Facet-dependent Fe(II) redox chemistry on iron oxide for organic pollutant transformation and mechanisms

Antimony and arsenic interactions with iron oxides and aluminum oxides in surface environment: A review focused on processes and mechanisms

It has been assumed and widely reported that arsenic (As) and antimony (Sb) share some similarities but also exhibit significant differences in their geochemical behaviors. Their environmental fates are generally controlled by iron (Fe) oxides and aluminum (Al) oxides. The mechanistic differences in their interactions, especially under dynamic environmental conditions, remain poorly understood, which hinders the development and implementation of effective pollution prevention and control measures. Therefore, this review focuses on the processes and mechanisms of interactions between As/Sb and Fe oxides/Al oxides. Antimony exhibits a higher susceptibility to oxidation than As due to its larger atomic radius and lower electronegativity. The property is an important basis for explaining the differences in their interactions in the environment. To obtain a clearer understanding of interactions, a detailed adsorption theory (charge distribution multi-site ion complexation) for the Fe oxides and Al oxides and three primary adsorption mechanisms (electrostatic adsorption, chemical adsorption, and coprecipitation) were explored. Furthermore, the effects of various factors (pH, redox, surface coverage, competing ions, and types of Fe oxides and Al oxides) on the adsorption efficiency were evaluated. We discussed the mechanisms and efficiency of Sb and As adsorption on Fe oxides and Al oxides, and the differences in Sb and As adsorption for various valence states. To efficiently control Sb and As pollution, some differences between Sb and As need to be taken into account.

Oxygen adsorption and activation control the photochemical activity of common iron oxyhydroxide polymorphs in mediating oxytetracycline degradation under visible light

The natural minerals with semiconducting properties possess photochemical activity through generating reactive oxygen species (ROSs) and affect the fate of adsorbed organic pollutants. Iron oxyhydroxides occur in different polymorphic structures under various geological and climatic conditions in natural environment. However, the difference in their photoactivity has not been well understood. This work elucidates the mechanism of light-induced generation of ROSs by common iron oxyhydroxide polymorphs, including goethite (α-FeOOH), akaganeite (β-FeOOH), lepidocrocite (γ-FeOOH), and feroxyhyte (δ-FeOOH), and the degradation of oxytetracycline (OTC) mediated by them. Under visible light irradiation, all these iron oxyhydroxide polymorphs generated superoxide radicals (O2-•), hydroxyl radicals (•OH), and H2O2. Among the ROSs, O2-•, whose concentrations in the photochemical systems of α-, β-, γ- and δ-FeOOH reached 4.25 × 10-4 ± 5.67 × 10-5, 2.29 × 10-4 ± 2.55 × 10-5, 6.2 × 10-4 ± 8.5 × 10-5 and 7.65 × 10-4 ± 5.67 × 10-5 μmol/L, respectively, played a dominant role in the degradation of OTC. The corresponding rate of OTC degradation in these photochemical systems were 0.054 ± 0.010, 0.015 ± 0.001, 0.338 ± 0.073, and 0.404 ± 0.016 min-1, respectively. A good linear relationship was observed between the steady-state concentrations of O2-• and OTC degradation rates in the photochemical systems of iron oxyhydroxide polymorphs. O2-• was primarily generated by the electron reduction of O2 on the conduction bands (CBs) of iron oxyhydroxide polymorphs, which was controlled by the surface adsorption and activation of O2. H2O2 and surface Fe2+ on iron oxyhydroxide polymorphs synergistically contributed to •OH generation through Fenton reaction. The generation of surface Fe2+ was determined by the geometrical configuration, crystallinity, and surface oxygen vacancies of iron oxyhydroxide polymorphs. These findings demonstrate the photochemical generation of ROSs in the presence of major iron oxyhydroxide polymorphs was primarily controlled by O2 adsorption and activation, leading to different activity in mediating the degradation of adsorbed organic pollutants.

Quantitative Enhancement of Arsenate Immobilization Induced by Vacancy Defects on Various Exposed Lattice Facets of Hematite

Defects are common features in hematite that arise from deviations from the perfect mineral crystal structure. Vacancy defects have been shown to significantly enhance arsenate (As) immobilization by hematite. However, the contributions from vacancy defects on different exposed facets of hematite have not been fully quantified. In this study, hematite samples with various morphologies were pretreated with sodium borohydride (NaBH4) to generate oxygen vacancy defects (OVDs), analyzed quantitatively using extended X-ray absorption fine structure (EXAFS) and thermogravimetric analysis (TG). Batch experiments revealed that the OVDs on different exposed facets showed significant variations in improving arsenate adsorption, i.e., the quantitative enhancement of arsenate adsorption amount per unit OVD concentration (ΔQm/Cdefect) followed the sequence of (110) facet (80.05 μmol/mmoldef) > (001) facet (31.85 μmol/mmoldef) > (012) facet (13.14 μmol/mmoldef). The underlying mechanism by which OVDs affect arsenate adsorption across different exposed facets of hematite was studied. The results reveal that the tremendous improvement of arsenate adsorption caused by OVDs on the (110) facet compared to (001) and (012) facets was attributed to their stronger bonding strength of As to under-coordinated Fe atoms, thus significantly promoting the immobilization of arsenate. The findings of this study enhance our ability to precisely understand the migration and fate of As while also aiding in the design of highly efficient iron mineral materials for mitigating As pollution.

Nano boron carbide effectively boost Fenton-like performance of hematite mediated systems: Roles of hematite exposed facets and synergistic catalysis between Fe and B

The inherent properties of exposed facets of iron minerals played key roles in heterogeneous reactions at the mineral interface, and the addition of co-catalysts has been elucidated to further enhance the reactions for contaminants degradation. Here, synergistic Fenton-like catalytic reactivity of different hematite dominant exposed facets ({001}, {012}, {100}, and {113}) with nano boron carbide (B4C) was revealed. In 5 h, as compared with the cumulative •OH in the B4C/H2O2 system (96.9 μM), while that in the {001}/B4C/H2O2 system was decreased by 19.6%, those in the {012}/B4C/H2O2, {100}/B4C/H2O2, and {113}/B4C/H2O2 systems were increased by 53.8%, 75.9%, and 84.0%, respectively. Significantly, {113}/B4C/H2O2 system exhibited strong capability for degradation of a broad spectrum of organic pollutants, including typical phenol, endocrine disruptor (bisphenol A), antibiotic (sulfanilamide), dyes (Rhodamine B and methylene blue), and pesticide (atrazine). During the Fenton-like reactions, higher synergy factor, Fe(III)/Fe(II) cycling rate, and amount of Fe-O-B bond in the {113}/B4C/H2O2 system were shown than those in other systems, thus exhibiting its desirable catalytic performance for •OH production and pollutants oxidation. Iron species and X-ray photoelectron spectroscopy (XPS) analyses indicated that B-B bond and interfacial suboxide boron (e.g., B-O) could provide electrons to facilitate Fe(III) reduction for boosting the Fe(III)/Fe(II) cycling. Density functional theory (DFT) results demonstrated the formation of Fe-O-B bond on hematite {113}, {100}, and {012} facets, which were beneficial to the breakage of O-O bond of bound H2O2 molecule and thus improved the generation of •OH. This study emphasized the essential role of B4C in developing tailored hematite facets as a contaminant remediation substrate, and provided important insights into the design of efficient heterogeneous Fenton-like systems.

Unveiling the impact of exposed mineral facets on chromium repartitioning in aging lepidocrocite

As a typical intermediate product of metastable iron oxide phase transformation, lepidocrocite is a critical player in migrating and transforming heavy metals (HMs) in soils and sediments. However, the repartitioning behavior of its associated HMs during the aging of lepidocrocite is not fully understood. We investigated the phase transformation of Cr(VI)-lepidocrocite with different exposed facets at various aging conditions (pH: 7 and 10; temperature: 25 °C and 75 °C). The results indicated that the phase transformation of lepidocrocite is facet-dependent. The rod-like lepidocrocite (R-LEP), characterized by a high ratio of (001)/(010) facets, demonstrated significant structural stability, with only minimal transformation to goethite observed over 9 days. In contrast, the dominant (010) facet in plate-like lepidocrocite (P-LEP) directly underwent the phase transformation from lepidocrocite to hematite. Meanwhile, the coordination of Cr(VI) onto the facets of lepidocrocite notably strengthened the resistance to its phase transformation. Specifically, the interaction between Cr(VI) and the (001) facets was particularly effective in inhibiting the phase transformation of lepidocrocite. In addition, the release behavior of Cr(VI) also showed lepidocrocite facet dependence. For instance, at pH = 7 and 75 °C, the release percentage of Cr(VI) on P-LEP reached up to 81 %, 1.7 times that of the release on R-LEP (46.7 %). Moreover, more non-extractable Cr speciation in P-LEP transformed into aqueous speciation and was partially redistributed on hematite. These findings provide novel insights into the role that the mineral exposed facets play in regulating the environmental behavior of Cr during the iron oxide phase transformation.

Iron coupled with hydroxylamine turns on the "switch" for free radical degradation of organic pollutants under high pH conditions

Reducing iron by hydroxylamine (HA) can promote the generation of reactive oxygen species (ROS) in the Fenton reaction and play a crucial role in the degradation of organic pollutants. However, the performance of this system at wider environmental thresholds is still not sufficiently understood, especially in the highly alkaline environments resulting from human activities. Here, we assessed the impact of solution pH on organic pollutant degradation by goethite with the addition of HA and H2O2. The solid phase variation and ROS generation were analyzed using Mössbauer spectroscopy, X-ray absorption near edge structure spectroscopy, and electron paramagnetic resonance analysis. This study found that under alkaline conditions, the system can continuously scavenge organic pollutants through oxygen-mediated generation of free radicals. At lower pH levels, organic pollutant decomposition, exemplified by the breakdown of bisphenol A (BPA), is primarily driven by the Fenton reaction facilitated by iron. As pH increases, hydroxyl radical (•OH) production decreases, accompanied by decreased BPA removal efficiency. However, the removal efficiency of BPA increased significantly at pH > 9. At pH 12, the removal of BPA exceeded that of the acidic condition after one hour, which is consistent with observations in soil system studies. Unlike the Fenton reaction, which is not sensitive to oxygen content, the removal of BPA under alkaline conditions occurs only under aerobic conditions. H2O2 is hardly involved in the reaction, and the depletion of HA becomes a critical factor in the decomposition of BPA. Importantly, in contrast to acidic conditions, where the dramatic decomposition of BPA occurs mainly in the first 10 min, the decomposition of BPA under alkaline conditions continued to occur over the 2 h of observation until complete removal. For natural systems, the remediation of pollutants depends more on the active time of ROS than on their reactivity. Therefore, this idea can reference pollution remediation strategies in anthropogenically disturbed environments.

Facet-dependent activation of oxalic acid over hematite nanocrystals under the irradiation of visible light for efficient degradation of pollutants

Naturally occurring hematite has been widely studied in the Fenton-like system for water pollutant remediation due to its abundance and non-toxicity. However, its inadequate catalytic activity results in difficulty in effectively degrading pollutants in the catalytic degradation system that it constitutes. Thus, we constructed a photochemical system composed of hematite with {001} facet of high activity facet and low-cost and non-toxic oxalic acid (OA) for the removal of various types of pollutants. The removal rate for the degradation of metronidazole, tetracycline hydrochloride, Rhodamine B, and hexavalent chromium by hematite nanoplate with the exposed {001} facet activating OA under visible light irradiation was 4.75, 2.25, 2.33, and 2.74 times than that by the exposed {110} facet, respectively. Density functional theory (DFT) calculation proved that the OA molecule was more easily adsorbed on the {001} facet of hematite than that on the {110} facet, which would favor the formation of the more Fe(III)-OA complex and reactive species. In addition, the reactive site of metronidazole for the attraction of radicals was identified on the basis of the DFT calculation on the molecular occupied orbitals, and the possible degradation pathway for metronidazole included carbon chain fracture, hydroxyethyl-cleavage, denitrogenation, and hydroxylation. Thus, this finding may offer a valuable direction in designing an efficient iron-based catalyst based on facet engineering for the improved activity of Fenton-like systems such as OA activation.

Migration of l-Selenomethionine in the Water-Soil Interface Dominated by Iron Oxides

Organic selenium (Se) accounts for up to 10-80% of total Se in soils, and l-selenomethionine (SeMet) is a typical organic Se species. However, the migration of SeMet in soils remains elusive. This study investigated the solid-liquid distribution, adsorption, desorption by phosphate, and self-oxidization of SeMet in solution under the influence of ferrihydrite, goethite, and hematite through batch experiments. Iron oxides could adsorb a much larger amount of SeMet than inorganic Se. At the initial Se element concentrations of 0-200 mg/L, the solid/liquid partition coefficient of SeMet was constant, which was 0.41, 0.43, and 0.50 on ferrihydrite, goethite, and hematite, respectively. In addition, the adsorption process of SeMet on the three iron oxides could be well described by the linear driving force model. Accordingly, the intraparticle diffusion coefficient of SeMet in ferrihydrite, goethite, and hematite was 1.4 × 103, 7.9 × 104, and 1.2 × 105 nm2/min, respectively. The adsorption of SeMet on the three iron oxides was slightly influenced by the pH and the coexisting ions, such as Cl-, NO3-, SO42-, and H2PO4-. The desorption ratio of SeMet on the three iron oxides by phosphate was lower than 2.5%. SeMet would aggregate the nanoparticles of iron oxides, resulting in a synergistic effect on the adsorption of phosphate. The oxidization ratio of SeMet was 23.9% in the solution, while it decreased to 17.1-17.5% in iron oxide suspensions. For this oxidization process, the three iron oxides exhibited varying effects to decelerate SeMet oxidation, as represented by the equivalent reaction. The findings of this study reveal the migration of SeMet in the water-soil interface under the influence of iron oxides, which can improve the understanding of Se cycling in the environment as well as provide some guidance for the better utilization of Se in soils and environmental remediation of Se pollution.

Influence of iron (hydr)oxide mineralogy and contents in aquifer sediments on dissolved organic carbon attenuations during aquifer storage and recovery

Aquifer storage and recovery (ASR) is a promising approach for managing water resources that enhances water quality through biogeochemical reactions occurring within aquifers. Iron (hydr)oxides, which are the predominant metallic oxides in soil, play a crucial role in degrading dissolved organic carbon (DOC), primarily through a process known as dissimilatory iron reduction (DIR). However, the efficiency of this reaction varies depending on the mineralogy and composition of the aquifer, and this understanding is essential for adequate water quality in ASR. The objective of this study is to investigate the impact of iron (hydr)oxide on acetate, as an organic carbon source, attenuation during the ASR. To achieve this, three sets of laboratory sediment columns were prepared, each containing a different type of iron (hydr)oxide minerals: ferrihydrite, goethite, and hematite. Following an acclimation period of 28 days to simulate the microcosm within an aquifer, the columns were continuously supplied with the simulated river water spiked with acetate (DOC 40-60 mg L-1), and the acetate concentration in the effluent was monitored. The result revealed that the column containing ferrihydrite achieved 97% acetate attenuation through DIR with anoxic conditions (DO < 0.1 mg L-1), while the goethite and hematite columns exhibited limited attenuation rates of 40 and 50%, respectively. Furthermore, the efficiency of acetate attenuation in the ferrihydrite columns increased with the content of ferrihydrite but experienced a rapidly declined at higher contents (3-4%), possibly due to the partial conversion of ferrihydrite to goethite as a result of the interaction between ferrihydrite and the Fe(II) produced during DIR. Additionally, an analysis of the microbial community demonstrated that microorganisms known to possess the ability to reduce iron (hydr)oxides under anaerobic conditions were abundant in the ferrihydrite columns.

Facet-dependent adsorption of aromatic organoarsenicals on hematite: The mechanism and environmental impact

Aromatic organoarsenic feed additives have been extensively used in poultry and livestock farming; however, a risk of releasing toxic inorganic arsenic exists when they are exposed to the environment. An in-depth understanding of the adsorption -migration behavior of aromatic organoarsenicals on environmental media is limited. In this study, p-arsanilic acid (p-ASA) and roxarsone (ROX) were considered as examples to systematically study their adsorption behaviors on the surface of hematite, a representative iron oxide in soil. By comparing the adsorption abilities and adsorption kinetics of hematite exposed with different facets (hexagonal nanoplates, HNPs, mainly exposed with {001} facets and hexagonal nanocubes, HNCs, exposed with {012} facets), combined with in situ shell-isolated nanoparticle enhanced Raman spectroscopy characterization and density functional theory simulation, the facet-dependent adsorption performance was observed and the mechanism revealed. The results showed that p-ASA formed a bidentate binuclear complex on HNCs and HNPs, whereas ROX formed monodentate mononuclear and bidentate binuclear configurations on the {001} and {012} facets, respectively. These differences not only lead to facet-dependent adsorption capacities but also affect their stability, as verified by sequential extraction experiments, affecting the environmental behavior and fate of aromatic organoarsenicals. This study not only provides insights into the environmental behavior of aromatic organoarsenicals but also offers theoretical support for the development of functional adsorbents and remediation strategies.

Synergetic effect of nitrate on dissolved organic carbon attenuation through dissimilatory iron reduction during aquifer storage and recovery

Aquifer storage and recovery (ASR) is a promising water management technique in terms of quantity and quality. During ASR, iron (Fe) (hydr)oxides contained in the aquifer play a crucial role as electron acceptors in attenuating dissolved organic carbon (DOC) in recharging water through dissimilatory iron reduction (DIR). Considering the preference of electron acceptors, nitrate (NO3⁻), possibly coexisting with DOC as the prior electron acceptor to Fe (hydr)oxides, might influence DIR by interrupting electron transfer. However, this phenomenon is yet to be clarified. In this study, we systematically investigated the potential effect of NO3⁻ on DOC attenuation during ASR using a series of sediment columns representing typical aquifer conditions. The results suggest that DOC attenuation could be enhanced by the presence of NO3⁻. Specifically, total DOC attenuation was notably higher than that from the stoichiometric calculation simply employing NO3⁻ as the additional electron acceptor to Fe (hydr)oxides, implying a synergetic effect of NO3⁻ in the overall reactions. X-ray photoelectron spectroscopy analyzes revealed that the Fe(II) ions released from DIR transformed the Fe (hydr)oxides into a less bioavailable form, inhibiting further DIR. In the presence of NO3⁻, however, no aqueous Fe(II) was detected, and another form of Fe (hydr)oxide appeared on the sediment surface. This may be attributed to nitrate-dependent Fe(II) oxidation (NDFO), in which Fe(II) is (re)oxidized into Fe (hydr)oxide, which is available for the subsequent DOC attenuation. These mechanisms were supported by the dominance of DIR-relevant bacteria and the growth of NDFO-related bacteria in the presence of NO3⁻.

Electrodeposited copper enhanced removal of 2,4-dichlorophenol in batch and flow reaction in Cu@CC-PS-MFC system

Combination of microbial fuel cell (MFC) and advanced oxidation process (AOP) is promising for pollutant removal. In this paper, Cu0-loaded carbon cloth cathode by electrodeposition (Cu@CC-PS-MFC) was applied to enhance 2,4-dichlorophenol (2,4-DCP) degradation based on persulfate (PS) activation in microbial fuel cell. Cu0 exhibited a typical structure of face-centered cubic metal polyhedron on carbon cloth. The removal of 2,4-DCP by Cu@CC-PS-MFC (75.6%) was enhanced by more than 50% compared to CC-PS-MFC (49.2%) after 1 h of reaction. 30 mg/L 2,4-DCP in Cu@CC-PS-MFC was completely removed and achieved a high mineralization (80.6%) after 9 h of reaction under optimized condition with low dissolved copper ion concentration (0.615 mg/L). Meanwhile, more than 90% removal of 2,4-DCP was stably achieved with flow operation condition (hydraulic residence time of 7.2 h). The change of copper valent state Cu0/Cu2O/CuO was the main mechanism of PS activation with main reactive species of O•H and O21. The bioanode of MFC enhanced the in-situ regeneration of ≡Cu+ and ≡Cu0 on the catalyst surface by transporting electrons, which was believed to contribute to good catalyst lifetime and excellent 2,4-DCP removal. Electrodeposited copper contributes to the enhanced degradation of 2,4-DCP with energy recovery at the same time which can further broaden the application MFC.

Nalidixic Acid and Fe(II)/Cu(II) Coadsorption at Goethite and Akaganéite Surfaces

Interactions between aqueous Fe(II) and solid Fe(III) oxy(hydr)oxide surfaces play determining roles in the fate of organic contaminants in nature. In this study, the adsorption of nalidixic acid (NA), a representative redox-inactive quinolone antibiotic, on synthetic goethite (α-FeOOH) and akaganéite (β-FeOOH) was examined under varying conditions of pH and cation type and concentration, by means of adsorption experiments, attenuated total reflectance-Fourier transform infrared spectroscopy, surface complexation modeling (SCM), and powder X-ray diffraction. Batch adsorption experiments showed that Fe(II) had marginal effects on NA adsorption onto akaganéite but enhanced NA adsorption on goethite. This enhancement is attributed to the formation of goethite-Fe(II)-NA ternary complexes, without the need for heterogeneous Fe(II)-Fe(III) electron transfer at low Fe(II) loadings (2 Fe/nm2), as confirmed by SCM. However, higher Fe(II) loadings required a goethite-magnetite composite in the SCM to explain Fe(II)-driven recrystallization and its impact on NA binding. The use of a surface ternary complex by SCM was supported further in experiments involving Cu(II), a prevalent environmental metal incapable of transforming Fe(III) oxy(hydr)oxides, which was observed to enhance NA loadings on goethite. However, Cu(II)-NA aqueous complexation and potential Cu(OH)2 precipitates counteracted the formation of ternary surface complexes, leading to decreased NA loadings on akaganéite. These results have direct implications for the fate of organic contaminants, especially those at oxic-anoxic boundaries.

Facet-dependent electron transfer induces distinct arsenic reallocations on hematite

The interfacial electron transfer (ET) between electron shuttling compounds and iron (Fe) oxyhydroxides plays a crucial role in the reductive dissolution of Fe minerals and the fate of surface-bound arsenic (As). However, the impact of exposed facets of highly crystalline hematite on reductive dissolution and As immobilization is poorly understood. In this study, we systematically investigated the interfacial processes of the electron shuttling compound cysteine (Cys) on various facets of hematite and the reallocations of surface-bound As(III) or As(V) on the respective surfaces. Our results demonstrate that the ET process between Cys and hematite generates Fe(II) and leads to reductive dissolution, with more Fe(II) generated on {001} facets of exposed hematite nanoplates (HNPs). Reductive dissolution of hematite leads to significantly enhanced As(V) reallocations on hematite. Nevertheless, upon the addition of Cys, a raipd release of As(III) can be halted by its prompt re-adsorption, leaving the extent of As(III) immobilization on hematite unchanged throughout the course of reductive dissolution. This is due to that Fe(II) can form new precipitates with As(V), a process that is facet-dependent and influenced by water chemistry. Electrochemical analysis reveals that HNPs exhibit higher conductivity and ET ability, which is beneficial for reductive dissolution and As reallocations on hematite. These findings highlight the facet-dependent reallocations of As(III) and As(V) facilitated by electron shuttling compounds and have implications for the biogeochemical processes of As in soil and subsurface environments.

Oxidation of 1,3-diphenylguanidine (DPG) by goethite activated persulfate: Mechanisms, products identification and reaction sites prediction

As emerging pollutants continue to be discovered, studies on the degradation behavior of emerging pollutants have proliferated, but few studies have focused on the reactivity of the new pollutants themselves. The work investigated the oxidation of a representative roadway runoff-derived organic contaminant, 1,3-diphenylguanidine (DPG) by goethite activated persulfate (PS). DPG exhibited the highest degradation rate (k_d = 0.42 h^-1) with present of PS and goethite at pH 5.0, then started to decrease with increasing pH. Chloride ion inhibited DPG degradation by scavenging HO·. Both HO· and SO₄^-· were generated in goethite activated PS system. Competitive kinetic experiments and flash photolysis experiments were conducted to investigate free radical reaction rate. The second-order reaction rate constants for DPG reacting with HO· and SO₄^-· were quantified (k_{DPG + HO·,}k_DPG + SO4-_·), which both reached above 10⁹ M^-1 s^-1. Chemical structures of five products were identified, four of them were previously detected in DPG photodegradation, bromination and chlorination processes. By density functional theory (DFT) calculations, ortho- and para- C were more easily attacked by both HO· and SO₄^-·. Abstraction of H on N by HO· and SO₄^-· were the favorable pathways, and the product TP-210 might be generated by cyclization of DPG radical from abstraction of H on N (3). The results of this study help us to better understand the reactivity of DPG with SO₄^-· and HO·.

Iron Vacancy Accelerates Fe(II)-Induced Anoxic As(III) Oxidation Coupled to Iron Reduction

Chemical oxidation of As(III) by iron (Fe) oxyhydroxides has been proposed to occur under anoxic conditions and may play an important role in stabilization and detoxification of As in subsurface environments. However, this reaction remains controversial due to lack of direct evidence and poorly understood mechanisms. In this study, we show that As(III) oxidation can be facilitated by Fe oxyhydroxides (i.e., goethite) under anoxic conditions coupled with the reduction of structural Fe(III). An excellent electron balance between As(V) production and Fe(III) reduction is obtained. The formation of an active metastable Fe(III) phase at the defective surface of goethite due to atom exchange is responsible for the oxidation of As(III). Furthermore, the presence of defects (i.e., Fe vacancies) in goethite can noticeably enhance the electron transfer (ET) and atom exchange between the surface-bound Fe(II) and the structural Fe(III) resulting in a two time increase in As(III) oxidation. Atom exchange-induced regeneration of active goethite sites is likely to facilitate As(III) coordination and ET with structural Fe(III) based on electrochemical analysis and theoretical calculations showing that this reaction pathway is thermodynamically and kinetically favorable. Our findings highlight the synergetic effects of defects in the Fe crystal structure and Fe(II)-induced catalytic processes on anoxic As(III) oxidation, shedding a new light on As risk management in soils and subsurface environments.

The nature of metal atoms incorporated in hematite determines oxygen activation by surface-bound Fe(II) for As(III) oxidation

The incorporation of secondary metal atoms into iron oxyhydroxides may regulate the surface chemistry of mediating electron transfer (ET) and, therefore, the biogeochemical pollutant processes such as arsenic (As) in the subsurface and soils. The influence of incorporating two typical metals (Cu and Zn) into a specific {001} hematite facet on O₂ activation by surface-bound Fe(II) was addressed. The results showed that Cu-incorporated hematite enhances As(III) oxidation in the presence of Fe(II) under oxic conditions and increases with increasing Cu content. Conversely, Zn incorporation leads to the opposite trend. The As(III) oxidation induced by surface-bound Fe(II) is positively related to the Fe(II) content and is favorable under acidic conditions. Reactive oxygen species (ROS), such as superoxide (·O^2-) and H₂O₂, predominantly contribute to As(III) oxidation as a result of 1-electron transfer from bound Fe(II) to surface O₂ on hematite and radical propagation. Electrochemical analysis demonstrates that Cu incorporation significantly lower the oxidation potential of Fe(II) on hematite, whereas Zn led to a higher reaction potential for Fe(II) oxidation. Subsequently, distinct surface reactivities of hematite for the activation of O₂ to form ROS by surface-bound Fe(II) are evidenced by metal incorporation. Our study provides a new understanding of the changes in the surface chemistry of iron oxyhydroxides because of incorporating metals (Zn and Cu), and therefore impact the biogeochemical processes of pollutants in soils and subsurface environments.