidative electricity, low value, non-toxicity, and extremely good chemical stability. The proposed degradation mechanism for CIPs followed by TiO₂ photocatalyst are illustrated in eqn (1)–(8).^84,85 Mechanistically presented in Fig. 18A.⁸⁶ However, its wide bandgap (about 3.2 eV for anatase) limits its pastime to the ultraviolet (UV) location, which represents simplest a small fraction of the solar spectrum.⁸⁷ Scientists have used a variety of methods, including heterojunction formation, surface sensitization, and metal and non-metal doping, to drastically change TiO₂ in order to overcome this limitation. Fig. 18B, demonstrated the visual representation of bandgap with pure TiO_2, with metal and non-metal doped.⁸⁸ For example, nitrogen-doped TiO₂ has demonstrated notably enhanced activity under visible light by narrowing the bandgap.⁸⁹ Moreover, combining TiO₂ with different semiconductors or carbon-based materials, together with graphene oxide or graphitic carbon nitride (g-C₃N₄), enhances price separation and broadens its light absorption range. Fig. 18C demonstrated CIPs degradation via BGCN-TiO₂.⁹⁰

TiO₂ + hv → h⁺ + e⁻ (1)

O₂ + e⁻ → O₂˙⁻ (2)

H₂O + h⁺ → H⁺ + OH˙ (3)

OH + h⁺ → OH˙ (4)

2O₂˙⁻ + 2H⁺ → H₂O₂ + O₂ (5)

OH˙ + OH˙ → H₂O₂ (6)

H₂O₂ + O₂˙⁻ → OH˙ + OH⁻ + O₂ (7)

CIPs + O₂˙⁻ + OH˙ → CO₂ + O₂ + by-product (8)

Fig. 18 (A) Photocatalytic degradation mechanism of ciprofloxacin and amoxicillin using TiO₂ photocatalyst under sunlight, reused with permission from ref. 86 Copyright 2024, American Chemical Society. (B) Illustration of photocatalysis process of TiO₂. hν₁: pristine TiO₂; hν₂: metal doped TiO₂; and hν₃: nonmetal doped TiO₂ retaken with permission from ref. 88 Copyright 2022, mdpi. (C) Suggested degradation mechanism of removal of CIP micropollutant via BGCN-TiO₂ adopted with permission from ref. 90, Copyright 2025, IOPscience.

Titanium dioxide (TiO₂) has tested considerable efficacy in ciprofloxacin (CIP) degradation, achieving a 92.91% elimination efficiency and following pseudo-2nd order kinetics with a price steady (k) of 0.056 min⁻¹ after 240 minutes of irradiation.⁹¹ Buu et al. reported 99.7% CIP elimination in just one hundred mins under seen mild the use of a TiO₂–ZnO heterojunction with a Z-scheme configuration incorporated into an oxygen-doped graphitic carbon nitride (g-C₃N₄) matrix, following pseudo-first-order kinetics with a fee steady of 0.04796 min⁻¹.⁹² In pursuit of developing S-scheme photocatalysts for water purification, heterojunction composites were designed, including Fe-based metal–organic framework (MIL-101(Fe))/BiOBr and Bi₂WO₆/C₃N₄/carbon fiber systems.^93,94 They improved the photocatalytic performance of TiO₂/AgBr/Ag by designing S-scheme heterojunctions, achieving an excellent removal efficiency of 86.18% following pseudo-first-order kinetics. In addition, they developed a ternary film composed of rutile–anatase TiO₂ (B-TiO₂), bismuth sulfide, and photonic crystals (B-TiO₂/Bi₂S₃/PCs). This system significantly outperformed pristine B-TiO₂ in ciprofloxacin (CIP) degradation, attaining a removal efficiency of 72.79% with outstanding reusability.⁹⁵ It was demonstrated that the TiO₂/NiFe₂O₄@MWCNTs nanocomposite is a highly efficient and sustainable photocatalyst, achieving up to 97.2% degradation of ciprofloxacin (CIP) under pseudo-first-order kinetics. Notably, its photocatalytic performance decreased by less than 3.2% after four consecutive cycles, indicating excellent stability and long-term applicability for water purification.¹³² Surface change through dye sensitization—the usage of herbal or artificial dyes to create catalysts responsive to visible or sun mild—is another environmentally pleasant approach.⁹⁶ Carbon quantum dots embellished on nitrogen- and copper-co-doped TiO₂ (NCuTCQD) have shown superior overall performance, attaining entire (100%) CIP degradation at a wonderful charge with minimum loss of activity after six cycles ⁹⁷ outperforming traditional TiO₂-primarily based photocatalysts. Table 1 illustrated CIPs degradation via TiO₂ based photocatalyst.

Table 1 TiO₂ based photocatalysts for ciprofloxacin degradation concentration efficiency and kinetic parameters

S/n	Catalyst	Dosage (g L^−1)	Light source	Concentration of CIPs (mg L^−1)	Kinetics model	Rate constant (min^−1)	Time (min)	Efficiency (%)	Ref.
1	TiO2 powder	0.1	16 W UV lamp	20	Pseudo 1st order	0.0063	120	57	98
2	TiO2/montmorillonite	0.1	16 W UV lamp	20	Pseudo 1st order	0.0069	120	62	98
3	TiO2 powder	0.5	UV (485 µW cm^−2) lamp	45.3 µM	Pseudo 1st order	0.137	40	—	99
4	TiO2/montmorillonite	0.04	UV lamp	20	—	—	30	90	100
5	TiO2 powder	0.5	UV (389 µW cm^−2) lamp	45.3 µM	Pseudo 1st order	163	40	—	99
6	g-C3N4/TiO2 powder	1	150 W tungsten lamp	10	—	—	60	95	101
7	TiO2 powder	1	HQI-T 250/daylight	50	Pseudo 1st order	0.04	120	—	102
8	TiO2 immobilized on glass plates	—	15 W UV lamp	60	Pseudo 1st order	0.024	120	97	103
9	PVA assisted TiO2/Ti film	—	300 W xenon lamp	10	Pseudo 1st order	0.0244	60	77	104
10	g-C3N4/TiO2/kaolinite	2	Xenon lamp (90 mW cm^−2)	10	Pseudo 1st order	0.0081	240	92	105
11	N-doped TiO2	1	400 W visible tungsten lamp	100	Pseudo 1st order	0.073	90	100	106
12	Fe3O4/TiO2/C-dot	0.75	Mercury vapor lamp	20	Pseudo 1st order	0.0154	150	90	107
13	P-25 TiO2	1	400 W tungsten lamp	100	Pseudo 1st order	0.059	180	—	106
14	P-25 TiO2	1	Solar radiation	100	Pseudo 1st order	0.108	180	—	106
15	TiO2 nanoparticles	1	UV lamp	300 µg L^−1	—	—	45	100	108

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3.2. ZnO-based photocatalysts

Zinc oxide (ZnO) remains a highly valued photocatalyst for CIPs degradation (eqn (9)–(18)) due to its low cost, high electron mobility, and short-term photostability.¹⁰⁹ However, ZnO's wide bandgap (∼3.3 eV) restricts its activation to UV light, limiting its use under solar irradiation.¹¹⁰ Moreover, photocorrosion and the rapid recombination of photogenerated e⁻/h⁺ pairs reduce its long-term applicability and efficiency.¹¹¹ To address these limitations, various modification strategies—such as metal/non-metal doping, co-doping, composite formation, and sensitization techniques—have been explored to improve ZnO's visible-light responsiveness and suppress charge recombination. hence strategies are summarized in Fig. 19A–D.¹¹² With Fig. 19A and B demonstrating the photodegradation of CIPs by doped Cu-ZnO/g-C₃N₄ (ref. 113) and Ag-doped ZnO (ZnO/Ag(3))¹¹⁴ respectively (Fig. 20).

ZnO + hv → ZnO (h⁺ + e⁻) (9)

e⁻ + O₂ → O₂˙⁻ (10)

H₂O₂ + hv → 2OH˙ (14)

H₂O₂ + e⁻ → OH˙ + OH⁻ (15)

H₂O + h⁺ → OH˙ + H⁺ (16)

OH⁻ + h⁺ → OH˙ (17)

OH˙ + CIPs → intermediates → CO₂ + H₂O (18)

Fig. 19 The proposed photocatalytic degradation by (A) metal doping, (B) non-metal doping, (C) co-doping, and (D) coupling semiconductor on ZnO surface reproduced with permission from ref. 112 Copyright 2022, Elsevier.

Fig. 20 Schematic illustration of degradation of CIPs by doped ZnO (A) Cu-ZnO/g-C₃N₄ reused with permission from ref. 113 Copyright 2022, Elsevier. (B) ZnO/Ag(3) photocatalyst retaken with permission from ref. 114 Copyright 2023, Elsevier.

For instance, ZnO and iron-doped ZnO photocatalysts with doping concentrations of 0.1% and 0.2% have been synthesized using the sol–gel method and evaluated for ciprofloxacin degradation. The 0.2% Fe-doped ZnO exhibited the highest degradation performance of 90.49%. Kinetic analysis revealed that the response observed pseudo-first-order conduct, confirming the enhanced catalytic performance facilitated through iron doping. This catalyst has the potential to clean contaminated wastewater, as demonstrated by researchers' effective use of it to break down ciprofloxacin in commercial samples.¹¹⁵ Moreover, cobalt-doped ZnO nanoparticles showed strong CIP elimination. Furthermore, research have shown that redecorating zinc oxide nanoparticles with carbon dots substantially complements their photocatalytic performance, achieving a CIP elimination efficiency of up to 98%.¹¹⁶ As previously cited, building heterojunctions is any other broadly adopted method to improve visible-light pastime and rate separation in photocatalysts. For instance, Ye et al. developed a Z-scheme heterojunction composed of CaFe₂O₄, Ag, and ZnO nanoparticles, which established stepped forward seen-light absorption and more advantageous photocatalytic efficiency.¹¹⁷ Table 2. Summarizes ZnO based photocatalyst for CIPs degradation.

Table 2 ZnO based photocatalysts for ciprofloxacin degradation concentration, efficiency and kinetic parameters

S/n	Catalyst	Dosage (g L^−1)	Light source	Concentration of ciprofloxacin (mg L^−1)	Kinetics model	Rate constant (min^−1)	Time (min)	Efficiency (%)	Ref.
1	ZnO nanoparticles	0.2	UV light	5	Pseudo 1st order	0.0029	60	48	118
2	ZnO	0.35	300 W xenon lamp	20	Pseudo 1st order	0.042	60	95	119
3	ZnO/Au nanowire	0.0375	300 W xenon lamp	2 × 10^−5 mol L^−1	Pseudo 1st order	0.0013	120	16	120
4	ZnO/Ag2O	0.8	250 W UV lamp	10	Pseudo 1st order	0.043	60	31	121
5	ZnO nanotube	0.035	300 W xenon lamp	2 × 10^−5 mol L^−1	Pseudo 1st order	0.000961	120	12	120
6	γ-Fe2O3@ZnO	0.5	300 W xenon lamp	10	Pseudo 1st order	0.0419	60	92	122
7	ZnO/SnS2	0.66	200 W quartz tungsten halogen lamp	40	Pseudo 1st order	0.0571	120	100	123
8	ZnO-Ag2O/porous g-C3N4	0.5	500 W tungsten lamp	20	Pseudo 1st order	0.057	48	97	124
9	ZnO	0.15	9 W low pressure mercury UV lamp	10	Pseudo 1st order	0.032	140	100	125
10	N-ZnO/CdS/GO	0.5	300 W xenon lamp	15	Pseudo 2nd order	—	60	86	126

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3.3. Bismuth-based photocatalyst

Bismuth (Bi) covers a wide range of materials, as their categories are summarized in Fig. 21.¹²⁷

Fig. 21 Categories of bismuth (Bi) modified with permission from ref. 127 Copyright 2023, Viley-VcH.

Interest has been piqued by the unique layered structures of bismuth-based photocatalysts, particularly BiOCl, BiVO₄, and Bi₂WO₆, which generate internal electric fields that aid in charge carrier separation. These substances work remarkably well in visible light and have been successfully used to break down several pharmaceutical contaminants, including CIP.^128,129 For example, BiVO₄/g-C₃N₄ nanocomposites showed over 85% degradation efficiencies after 90 minutes of exposure to visible light. Proposed photocatalytic mechanism followed by BiVO₄/g-C₃N₄ for CIPs is shown in Fig. 22A and B respectively.¹³⁰ Their photocatalytic activity is further enhanced by doping with metals like molybdenum (Mo) or silver (Ag), which increase electron mobility and broaden the range of light absorption. Bandgaps for various bismuth-based photocatalysts are illustrated in Fig. 22C.¹³¹

Fig. 22 (A and B) Proposed CIPs degradation mechanism followed by BiVO₄/g-C₃N₄ photocatalyst adopted with permission from ref. 130 Copyright 2025, Elsevier. (C) Graphical demonstration of bandgap energy of various bismuth based photocatalyst reproduced with permission from ref. 131 Copyright 2022, Elsevier.

Since their bandgap is typically less than 3 eV, they can be used in situations where visible light is present. BBSc's photocatalytic qualities have attracted a lot of interest. For example, Hai et al. synthesized two bismuth oxide (BiOI) substances: BiOI-R through hydrolysis and BiOI-S through a solvothermal technique. Under seen mild, BiOI-S exhibited incredible adsorption and photocatalytic degradation overall performance for ciprofloxacin, accomplishing an outstanding elimination efficiency of 99.89%. Fig. 23A demonstrated the bandgap energy and degradation pathway for CIP by proposed material.¹³² The formation of heterojunctions in bismuth-based totally photocatalysts has further greater their photocatalytic pastime. Examples encompass nitrogen-doped carbon quantum dots mixed with bismuth tungstate (NCQDs/Bi₂WO₆),¹³³ SiO₂/Bi₂O₃/Ag, Bi/BiOF/Bi₂O₂CO₃, and WS₂/BiOCl with oxygen vacancies (4-WS₂/BiOCl_OV).¹³⁴ These heterojunctions enhance photocatalytic overall performance by way of enhancing light absorption, promoting efficient fee separation, and strengthening redox competencies. Bismuth-based semiconductors (BBSc) with metallic doping exhibit increased photocatalytic activity.¹³⁵ Research has indicated that metal-doped BBScs, including Zn-doped BiOBr, Cu-doped Bi₂O₃, and Gd³⁺-doped Bi₄O₅ (ref. 136) have enhanced photocatalytic performance. These metal-doped semiconductors have demonstrated better redox characteristics, more light absorption, and greater charge separation. Numerous attempts have been made to create and employ bismuth-based photocatalysts that are durable. For example, an α-Fe₂O₃–modified Bi₂WO₆ heterostructure photocatalyst was developed, which exhibited excellent photocatalytic activity and maintained stable performance even after six consecutive cycles, with no discernible loss in activity.¹³⁷ Table 3 presented Bi-based photocatalysts for ciprofloxacin degradation.

Fig. 23 (A) Scheme for BiOI-S photocatalyst for CIPs degradation reused with permission from ref. 132 Copyright 2023, Viley-VcH. (B) Advantages of Fe-photocatalyst & (C) possible degradation pathway of CIP in the A-MnO₂/Fe₂O₃@GS/H₂O₂/vis system with permission from ref. 155 Copyright 2023, Elsevier.

Table 3 Bismuth based photocatalysts for ciprofloxacin degradation concentration, efficiency and kinetic parameters

S/n	Catalyst	Dosage (g L^−1)	Light source	Concentration of ciprofloxacin (mg L^−1)	Kinetics model	Rate constant (min^−1)	Time (min)	Efficiency (%)	Ref.
1	BiOBr	0.5	400 W halogen lamp	5	Pseudo 1st order	0.0272	140	100	138
2	RGO/BiVO4	0.2	300 W xenon lamp	10	—	—	60	68	139
3	BiPO4	0.3	250 W high pressure Hg lamp	10	—	—	90	100	140
4	Er/BiOBr	0.1	300 W xenon lamp	10	—	—	360	61	141
5	BiOCl/titanium phosphate	0.4	300 W xenon lamp	5	—	—	5	100	142
6	BiOBr	0.6	300 W xenon lamp	20	Pseudo 1st order	0.0164	180	95	143
7	Bi3TaO7 QDs/g-C3N4	0.5	86 W blue LED lamp	10	Pseudo 1st order	0.198	120	91	144
8	BiOBr nanoflake/covalent triazine framework	0.2	500 W xenon lamp	10	Pseudo 1st order	0.0102	50	62	145
9	Ag modified P-doped ultrathin g-C3N4 nanosheets/BiVO4	1	300 W xenon lamp	10	Pseudo 1st order	0.0203	120	93	146
10	Bi2S3/g-C3N4	0.625	350 W xenon lamp	20	—	—	120	65	147
11	Pt-BiVO4	1.5	150 W tungsten halogen lamp	10	—	—	60	92	148
12	BiOCl half shells	1	500 W xenon lamp	10	Pseudo 1st order	0.0569	60	91	149
13	BiOCl nanosheets	1	500 W xenon lamp	10	Pseudo 1st order	0.0173	60	50	149
14	BiOBr-Bi2MoO6	—	300 W xenon lamp	10	—	—	120	85	144
15	Graphene-like BN/BiOBr	0.5	300 W xenon lamp	10	—	—	300	81	150

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3.4. Iron based photocatalysts

Iron oxide semiconductors have proven extraordinary potential for photocatalytic programs in environmental remediation due to exceptional features.¹⁵¹ As illustrated in Fig. 23B, iron oxide nanoparticles provide numerous benefits as photocatalysts. Naturally, iron oxides occur in 3 foremost paperwork: magnetite (Fe₃O₄), hematite (α-Fe₂O₃), and maghemite (γ-Fe₂O₃).¹⁵² Among those, hematite (α-Fe₂O₃) is a crimson-colored n-type semiconductor acknowledged for its unique homes, together with an indirect bandgap of 2.1 eV, herbal abundance, low-cost synthesis, excessive chemical balance, environmental friendliness, and ease of recovery.¹⁵³

Research has shown that editing iron-based totally photocatalysts can substantially beautify their photocatalytic overall performance. For example, researchers investigated the degradation of 10 mg L⁻¹ ciprofloxacin under solar irradiation using both pristine and composite photocatalysts, including Fe₂O₃ and Ni-doped Fe₂O₃. The Ni-doped Fe₂O₃ exhibited a 42.1% enhancement in photocatalytic activity compared with the pristine material.¹⁵⁴ Additionally, a Ni-doped α-Fe₂O₃/g-C₃N₄ nanocomposite synthesized through an easy co-precipitation technique tested advanced pastime, reaching an 82.1% removal efficiency. This improved overall performance was attributed to improved charge separation and decreased electron–hole recombination. Heterojunction formation similarly improves the photocatalytic efficiency of iron-primarily based materials. Cai et al. developed a Z-scheme heterojunction composed of α-Fe₂O₃@geopolymer spheres decorated with MnO₂, accomplishing 91% degradation of 20 mg per L ciprofloxacin below visible mild. The proposed degradation pathways is depicted in Fig. 23C.¹⁵⁵ Table 4, summarizes CIPs degradation via Fe-photocatalyst.

Table 4 Iron-based photocatalysts for ciprofloxacin degradation concentration, efficiency and kinetic parameters

Photocatalyst	Irradiation source	Catalyst dosage	Concentration of ciprofloxacin	Time (min)	Reaction kinetics	Rate constant	Degradation efficiency (%)	References
Ni-doped α-Fe2O3/g-C3N4	Solar light	15 mg	10 mg L^−1	120	Pseudo-1st order	0.015 min^−1	82.1	156
FeS/FeO@10% 2D TiO2-x	Visible light	0.1 g	20 mg L^−1	180	—	0.0121 min^−1 (PPS)	90 (PPS)	157
						0.0176 min^−1 (H2O2)	100 (H2O2)
NiFe2O4@chitosin	Visible light	1.0 g L^−1	5 mg L^−1	120	Pseudo-1st order	0.032 min^−1	100	158
				120
FeTiO3/Fe-doped g-C3N4	Visible light	0.5 g L^−1	30 mg L^−1	120	Pseudo-1st order	0.0296 min^−1	93.4	159

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3.5. Carbon based photocatalyst

Materials based on carbon, such as graphene, graphene oxide, reduced graphene oxide, and carbon nanotubes, have greatly enhanced photocatalytic performance. These materials have a high surface area, good electrical conductivity, and a strong adsorption capacity for organic pollutants like CIP. When used as supports or co-catalysts, carbon-based materials reduce electron–hole recombination and enable effective charge carrier separation. For instance, TiO₂-rGO composites have shown a degradation efficiency of more than 95% for CIP under both UV and visible light. Similarly, CNT-ZnO hybrids' improved recyclability and long-term photostability make them suitable for continuous water treatment applications.¹⁶⁰ Recent studies show that carbon-based photocatalysts effectively degrade CIP. Ciprofloxacin (CIP) degradation has been mentioned the usage of both pristine g-C₃N₄ and phosphorus-doped g-C₃N₄ (P-g-C₃N₄), accomplishing removal efficiencies of 50% and 60%, respectively. The more advantageous photocatalytic hobby turned into attributed to stepped forward era of photogenerated electrons, holes, and hydroxyl radicals, at the same time as phosphorus doping reduced the bandgap from 2.70 to 2.48 eV, thereby growing the degradation performance, Fig. 24A and B demonstrate degradation mechanism for CIPs with 2% P-g-C₃N₄ photocatalyst.¹⁶¹ Further upgrades were found in nanohybrids combining g-C₃N₄ with V₂O₅ nanoparticles through easy calcination technique, attaining CIP removal as much as 97%.¹⁶²

Fig. 24 (A and B) Proposed CIPs degradation mechanism by 2% P-g-C₃N₄ photocatalyst retaken with permission from ref. 161 Copyright 2024, Springer. (C and D) Z-scheme mechanism and mechanistic degradation of CIPs by NCD@BMCN under visible-light irradiation respectively reproduced with permission from ref. 163 Copyright 2021, Elsevier.

A novel nanohybrid composed of nitrogen-doped carbon dots (NCD) deposited on bismuth molybdate and g-C₃N₄ called Bi₂MoO₆/g-C₃N₄ proven green photocatalytic degradation of CIP underneath visible mild. This photocatalyst exhibited high activity, balance, and reusability, effective degradation, as schematically and mechanistically illustrated in Fig. 24C and D respectively.¹⁶³ Similarly, a carbon-primarily based photocatalyst, g-C₃N₄/CQD/CC, was prepared by means of embedding 0-dimensional carbon quantum dots (CQDs) into one-dimensional porous tubular g-C₃N₄ on carbon fabric (CC). This material degraded 98% of CIP within 60 min under seen-light irradiation, attributed to more advantageous light absorption.¹⁶⁴ Wei et al. Synthesized round CdS nanoparticles within graphene aerogel (GA) thru a one-pot hydrothermal technique. The CdS-GA hybrids exhibited a porous shape and sturdy electric interactions, achieving CIP elimination of as much as 85.8%, demonstrating the capacity of semiconductor-GA composites for combined adsorption and photocatalytic interest.¹⁶⁵ Another system, CNT@MIL-115 (Fe), blended multi-walled carbon nanotubes (CNTs) with the metallic–organic framework MIL-one zero one (Fe), providing quicker charge switch and activation, ensuing in 90% CIP degradation underneath seen light. Fig. 25A depicted proposed degradation mechanism by these materials.¹⁶⁶ Table 5, discussing some carbon-based photocatalyst for CIPs.

Fig. 25 (A) CNT@MIL-Fe based catalytic CIPs degradation & (B) Ag-doped-graphite derived photocatalyst adopted with permission from ref. 172 Copyright 2024, Elsevier. (C and D) Schematic and mechanistic representation of CIPs photodegradation Cu_xO/MOF reproduced with permission from ref. 180 Copyright 2023, mdpi.

Table 5 Carbon based photocatalysts for ciprofloxacin degradation concentration, efficiency and kinetic parameters

Photocatalyst	Irradiation source	Catalyst dosage	Concentration of ciprofloxacin	Time (min)	Reaction kinetics	Rate constant	Degradation efficiency (%)	Ref.
NCD@HMCN	Visible light	1.0 g L^−1	1.0 mg L^−1	30	Pseudo 1st order	0.16 min^−1	99.5	163
CNT@MIL-101(Fe)	Visible light	0.5 g L^−1	3.02	45	Pseudo 1st order	0.000685 s^−1	90	166
Cds-GA	Visible light	5.0 mg	20.0 mg L^−1	60	—	—	85.8	165
g-C3N4/CQD/CC	Visible light	—	5.0 mg L^−1	60	—	—	99	167
V2O5/g-C3N4	Visible light	0.1 g	10.0 mg L^−1	150	Pseudo 1st order	0.0152 min^−1	90.17	162

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3.6. Graphite

Graphitic carbon nitride (g-C₃N₄), a metal-free photocatalyst active under visible light, has gained significant attention due to its moderate bandgap (∼2.7 eV).¹⁶⁸ Its unique layered structure and ease of synthesis from nitrogen-rich precursors make it an attractive alternative to metal oxide photocatalysts. However, pristine g-C₃N₄ suffers from challenges such as photogenerated electron–hole pair recombination and a modest surface area.¹⁶⁹ To enhance its photocatalytic performance, various strategies have been employed, including the formation of composites with semiconductors like TiO₂, ZnO, and BiVO₃, which promote charge separation and improve efficiency.¹⁷⁰ Porous and doped forms of g-C₃N₄ have also been studied to enhance surface reactivity. A g-C₃N₄/TiO₂ nanocomposite, demonstrated 95% CIP removal under visible light in less than 90 minutes, outperforming the component parts.¹⁷¹ Fig. 25B demonstrated mechanism of CIPs photodegradation by Ag-doped-graphite-derived photocatalyst.¹⁷²

3.7. Metal organic frameworks (MOF)

In the photocatalytic degradation of antibiotics, metal–organic frameworks (MOFs) have also been employed due to their high porosity, tunable structures, and large surface areas.¹⁷³ When used as stand-alone photocatalysts or as parts of hybrid systems, MOFs like MIL-53(Fe), UiO-66, and ZIF-8 have demonstrated encouraging outcomes. MOFs have the ability to absorb light or serve as support that improve the activity and dispersion of conventional semiconductors.¹⁷⁴ Under visible light, it has been demonstrated that MIL-53(Fe) and g-C₃N₄ can degrade more than 90% of CIP in 90 minutes. However, MOFs' practical application is still limited by their moisture sensitivity and relatively high synthesis costs.¹⁷⁵

Metal–natural frameworks (MOFs) have emerged as surprisingly versatile and efficient photocatalysts for the degradation of ciprofloxacin (CIP) due to their rather excessive surface place, tunable porosity, and bendy structural layout.¹⁷⁶ Composed of metal ions or clusters coordinated to natural linkers, MOFs provide abundant lively sites for pollutant adsorption and facilitate speedy mass transfer at some stage in photocatalysis.¹⁷⁷ Their optical and digital residences may be precisely changed through selecting suitable metal centers (along with Fe, Ti, Zr, or Co) and natural ligands, enabling sturdy absorption in the UV-seen area. Among them, Fe-primarily based MOFs (e.g., MIL-100-Fe) showcase strong redox behavior and wonderful seen-mild responsiveness, whilst Zr-MOFs like UiO-66 and its derivatives provide notable aqueous balance and more advantageous ligand-to-steel charge transfer, making them appropriate platforms for photocatalytic change. Ti-MOFs such as MIL-125-(Ti) also mimic the photoactivity of TiO₂ however with stepped forward seen-mild absorption.¹⁷⁸ To similarly improve their photocatalytic efficiency, numerous change techniques were implemented to MOFs, which includes steel doping, ligand functionalization, and the formation of heterojunctions with semiconductors like TiO₂, ZnO, BiOBr, or g-C₃N₄. These mixtures create green Z-scheme, S-scheme, or kind-II heterojunctions that beautify rate separation and suppress electron–hole recombination. Additionally, integrating MOFs with carbon-based totally substances inclusive of graphene oxide, decreased graphene oxide, or carbon nanotubes substantially improve conductivity, light harvesting, and structural stability. MOF-derived metallic oxide nanostructures and carbon-doped substances produced via controlled calcination have additionally proven outstanding photocatalytic activity towards CIP, combining the structural advantages of MOFs with the robustness of metallic oxides.¹⁷⁹ Fig. 25C and D depict schematic and mechanistic photodegradation of CIPs via MOF-derived photocatalyst called Cu_xO/MOF.¹⁸⁰

During photocatalysis, MOFs generate electron–hollow pairs underneath mild irradiation, which sooner or later form reactive oxygen species (ROS) which include hydroxyl radicals (˙OH), superoxide radicals (˙O₂⁻), and singlet oxygen (¹O₂). These reactive species assault the CIP molecule, initiating piperazine ring cleavage, decarboxylation, defluorination, and in the long run mineralization into CO₂ and H₂O.¹⁸¹ Several MOF-based composites have demonstrated high CIP removal efficiencies, often followed pseudo-first-order kinetics and shown advanced performance compared to conventional semiconductors. Although demanding situations remain—including lengthy-term water stability, potential metal leaching, and the value of sure linkers—MOFs retain to draw large attention as promising photocatalysts for advanced water treatment packages.¹⁸²

3.8. Photocatalytic degradation kinetics of ciprofloxacin

The efficiency and mechanism of pollutant removal under light irradiation are greatly influenced by the kinetics of photocatalytic degradation. The degradation process typically adheres to Langmuir–Hinshelwood (L–H) kinetics, which characterizes the interaction between the organic molecules and the photocatalyst surface's active sites.¹⁸³ This model states that the rate of degradation is determined by the adsorption of reactant molecules onto the catalyst and their subsequent reaction with reactive species produced by photolysis, such as superoxide anion radicals (O₂˙⁻) and hydroxyl radicals (˙OH).¹⁸⁴ Degradation frequently occurs at low pollutant concentrations according to pseudo-first-order kinetics, in which the rate of reaction is exactly proportional to the pollutant's initial concentration.¹⁸⁵ Higher concentrations, however, cause the photocatalyst surface's active sites to become saturated, deviating from first-order behavior and possibly shifting the reaction toward zero-order kinetics. The apparent rate constant is greatly influenced by variables like pH, light intensity, catalyst dosage, and the presence of electron acceptors.^186,187 In addition to offering insights into the pathways of degradation, kinetic studies aid in the optimization of operating conditions to maximize photocatalytic efficiency in environmental applications. Table 6 demonstrated various kinetic models for photocatalytic system.

Table 6 Kinetic models of photocatalyst system

No	Phocatalyst system	Light source	Kinetic model	Rate constant (K)	Degradation efficiency	Reference
1	Fe3−xO4–TiO2 + H2O2	UV	Pseudo-1st order	0.05 min^−1	∼50% Reduction in ∼17 min	188
2	Ti2 (bare)	UV (16 W lamp)	Pseudo-1st order	0.0063 min^−1	57% (120 min)	189
3	ZnO/g-C3N4	Visible	Not quantified	—	∼93.8% (∼5× faster than g-C3N4)	190
4	TiO2@g-C3N4@biochar (Z-scheme)	UV-visible	Not quantified	—	99.3% (UV-vis), 89.2% (VIS)	191
5	SiC/g-C3N4 + persulfate (SCN/PS)	Visible	Pseudo-1st order	0.132 min^−1 (SCN/PS), 0.0102 min^−1 (SCN only)	∼95% (30 min)	192
6	TiO2/SnO2/g-C3N4 (ternary heterojunction)	Solar simulated	Not quantified	—	>97%	193

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3.9. Factors affecting the photocatalytic degradation efficiency of ciprofloxacin (CIP)

The efficiency of ciprofloxacin (CIP) photocatalytic degradation is strongly tormented by various operational and environmental elements. Key effects encompass the overall mineralization charge, the manufacturing of reactive oxygen species (ROS), and the interactions among the pollutant and the photocatalyst. These factors must be understood in order to optimize experimental conditions and scale up photocatalytic processes in real water treatment systems.¹⁹⁴

3.9.1. pH and ionic strength. The pH impacts both the surface charge of the photocatalyst and the speciation of CIP in water. Due to its carboxylic and amino practical agencies, CIP can exist in distinct ionic forms relying on the pH: cationic at low pH, zwitterionic near neutral pH, and anionic at high pH, in line with its acid–base equilibria.¹⁹⁵ Adsorption, a requirement for effective photocatalytic degradation, is directly impacted by electrostatic interactions between these species and the photocatalyst surface. Additionally, pH affects ROS production; for example, alkaline environments produce more surface hydroxide ions, which photogenerated holes can easily oxidize to ˙OH radicals. Certain anions, such as Cl⁻ and HCO₃⁻, can scavenge radicals, while others, such as SO₄²⁻, can contribute to the production of secondary radicals' pH and ionic strength. Ionic strength is influenced by dissolved salts and can either promote or inhibit degradation, depending on the type of ions. The solution since pH affects the generation of radicals and the mechanism by which the contaminants are degraded, it is an important factor in the advanced oxidation process¹⁹⁶ The degradation fee of CIP is ruled by the blended actions of each kinds of radicals, with their relative contributions various consistent with the answer pH.¹⁹⁷ Additionally, chemical speciation and ionization significantly impact pollutant degradation.¹⁹⁸ CIP is a zwitterionic molecule, containing a charged piperazine ring and a negatively charged carboxyl institution. At pH values beneath its first pK_a (6.16), protonation takes place, that can inhibit reactions with electrophilic radicals. Conversely, while the pH exceeds its 2nd pK_a (8.23), deprotonation enhances the novel-mediated degradation of CIP.¹⁹⁹ This twin conduct highlights the full-size impact of protonation and deprotonation on CIP's interaction with reactive radicals. Importantly, the combined action of sulfate and hydroxyl radicals drives CIP degradation, which accounts for the highly strong rate regular throughout different pH tiers. Consequently, thermally activated persulfate-based totally AOPs can effectively degrade pollution in wastewater over a huge pH variety.²⁰⁰ As a result, thermally activated PS-based AOP can break down pollutants in wastewater across a wide pH range. The overall scenario of the given description is proposed in Fig. 26A and B show pH effect on CIPs degradation.²⁰¹

Fig. 26 Effect of various parameter on CIPs degradation. (A and B) pH effect reused with permission from ref. 201 Copyright 2025, Elsevier. (C) CIPs initial conc. Effect retaken with permission from ref. 204 Copyright 2024, nature. (D) Catalyst dosage effect adopted with permission from ref. 207 Copyright 2025, nature. (E and F) Effect of temperature and light intensity respectively reproduced with permission from ref. 204 Copyright 2024, nature.

3.9.2. Initial concentration of ciprofloxacin. The initial concentration of CIP affects degradation kinetics by determining the number of molecules competing for ROS. While ROS depletion and the buildup of intermediate products at high concentrations may stop further degradation, abundant ROS in relation to pollutant molecules at low concentrations cause higher degradation rates.^202,203 Moreover, high concentrations of CIP may reduce the effective photon flux that reaches the photocatalyst surface by obstructing light transmission through the solution. To look at the effect of the preliminary CIPF attention at the photocatalytic overall performance of green 5% Hy-Co-ZnO NPs, a series of experiments were conducted at a catalyst dosage of 1.0 g L⁻¹ and pH 7, the usage of varying CIPF starting concentrations (C = 10, 20, 30, 40, and 50 ppm). As the initial concentration of CIPF increases, Fig. 26C shows a clear decline in degrading efficiency. The findings suggest that at higher starting concentrations, CIPF photodegradation may be inhibited by the saturation of the Co-ZnO surface. This is because fewer photons would reach the catalyst surface at greater CIPF concentrations due to increased competition for the active sites. Consequently, fewer electron–hole pairs and hydroxyl radicals (˙OH) are produced.²⁰⁴

3.9.3. Photocatalyst dosage. The amount of photocatalyst determines the total number of active sites available to produce ROS. An increase in dosage usually improves degradation rates until a certain point, when light scattering and particle aggregation reduce effective irradiation and hinder photocatalytic efficiency.²⁰⁵ Optimizing the catalyst loading is important to balance surface reactivity and light utilization. At high dosages, catalyst particles tend to agglomerate all through photodegradation, which reduces the to be had floor area and diminishes the efficiency of drug elimination. Additionally, exceeding the surest catalyst amount can growth answer turbidity, limiting UV mild penetration and in addition lowering degradation overall performance. Conversely, growing the catalyst inside the best range complements the quantity of lively websites and promotes the generation of radicals that power the degradation procedure.²⁰⁶ Fig. 26D, depicted the catalytic dosage effect on CIPs degradation.²⁰⁷

3.9.4. Temperature. Temperature has very little effect on photocatalytic processes because photons, not heat, are the primary driving force.²⁰⁸ Nevertheless, moderate temperature increases can enhance diffusion rates and adsorption kinetics, whereas extremely high temperatures may promote the desorption of CIP and intermediates, reducing surface contact with ROSA collection of experiments were finished using green 5% Hy-Co-ZnO NPs at temperatures starting from 25 to 45 °C to research the effect of temperature on the photodegradation price of ciprofloxacin. Fig. 26E illustrates the effect on temperature on ciprofloxacin photocatalytic degradation at an initial attention of 30 ppm and a catalyst dose of 1.0 g L⁻¹. The consequences display that the degradation fee increases with temperature, as higher temperatures enhance the technology of unfastened radicals and reduce the recombination of electron–hollow pairs. As the temperature increased up to 45 °C, the kinetics of deterioration slowed. One possibility could be that heat stress increases the stability of fluoroquinolones.²⁰⁴

3.9.5. Light intensity. For efficient excitation of electrons from the valence band to the conduction band, the energy and spectral distribution of the light supply need to correspond to the bandgap of the photocatalyst. While UV mild is powerful for activating wide bandgap semiconductors like TiO₂, seen-light-responsive materials—along with doped TiO₂, g-C₃N₄, or heterojunction composites—enable the utilization of sun energy. The production of electrons and holes is accelerated by increasing light intensity, but excessive intensities can lead to recombination because they produce carriers too rapidly without providing adequate reaction time.²⁰⁹ The study examined how light intensity affects photocatalytic degradation of CIP (C₀ = 10 mg L⁻¹, pH = 6.8). Light intensities ranging from 7.7 to 42 W m⁻² were tested. Fig. 26F (ref. 204) shows that higher light intensity accelerates photocatalytic breakdown. As light intensity increases, photoactive species (electron–hole pairs) are generated on the semiconductor surface, leading to faster photocatalytic destruction of CIP.²¹⁰

3.10. Comparison with other advanced oxidation processes (AOPs)

Photocatalysis has gained widespread recognition due to its affordability, sustainability, and ease of integration into water treatment systems. However, when compared to other advanced oxidation processes (AOPs), several kinetic, operational, and matrix sensitivity differences are particularly noticeable. For example, ciprofloxacin degradation is typically higher in UV-based AOPs such as UV/chlorine and UV/persulfate (UV/PS), especially in reclaimed water. These systems outperform conventional UV and UV/H₂O₂ setups due to their ability to generate potent reactive species, such as sulfate and chlorine radicals. Common water contaminants like organic matter, bicarbonates, and chlorides, however, can significantly reduce their effectiveness. Compared to UV/PMS systems (0.145 min⁻¹) and UV/sulfite systems (0.269 min⁻¹), UV/PS systems have shown pseudo-first-order rate constants of up to 0.752 min⁻¹ under ideal conditions. The latter also facilitate effective defluorination at alkaline pH through hydrated electron pathways. Attention has also been drawn to the ability of catalytic sulfate radical AOPs, such as MnFeO₄-peroxymonosulphate (MPS/MFO) systems, to achieve high CIP removal (∼92%) over a broad pH range. However, these systems have the potential to increase ionic strength and introduce secondary sulfate ions, which could limit the environmental acceptability of treated water.²¹¹ Ozone-based processes represent yet another competitive alternative. Ozone alone can remove more than 98% of CIP, and ozone and H₂O₂ together significantly increase TOC removal (>90%) compared to ozone alone (∼15%). Energy-efficient variants, such as ozone microbubble diffusion systems with aerator pumps, have shown improved degradation efficiency (83.5% vs. 60.9%) and energy utilization (∼0.146 g kW⁻¹ h⁻¹).²¹² Several studies have demonstrated the high mineralization capacity of electrochemical AOPs, particularly the electro-Fenton (EF) process, which removes more than 94% of the total organic carbon (TOC). EF systems produce hydroxyl radicals in situ by catalyzing reactions between Fe²⁺ and electrochemically produced H₂O₂. Other electrochemical technologies like photo electrocatalysis, anodic oxidation, and combined techniques using persulfate or ultrasound often yield mineralization rates above 90%, despite the fact that these systems typically require high operating energy and maintenance.²¹³ An additional useful technique that commonly makes use of nano-CuO_x and H₂O₂ is catalytic wet peroxide oxidation (CWPO). At high temperatures (∼95 °C), it has been demonstrated to remove CIP significantly (∼86.8%) and reduce COD moderately (∼54.9%). Although CWPO can effectively break down complex molecules like ciprofloxacin through processes like decarboxylation and ring hydroxylation, its use may be limited due to its need for high temperatures and metallic catalysts.²¹⁴ However, photocatalysis offers a more flexible and eco-friendlier platform, especially when using visible-light active materials like doped ZnO composites or TiO₂/g-C₃N₄ as shown in Fig. 27.²¹⁵ Although its degradation rates may be slightly lower than those of radical-based AOPs under controlled lab conditions, photocatalysis has the advantages of operating in mild conditions, using solar energy, and being compatible with catalyst immobilization strategies for easy reuse. However, issues like slower kinetics in turbid or colored water, catalyst deactivation, and occasionally limited mineralization potential continue to be challenges. Hybrid systems that combine photocatalysis with other AOPs, like photocatalysis plus persulfate or photo electrocatalysis, may offer the most promising path forward to balance efficiency, cost, and environmental sustainability.

Fig. 27 Photocatalytic degradation mechanism of CIP on the TiO₂@g-C₃N₄@BC under UV-vis light irradiation reproduced with permission from ref. 215 Copyright 2025, nature.

3.11. Future prospects and research gaps

Effective photocatalysts for the degradation of ciprofloxacin (CIP) have been developed to a great extent, but several significant problems and research gaps still limit their practical applications.²¹⁶ These limitations must be overcome to advance photocatalytic water treatment technologies from laboratory research to broad adoption. To function properly, most conventional photocatalysts, such as ZnO and TiO₂, require strong solar irradiation or artificial UV light sources. Since UV only accounts for around 5% of solar energy, the systems' overall energy efficiency and viability are severely limited.²¹⁷ For environmentally friendly treatment processes, future research must focus on developing visible-light-active photocatalysts that can effectively capture the broader solar spectrum (for example, by band gap engineering, doping, or heterojunction formation). Although many photocatalysts achieve high removal of CIP (above 90%), total mineralization, as measured by the removal of total organic carbon (TOC), is often much lower. This leads to the creation of potentially dangerous or biologically active intermediates.²¹⁸ Future studies should investigate combining photocatalysis with other procedures (like ozonation, biological treatment, or electrochemical methods) to fully detoxify and mineralize antibiotics. In slurry-based systems, catalyst separation remains a major operational challenge, especially for large-scale applications. Magnetic recovery, membrane filtration, and immobilization techniques (like coating on glass, membranes, or ceramics) have shown promise, but they usually lead to a trade-off between surface area and light exposure.²¹⁹ Future designs should prioritize scalable reactor configurations that allow for easy catalyst reuse and effective light penetration without compromising photocatalytic activity. Numerous reactor designs, including batch, packed-bed, and tubular reactors, have been studied; however, few of these studies have connected lab-scale performance to pilot or full-scale deployment.²²⁰ Examples of parameters that are frequently disregarded in early-stage research include photon flux distribution, flow dynamics, and hydraulic retention time (HRT). To evaluate viability at the municipal or industrial scales, future research must prioritize techno-economic analyses, pilot testing under realistic water matrices, and scale-up modeling.

Real wastewater contains ions (like Cl⁻, SO₄²⁻, and HCO₃⁻), natural organic matter (NOM), and other micropollutants that can either inhibit photocatalytic activity or compete with target pollutants for reactive species. The artificial or ultrapure solutions used in many published studies are not representative of the environment. Thus, photocatalyst performance in complex water matrices, including surface waters, hospital wastewater, and pharmaceutical effluents, should be systematically investigated in future research.²²¹ Although photocatalysis can degrade CIP, the nature and toxicity of the intermediate products are not always well understood. Sometimes, intermediates can retain their antimicrobial qualities or even become more toxic than the parent compound. Comprehensive identification of degradation products using state-of-the-art techniques (LC-MS/MS, FTIR, and NMR) in combination with bioassays or ecotoxicological evaluations is essential to confirming water safety following treatment.²²² The lack of standardized methods for assessing photocatalytic efficiency, which include variations in light intensity, reactor volume, and catalyst dosage, makes it difficult to compare results from various investigations. Additionally, many reports employ simple pseudo-first-order kinetics without accounting for mass transfer limitations, catalyst deactivation, or radical scavenging effects.²²³ Future studies should focus on standardized testing conditions in addition to comprehensive kinetic and mechanistic modeling.

4. Conclusion

The increasing detection of ciprofloxacin (CIP) in aquatic environments poses a serious threat to public safety and ecological health due to its persistence, bioaccumulation potential, and role in antibiotic resistance. Photocatalytic degradation has emerged as a practical and sustainable way to remove CIP from water systems, as it generates highly reactive species when exposed to light, breaking down complex pharmaceutical molecules. Various photocatalytic materials, including metal oxides (like TiO₂ and ZnO), doped semiconductors, heterojunction composites, and magnetic nanostructures, have demonstrated effective CIP removal under UV and visible light, with multiple studies achieving degradation efficiencies above 90%. Among these, the most promising catalyst design strategies identified in this review include:

➢ Doping with metal or non-metal elements such as Fe, Ag, N, or C to modify the bandgap and enhance light absorption, allowing for visible-light activation and improving photocatalytic efficiency.

➢ Heterojunction formation, which effectively improves charge carrier separation and reduces recombination rates, thus enhancing the efficiency of photocatalytic degradation.

➢ Integration with carbon-based materials, such as graphene oxide or reduced graphene oxide (rGO), which further enhances photocatalyst performance by improving conductivity and light harvesting capabilities.

However, significant barriers to practical implementation remain, including low mineralization rates, limited visible-light response, issues with catalyst recovery, and poor scalability in real wastewater conditions. Advanced photocatalysts with enhanced light absorption, stability, and reusability must be developed alongside optimized reactor configurations to bring this technology closer to full-scale deployment. Future research must focus on performance validation in realistic environmental matrices and thorough assessment of degradation intermediates, ensuring safe and effective treatment of pharmaceutical pollutants.

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Data availability

Since this manuscript is a review paper, no new experimental data were generated or analyzed in the course of this work. All data supporting the findings of this study are available within the cited references, which have been duly acknowledged in the manuscript.

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