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Abbreviation (ISO4): Prog Chem      Editor in chief: Jincai ZHAO

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Progress in Chemistry >

2025 , Vol. 37 >Issue 7: 1011 - 1024

DOI: https://doi.org/10.7536/PC241109

Review

Reduction Pathways in Zero-Valent Iron Systems： Discussions Triggered by Research Approaches and Detection Methods

  • Shiying Yang , 1, 2, 3, * ,
  • Wenjun Kuang 3
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  • 1 The Key Laboratory of Marine Environment & Ecology， Ministry of Education， Qingdao 266100， China
  • 2 Shandong Provincial Key Laboratory of Marine Environment and Geological Engineering （MEGE）， Qingdao 266100， China
  • 3 College of Environmental Science and Engineering， Ocean University of China， Qingdao 266100， China
*

Received date: 2024-11-11

  Revised date: 2025-01-08

  Online published: 2025-05-15

Supported by

the National Natural Science Foundation of China， CN(52370089)

the Natural Science Foundation of Shandong Province， CN(ZR2024ME131)

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Abstract

Zero-valent iron （ZVI） and its surface-modified materials have been widely used for the removal of various pollutants due to their excellent reduction properties. The three recognized potential reduction pathways of ZVI include direct electron transfer reduction， Fe（II） reduction， and atomic hydrogen reduction. Due to varying interpretations among researchers regarding the three reduction pathways and the diverse methods employed to detect them， recent studies have yielded differing conclusions on several critical aspects， including： （1） the dominant reduction pathway for pristine ZVI materials； （2） the impact of sulfur modification on the generation or recombination of atomic hydrogen； （3） the role of carbon modification in enhancing the reduction performance of ZVI through accelerated direct electron transfer or atomic hydrogen production； （4） and the underlying mechanisms by which different transition metal modifications influence the dominant reduction pathways of ZVI， among others. These discrepancies have sparked debates concerning the predominant reduction pathways involved in the removal of pollutants by ZVI and its surface-modified materials. This review systematically summarizes the following points： （1） the structure and modification principles of ZVI and its surface-modified materials； （2） the mechanisms and detection methods of the three reduction pathways in ZVI reduction systems； （3） the influence of different surface modification techniques （sulfur modification， carbon material modification， and transition metal modification） on the reduction pathways and the existing controversies； （4） the interference of environmental conditions （pH， coexisting ions， and natural organic matter） on the reduction pathways. Based on the reduction pathways， the review also presents prospects for future research directions， with the aim of addressing some of the current uncertainties in reduction pathway research and promoting a unified understanding of ZVI reduction pathways， thereby advancing scientific research and development in ZVI and its surface-modified materials.

Contents

1 Introduction

2 Iron as a starting point： ZVI and its surface modified materials

2.1 ZVI structure and pollutants removal

2.2 Structure and modification principles of sulfur-modified ZVI

2.3 Structure and modification principles of carbon-modified ZVI

2.4 Structure and modification principles of transition metal-modified ZVI

3 First glimpses： reduction pathways and its detection methods

3.1 Direct electron transfer reduction pathway

3.2 Fe（Ⅱ） reduction pathway

3.3 Atomic hydrogen reduction pathway

4 Manifestation of conflicts： the impact of surface modification on reduction pathways

4.1 Sulfurization modification

4.2 Carbon material modification

4.3 Transition metal modification

5 Echoes of the tributary： interference of environmental conditions

5.1 pH

5.2 Coexisting ions

5.3 Natural organic contaminants

6 Conclusion and outlook

Key words： zero-valent iron; surface modification technology; reduction; direct electron transfer; Fe（Ⅱ）; atomic hydrogen

Cite this article

Shiying Yang , Wenjun Kuang . Reduction Pathways in Zero-Valent Iron Systems： Discussions Triggered by Research Approaches and Detection Methods[J]. Progress in Chemistry, 2025 , 37(7) : 1011 -1024 . DOI: 10.7536/PC241109

1 Introduction

Iron, as one of the core elements of the Earth, exists in various forms throughout nature and is widely utilized in many fields. In the context of environmental pollution control, zero-valent iron (Fe0, ZVI) has garnered significant attention for pollutant removal due to its advantages such as non-toxicity, low cost, and easy availability. ZVI possesses strong reducing properties and can serve as an excellent electron donor for pollutant reduction, with standard redox potentials of E 0(Fe2+/Fe0) = -0.447 V and E 0(Fe3+/Fe2+) = +0.771 V[1]. In recent years, ZVI and its surface-modified materials have been a research hotspot in the reduction-based removal of heavy metals, nitrates, and organic pollutants from water[2-11].
Currently, regarding the reduction pathways of pollutants by ZVI, the academic community generally recognizes three main pathways: ① direct electron transfer (DET) reduction of pollutants; ② reduction of pollutants by Fe(II) generated from ZVI corrosion; and ③ atomic hydrogen (H*) produced by the uptake of electrons by H⁺/H₂O participating in pollutant reduction[5,10,12-18]. Since the 1990s, zero-valent iron reduction technology has been applied to the removal of halogenated organic compounds from water, and the DET pathway was identified as the primary reduction route in some studies[19]. As research deepened, by the early 21st century, studies had revealed that Fe(II) and H* reduction pathways also exist in zero-valent iron systems[20-21]. In 2000, Li et al.[22] introduced transition metals into the ZVI system and found that H* reduction was the dominant pathway for trichloroethylene dechlorination. In 2011, Kim et al.[23] first synthesized sulfur-modified ZVI, which significantly enhanced trichloroethylene removal by strengthening DET reduction. With the rapid development of carbon materials, carbon-modified ZVI also gained widespread attention in the early 21st century. Carbon modification improved ZVI's reduction performance by promoting DET; however, with the emergence of the H* spillover mechanism, the contribution of the H* pathway in carbon-modified ZVI systems has also been extensively studied[24].
There is currently ongoing debate regarding the three different reduction pathways. Some scholars argue that in the ZVI/H2O corrosion system, due to the non-conductive nature of the ZVI oxidation layer, it is impossible for electrons from the Fe0 core to directly reach the surface of the oxidation layer and come into contact with pollutants for electrochemical reactions. Instead, pollutant reduction can only occur through iron corrosion products (such as H*/H2, Fe(Ⅱ), Fe3O4, green rust, etc.)[25-26]. Numerous studies have also confirmed that during the reduction of heavy metals[2-3], halogenated hydrocarbons[4-7], or nitrates[8-9], ZVI generates reduction-active species such as H* or Fe(Ⅱ). However, these studies do not deny the contribution of DET to pollutant reduction. Moreover, some scholars have indirectly demonstrated that pollutant reduction can indeed occur via the DET pathway[10,27]. The varying interpretations of the three reduction pathways and the differences in detection methods have led to confusion regarding the understanding of ZVI reduction pathways. Therefore, clarifying the relationships among the three reduction pathways and summarizing the discrepancies arising from various detection methods has become increasingly important for gaining a deeper understanding of the ZVI-mediated pollutant reduction process.
In addition, ZVI itself tends to passivate easily. To optimize the pollutant removal capability of ZVI, modified ZVI materials have attracted considerable attention from researchers. The modification methods for zero-valent iron are diverse and can be broadly categorized into carrier material modification (using carbon materials or natural minerals), coating modification (with biopolymers or inorganic compounds), sulfur modification, transition metal modification, and multi-modification composites. Among these, carbon modification, sulfur modification, and transition metal modification have received significant attention and in-depth research in recent studies on zero-valent iron reduction systems[28]. Different sulfur species, carbon materials, and transition metals can influence various aspects of ZVI, including corrosion rate, electron transfer pathways, catalytic hydrogen production capacity, hydrophilicity/hydrophobicity, and pollutant enrichment ability[29-35], thereby differently affecting the contribution of the three reduction pathways in pollutant removal. There is ongoing debate regarding sulfur modification: whether it inhibits H* generation or H* recombination. For carbon modification, it remains unclear whether it accelerates DET reduction or H* reduction. As for transition metal modification, the underlying mechanisms are still poorly understood. Therefore, clarifying the impact of various surface modification techniques on pollutant reduction pathways will help researchers select appropriate surface modification methods to enhance the reduction and removal efficiency of target pollutants.
In summary, there is currently no unified summary of the three reduction pathways in ZVI reduction systems, both domestically and internationally. Therefore, this article will conduct an in-depth analysis of ZVI reduction systems and systematically summarize: (1) the structure of ZVI and its surface-modified materials, as well as the principles behind different modification methods; (2) the three mechanisms of action of reduction pathways in ZVI reduction systems and the various detection methods used; (3) the different mechanisms by which surface modification techniques influence reduction pathways in ZVI systems; and (4) the interference of environmental conditions on the existence of different reduction pathways in ZVI reduction systems. By summarizing these four aspects, we identify unresolved issues regarding pollutant removal pathways in ZVI reduction systems, provide a research outlook for ZVI and its surface-modified materials in terms of reduction pathways, and hope to achieve a unified understanding of the three reduction pathways in ZVI systems, thereby promoting the scientific research and development of ZVI and its surface-modified materials.

2 Starting with Iron—ZVI and Its Modified Materials

ZVI has been widely used for the reductive removal of pollutants due to its strong adsorption and reduction capabilities. However, during use, ZVI materials are prone to oxidative passivation and aggregation, which reduces their reactivity and limits their effectiveness in pollutant removal[36]. Therefore, numerous studies have employed various surface modification techniques to enhance the reducing capacity of ZVI and extend its service life.

2.1 ZVI Structure and Pollutant Removal

The original ZVI material exhibits a typical core-shell structure (Figure 1), consisting of an Fe0core and an iron oxide shell[37]. The Fe0core, as an excellent electron donor, continuously releases electrons to reduce and remove pollutants. Meanwhile, the iron oxide shell, with its abundant Lewis acidic or basic sites, can adsorb various pollutants[38]. To enhance electron release and pollutant adsorption, methods such as optimizing material synthesis and pretreatment are widely employed in ZVI material modification to improve its performance. Mathew et al.[39]successfully reduced the thickness of the passivation layer by liquid nitrogen treatment during the chemical reduction synthesis of nZVI, significantly improving its Cr(VI) reduction capability. Lan et al.[12]co-milled mZVI with oxalic acid, forming Fe(II) coordination sites and FeC2O4on the surface of mZVI, thereby enhancing its adsorption and reductive dechlorination effects on trichloroethylene.
图1 铁/氧化物核壳结构：（a）铁纳米颗粒的HAADF-STEM图像；（b）Fe和（c）O的EDS元素映射；（d）Fe和O叠加的EDS元素映射[37]

Fig.1 Fe/oxide core-shell structure. （a） The HAADF-STEM image of an iron NP and the corresponding EDS elemental mapping of （b） Fe K and （c） O K. （d） The overlay of （b） and （c）[37]. Copyright 2018， American Chemical Society

2.2 Sulfur-modified ZVI structure and modification principle

Low electron utilization and passivation have always been the two major challenges hindering the effective use of ZVI. To enhance the performance of ZVI, sulfur modification technology has been introduced into the field of ZVI materials[24,40]. By employing a simple surface modification technique, core-shell structured S-ZVI materials with Fe0as the core and FeS xas the shell (Figure 2) have received considerable attention from researchers[41]. Sulfur modification improves electron transfer pathways and enhances material hydrophobicity, thereby improving the anti-passivation performance, pollutant selectivity, and electron utilization of ZVI materials[11,42]. However, the commonly used sulfur modification methods—mechanical ball milling and chemical synthesis—cannot achieve a perfect core-shell structure for S-ZVI[41]. Li et al.[43]compared S-ZVI prepared by mechanical ball milling and a two-step chemical synthesis method and found that the material surface consisted of a mixed shell of iron (hydro)oxides and FeS x. Chen et al.[44]prepared S-ZVI via a one-step chemical synthesis method and discovered that sulfur atoms were incorporated within the ZVI, without forming a distinct core-shell structure.
图2 S-mZVI的SEM-EDS映射图像[45]

Fig.2 SEM-EDS mapping image of S-mZVI[45]. Copyright 2020， American Chemical Society

Although the synthesis of S-ZVI materials with a perfect core-shell structure has not yet been achieved, the mechanism by which sulfur modification affects ZVI has been thoroughly understood. S-ZVI prepared by mechanical ball milling and two-step chemical synthesis, due to the presence of FeS xon its surface, exhibits superior conductivity compared to iron (hydro)oxides. Electrons tend to be conducted more readily through FeS x, and the hydrophobic nature of sulfur sites inhibits contact between the material and H2O, enhancing interaction with pollutants and optimizing the electron transfer pathway from the Fe0core to pollutants, thereby achieving efficient pollutant degradation[11,24,40-43,45]. On the other hand, S-ZVI prepared by one-step chemical synthesis, due to the strong affinity of sulfur atoms for iron atoms, results in elongated Fe—Fe bond lengths. Iron atoms not attracted by sulfur are more likely to be released into the solution through reactions, forming mass transfer channels from the Fe0core to the external environment, enabling rapid reduction of pollutants and even encapsulating heavy metals within the ZVI[44].

2.3 Carbon-modified ZVI structure and modification principles

Carbon materials can modify ZVI in two ways: loading ZVI onto carbon materials and coating the surface of ZVI with a layer of carbon material. The growth of ZVI on carbon materials can be achieved through liquid-phase chemical reduction, which effectively disperses ZVI particles and alleviates the aggregation phenomenon caused by magnetic effects[27]. Mechanical ball milling can be used to attach a layer of carbon material to the surface of ZVI, reducing the contact area between ZVI and H2O and mitigating the passivation phenomenon[34].
Regardless of the modification method, carbon materials share similar effects on ZVI modification. First, carbon materials themselves possess excellent electrical conductivity, enabling them to replace the iron oxide layer in electron transfer[29]. Second, carbon materials can effectively disperse ZVI particles, inhibiting their aggregation[32]. Third, due to their high specific surface area and abundant functional groups, carbon materials can adsorb pollutants, increasing the contact opportunities between reactants and enhancing electron utilization[46-47]. Finally, the encapsulation of ZVI by carbon materials creates a microelectrolytic system, accelerating the rate of electron release (Figure 3)[48]. Additionally, differences among various carbon materials in particle size, specific surface area, structural characteristics, and types and quantities of functional groups also have varying impacts on ZVI performance[49].
图3 球磨法制备的N（C）改性的ZVI材料结构及其对三氯乙烯的去除机制[13]

Fig.3 Structure of N（C）-modified zero-valent iron materials prepared by ball milling and its mechanism for the removal of trichloroethylene[13]. Copyright 2024， Elsevier

2.4 Transition Metal-Modified ZVI Structure and Modification Principles

The combination of transition metals with ZVI can enhance the pollutant reduction performance of ZVI in two ways: the enhancement of reaction activity by the bimetallic system and the catalytic effect of transition metals[5]. The attachment of transition metals on the surface of ZVI can form a galvanic cell system. At the ZVI end, the Fe0 core releases electrons to generate Fe(Ⅱ)/Fe(Ⅲ); at the transition metal end, pollutants or H2O accept electrons and are reduced. This galvanic cell reaction accelerates the electron release rate, effectively alleviating aggregation and passivation phenomena, thereby significantly enhancing the reduction activity of ZVI[31]. Additionally, transition metal modification can cover some active sites on the ZVI surface, converting them into transition metal active sites. These active sites play a catalytic role in the Fe(Ⅱ)/Fe(Ⅲ) cycle and the generation of H*[50-51]. The study by Yang et al.[5] illustrates the dual effects of transition metal modification (Figure 4): at low Pd loading, the enhanced activity of ZVI makes halogenated hydrocarbons more likely to be reduced via direct electron transfer; whereas at high Pd loading, the high catalytic activity of Pd for H* causes halogenated hydrocarbons to be reduced more through the H* pathway.
图4 不同Pd负载量改性ZVI对污染物的还原机制[5]

Fig.4 Reduction mechanism of pollutants by ZVI modified with different palladium loadings[5]. Copyright 2013， American Chemical Society

3 The First Glimpse—Reconstructing the Pathway and Its Detection Methods

For redox reactions to occur, both an electron donor and an electron acceptor must be present. Clearly, ZVI serves as the electron donor in pollutant removal reactions, while pollutants act as the target electron acceptors. The transfer of electrons from ZVI to pollutants involves a complex process, primarily consisting of three pathways: direct electron transfer (DET) reduction, Fe(Ⅱ) reduction, and H* reduction.

3.1 DET reduction pathway

3.1.1 Electronic transmission

Due to the presence of an oxide layer on the surface of ZVI, electrons released from the Fe0 core must overcome the Schottky barrier and Ohmic contact through the iron-iron oxide heterojunction to reach the iron oxide surface for chemical reactions[38]. The native oxide layer of ZVI can be divided into inner and outer layers: the inner layer consists of Fe3O4, while the outer layer is composed of Fe2O3 [52]. The inner Fe3O4layer is a semiconductor with a band gap of 0.11 eV, allowing electrons to pass through easily, whereas the outer Fe2O3layer has a band gap of approximately 2.0 eV, making it difficult for most electrons to penetrate[53-54]. Although in aqueous systems, the outer Fe2O3layer may receive electrons from the Fe0 core and transform into (hydro)oxides of Fe(II)/Fe(III), represented by magnetite (Fe3O4) or green rust, according to equations (1 and 2)[55], thereby reducing the difficulty of electron transfer[56]. However, as the reaction proceeds, the increasing thickness of the oxide layer and the formation of Fe(III) (hydro)oxides, such as goethite (α-FeOOH), maghemite (γ-Fe2O3), and hematite (α-Fe2O3), lead to surface passivation, inhibiting the release of electrons from the Fe0 core to the outer layer for participation in reduction reactions[56-58].
F e   →   F e 2 +   +   2 e -
F e 2 O 3   +   2 H +   +   2 e -   →   F e 3 O 4   +   H 2 O

3.1.2 Electronic Reception and Reduction

When electrons released from the Fe0 core are transferred to the surface of the oxide layer, direct contact with pollutants is required to achieve direct reduction of the pollutants. Therefore, adsorption of pollutants is a prerequisite for achieving DET reduction[36,59]. For example, Lin et al.[60] and Gao et al.[61] used amorphous ZVI and hollow ZVI, respectively, to reduce and remove Cr(Ⅵ). Compared to the original ZVI, both materials exhibited enhanced hydrophilicity, promoting the adsorption of Cr(Ⅵ) onto the ZVI surface and thereby improving the reduction and removal efficiency. Theoretically, during corrosion, ZVI forms a layer of Fe(Ⅲ) (hydro)oxide on its surface, creating more abundant adsorption sites and enhancing pollutant removal. However, the formation of the passivation layer interferes with electron transfer, so it is necessary to balance pollutant adsorption and electron release[62].
The DET pathway generally involves the transformation from Fe0to Fe2+, with standard redox potentials of E 0(Fe2+/Fe0) = -0.447 V. The DET process can achieve the reduction of most pollutants; however, for some refractory pollutants, such as per- and polyfluoroalkyl substances (PFAS), due to their inherently stable chemical properties, the DET pathway typically struggles to facilitate their reductive defluorination[63]. Additionally, whether the reducible sites of pollutants can directly accept electrons also affects the direct electron transfer pathway. For instance, Cu(Ⅱ)-EDTA, as a heavy metal complex, cannot directly receive electrons for reduction because Cu(Ⅱ) is encapsulated by the ligand. Instead, nZVI must first corrode to form Fe(Ⅲ), which rapidly complexes with EDTA to release Cu(Ⅱ), followed by DET-mediated displacement reduction to generate Cu0 [64]. In the case of florfenicol, a pollutant containing both chlorine and fluorine substituents, the chlorine substituent, after adsorption, is closer to the ZVI surface, while the fluorine substituent is farther away. Therefore, DET reduction tends to preferentially remove chlorine before fluorine[10].

3.1.3 Detection methods

The detection of the DET reduction pathway can be performed through electrochemical tests and electron quenching experiments. Electrochemical impedance spectroscopy (EIS) can reflect the ZVI corrosion mechanism and surface state changes under pollutant adsorption conditions[4,53]; linear sweep voltammetry (LSV) extrapolation fitting of Tafel polarization curves allows for a detailed analysis of the electron transfer rate during the reaction process[4,65]; adding KBrO3as an electron quencher to the ZVI reaction system can assess the contribution of electrons released by ZVI to the pollutant reduction process[66]. However, electrochemical methods only measure the ability of electrons to transfer from the Fe0core to the oxide layer surface, and the competition from electron quenchers for these transferred electrons may also interfere with the production of H* generated by electron acceptance on the ZVI surface. Therefore, there is currently no direct and robust method for detecting the DET reduction pathway of pollutants by ZVI; instead, it is often supported by excluding other reduction pathways.

3.2 Fe(II) reduction pathway

3.2.1 Fe(II) source

The electron release from ZVI can be divided into two steps. The first step involves the core Fe0 releasing electrons to transform into Fe(Ⅱ) (Equation (1)), and the second step is the re-oxidation of Fe(Ⅱ) to form Fe(Ⅲ) (Equation (3))[67-68]. Since the electrochemical corrosion of ZVI in water primarily proceeds through the first step, the generation of Fe(Ⅱ) during reactions with pollutants is inevitable[69]. Additionally, Fe(Ⅲ), which has fully released its electrons, may also accept electrons released from the Fe0 core and be reduced back to Fe(Ⅱ) (Equation (4))[68]. As Fe(Ⅱ) has not yet completely released its electrons and still retains certain reducing properties, the Fe(Ⅱ) present in the system also plays a role in pollutant removal.
F e 2 +   -   e -   →   F e 3 +
F e 0   +   2 F e 3 +   →   3 F e 2 +

3.2.2 Reduction process

Based on the pH differences in the system during the reaction between ZVI and pollutants, Fe(II) may exist in two forms: free state and adsorbed state[70-71]. When Fe(II) in the free state in solution comes into contact with pollutants, it acts as an electron donor, releasing electrons to reduce the pollutants. However, due to its limited electron-releasing capacity (1 mol of Fe(II) can release only 1 mol of electrons), and its standard redox potential E 0(Fe3+/Fe2+) = +0.771 V, the reducing ability of Fe(II) toward pollutants is relatively weak[71]. In contrast, Fe(II) adsorbed on the surface of ZVI can rapidly undergo Fe(II)/Fe(III) cyclic conversion due to electron release from the Fe0 core. Additionally, the adsorption and enrichment effect of iron (hydro)oxides on pollutants further enhances the reductive activity of Fe(II) toward pollutants that can be reduced by Fe(II)[2,71]. As shown in Figure 5, when adsorbed Fe(II) comes into contact with Cr(VI), it quickly reduces Cr(VI) to Cr(III), and the resulting Fe(III) is then reduced back to Fe(II) by receiving electrons released from the Fe0 core, thus completing the cycle.
图5 生物炭负载的nZVI对Cr（Ⅵ）去除机制[70]

Fig.5 Removal mechanism of Cr（Ⅵ） by biochar-supported nZVI[70]. Copyright 2024， Elsevier

3.2.3 Detection methods

Based on the diffraction peaks that differentiate between Fe(II) and Fe(III), XPS (X-ray photoelectron spectroscopy) and XRD (X-ray diffraction) are commonly used to determine whether Fe(II) participates in pollutant reduction reactions. By examining changes in the Fe(II) and Fe(III) content of the material before and after the reaction, it is possible to qualitatively analyze whether Fe(II) contributes to the pollutant reduction process[53]. However, due to the presence of the Fe(II)/Fe(III) cycle, these methods cannot effectively conclude whether Fe(II) reduction is the primary reduction pathway. 1,10-Phenanthroline, because of its strong complexation with Fe(II) and its ability to inhibit Fe(II) from participating in pollutant reduction, is used for quantitative analysis of the contribution of Fe(II) to pollutant reduction[53,72]. By comparing the interference of pollutant removal efficiency with and without the addition of 1,10-phenanthroline, it is possible to confirm the proportion of Fe(II) reduction in pollutant removal[2].

3.3 H* reduction pathway

3.3.1 Hydrogen evolution active sites

The electrons released by the Fe0core are accepted by H+/H2O on the ZVI surface, resulting in the formation of H* adsorbed onto the material surface (Equations (5, 6)). The generation of H* is limited by the hydrogen adsorption free energy (ΔG H*) on the material surface. When ΔG H*is too large, greater energy is required for H* formation, which is unfavorable for hydrogen bonding with active sites to generate H*; when ΔG H*is too small, the interaction between H* and active sites becomes too strong, hindering H* from participating in subsequent reactions[73-75]. Therefore, the closer ΔG H*is to zero, the more favorable it is for H* generation and its participation in subsequent reduction reactions.
H 3 O +   +   e -   →   H *   +   H 2 O
H 2 O   +   e -   →   O H -   +   H *
theoretically, the ΔG H*for pure α-Fe0is -0.31 eV, indicating that rapid generation and adsorption of H* can be achieved at Fe sites on the ZVI surface. However, due to the formation of a γ-FeOOH layer on the surface during ZVI corrosion, the actual ΔG H*for ZVI is 0.48 eV. This suggests that the formation of iron corrosion products inhibits the production of H*, thereby weakening the reducing ability of ZVI[76].

3.3.2 Hydrogenation Reduction and Hydrogen Production

H* possesses strong reducing power, with a standard redox potential E 0(H*/H2O) = -2.30 V. Even for highly recalcitrant PFAS, H* exhibits a certain degree of reductive capability[63,77]. On the material surface, H* can achieve the reductive removal of pollutants through three pathways: hydrogenation dehalogenation (using chlorinated hydrocarbons as an example[5], Equation (7)), hydrogenation deoxygenation (using nitrate as an example[78], Equation (8)), and hydrogenolysis (using N-nitrosodimethylamine (NDMA) as an example[79], Equation (9)). Additionally, it can be converted into H2evolution via the Tafel pathway (Equation (10)) and the Heyrovsky pathway (Equation (11)).
R - C l   +   H *   +   e -   →   R - H   +   C l -
N O 3 -   +   H *   +   e -   →   N H 3 / N 2   +   O H -
N D M A   +   H *   +   e -   →   D M A   +   N H 3
H *   +   H *   →   H 2   (   T a f e l   )
H *   +   H +   +   e -   →   H 2   (   H e y r o v s k y   )
h* has a certain conflict with pollutant reduction and H2 production. For both the Tafel pathway and the Heyrovsky pathway of H2 generation, they are controlled by ΔG H*. The stronger the bond energy at the H* adsorption site, the more unfavorable it is for H2 production, and the longer the lifespan of H*. In the reductive dehalogenation process involving H*, it is necessary to first accumulate H* on the material surface before proceeding with the hydrogenation reduction of halogenated pollutants[80]. Therefore, prolonging the lifespan of H* can enhance its accumulation, which is beneficial for the reduction of halogenated pollutants. Additionally, for hydrogenation deoxygenation and hydrogenolysis, enhancing the contact between H* and pollutants is an important factor affecting the effective utilization of H*[81].

3.3.3 Detection methods

The quenching experiment, a routine method for free radical detection, is also used for the identification of H*. Currently, tert-butanol is the most commonly used H* quencher, with a reaction rate constant for H* of (1.0 ± 0.15) × 105 M-1·s-1, enabling a rapid response to H*[82]. However, it should be noted that tert-butanol also acts as a quencher for ·OH, and the difference in affinity between H* and ·OH remains unclear; thus, whether the quenching experiment has strong persuasive power still needs further investigation[83]. 5,5-Dimethyl-1-pyrroline-N-oxide (DMPO), due to its high affinity for H*, can capture H* and extend its lifetime, making it frequently used for qualitative analysis of H* in electron paramagnetic resonance (EPR). In the ZVI reduction system, the H* generated will react with DMPO, and the resulting product exhibits nine characteristic peaks in the EPR spectrum. Changes in these characteristic peaks can be used to determine the contribution of H* to pollutant reduction[84]. Additionally, the solvent kinetic isotope effect is also employed for the identification of H*. Based on the difference in relative atomic masses of hydrogen atoms in D2O and H2O, the chemical reaction rate involving D2O is significantly lower than that involving H2O. Therefore, by altering the solvent in the reaction system and comparing the effects of D2O and H2O on pollutant reduction efficiency, the contribution of H* to pollutant reduction can be confirmed[5,27].

4 Conflict Manifestation—The Impact of Surface Modification on the Reduction Path

4.1 sulfur modification

Sulfur-modified ZVI (S-ZVI) was initially widely studied due to its high reactivity toward chlorinated compounds and heavy metals[85]. Early studies suggested that this enhanced reactivity might stem from two aspects: conductivity and hydrophobicity. However, there has been considerable debate regarding the relative contributions of these two factors in the ZVI reduction pathway. Some studies have indicated that sulfur modification enhances the hydrophobicity of the ZVI surface, thereby inhibiting the reaction between ZVI and water to produce H*, and increasing the contribution of DET in the reductive removal of pollutants[11,14,86]. Other studies, however, argue that sulfur modification does not affect the generation of H* but instead enhances the strong affinity of S sites for H*, which suppresses the formation of H2 from H*, thus boosting the role of H* in the reductive removal of pollutants[35,87]. Furthermore, numerous studies on sulfur-modified ZVI systems (Table 1) have indeed confirmed that the primary reduction pathways in sulfur-modified ZVI systems are DET and the H* pathway. Therefore, it is necessary to conduct an in-depth analysis of how sulfur modification affects the ZVI reduction pathway.
表1 ZVI基材料的主导还原路径及其检测方法汇总

Table 1 Summary of dominant reduction pathways and their detection methods for ZVI-based materials

Materials Dominant
Reduction
Pathways		 Detection Methods Pollutants Condition parameters Ref
ZVI/HA DET TBA Quenching Experiment， Phen Experiment， KBrO3 Quenching Experiment PNP pH = 5， T = 25 ℃， 10 mg/L PNP， 0.5 g/L ZVI 66
LZVI Fe（II） Phen Experiment Cr（VI） pH = 6.28， T = 25 ℃， 2 mg/L Cr（VI）， 5 g/L LZVI 2
ZVI H* Tafel， EIS， CV CT pH = 0.4， 30 mg/L CT 4
nZVI H* TBA Quenching Experiment FF pH = 7.0， T = 25 ℃， 0.28 mmol/L FF， 1.0 g/L nZVI 10
Fe0-Fe3O4-BM DET and Fe（II） Tafel， EIS， CV， Phen Experiment Cr（VI） pH = 3， 30 mg/L Cr（VI）， 1.0 g/L Fe0-Fe3O4-BM 53
OX-ZVIbm DET and Fe（II） Tafel， EIS， EPR， Methanol Quenching Experiment， Phen Experiment TCE T = 25 ℃， 20 mg/L TCE， 4 g/L OX-ZVIbm 12
nZVI DET and Fe（II） Phen Experiment Cr（VI） pH = 6.28， T = 35 ℃， 8 mg/L Cr（VI）， 0.15 g/L nZVI 3
S-nZVI DET Tafel PCE pH = 6.5， T = 25 ℃， 64 μmol/L CE， 1 g/L S-nZVI（S/Fe = 0.007） 88
S-nZVI DET Tafel， Kinetic Analysis TCE pH = 8， 10 mg/L TCE， 1 g/L S-nZVI（S/Fe = 0.20） 6
S-ZVI DET Solvent Kinetic Isotope Experiment c-DCE，
TCE		 pH = 7.0， 0.1 mmol/L c-DCE/TCE， 5 g/L S-ZVI（S/Fe = 0.10） 14
S-mZVI DET Kinetic Analysis TCE pH = 7.0， 100 μmol/L CE， 0.26 g S-mZVI（S/Fe = 0.10） 42
S-mZVI Fe（II） Phen Experiment Cr（VI） pH = 6.00， 20 mg/L Cr（VI）， 0.4 g/L S-mZVI（S/Fe = 0.112） 105
S-nZVI H* Tafel VC，
c-DCE		 pH = 6.5， T = 25 ℃， 64 μmol/L CE， 1 g/L S-nZVI（S/Fe = 0.007） 88
S-nZVI H* CV， EPR， Solvent Kinetic Isotope Experiment DTA pH = 7.0， T = 25℃， 30 μmol/L DTA， 2.0 g/L S-nZVI（S/Fe = 0.25） 35
S-mZVI H* Kinetic Analysis c-DCE， t-DCE， VC pH = 7.0， 100 μmol/L CE， 0.26 g S-mZVI（S/Fe = 0.10） 42
S-nZVI H* Kinetic Analysis TCE pH = 7.8-8.2， T = 22 ℃， 25 mg/L TCE， 5 g/L S-nZVI（S/Fe = 0.05） 87
S-ZVI DET and Fe（II） Tafel， CV c-DCE pH = 3.0， 2 mg/L c-DCE， 0.25 g/L S-ZVI（S/Fe=0.11） 33
ZVI/FeS2 DET and Fe（II） Phen Experiment NB pH = 6.0， 25.0 mg/L NB， 0.5 g/L ZVI + 2.0 g/L FeS2 106
S-nZVI DET and H* TBA Quenching Experiment FF pH = 7.0， T = 25 ℃， 0.28 mmol/L FF， 1.0 g/L S-nZVI（S/Fe = 0.07） 10
S-mZVI DET and H* TBA Quenching Experiment CAP T = 25 ℃， 40 mg/L CAP， 0.4 g/L S-mZVI（S/Fe = 0.112） 11
S-mZVI DET and H* Kinetic Analysis PCE，
1，1-DCE		 pH = 7.0， 100 μmol/L CE， 0.26 g S-mZVI（S/Fe = 0.1） 42
nZVI/CNT H* CV， EPR， Solvent Kinetic Isotope Experiment Cr（VI） pH = 6.5， T = 25 ℃， 10 mg/L Cr（VI）， 0.2 g/L nZVI/CNT 35
AC-ZVI H* Electrochemical Analysis TCE pH = 8.7~9.2， T = 22 ℃， 6 mg/L TCE， 5 g/L AC-ZVI 96
AC-ZVI H* Electrochemical Analysis DDT pH = 8.8， 5 mg/L DDT， 667 g/L AC-mZVI 15
nZVI@CP-BC DET and Fe（II） pH Analysis， ORP Analysis Cr（VI） pH = 5.1， T = 25 ℃， 20 mg/L Cr（VI）， 37.5 mg nZVI@CP-BC 70
BC-nZVI DET and H* Tafel， EIS， HER Analysis TCE 25 mg/L TCE， 2 g/L BC-nZVI 29
MNBC-ZVI DET and H* CV， TBA Quenching Experiment TMX T = 25 ℃， 10 mg/L TMX， 0.75 g/L MNBC-ZVI 34
nZVI/BC DET and H* Solvent Kinetic Isotope Experiment TBBPA pH = 7， T = 25 ℃， 10 mg/L TBBPA， 2.0 g/L nZVI/BC 27
nZVI@CBC Fe（II） and H* XPS Analysis Cr（VI） pH = 7.0， 4 mg/L Cr（VI）， 2.5 g/L nZVI@CBC 16
Pd/Ni-nZVI H* Tafel， Kinetic Analysis TCE pH = 8， 10 mg/L TCE， 1 g/L Pd/Ni-nZVI 6
Cu-Febm（CuSO4） H* CV， ESR， Methanol Quenching Experiment TCE pH = 7， T = 25 ℃， 10 mg/L TCE， 2.0 g/L Cu-Febm（CuSO4） 103
Fe-Pd-Cu H* ESR DCF T = 25 ℃， 20 mg/L DCF， 30 g/L Fe-Pd-Cu 51
Pd/nZVI H* Solvent Kinetic Isotope Experiment 1，1，1，2，5-TeCA pH = 8， 75 μmol/L 1，1，1，2，5-TeCA， 0.22 g/L Pd/nZVI 5
Pd/nZVI H* Solvent Kinetic Isotope Experiment c-DCE pH = 8， 75 μmol/L c-DCE， 0.22 g/L Pd/nZVI 5
N（C）-mZVI DET Tafel， EIS， CV， TBA Quenching Experiment， TCE pH = 7， T = 25℃， 76 μmol/L TCE， 10 g/L mZVI 13
MoS-mZVI DET CV TCE T = 25℃， 10 mg/L TCE， 5.2 g/L MoS-ZVI 107
S-nZVI/BC Fe（II） Phen Experiment Cd（II）， As（III） pH = 5.0， 20 mg/L Cd（II）， 40 mg/L As（iii），
0.2 g/L S-nZVI/BC		 18
Ni-nZVI/BC H* EIS， CV， EPR， Methanol Quenching Experiment TCE pH = 7.0， T = 25 ℃， 20 mg/L TCE， 1.0 g/L Ni-nZVI/BC 17
S-mZVI/GO H* CV TCE T = 25 ℃， 10 mg/L TCE， 4 g/L S-mZVI/rGO 24
Fe0@Fe-N4-C H* EPR TCE pH = 6.5， T = 25 ℃， 1 mg/L TCE， 1 g/L Fe0@Fe-N4-C 76
Ni/S-mZVI H* ESR ATZ T = 12 ℃， 4 mg/L ATZ， 24 g/L Ni/S-mZVI 101
S-mZVI/BC DET and Fe（II） Phen Experiment， ICP Analysis Cr（VI） pH = 2， 50 mg/L Cr（VI）， 1 g/L S-mZVI/BC 40
NG/nZVI DET and H* TBA Quenching Experiment VC 44.8 μmol/L VC， 2 g/L nZVI， 4 g/L NG 47
S-nZVI/BC DET and H* TBA Quenching Experiment， Solvent Kinetic Isotope Experiment TBBPA pH = 7， T = 25 ℃， 10 mg/L TBBPA， 2.0 g/L S-nZVI/BC（S/Fe = 0.090） 108
S-nZVI/BC DET and H* Solvent Kinetic Isotope Experiment TBBPA pH = 7， T = 25 ℃， 10 mg/L TBBPA， 2.0 g/L S-nZVI/BC 27
S-nZVI@NBC DET and H* DFT Analysis， PDOS Analysis NOR 10 mg/L NOR， 0.8 g/L S-nZVI@NBC 81
BC@Fe/Ni DET and H* XPS Analysis 2，4-DCP T = 25 ℃， 50 mg/L 2， 4-DCP， 2 g/L BC@Fe/Ni 109

*Full form of partial abbreviations： activated carbon （AC）， carbon nanotube （CNT）， biochar （BC）， tertiary-butyl alcohol （TBA）， oxidation-reduction potential （ORP）， inductively coupled plasma （ICP）， density functional theory （DFT）， partial density of states （PDOS）， p-nitrophenol （PNP）， carbon tetrachloride （CT）， florfenicol （FF）， trichloroethylene （TCE）， perchlorethylene （PCE）， dichloroethylene （DCE）， vinyl chloride （VC）， diatrizoic acid （DTA）， nitrobenzene （NB）， chloramphenicol （CAP）， dichlorodiphenyltrichloroethane （DDT）， thiamethoxam （TMX）， tetrabromobisphenol A （TBBPA）， tetrachloroethane （TeCA）， atrazine （ATZ）， norfloxacin （NOR）， dichlorophenols （DCP）

4.1.1 Conductivity Optimization

Sulfur-modified ZVI generates two iron sulfide species—FeS and FeS2. These iron sulfides partially replace the oxide layer on the surface of ZVI, forming S-active sites[11]. The band gaps of FeS and FeS2 are 0.1 and 0.95 eV, respectively, while that of Fe2O3 is approximately 2.0 eV[53-54]. A lower band gap implies enhanced conductivity; thus, a large number of electrons can be transferred through FeS x, leading to increased electron release from ZVI and consequently enhancing DET reduction and H* generation[88]. Dai et al.[11] tested the conductivity of ZVI and various S-ZVI samples. The electrochemical impedance test results (Figure 6) indicate that sulfur modification of ZVI accelerates electron transfer. Furthermore, the enhanced conductivity of ZVI also promotes the formation of more Fe(II), thereby improving its reduction capability[71,89-90].
图6 ZVI和S-ZVI材料的电化学阻抗测试[11]

Fig.6 Electrochemical impedance tests of ZVI and S-ZVI[11]. Copyright 2023， Elsevier

4.1.2 Selective enhancement

Selective pollutant reduction refers to the preferential use of electrons released by ZVI for pollutant reduction rather than for other side reactions such as hydrogen production. The increased hydrophobicity caused by sulfidation is considered the source of this selective pollutant reduction. Li et al.[30]found that the water contact angle of S-ZVI can reach 60° or even higher, more than three times that of ZVI. High hydrophobicity means that electrons released by ZVI through S sites are less likely to react with H+/H2O, thereby increasing the likelihood that pollutants directly receive electrons from S sites and are reduced[11,14,86]. However, some studies have also found that the hydrophobicity of S-ZVI is related to its sulfur content: when S/Fe is less than 0.05, the water contact angle of S-ZVI increases with increasing sulfur content; when S/Fe exceeds 0.05, the water contact angle decreases as sulfur content rises, indicating that the hydrophobicity of S-ZVI initially increases and then decreases with increasing sulfur content[91]. This may explain why Zhou et al.[35]observed that when using S-ZVI with an S/Fe ratio of 0.088 for pollutant degradation, pollutants were not reduced via DET, but rather H* was the primary active species. Nevertheless, many studies have also found that when S/Fe is within a high hydrophobic range, the active S sites can adsorb H*, inhibiting the Tafel and Heyrovsky reactions, enhancing stability, and facilitating H* accumulation, thus enabling pollutant removal through H* reduction[35,87]. It remains unclear whether this phenomenon is due to differences in detection methods or inherent material properties. Therefore, further research is needed to explore how sulfur modification alters the reduction pathways of ZVI.

4.2 Carbon Material Modification

Carbon-modified ZVI (C-ZVI) shares similarities with S-ZVI, both of which can accelerate electron release from ZVI by enhancing conductivity[92]. Unlike sulfur modification, carbon materials themselves have an enriching effect on pollutants due to their porous structure, enabling them to adsorb large amounts of contaminants[93]. Moreover, carbon materials can undergo surface functional group modifications to achieve transformations between hydrophilicity and hydrophobicity, resulting in different removal efficiencies for various pollutants[36,94]. Since carbon materials can serve as bridges for electron transfer, many studies suggest that the enhancement of ZVI reduction performance by carbon modification stems from the strengthened DET pathway. However, the discovery of the H* spillover mechanism provides strong evidence supporting carbon modification's promotion of the H* reduction pathway. Due to differences among various carbon materials in particle size, specific surface area, structural characteristics, and types and quantities of functional groups, their impact on the ZVI reduction pathway varies to some extent; yet, there are also similarities in their mechanisms affecting the reduction pathway[49]. According to numerous studies on carbon-modified ZVI systems (Table 1), the DET and H* pathways are the primary reduction routes. Therefore, it is necessary to deeply analyze the similar mechanisms by which most carbon materials modify the ZVI reduction pathway.

4.2.1 Enhanced reactivity

C-ZVI possesses an iron-carbon microelectrolysis structure, with ZVI serving as the anode to release electrons and generate Fe(Ⅱ)/Fe(Ⅲ), while carbon materials act as the cathode, where pollutants and H+/H2O accept electrons and are reduced. This electrochemical reaction process significantly promotes electron release from ZVI, and the electron transfer mediated by carbon materials also alleviates the decline in material reactivity caused by ZVI passivation[95]. Therefore, carbon material modification enhances DET reduction, Fe(Ⅱ) reduction, and H* reduction to some extent. Studies by Xu et al.[81]and Hou et al.[29]have found that biochar-modified ZVI can achieve the reductive degradation of norfloxacin and trichloroethylene through both DET reduction and H* reduction. Research by Deng et al.[70]indicates that Chlorella biochar can enhance the reductive capacity of this biochar-modified ZVI material toward Cr(Ⅵ) by promoting the Fe(Ⅱ)/Fe(Ⅲ) cycle.

4.2.2 H* overflow mechanism

Due to inherent structural reasons, carbon materials have certain defect sites, through which H* can shuttle and adsorb within the carbon material[47]. Based on this characteristic, researchers have proposed an H* spillover mechanism for the C-ZVI reduction pathway. The H* spillover mechanism can be divided into the following three steps: ① H* generated on the ZVI surface is adsorbed by defect sites in the carbon material, leading to the accumulation of H* inside the carbon material; ② when the internal sites become saturated with adsorbed H*, H* begins to spill over onto the surface of the carbon material, forming surface adsorption; ③ the H* adsorbed on the surface reacts with pollutants, initiating the reduction process[15,24,96]. The presence of carbon materials allows pollutants to be removed without direct contact with ZVI, and the H* spillover mechanism facilitates long-distance transport of H*, enhancing its contribution to the reduction and removal of pollutants, especially in reductive dehalogenation.

4.2.3 Adsorption and Catalytic Reduction

Carbon materials can adsorb heavy metals, inorganic anions, and organic pollutants through electrostatic adsorption, hydrogen bonding, and π-π interactions. Pollutants adsorbed onto the material can be reduced via a DET pathway mediated by carbon materials[32]. Additionally, carbon materials themselves possess certain catalytic active sites and exhibit catalytic activity for some reduction reactions[97]. It is generally believed that adsorption of pollutants promotes the progress of reduction processes; however, for certain catalytic reduction processes, strong adsorption can inhibit the transfer of pollutants to catalytic active sites, thereby suppressing catalytic reduction[98]. For some heavy metals and inorganic anions, adsorption by carbon materials facilitates reduction via the DET pathway[32,99-100]. As for certain halogenated organic pollutants, hydrophobic carbon materials can effectively enrich them, enabling their reduction via either the DET or H* pathway[27,33].

4.3 Transition metal modification

Most transition metal modifications can effectively enhance the electron release capability of ZVI and promote the hydrogen evolution reaction between Fe0 and H2O, thereby accelerating the generation of H*[101]. Therefore, transition metal modification can optimize the H* reduction pathway, thus enhancing the pollutant reduction performance of ZVI. Controlling the reaction between H* and pollutants and inhibiting the Tafel and Heyrovsky reactions are key to improving the electron selectivity of transition metal-modified ZVI[102]. Additionally, similar to carbon materials, transition metals can also directly transfer electrons to ZVI, enabling DET reduction. However, whether the modified ZVI is dominated by DET reduction or H* reduction depends on the type and content of the modifying metal[5]. Most studies (Table 1Table 1) indicate that the H* pathway is the primary reduction mechanism in transition metal-modified ZVI systems. Therefore, it is necessary to thoroughly analyze the underlying mechanisms of transition metal modification.

4.3.1 Enhanced bimetallic reaction

The addition of heterometallic elements to construct iron-based bimetallic materials significantly enhances the reactivity of ZVI. Firstly, the bimetallic material can form a galvanic cell system, enhancing galvanic corrosion and promoting electron release from ZVI[103]. Secondly, the transition metal coating on the surface of ZVI not only creates an excellent electron transport pathway but also inhibits the formation of iron (hydro)oxides, thereby improving the anti-passivation performance of ZVI. Finally, the incorporation of transition metals improves the surface properties and pore structure of ZVI, increasing the number of reactive sites[104].

4.3.2 Catalytic hydrogen production

The generation and reaction of H* are both related to ΔG H*, and the enhancement of hydrogen evolution reaction by transition metals is attributed to their ΔG H*values being closer to zero. As shown in Figure 7, Du et al.[73] calculated the ΔG H*of 26 single-atom transition metals supported on onion-like carbon. Among them, Co, Ni, Pd, Mo, and Pt, commonly used for ZVI transition metal modification, often exhibit ΔG H*values superior to Fe. This indicates that these transition metals possess better catalytic hydrogen production performance, and their modification of ZVI can increase more active hydrogen evolution sites, thereby promoting the generation of H* and its participation in reduction. Although transition metals with ΔG H*worse than Fe may have adverse effects on the H* reduction pathway, they could potentially enhance ZVI reduction performance by strengthening the DET pathway or the Fe(Ⅱ)/Fe(Ⅲ) cycle.
图7 洋葱状碳负载的过渡金属（Mn~Ni（a）； Y~Mo（b）； Hf~W（c））对单个H原子的氢吸附自由能[73]

Fig. 7 ΔGH* of TM1/OLC （TM = Mn~Ni（a）； Y~Mo（b）； Hf~W（c）） with a single H atom adsorbed[73]. Copyright 2023， American Chemical Society

5 Subtle Echoes—Interference from Environmental Conditions

5.1 pH

The environmental pH has a significant impact on the speciation of iron species. As is well known, Fe(II)/Fe(III) species generally exist in ionic form under acidic conditions, while forming iron (hydro)oxides under alkaline conditions. This means that ZVI materials will become passivated due to the increased thickness of the surface oxide layer under alkaline conditions, resulting in a loss of electron-releasing capability. The formation of the passivation layer hinders DET reduction, as hydrogen-evolving sites on the surface are covered by iron (hydro)oxides, and the decrease in H⁺ concentration in the solution negatively affects the generation of H*. Meanwhile, Fe(II)/Fe(III) existing as precipitates under alkaline conditions find it difficult to complete cyclic transformations due to hindered electron transfer. Therefore, an alkaline environment poses significant resistance to the pollutant removal via ZVI reduction. In addition, differences in the isoelectric points of materials under varying pH conditions lead to different surface charge distributions, which can interfere with pollutant adsorption and thus affect pollutant reduction. Dai et al.[11]tested the degradation of chlorotoxin by S-ZVI under different pH conditions and confirmed that, from acidic to alkaline conditions, the interaction between chlorotoxin and ZVI/S-ZVI shifts from electrostatic adsorption to electrostatic repulsion, thereby affecting the reductive removal of chlorotoxin.

5.2 coexisting ions

The effects of coexisting ions on reduction can be categorized into several aspects: corrosion enhancement, competitive adsorption, co-precipitation, and competitive reduction. Cl-, due to its small ionic radius, strong penetrating power, and ability to form iron chlorides on the iron surface under acidic conditions instead of iron (hydro)oxide protective layers, has a certain enhancing effect on electron release from ZVI material corrosion. Depending on the distribution of surface charges on ZVI, when electrostatic adsorption occurs between ZVI material and target pollutants, ions with the same charge as the target pollutants will compete for adsorption sites, which is detrimental to the DET reduction pathway. Some heavy metal pollutants, after reduction, tend to co-precipitate with iron species and cover the ZVI surface, thereby hindering the three reduction pathways. For example, using ZVI to reduce Cr(Ⅵ) results in the formation of Cr xFe1- xOOH precipitates on the ZVI surface[16]. Additionally, certain reducible anions (such as NO3 -), as non-target pollutants present in the ZVI reduction system, may compete with target pollutants for H*, thus weakening the contribution of the H* reduction pathway to pollutant reduction[110].

5.3 Natural Organic Matter

Natural organic matter represented by humic acid (HA) is widely present in various natural water bodies and can interfere with the removal of pollutants via ZVI reduction. HA interacts with metal ions through exchange, chelation, and adsorption. First, it can react with Fe(Ⅱ)/Fe(Ⅲ), inhibiting their cycling and thus reducing the reduction performance. Second, when heavy metal ions are the target pollutants, HA's adsorption of these ions prevents them from participating in the reduction reaction. Finally, HA itself competes for adsorption sites on the ZVI material, thereby reducing the available adsorption space for target pollutants[110]. Additionally, studies have shown that HA has different effects on ZVI particles of varying sizes: HA has a passivation-removing effect on mZVI, enhancing H* generation, while on nZVI, it forms HA-Fe(Ⅲ) complexes that coat the material surface, inhibiting H* production[111].

6 Conclusion and Outlook

The reduction system involving ZVI is characterized by highly complex reducing active substances. In recent years, studies on these reducing active substances have confirmed the presence of DET reduction, H* reduction, and Fe(II) reduction through various detection methods. The primary reducing active substances and the associated reduction pathways in the reduction process of different pollutants have been clearly identified. Moreover, the variety of detection methods for these reducing active substances continues to expand, aiming to enhance the credibility of the proposed reduction pathways. Additionally, the application of various surface modification techniques has significantly improved the reduction performance and applicability of ZVI materials, enabling their widespread use under diverse aquatic conditions.
However, the continuous deepening of research on ZVI and its surface-modified materials has also brought about some other issues.
(1) Feasibility of the detection method for reducing active substances
Currently, it is difficult to directly observe the reduction of DET. Testing the material's conductivity and potential changes during the reaction process through electrochemical methods only provides an indication of electron release; whether pollutants directly receive electrons and undergo reduction still requires further discussion.
For the detection of Fe(Ⅱ), a strong complexing agent quenching experiment is typically used. In recent years, studies on complexed heavy metals have shown that ZVI materials possess the ability to decomplex heavy metals via redox pathways[64]. Therefore, whether the complexing quenching experiment will be affected by ZVI requires further investigation.
The identification of H* is generally performed through EPR, quenching experiments, and electrochemical experiments. However, these methods cannot avoid false-positive interference caused by other hydrogen-containing reducing substances. Moreover, in recent years, some scholars have questioned the reliability of quenching experiments, arguing that a single quenching experiment lacks persuasiveness in detecting active species and requires multiple methods to corroborate the results[83]. Therefore, the feasibility of detection methods and sources of interference need further discussion to ensure consistent conclusions can be drawn for the same reduction system.
(2) In-depth study of surface modification techniques on the reduction pathway
With the continuous advancement of surface modification technology research, various novel surface modification techniques or composite modification methods have garnered widespread attention. These ZVI surface modification techniques either optimize existing methods or combine the advantages of multiple surface modification approaches, significantly enhancing the reduction performance of ZVI. However, there are still many questions regarding how these surface modification techniques influence pollutant reduction at the reduction pathway level and how different surface modification techniques interact with each other during the reduction process. Therefore, research on surface modification technologies should not be limited to performance optimization alone; instead, deeper mechanistic studies are needed to derive systematic conclusions.
(3) Systematic study on the selectivity of pollutant-reducible structures toward reduction pathways
The differences in reducible structures of pollutants can interfere with the reduction pathways. Reduction is a process jointly involving ZVI materials and pollutants; in addition to the intrinsic properties of the material affecting the generation of reducing active species, pollutants may also exhibit certain selectivity toward these reducing active species. For different reducible structures on a single pollutant, the participating reducing active species and reduction pathways may vary. Similarly, for similar reducible structures on different pollutants, the participating reducing active species and reduction pathways may also differ. These variations in reduction pathways could be determined by the redox potentials of the reducible structures or influenced by the spatial configurations of the pollutants. However, most current studies tend to focus on the removal of one or several pollutants with similar structures using ZVI and its surface-modified materials. From the perspective of pollutant reduction, there is still a lack of systematic research exploring the patterns governing how different reducible structures influence the selection of reduction pathways. Therefore, investigating the influence of pollutant structures on the preference for reduction pathways is crucial for selecting appropriate modification strategies.

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