Synergistic integration of nanoscale zero-valent Iron and biological treatment for environmental remediation: mechanisms, system configurations, and performance optimization

Abstract

In this review, we explore recent advances in coupling nanoscale zero valent iron (nZVI) with biological treatments for environmental remediation, emphasizing mechanisms, system configurations (direct vs. indirect contact), microbial interactions, and key factors that govern performance. We first provide an overview of the current literature pertaining to nZVI- and or biological-mediated reductive treatment of organic/inorganic pollutants and compare the pros and cons of individual treatment methods. We emphasize the need for combined processes and explore the mechanisms driving hybrid systems, examining various system configurations. We then conduct a comprehensive evaluation of microbial–nZVI interactions and the environmental/material parameters, paired with engineering control strategies for enhanced performance. We also highlight the influential parameters that affect treatment efficiency, providing a critical analysis of the factors that can either enhance or impede the remediation process. In summary, we prioritize practical optimization, risk considerations, and pathways for scaling from laboratory to field applications, offering guidance for future research and practical applications.

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Nuo Liu

Dr. Nuo Liu is currently working as an Assistant Professor at Shanghai Polytechnic University. He conducts scientific research in the field of nanoscale iron materials with applications in environmental remediation.

Tang Chenliu

Tang Chenliu currently serves as an Associate Professor at Beijing University of Chemical Technology. Her research focuses on microscale surface interaction, soil remediation, and nano-agricultural technology.

Yaoguang Guo

Dr. Yaoguang Guo serves as a Professor at Shanghai Polytechnic University. His research focuses on the comprehensive treatment technology assessment of VOCs.

Chunli Zheng

Dr. Chunli Zheng currently serves as a Professor at Shanghai Polytechnic University. Her research focuses on bioremediation and phytoremediation of contaminated soil.

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Environmental significance

Integrating nanoscale zero-valent iron (nZVI) with biological treatment offers a promising, versatile strategy for remediating contaminated soil and water by combining rapid abiotic reduction with biologically driven degradation. This review evaluates mechanisms, reactor configurations, and operational strategies to enhance contaminant removal while addressing challenges such as nZVI aggregation, transport, persistence, and ecological impacts on microbial communities. Emphasis is placed on low-cost, green synthesis and stabilization approaches, selective electron transfer to microbes, and system optimization informed by advanced molecular tools such as emerging genetic techniques like CRISPR-enabled bioaugmentation, providing safer, more effective combined remediation technologies. This approach aims to balance efficacy with environmental safety.

1. Introduction

Wastewater characteristics vary depending on numerous factors, comprising natural and anthropogenic factors. Many industrial plants discharge wastewater without pretreatment, rendering it toxic and difficult to decontaminate.¹ Wastewater co-contaminated with halogenated organic compounds (HOCs), organic dyes, nitroaromatic compounds, and/or other chemical compounds needs to be detoxified and the chemical oxygen demand (COD) needs to be reduced to meet permissible limits.² Traditional treatment such as coagulation, flotation and evaporation was proven to be insufficient owing to incomplete degradation, energy input and cost.³

The selection of appropriate techniques requires a comprehensive consideration of factors such as the type of contamination, the degree of contamination and site conditions. In situ techniques, such as chemical oxidation,⁴ bioremediation,⁵ and permeable reactive barriers (PRBs),⁶ make direct contact with contaminants, reducing costs. Ex situ methods, like pump-and-treat⁷ and multi-phase extraction,⁸ are used when in situ methods are ineffective, involving the extraction of groundwater for treatment. Actually, some new remediation technologies such as nanozyme technology, microbial electrochemical systems and metal–organic frameworks (MOFs)⁹ are continuously emerging, providing more possibilities for solving complex contamination problems.

Soil remediation involves physical methods,^10,11 chemical methods,¹¹ and biological methods.¹² In practice, a combination of techniques is often employed for optimal results, such as integrated and multiphase remediation approaches (e.g., nano-bio technology and nano-bio systems driven by electricity and/or photo-¹³), to address soil contaminants. Advances in technology continue to introduce new remediation solutions, offering greater potential for addressing complex pollution challenges.

Bioremediation has gained a lot of attention in the treatment of mixed contaminated wastewater due to its low cost and eco-friendliness.^14,15 The mechanism involves the transformation and conversion of contaminant compounds to biomass or gases (CO₂ and CH₄) by microbial activity.¹⁶ However, the drawback of the long period required indicated that this technology is also in need of improvement.

Nanoscale zero-valent iron (nZVI) is a strong nonspecific reducing agent that is used for recalcitrant pollutant treatment.^17,18 nZVI particles can be directly injected into the subsurface¹⁹ or in the form of a permeable reactive barrier (PRB), which is considered as an effective and promising method for wastewater remediation.²⁰ The high specific surface area, surface chemistry characteristics and unique core–shell structure enhance the degradation efficiency of pollutants.²¹ However, nZVI tends to aggregate and the corrosion products may coat on the surface of the particles, which can inhibit the activity of nZVI.²² These problems would largely restrict the applications of nZVI.

Recently, the combinational use of nZVI and biological treatments has been recognized as an effective method for recalcitrant wastewater treatment.^23,24 A combined system of anaerobic sludge and nZVI was designed, achieving efficient decolorization of reactive blue 13. The decolorization ratio was enhanced compared with individual methods, indicating the synergy effect of the combined system.²⁵ Choi et al. described a remediation test integrating an anaerobic–aerobic-ZVI system to enhance the biological treatment of dye wastewater.²⁶ The combined method presents several advantages. First, nZVI reduces pollutants, which creates favorable conditions for microbes.²⁷ Additionally, the corrosion-derived electron supply induces some metabolic pathways that would have otherwise been impossible.²⁸ Furthermore, the combined method can completely degrade contaminants into nontoxic products, and nZVI nanoparticles (NPs) can be a potentially rich source of Fe for the metabolism of microorganisms.^24,29 However, this technology faces certain challenges, for example, various combination types and their efficiencies on recalcitrant pollutant treatment, the interactions between nZVI and microbes, and challenges of the combined nZVI–bio system in further applications.

This review aimed to provide a detailed understanding of the combined nZVI–bio system and a perspective on engineering applications. Two common hybrid systems of nZVI and biological methods for recalcitrant wastewater treatment and the coupled effects of the integrated system and degradation mechanism were summarized. Additionally, the factors influencing treatment efficiency were discussed, and the challenges of the combined nZVI–bio system were highlighted.

2. Biological processes for wastewater treatment: pros and cons

Biological processes remove organic and inorganic matter, such as nitrogen, phosphorus, and heavy metals, by biological metabolism.^30,31 Bioremediation rapidly converts aqueous organic compounds into biomass, which can be separated from the aqueous phase. Bioremediation is a promising method for reducing sludge production and lowering energy input.³²

2.1 Degradation of organic contaminants

Owing to societal development, several highly toxic compounds, including polychlorinated biphenyls (PCBs), chlorinated aliphatic hydrocarbons (CAHs), and polycyclic aromatic hydrocarbons (PAHs), have been persistently discharged into soil or aquatic environments.^33,34 The successful removal of these compounds by biological treatment has been previously reported.^35–37 It had been reported that the bioremediation processes mainly occured through microbial degradation and sorption on the flocs, and volatilization process can also occurs during aeration.³⁸ Yuan et al. employed a mixed culture to biodegrade PAHs, and found that the mixed culture completely removed phenanthrene, acenaphthene, and pyrene at different intervals.³⁹ Another study focused on the biodegradation of high-molecular-weight PAHs and revealed that Beijerinckia spp. and Rhodococcus spp. were suitable for degrading various PAHs.⁴⁰

2.2 Remediation for inorganic contaminants

Biological processes have been widely proven to be effective for the removal of inorganic pollutants such as heavy metals, nitrates, and phosphates during wastewater treatment and soil remediation.

Toxic metals, including Cd(II), Cr(VI), Cu(II), Pb(II), and Zn(II), are widely distributed in the environment, and can pose health risks to humans even at low concentrations. Multiple microorganisms can reportedly bioremediate these toxic metals. Kang et al. tested mixed bacterial cultures to eliminate Pb, Cd, and Cu from contaminated soil, and found that mixed cultures exhibited a higher bacterial growth rate, enzyme activity, and tolerance than single microbial cultures.⁴¹ Ojuederie et al. demonstrated the synthetic effects of microorganisms and plants on toxic metals.⁴² The active antioxidant systems of the plants alleviated the toxic effects on tissues and cells. Shibasaki et al. proposed a strategy to enhance detoxification efficiency by controlling evolutionary dynamics.⁴³ Two types of mutants designed and periodically reintroduced were applied. Many studies have documented the biosorption of heavy metals using living and non-living algae.^44,45

Nitrate and phosphate pollution has become a global issue. Bioremediation processes have been extensively applied to remove nitrate and phosphate in wastewater treatment plants in the past decades.^46–48 During biological processes, NO₃⁻ was converted into nitrogen gas by microorganisms using anoxic–oxic (A/O) and anaerobic–anoxic–oxic (A²/O) technology. The complete denitrification by Paracoccus Denitrificans from wastewater was achieved after about nine days.⁴⁹ Similarly, biological phosphorus removal is based on phosphate accumulating organisms. Hu et al. found that anoxic phosphorus-accumulating organisms present a low phosphate removal rate compared to aerobic phosphorus-accumulating organisms under appropriate conditions.⁵⁰

2.3 Limitations of biological processes

Single biological treatment is typically accompanied by resistant bioavailability by-products. In addition, several limitations restrict the application of biodegradation. First, the complex components and harmful elements may retard bio-detoxification and cause low efficiency.⁵¹ Second, the capability of bacteria to eliminate bioavailable contaminants is insufficient, especially when applied for in situ remediation. Third, once pollutants enter the soil, they bind to the mineral or soil particles; therefore, the selectivity of microorganism suspensions determines their removal efficiency.⁵² Finally, microbial processes are generally slow and often less efficient than chemical reduction techniques and produce excessive biomass that requires further treatment before disposal. Moreover, microbial adaptation and action against pollutants with a high load are poor.

3. Abiotic degradation by nZVI: pros and cons

nZVI could serve as an efficient reducing agent or catalyst for the degradation of organics, heavy metals and nitrate and phosphate.^53,54

3.1 Degradation of organic contaminants

It has been reported that nZVI showed high trichloroethene (TCE) removal efficiency using modified nZVI, and that the dichlorination of nZVI played an important role.⁵⁵ Direct subsurface injection of nZVI slurry in a full-scale site successfully removed 95.7% of TCE in 60 days without producing chlorinated intermediates.⁵⁶ To further enhance the performance of nZVI particles, modified nZVI technologies such as biochar⁵⁷ and kaolin supported sulfurized nZVI⁵⁸ were applied to degrade organic pollutants.

The key degradation mechanisms include adsorption and coprecipitation on the nZVI surface by iron oxide/oxyhydroxide, surface reactions of nZVI, and a reductive chemical effect to break chemical bonds.

3.2 Remediation of inorganic contaminants

In recent years, nZVI has been exhibited to be suitable for the remediation of groundwater or soil polluted by heavy metals at pilot and field scales.^59–61 nZVI reactions with some toxic metals, such as As(III),⁶² Cd(II),⁶³ uranium (U),⁶⁴ Ni(II),⁶⁵ Cr(VI),⁶⁶ Cu(II), and Pd(II),⁶⁷ have been studied (Fig. 1). Li et al. demonstrated that nZVI could simultaneously remove wastewater containing Cu and As.⁶⁸ Long-term (over 12 months) tests indicated that the removal efficiency of nZVI exceeded 99.5% for all existing toxic metals. Fajardo et al. reported no obvious negative effects on the soil properties upon exposure to nZVI.⁶⁹ Moreover, Pb–nZVI enhanced Pb toxicity in soil, whereas Zn–nZVI decreased the toxicity. Yan et al. determined the core–shell structure of nZVI.⁷⁰ Shi et al. found that aggregation (influenced by the surface energy) triggered a substantial loss of the initial nZVI upon its exposure to pollutants, consequently requiring its reapplication.⁷¹ These drawbacks have led to a surge in research on bimetallic iron for enhancing nZVI stability. Recently, a combination of techniques has been often applied for better results, such as coupling electrokinetic remediation with modified nano zero-valent iron/nickel⁷² and bimetallic nano-zero-valent iron/copper synthesized with Ginkgo biloba leaves⁷³ to address Cr(VI).

Fig. 1 A schematic depicting the mechanism of the abiotic process. Reproduced from ref. 122 with permission from Elsevier,¹²² Copyright 2022.

Recently, there has been increasing interest in the removal of inorganic phosphate and nitrate pollutants using nZVI.^74,75 The surface chemistry and mechanism of nZVI corrosion between contaminants in solutions were described by ref. 76. The reductive effect of nZVI on nitrate species, including ammonia, nitrate, and nitrite, was analyzed by ref. 77. The nitrogen mass balance results revealed that nitrate was converted to ammonia ions and subsequently transformed into ammonia gas, and that it could modeled by pseudo-first-order kinetics. The removal mechanism of nZVI mainly consists of adsorption, redox reactions, and co-precipitation.⁷⁸

3.3 Limitations of the abiotic process

The removal of organic pollutants or toxic metals by nZVI has received considerable attention in recent decades. nZVI transforms hazardous compound structures into simple and bioavailable structures owing to its strong reductive capacity. However, several drawbacks restrict its further application.^79–81 First, the aggregation effect causes incomplete dehalogenation and can generate secondary toxic products. Second, side reactions can occur when nZVI is in contact with pollutants, increasing the required amount of nZVI.

These limitations about the biological and abiotic nZVI method promote the exploration of alternative approaches to enhance the efficiency and pollutant remediation. In this context, the combination of nanotechnology and biological systems has emerged as a promising solution. This synergistic approach overcomes the drawbacks in traditional biological and nZVI methods alone.

4. nZVI–biology combined system

Microbial adaptive responses occur mainly through following processes: (i) formation of biofilms and/or release of extracellular polymeric substances (EPS), (ii) direct electron transfer through cytochromes or nanowires and indirect electron transfer through mediators, and (iii) production or consumption of metabolites that chemically alter passivation layers such as organic acids, siderophores, H₂ or CO₂.⁸² For instance, some metal-reducing bacteria (e.g., Geobacter and Shewanella) can express certain cytochromes and conductive pili that can make contact with iron oxides and transfer electrons directly. Meanwhile, some microbes can utilize soluble redox mediators (quinones and humic substances) and hydrogen as electron carriers.

4.1 Biology techniques

Biological treatments have emerged as effective and sustainable solutions for addressing a wide range of environmental pollutants. These methods leverage the natural capabilities of microorganisms and biological processes to degrade, transform, or remove contaminants, offering environmentally friendly alternatives to conventional chemical or physical remediation techniques.

4.1.1 Activated sludge. The activated sludge process primarily removes nitrogen and phosphorus from wastewater through the synergistic action of microbial communities. For instance, aerobic granular sludge technology accelerates microorganisms to spontaneously form dense, spherical granules compared to traditional activated sludge flocs.⁸³

4.1.2 Biofilm technology. Biofilm technology forms a biofilm on the surface of solid substrates, combining the dual advantages of biological treatment and physical separation.⁸⁴ Compared to traditional activated sludge processes, biofilm methods offer higher biomass loading and lower sludge production. Recently, the development of supporting materials (e.g., graphene aerogels and catalysts) and additive-manufactured custom carriers (e.g., thermoplastic plastics and metals) has significantly improved the stability of biofilms and mass transfer efficiency.

4.1.3 Aerobic/anaerobic processes. Through the action of microorganisms, organic waste is converted into a stable, nutrient-rich material. Composting is widely used for treating agricultural and municipal solid waste.⁸⁵ In addition, the anaerobic digestion process, in which organic matter is broken down by microorganisms in the absence of oxygen, produces biogas.⁸⁶ This method is commonly used for treating organic waste from agriculture, food processing, and wastewater treatment plants. Furthermore, the direct addition of specific microbial strains, or stimulation of the activity of indigenous microbial communities, is also widely applied for treating complex organic compounds.

4.1.4 Bio-trickling filtration. Bio-trickling filter systems rely on microorganisms that attach to the surface, form biofilms, and degrade pollutants.⁸⁷ These systems are especially suitable for volatile organic compounds (VOCs) or other gas pollutants (e.g., H₂S). Moreover, the removal rate of nitrogen and sulfur compounds can be significantly enhanced by packing with zero-valent iron or nanocatalysts.

4.2 Combined strategies

nZVI and biological treatments have been extensively studied and successfully applied, suggesting that their combined use may be suitable for wastewater remediation.^29,88 Additionally, two hybrid models, with indirect and direct contact, have been suggested (Table 1). In remediation processes using hybrid systems, recalcitrant pollutants are first degraded by the chemical reduction of nZVI. Subsequently, residual pollutants with lower concentrations can be further reduced by the biological process, ultimately enabling complete degradation. Chemical reduction is more effective for pollutants at higher concentrations than for those at lower concentrations. However, hybrid systems could repress this limitation as the biological process could remove pollutants present at low concentrations.

Table 1 Summary of hybrid type studies on the removal of various pollutants

Pollutants	Combined types	Results	Ref.
Metalworking fluids	Direct contact (unanchored)	95.5% reduction in COD, 85% reduction in toxicity	2
Reactive blue 13	Direct contact (unanchored)	29.4% enhancement in decolorization (1.0 g L^−1 nZVI)	93
Azo dye	Direct contact (unanchored)	50% enhancement in COD	94
Decabromodiphenyl ether	Direct contact (unanchored)	63% enhancement in COD	95
Trichloroethylene	Direct contact (unanchored)	No inhibition to bacteria	96
Sulfate	Direct contact (unanchored)	95% of 1500 mg L^−1 sulphate was reduced in <50 h	97
ρ-Nitrophenyl phosphate	Direct contact (anchored)	1.41 times enhancement in activity, 31 times in stability	98
Lysozyme	Direct contact (anchored)	3-Times enhancement in adsorption	99
Azo dyes	Indirect contact sequential	Enhancement in removal	100
Dye wastewater	Indirect contact sequential	78–89% enhancement in decolorization	101
Organic dye wastewater	Indirect contact sequential	99% in decolorization, 58% in COD	27
Industry wastewater and urban sewage	Indirect contact sequential	No inhibition to bacteria	102
Acid rock drainage	Indirect contact sequential	99.7% in sulfate reduction	103
Coking wastewater	Indirect contact sequential	96.1% in COD, 99.2% in NH3–N, 92.3% in TIN	104

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4.2.1 nZVI coupled with sulfate reducing bacteria. Sulfate-reducing bacteria (SRB) interact with nanoscale zero-valent iron (nZVI) to degrade organohalides and remove heavy metals.⁸⁹ SRB can utilize nZVI as an electron donor, enhancing the reduction of sulfate to sulfide, which then precipitates heavy metals. This process not only removes heavy metals but also improves the redox environment, aiding organohalide degradation. Additionally, SRB can alter the surface properties of nZVI, enhancing its stability and reactivity.

4.2.2 nZVI coupled with anaerobic dechlorinating bacteria. A recent study explored the feasibility of combining anaerobic dechlorinating bacteria, such as Dehalobacterium and Dehalogenimonas, with nZVI to detoxify chlorinated methanes (e.g., chloroform and dichloromethane). The results indicated that nZVI could degrade chloroform to dichloromethane, which was then further degraded by Dehalobacterium to innocuous products. Moreover, soluble compounds released by nZVI can suppress bacterial activity, however, under field conditions, dilution is expected to mitigate this inhibitory effect.⁹⁰

4.2.3 nZVI coupled with denitrifying bacteria. Nanoscale zero-valent iron (nZVI) enhances the metabolic activity of denitrifying bacteria (DNB) through multiple pathways.⁹¹ Specifically, nZVI promotes the synthesis of key enzymes involved in the denitrification process, including nitrate reductase (NAR), nitrite reductase (NIR), nitric oxide reductase (NOR), and nitrous oxide reductase (N₂OR). The increased synthesis and catalytic capacity of these enzymes contribute significantly to improved denitrification efficiency. Furthermore, nZVI itself and the Fe²⁺ ions released during its corrosion can serve as electron donors for DNB, enhancing the activity of the electron transport system (ETS). Therefore, these effects substantially increase the nitrate-nitrogen (NO₃⁻-N) removal rate.

4.2.4 nZVI coupled with organohalide-respiring bacteria. Bio-nZVI-RD integrates the advantages of both bio-RD and nZVI-RD, making it a more promising approach for remediating organohalide pollutants.⁹² nZVI impacts organohalide-respiring bacteria in several ways: it modulates the bioavailability of organohalides through dynamic adsorption and desorption, provides a surface for bacterial attachment to support microbial growth, modifies microbial community structures and interactions, and directly influences bacteria through direct attachment.

4.3 Hybrid models

4.3.1 Direct contact system.
(1) Unanchored system. The unanchored direct-contact hybrid system has been widely applied owing to its easy design and operation. Feasibility studies on the inherent merits of nZVI and biotechnology have been conducted.² In a sequential study, Li et al. reported that the augmented efficient decolorization of reactive blue 13 was achieved by combining anaerobic digestion with nZVI.⁹³ The probable mechanisms were also elucidated: nZVI directly reduced reactive blue 13 and created favorable pH conditions for the subsequent anaerobic digestion process. Zhou et al. reported a sequential hybrid two-step system to achieve the complete degradation of p-chlorophenol.¹⁰⁵ Here, p-chlorophenol was reduced to phenol by Fe/Pd NPs and removed by adding a Bacillus fusiformis (BFN) suspension. Similarly,¹⁰² bimetallic Fe/Cu combined with an aerobic biological system was successfully applied to treat organic contaminants to enhance the treatment capacity, by retarding agglomeration and clogging phenomena. Bokare et al. reported a nano-bio system involving dichlorination using bimetallic NPs and subsequent degradation by bacteria under aerobic conditions.¹⁰⁶ Dong et al. observed enhanced remediation efficiency by combining nZVI with functional bacteria, such as sulfate-reducing bacteria (SBR) and iron reduction bacteria (IRB), which reduced the oxidation of Fe(III) to Fe(II).¹⁰⁷
(2) Anchored system. The immobilization of NPs on organic membranes is another promising approach for improving the efficiency of pollutant degradation. Wang et al. reported a new anchored hybrid system through the immobilization of nanomaterials in the extracellular matrix of bacterial biofilms and found that the removal efficiency, reusability,¹⁰⁸ and environmental perturbation tolerance could be significantly enhanced. Au-NPs were successfully fabricated in situ in G. sulfurreducens biofilms by Chen et al. and the functional biofilms increased the current density by 1.4-fold and the substrate removal 2.2 times.¹⁰⁹

The anchored hybrid system likely formed a bio-electrochemical system (BES) wherein the NPs significantly increased the microbial conductivity. The nZVI in the anchored system served as an electron donor to promote bacterial degradation efficiency. However, more effort is required to enable NPs to be anchored in mixed bacteria before wastewater treatment. Moreover, the NP reduction mechanism on the surface of biofilms and its influence on microbes remain unclear.

4.3.2 Indirect contact. Similar to the direct contact hybrid system (one-phase system), the indirect contact nZVI–bio system (multiphase system) has been investigated by researchers. The indirect model can alleviate the nZVI-induced negative effects on biological processes. Saxe et al. designed an elemental iron-activated sludge system for the biodegradation of azo dye wastewater and found that the reduction products, including aniline and sulfamic acid, were biodegradable by the activated sludge system.¹⁰⁰ In addition, the integrated system effluent had a lower total carbon concentration than that for the biological process. Choi et al. integrated nZVI and an anaerobic/aerobic (A/O) system to enhance the biological treatment of dye wastewater, and the results revealed an overall decolorization efficiency of 78–89% compared with the decolorization efficiency of 44–69% observed with the single biological treatment.¹⁰¹ nZVI destroyed the dye molecular structure and reduced its compounds to simpler fragments, improving their bioavailability and facilitating the subsequent A/O process. Liu et al. designed an indirect contact nZVI–bio hybrid system to treat Congo red wastewater.²⁷ An additional coagulation and sedimentation process was performed to remove excess nZVI or oxidation products (Fig. 2). The stability and superiority of the two-phase nano-bioreactor over a single biological batch for the long-term operation were confirmed. Fan et al. used Fe/Cu combined with an activated sludge system to treat mixed wastewater.¹⁰² The total biofilm increased by approximately 59% in the presence of Fe/Cu, which enhanced the overall removal efficiency. Sulfate reduction was significantly enhanced, and the heavy metal removal efficiency was 99.7%. The indirect contact model enhanced the system resistance to sudden loading shock and created suitable conditions for further biological processes.

Fig. 2 An example of nZVI and biological treatment of industrial wastewater: a) schematic of the nZVI–bio hybrid treatment system. The volume of the nZVI reactor and sedimentation reactor was 2.7 L. The HRT in the nZVI reactor was set at 2 h. The wastewater firstly entered in the nZVI recycled reactor. The flocculation process was achieved by adding polyferric sulfate (PFS) and polyacrylamide (PAM) and the dosage of PFS and PAM was 100 and 2 mg L⁻¹, respectively; b) efficiency of color removal and COD reduction. Reproduced from ref. 110 with permission from Elsevier,¹¹⁰ Copyright 2019.

4.4 Advantages of the combined system

The advantages of the current nZVI–bio combined system present a rapid reduction/adsorption reaction by nZVI while reaching total removal of pollutants by the following microbial degradation or biomineralization; the electron donation/transfer by nZVI corrosion additionally enhances metabolisms that sustain long-term effects; in all, the combined system has demonstrated advantages in terms of generating faster initial removal and total removal, longevity and sustained effects and lower cost when compared to either technology used alone.

4.4.1 Total removal of pollutants. The combined systems often achieve higher total removal/mineralization than nZVI or biological treatment alone because each technology alone fails to fully eliminate pollutants (nZVI breaks recalcitrant bonds; microbes mineralize intermediates). The nZVI reaction produces partially bioavailable intermediates (e.g., less-chlorinated compounds and smaller organics), and then microbes enable their complete degradation to CO₂, biomass or stable minerals.⁹¹

4.4.2 Longevity and sustained effects. The corrosion of nZVI generates electron donors that can stimulate the growth of microorganisms (e.g., organohalide-respiring bacteria, sulfate reducers, and methanogens). Moreover, nZVI can create conditions more favorable for microbial activity, by transforming highly toxic or biocidal species into less toxic forms, facilitating subsequent biological degradation.⁸⁹ In return, microbial reduction of passivating Fe(III) oxides can partially refresh reactive sites on their surface or enhance soluble Fe(II) production, thus increasing the longevity of the degradation process.

4.4.3 A wide range of contaminants and lower cost. The hybrid approach has been demonstrated to be effective in multiple contaminants (heavy metals and organic contaminants). Importantly, during the practical engineering applications, facilitating microbes to complete mineralization reduces the need for extra reagent inputs (e.g., chemical oxidant dosing), making the nano–bio combined technology promising for practical applications.

5. Mechanisms and influence factors

nZVI contributes to transmitting information and electrons, yielding various comparatively less toxic products, and may play an important role in microbial metabolism. The self-repair and reproduction nature of microorganisms potentially allows the nZVI–bio system to have high scalability.²⁸

5.1 Mechanism of the nZVI–bio hybrid system

The interactions are categorized into direct and indirect pathways between nZVI and microbes (Fig. 3). The mechanism involved the biotic effect mediated by microbes, the abiotic effect caused by nZVI and the synergistic interactions in the combined system.

Fig. 3 A schematic depicting the mechanism of the hybrid system: a) unanchored hybrid system; b) anchored system. Reproduced from ref. 108, 110, and 111, with permission from Elsevier,^108,110,111 Copyright 2016 and 2019.

5.1.1 Electron donation and transfer. Direct electron transfer and catalytic effect facilitate the reduction of pollutants. Electrons from Fe⁰ or Fe²⁺ on particle surfaces can be transferred to adsorbed pollutants, thereby promoting removal reactions. The surfaces of nZVI (nanoscale zero-valent iron) and iron oxides can also act as catalytic sites, enabling bond disruption and facilitating the overall reduction process. As shown in Fig. 3, given that nZVI has a standard redox potential of −0.44 V, it can serve as an electron donor to directly destroy the structural backbone of organic contaminants, thereby alleviating their inhibitory effect on subsequent biological processes. Such assistance in electron transfer induces metabolic pathways that would otherwise be impossible. The nZVI NPs can also be a potentially rich source of Fe for incorporation into enzymes.

5.1.2 Redox reactions by nZVI. nZVI is a strong nonspecific reducing agent that is used for recalcitrant pollutant treatment. The reduction of heavy metals by nZVI creates favorable conditions for microorganisms. Next, the Fe²⁺ produced from nZVI corrosion can bind with organic molecules. pH is an important parameter in microbial digestion, and avoiding acidification during digestion has become a research hotspot. The OH⁻ released during Fe⁰ corrosion can offset the decrease in pH caused by acidogenic activities to create a more suitable environment for bacteria.^112,113 Therefore, decontamination efficiency is augmented compared with that in single biological processes. In addition, biological processes can, in turn, prevent the passivation of Fe⁰. In a single nZVI system, the OH⁻ released and Fe²⁺ produced precipitate on the Fe⁰ surface in the form of ferrous hydroxide, thus requiring acid addition for neutralization. Moreover, in the presence of O₂, H₂O₂ or other peroxides, Fe²⁺ formed from corrosion can participate in Fenton or Fenton-like reactions to produce hydroxyl radicals (·OH), leading to non-selective oxidative degradation.

5.1.3 Sorption and precipitation. Fresh nZVI and its corrosion products (iron oxides/hydroxides) provide high-surface-area sorbents that concentrate hydrophobic organics and some polar compounds at reactive interfaces, increasing reactive sites and reaction rates. Precipitation and sequestration of co-contaminants: nZVI can reduce and precipitate heavy metals (e.g., Cr(VI), Cu(II), and Zn(II)), or facilitate sulfide precipitation, indirectly altering contaminant bioavailability and toxicity.

5.1.4 Surface sorption and complexation by microbes. The functional groups present on cell walls, including carboxyl, phosphoryl, and hydroxy, as well as extracellular polymeric substances (EPS), are capable of binding metal cations and polar organic compounds. Moreover, microbial redox activity alters contaminant speciation and solubility: e.g., sulfate-reducing bacteria (SRB) generate sulfide that precipitates many metals as insoluble sulfides (e.g., PbS, CdS, and ZnS). Moreover, the microbially induced carbonate or phosphate precipitation (MICP) can immobilize metals by trapping them in the carbonate lattice.¹¹⁴

5.1.5 Mitigation of side effects. The passivation effect in the nZVI–bio hybrid system owing to the generated OH⁻ can partially alleviate acidification during microbial digestion. Conclusively, the nZVI–bio hybrid system presents the advantages of both NPs and biological processes, thereby improving the self-buffering capability and enhancing biodegradability. Considering the potential cytotoxicity, the microbial metabolisms release enzymes such as superoxide dismutase (SOD) and catalase (CAT) or promote the secretion of specific proteins to encapsulate cytotoxic particles.¹¹⁵

5.1.6 Microbial–iron cycle. Microbial reduction the corrosion Fe(III) oxides can refresh reactive Fe(0)/Fe(II) surface properties or produce soluble Fe²⁺, then restore or regenerate the reactivity of aged nZVI particles. For instance, iron-reducing and iron-oxidizing microorganisms catalyze Fe(III)/Fe(II) transformations. Microbial reduction of iron oxides can remobilize Fe²⁺, refreshing some abiotic reactivity of aged nZVI surfaces.⁸² In return, nZVI surfaces can serve as substrates for attachment and biofilm development; biofilms facilitate the local concentration of enzymes and establish microenvironments (e.g., anoxic zones near particle surfaces) for immobilization of pollutants.

5.2 Influence factors

Parameters such as the nZVI dosage, size, operation pH, and co-existing components may influence the efficiency of hybrid systems.¹¹⁶

5.2.1 Effects of the nZVI size, dosage and pH. The size and dosage of nZVI and pH always interact with each other to influence both abiotic reactivity and microbial compatibility. Smaller nZVI particles exhibit higher specific surface areas, resulting in greater corrosion rates.⁹¹ Higher dosage accelerates the size effect, causing fast corrosion rates. The size effect can raise pH, depending on the dose and system characteristics. In contrast, the pH level also plays a crucial role in the reactivity of nZVI. At lower pH levels, there is an increase in redox reactions with nZVI, which leads to a higher corrosion rate. Consequently, this results in a greater number of reactive ions and accelerated reduction rates. Conversely, under higher pH conditions, nZVI tends to form iron hydroxides (such as Fe(OH)₂ and Fe(OH)₃), which can reduce its reactivity by forming a passive layer on the surface.

5.2.2 Effects of the co-existing components. Natural organic matter (NOM) has a dual effect on nZVI toxicity. NOM can alleviate toxicity and facilitate the growth of indigenous microbes. Chen et al. reported that the mitigation of nZVI toxicity to Gram-negative E coli and Gram-positive Bacillus was observed in the presence of Suwannee River humic acid (SRHA).¹¹⁷ However, the competition effect inhibited the reactivity against contaminants. The presence of NOM offers a tradeoff for nZVI-bioremediation, with high potential for sequential bioremediation while partially inhibiting abiotic reactivity for target contaminants.

Similar to inorganic components, nitrate, chloride, and sulfate can form complexes with iron hydroxides, which stimulate the nZVI passivation, thereby reducing its negative impacts. Inorganic components can also compete with active sites to reduce the degradation rate of contaminants.¹¹⁸

5.2.3 Effect of concentration of heavy metals. The presence of heavy metals within the integrated nano–bio combined system can exert cytotoxic effects on microbes. Moreover, heavy metals exhibit varying degrees of toxicity to bacteria due to their distinct chemical properties and mechanisms of action.¹¹⁹ For instance, mercury (Hg) is highly toxic to many bacterial strains compared with copper (Cu) and zinc (Zn) which become toxic when higher levels are reached. Metal ions can interact with cellular biomolecules, leading to protein denaturation and enzyme inhibition, disruption of the membrane structure and electron transport, and induction of oxidative stress. These effects inhibit bacterial activity, and consequently prolong the lag phase and reduce the overall contaminant removal efficiency.

5.2.4 Effect of microbes. The different types of microbes and the initial concentration also have an effect on the combined system. Microorganisms play their own critical roles in the combined nano–bio system, with some exhibiting the ability to degrade pollutants, while others may contribute to pollution under certain conditions.¹²⁰ The sulfate-reducing bacteria (SRB) favor degradation of organohalides and removal of heavy metals, while denitrifying bacteria (DNB) do better in nitrate-nitrogen (NO₃⁻-N) removal.

5.2.5 Effect of nZVI modification. Recently, nZVI modifications have been widely applied to enhance stability and dispersibility, and mitigate toxicity to microorganisms.¹²¹ The primary modification methods for nZVI and their respective impacts are as follows: sulfidation (S-nZVI) forms iron sulfide surface phases, which slow down passivation, alter corrosion and H₂ release, and reduce toxicity to microbes; however, over-sulfidation may reduce the initial reducing efficiency.¹²² Coatings (e.g., CMC) enhance dispersion and subsurface transport, reduce aggregation, and limit direct metal contact. These coatings increase spatial contact with microbes, but excessive coverage can hinder electron transfer.¹²³ Supported-nZVI (e.g., on biochar, activated carbon, and MOFs) offers high surface area, provides reactive sites, and adsorbs contaminants but there is a risk of secondary release. Bimetallic particles (e.g., Pd/Fe and Ni/Fe) catalytically accelerate hydrogenation reactions, enhancing reaction rates. However, they may suppress microbes or exhibit toxic metal effects.

5.3 Challenges for the combined system

Combined systems boast the combined benefits of individual technologies for the rapid and complete degradation of recalcitrant pollutants. However, some challenges need to be addressed before further application.

5.3.1 Cytotoxicity of nZVI to microbes. The presence of nZVI can be cytotoxic to microbes.²⁴ The observed microbial inhibition owing to nZVI is dose-dependent. The small size of the nZVI NPs further aids their penetration into cells through phagocytosis or membrane channels, leading to the production of intracellular reactive oxygen species (ROS) that can further react with lipids, proteins, or DNA and induce offset pathways in the microbes.

5.3.2 Lag period of biological treatment. In the combined nZVI–bio system, microbes typically present a lag phase in the initial stage, and the microbial reductive process via respiration is extremely slow. However, only a few minutes or hours are required for nZVI reduction. Therefore, efficiency gaps pose challenges for nZVI–bio applications.

5.3.3 Stability of the hybrid system. Stability is crucial for remediation processes, particularly in anchor systems where NPs are bound to biological materials via chemical bonds. For example, oxygen exposure can irreversibly inhibit the viability of some anaerobic microbes, such as organohalide respiring bacteria.

5.4 Performance optimization

Surface modifications of nZVI are widely employed to enhance particle stability and dispersibility while mitigating microbial toxicity.¹²⁴ Moreover, advances in recombinant DNA technology enhance nZVI–bio applications. Metagenomic and metatranscriptomic analyses reveal fundamental mechanisms of various microbes.¹²⁵ For instance, gene edit technology can insert metal-binding protein genes into microbial to create engineered strains with improved reactivity and selectivity for target pollutants.

6. Other NP–bio hybrid systems and applications

Previous studies have analyzed the feasibility of using other NPs, such as silver, gold, and TiO₂, as carriers to combine with organic components or using engineered bacterial biofilms as carriers for remediation applications.

6.1 Nanoparticles as carriers

The removal of heavy metals from wastewater through microorganism biosorption has been widely applied owing to its low cost and applicability with various heavy metals. However, the nature of the suspension restricts further applications in industrial operations. Efforts to resolve this drawback have been made using immobilization technology.^126–128 Researchers have proven that immobilization enhances reactivity, benefits resistance to environmental conditions, and facilitates separation and reuse. Recently, owing to the advantage of chemical inertness and favorable biocompatibility, NPs have been designed as carriers.¹²⁹ Peng et al. successfully immobilized Saccharomyces cerevisiae on the surface of chitosan-modified NPs to adsorb Cu(II) in an aqueous solution.¹³⁰ The results indicated that over 90% of Cu(II) was adsorbed in the first 10 min, and that the adsorption capacity was 144.9 mg g⁻¹, several times higher than that for conventional adsorbents, including chitosan beads and microorganisms immobilized on polyurethane.

6.2 Engineered bacterial biofilms as carriers

NPs have been considered powerful catalysts owing to their high reactivity and selectivity compared with conventional chemical catalysts. NP catalysis has recently become a highly active field.^131,132 However, its characteristics present some drawbacks, including recycling and potential secondary pollution when applied at pilot or in situ scales. Efforts have been made to address these challenges by binding NPs to their substrates. Sharifi et al. introduced a strategy to enhance electron transfer properties by depositing nickel oxide NPs (NiO-NPs) on DNA-modified glassy carbon electrons.¹³³ The results revealed that triangular NPs were formed, which improved superior electron transfer properties and electrocatalytic activity compared with spherical NPs. Therefore, various biological components have been considered to enhance electrocatalytic activity.¹³⁴ Wang et al. designed engineered Escherichia coli secreted biofilm-anchored gold nanoparticles to address the challenges.¹⁰⁸ Hybrid systems can thus acquire the advantages of engineered living systems and NPs. In lab-scale operation, hybrid systems have shown high efficiency for p-nitrophenol degradation.

7. Conclusions and outlook

This review describes the mechanisms of two nZVI–bio models, the interaction between nZVI and microbial communities and its influencing factors. The sole use of nZVI or biological technologies to remove toxic metals and organic compounds and their limitations were discussed to highlight the need for combined processes. Integrating nZVI technology into conventional biological processes has proven to be effective for wastewater treatment and environmental remediation. Combined schemes can minimize the major drawbacks of biological processes, such as metal cytotoxicity, low bioavailability, and toxic by-products of some organic pollutants.

Future research should pay attention to the fate of nZVI in the environment before the in situ applications because the size effect could pose high ecological risks. The transport of nZVI also affects the microbial community upon contact, thereby determining its subsequent ecological consequences. Moreover, low cost and “green” synthesis methods should be explored and more efforts should be made on the stabilized nZVI–biology hybrid system to avoid the aggregation effect and enhance the electron selectivity. In addition, ongoing developments from both nanotechnology and recombinant DNA techniques such as metagenomics and meta-transcriptomics can further expand the advantages of the nZVI–bio technology. For example, the clustered regularly interspaced short palindromic repeats (CRISPR) may trigger high scalability of bacteria with specific metal-binding genes for remediating recalcitrant pollutants.

Author contributions

Nuo Liu: methodology, data curation, writing – original draft, Chenliu Tang: visualization, Yaoguang Guo: software, Chunli Zheng: supervision, conceptualization, funding acquisition.

Conflicts of interest

The authors declared that they have no conflicts of interest to this work.

Data availability

No new data were generated or analyzed as part of this review.

Acknowledgements

This work was supported financially by the Shanghai University Youth Teacher Training and Funding Program (ZZEGD202408), the Natural Science Foundation of China (52270129), Oriental Talent Youth Program, Shanghai Shuguang Program (23SG52) and The Central Government Guidance Fund for Local Science and Technology Development (2022ZY027).

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