Redox reactions involving Fe-bearing clay minerals play a significant role in the cycling of elements such as carbon, and for the mobility of inorganic and organic contaminants in subsurface environments. The layered alumo-silicate structure of clay minerals can accommodate up to 30 wt% Fe in both octahedral and tetrahedral coordination. Tetrahedral Fe is naturally present in Fe-rich clay minerals like nontronite, yet the behavior and fate of tetrahedral Fe during redox reactions remains poorly constrained. Here, we synthesized a series of nontronites with 0 to 27% of the total Fe present as tetrahedral Fe(III) that we subjected to chemical reduction, re-oxidation, and re-reduction with dithionite and hydrogen peroxide, respectively. We used FT-IR and Mössbauer spectroscopies, SEM-EDS, HRTEM and XRD to monitor changes in crystal chemistry and the fate of tetrahedral Fe during the redox manipulation. Reduction of all nontronites resulted in almost all structural Fe becoming reduced (77-100% Fe(II)/Fe total ), including a portion of the initially present tetrahedral Fe, and re-oxidation restored all Fe to its ferric form. The initial 2:1 smectite structure was conserved throughout all redox manipulations, with the typical structural changes during Fe reduction, mainly trioctahedral domain formation and structural OH-group loss, being largely reverted by re-oxidation. However, the initial reduction led to significant loss of tetrahedral Fe from the nontronite structure, and the tetrahedral Fe content in all nontronites converged to the same value of ~0.2 per half formula unit. The concomitant decrease in particle size and number of stacked layers in a significant portion of each nontronite led to the presence of two distinct populations of particles and suggests partial reductive dissolution as a plausible mechanism for tetrahedral Fe release. While some of the Fe was present in the aqueous phase, increased interlayer distances in reduced and re-oxidized nontronites implies that 48-63% of the released Fe became intercalated, presumably in the form of hydroxylated pillars. Overall, highly similar structures, crystal chemistry, and tetrahedral Fe content resulted for all nontronites irrespective of their initial tetrahedral Fe content and remained stable in subsequent redox manipulations, indicating the reversible reduction and re-oxidation of both octahedral and tetrahedral Fe in the initially altered clay mineral structure. Our results provide compelling evidence for why naturally occurring clay minerals contain only low amounts of tetrahedral Fe and that both octahedral and tetrahedral Fe in clay minerals may contribute to electron transfer reactions in natural environments.

Metal pollution can increase in situ antibiotic resistance (AR), leading to elevated AR exposures and the potential evolution of new resistance. Metal-driven AR selection has been observed downstream of historic zinc (Zn) mining areas, such as the River Tyne catchment in Northeast England. The South Tyne was heavily mined until the early 20th century, leaving its waters and sediments with high Zn levels. Conversely, the North Tyne basin was unmined, providing a stark contrast in environmental Zn levels. Replicate sediment and water samples were collected six times from two North Tyne (Reaverhill, RH; Warks Burn, WB) and three South Tyne (Featherstone, FS; River West Allen, WA; River Nent, RN) sites, and phenotypic Zn, meropenem, and combined meropenem-Zn resistance were quantified relative to total and dissolved Zn levels and water quality conditions. In parallel, a 309-day batch experiment was performed on sediments from WB or WA, amended with inorganic Zn, and meropenem and Zn resistance were quantified over time. Sediment plating showed Zn-resistant and combined-resistant bacteria were significantly more abundant than meropenem-resistant bacteria in WA and RN (p < 0.05), suggesting Zn-dependent meropenem resistance prevailed at elevated Zn levels. Parallel batch studies, however, showed that only dissolved Zn levels correlated with phenotypic meropenem co-resistance, suggesting resistance was probably driven by dissolved rather than total Zn. Subsequent qPCR analysis on sediment samples showed Zn efflux and porin genes, especially oprD, associated with phenotypic Zn and combined Zn-meropenem resistance. Overall, results suggest meropenem co-resistance exists in the Tyne, but it is Zn-dependent, possibly linked to oprD-mediated uptake repression. However, dissolved Zn levels, not total Zn, drive meropenem resistance co-selection in the Tyne. In the future, dissolved and total metal levels should always be quantified in AR risk assessments on metal-polluted sites.

Iron-bearing smectite clay minerals are important redox-active phases in soils and sediments. Yet, the influence of iron substitution in the tetrahedral sheets ( Tet Fe) of smectites on their redox properties remains poorly understood. Here, we investigated the repeated reduction and re-oxidation of synthetic Fe-rich smectites, known as synthetic nontronites (SyN), with constant content of Fe in the octahedral sheet, Oct Fe, but variable initial content of Tet Fe. The redox properties of redox-cycled smectites were characterized using mediated electrochemical analysis. The resulting Fe 2+ /Fe Total ratios determined over a wide range of reduction potentials were analyzed using a process-based model to extract thermodynamic and kinetic descriptors of the interfacial electron transfer and subsequent charge redistribution processes that occur during the redox reaction. The fitted standard reduction potentials and apparent diffusion coefficients for charge redistribution were insensitive to Tet Fe content, indicating that thermodynamics and bulk charge transport within the smectite crystal are predominantly governed by the Fe–O–Fe networks in the octahedral sheet. Conversely, samples containing Tet Fe exhibited fitted interfacial electron-transfer rate constants approximately an order of magnitude higher than the Tet Fe-free sample. Tet Fe accelerated the rate constant of smectite redox reactions by approximately an order of magnitude. Comparison of these thermodynamic and kinetic descriptors between synthetic nontronites and natural reference smectites showed that parameter values converge within a narrow range across iron-rich dioctahedral smectites. These model-based results highlight the distinct roles of Oct Fe and Tet Fe in controlling smectite redox reactivity in subsurface environments, with Oct Fe governing the thermodynamics and Tet Fe primarily influencing interfacial electron transfer kinetics.

Mine drainage from abandoned mines is a major source of Arsenic (As); a ubiquitous, toxic and carcinogenic metalloid affecting over 200 million people worldwide. Recently, we observed extensive (up to 90%) co-removal of As in a vertical flow pond (VFP) passive treatment system that was designed to remove zinc from mine water drainage by precipitating ZnS following microbial sulphate reduction. However, the mechanism of As removal in the passive treatment system was unclear, even as microbial sulphate reduction is an emerging and cost-effective innovation for treating As contamination yet has received limited attention. Hence, the aim of this research was to investigate the main mechanism of As removal in the passive treatment system.To understand the complex biogeochemical interactions of As with redox sensitive elements (Fe, S) and dissolved organic carbon (DOC), we conducted monthly field sampling over one year at the passive treatment system at the Force Crag abandoned mine site, Cumbria, UK. Aqueous sample and porewater of three depth profiles including overlying water in the VFP were collected and analysed for total element concentration, speciation (As, Fe) and DOC. Elemental composition was determined with ICP-MS. Speciation of As and Fe in aqueous phase were determined using solid phase extraction cartridges and phenanthroline method respectively, where DOC was determined with TOC Analyser.The concentration of As (total, dissolved and colloidal) were consistently positively correlated with total, dissolved and colloidal Fe at the influent and four effluents, with concomitant decrease of both elements at the four effluents indicating potential influence of Fe on As mobility. Highest concentration of dissolved As and Fe were recorded in the porewater, which increased with depths, possibly due to vertical transportation and accumulation through the VFP, although highest level of DOC and sulphate in porewater may have caused competitive adsorption with As, resulting to weak retention of As on the binding sites. As(III) and Fe(II) were predominant in all aqueous samples, including the porewater, suggesting, to our surprise, the absence of redox transformations of As and Fe in the VFP. Decreased As concentrations at the four effluents coincided with decreased redox potentials (anaerobic), decreased sulphate and increased DOC, indicating that organic substrates were available as electron donor and may have fuelled microbial sulphate reduction, and subsequently generating sulphide. Combined with geochemical modelling of mineral saturation indices, our results point to the precipitation of As sulphides and/or co-precipitation with Fe sulphides as the likely mechanism(s) through which As was scavenged in the treatment system. We suggest that this passive treatment system relying on microbial sulphate reduction could be further developed for treatment of As contamination in mine water effluents.

Wastewater treatment plants (WWTPs) release antibiotic-resistant bacteria (ARB) and antibiotic-resistant genes (ARGs) into the environment. Advanced oxidation processes (AOPs) can remove ARB and ARGs, but they often require impractically high chemical or energy use. Here, we explore a low-energy AOP that uses Fe-bearing clay mineral (NAu-1) either combined with H2O2 (H2O2/NAu-1) or as prereduced structural Fe (rNAu-1) to degrade selected ARGs (i.e., tetM, tetQ, and bla OXA-10), int1 (a mobile genetic element), and the 16S rRNA gene in postsecondary WWTP effluents. Addition of H2O2/NAu-1 significantly increased tetM and int1 removals relative to UV irradiation and H2O2/UV (p ≤ 0.02). Removals increased with greater H2O2 doses and contact times, reaching maximum values of 1.2 and 2.3 log units at H2O2 doses of 0.26 and 10 mM and contact times of 4 and 8 h, respectively. Bacterial regrowth after 24 h of contact was probably due to H2O2 depletion. However, the addition of rNAu-1 achieved the highest removals, up to 2.9 log units after 0.5 h, and suppressed bacterial regrowth over 24 h. Similar removals were observed with rNAu-1 under oxic and anoxic conditions. Results show that mineral-based AOPs offer the potential for elevated ARG removal and lower chemical and energy demands in tertiary wastewater treatment.

Environmental scientists are increasingly returning to Mössbauer spectroscopy (MBS) to reveal details about iron (Fe)-bearing phases in soils and sediments. MBS is particularly powerful at distinguishing between Fe(II) and Fe(III) and, given appropriate background information, can offer exceptionally precise information on Fe speciation in compositionally complex environmental samples. However, there are relatively few accessible guides for analyzing environmental samples by MBS. In this review, we seek to distill the essential understanding of MBS for earth scientists and provide guidance on analysis, spectral fitting, and interpretation for new practitioners and a consolidation of approaches for experienced users. As a rule, Fe phases in soils and sediments are more disordered and complex than synthetic or geogenic Fe minerals. We cover the most successful ways MBS can be applied to soils, including the determination of Fe(II)/Fe(III) ratios, characterization of Fe (oxyhydr)oxide crystallinity, and the use of 57 Fe isotope spikes, as well as highlighting how to avoid common pitfalls and arrive at Fe phase identification and quantification by leveraging complimentary data and environment context. We outline procedures for sample preparation, analysis, and spectral fitting using decision trees based on the analytical goals and sample conditions. The fitting and interpretation of magnetically ordered ferrous phases at low temperature is lacking in the literature and so we offer an expanded discussion of approaches to these challenging spectra. We provide a discussion and fitting guidance for the most common Fe phases in soils and sediments organized around environmental contexts: young soils (and sediments derived from them) dominated by aluminosilicates, highly weathered soils rich in Fe oxides, organic-rich soils, soils in sulfur-rich environments, and soils exposed to anoxia. For each context, we describe expected Fe phases and their characteristic spectral features while emphasizing the importance of complementary analyses for reliable interpretation. Finally, we identify two critical needs in the field: improved theoretical frameworks for fitting low-temperature ferrous octets and Fe–sulfur phases and a need for standardization of parameter reporting and data sharing within the environmental MBS community. This review aims to both facilitate broader adoption of MBS in the environmental sciences and advance the technique's application to complex natural samples.

Chlorinated ethenes (CEs) are some of the most commonly found groundwater contaminants, and their clean-up still relies heavily on energy intensive clean-up practices such as pump and treat. As a sustainable alternative, abiotic natural attenuation by Fe(II) species would be preferable. While data is available on reduction of some CEs by stable Fe(II) phases, these reactions appear to be slower than reduction by freshly precipitated, transient Fe(II) phases (i.e., reactive mineral intermediates, RMIs). Here, we evaluated cis-1,2-dichloroethene (cDCE) reduction by stable and transient Fe(II)-containing phases, and characterized the transient phases formed. In the absence of aqueous Fe(II) (Fe(II)aq), magnetite, chloride green rust, hematite, mackinawite, and clay minerals did not reduce cDCE. When Fe(II)aq was present with these minerals, reduction usually occurred when conditions favored precipitation of ferrous hydroxide (Fe(OH)2). Additionally, we observed cDCE reduction by Fe(II) precipitates made from FeCl2 and ferrous ammonium sulfate (FAS), but never with FeSO4 present. Under no conditions, with or without Fe(II)aq, was cDCE reduced by goethite, chukanovite, sulfate green rust, or aluminum oxide. Mössbauer spectra of the transient phases indicate that ferrous (oxy)hydroxides such as Fe(OH)2 formed from FeCl2 and FeSO4 and a green rust-like precipitate formed from FAS. These spectra suggest that reduction is faster when the phases are less ordered, possibly because the Fe(II) precipitates are less crystalline or form smaller particles. Our work suggests that although most stable Fe(II) phases do not reduce cDCE sufficiently fast for significant abiotic natural attenuation, Fe(II) RMI phases may contribute to attenuation of cDCE plumes.

Clay minerals contain significant amounts of Fe in their alumosilicate framework, and this structural Fe can be reduced and re-oxidized, constituting a potentially renewable source of reduction equivalents in sedimentary environments. However, dissolution and/or clay mineral transformations during microbial Fe reduction contradict this concept. Here, we investigate how Fe reduction and re-oxidation affect the propensity of Fe to be released from the clay mineral structure and use selective sequential extractions in combination with Mössbauer spectroscopy. Negligible amounts of Fe were released in the sequential extraction of high Fe content clay minerals NAu-1 and NAu-2. Once aqueous Fe(II) was added as a reductant, the extraction procedure recovered the initially added Fe amount and up to 30% of the Fe from the clay mineral structure as both Fe(II) and Fe(III). Similar extents of Fe mobilization were found for clay minerals partly reduced (7%–20%) with dithionite, suggesting that mobilization was reduction-induced and independent of the source of reduction equivalents (Fe(II), dithionite). Although higher Fe reduction extents mobilized more structural Fe, i.e., >90% in fully reduced clay minerals, re-oxidation largely reverted the reduction-induced Fe mobilization in clay minerals. Our finding of reduction-driven Fe mobilization provides a plausible explanation for conflicting reports on Fe release from clay minerals and how extensive Fe atom exchange between aqueous and clay mineral Fe occurs.

One potential solution to the rising threat of antibacterial drug resistance is the application of therapeutic clays to treat wound infections. Clays with antibacterial activity have been identified from a range of sources with their antibacterial properties often attributed to the release of toxic metal ions such as Fe(II) and Al(III). Here, clays from Afghanistan, Azerbaijan and Bangladesh that are utilized for washing and healing purposes were examined. Their antibacterial activities were assessed in suspension and as aqueous leachates against representative Gram-negative, Escherichia coli, and Gram-positive, Bacillus subtilis, bacteria. The majority of the clays conferred no deleterious effect and, in fact, tended to promote bacterial growth, likely as a result of released organic and inorganic nutrients. However, one of the clays, obtained from the Dhaka region of Bangladesh, displayed significant bactericidal activity against E. coli and B. subtilis as a clay suspension but not as an aqueous leachate. Further experiments confirmed that contact between clay and the bacteria was necessary for most of the antibacterial effects. Detailed analysis of bulk and <2 mu m clay fraction mineralogy and geochemistry revealed no single defining parameter or mineral component that could be used to easily distinguish natural clays with antibacterial properties from those without. Overall, the results suggest a mechanism of antibacterial action of the Dhaka clay that arises from acidic conditions, likely enabled by the absence of calcite in the bulk clay, metal release, the presence of interstratified chlorite-smectite, and direct clay-bacteria interactions.

Many studies have characterised resistomes in river microbial communities. However, few have compared resistomes in parallel rural catchments that have few point-source inputs of antimicrobial genes (ARGs) and organisms (i.e., AMR) - catchments where one can contrast more nebulous drivers of AMR in rural rivers. Here, we used quantitative microbial profiling (QMP) to compare resistomes and microbiomes in two rural river catchments in Northern England, the Coquet and Eden in Northumberland and Cumbria, respectively, with different hydrological and geographical conditions. The Eden has higher flow rates, higher annual surface runoff, and longer periods of soil saturation, whereas the Coquet is drier and has lower flowrates. QMP analysis showed the Eden contained significantly more abundant microbes associated with soil sources, animal faeces, and wastewater than the Coquet, which had microbiomes like less polluted rivers (Wilcoxon test, p < 0.01). The Eden also had greater ARG abundances and resistome diversity (Kruskal Wallis, p < 0.05), and higher levels of potentially clinically relevant ARGs. The Eden catchment had greater and flashier runoff and more extensive agricultural land use in its middle reach, which explains higher levels of AMR in the river. Hydrological and geographic factors drive AMR in rural rivers, which must be considered in environmental monitoring programmes.

Structural Fe in clay minerals is an important, potentially renewable source of electron equivalents for contaminant reduction, yet our knowledge of how clay mineral Fe reduction pathways and Fe reduction extent affect clay mineral Fe(II) reactivity is limited. Here, we used a nitroaromatic compound (NAC) as a reactive probe molecule to assess the reactivity of chemically reduced (dithionite) and Fe(II)-reduced nontronite across a range of reduction extents. We observed biphasic transformation kinetics for all nontronite reduction extents of ≥5% Fe(II)/Fe(total) regardless of the reduction pathway, indicating that two Fe(II) sites of different reactivities form in nontronite at environmentally relevant reduction extents. At even lower reduction extents, Fe(II)-reduced nontronite completely reduced the NAC whereas dithionite-reduced nontronite could not. Our 57Fe Mössbauer spectroscopy, ultraviolet-visible spectroscopy, and kinetic modeling results suggest that the highly reactive Fe(II) entities likely comprise di/trioctahedral Fe(II) domains in the nontronite structure regardless of the reduction mechanism. However, the second Fe(II) species, of lower reactivity, varies and for Fe(II)-reacted NAu-1 likely comprises Fe(II) associated with an Fe-bearing precipitate formed during electron transfer from aqueous to nontronite Fe. Both our observation of biphasic reduction kinetics and the nonlinear relationship of rate constant and clay mineral reduction potential EH have major implications for contaminant fate and remediation.

Ferruginous clay minerals in saturated soils and within hydrocarbon deposits often exist in a reduced state. Upon introduction of dissolved oxygen, or other oxidants, the clay minerals oxidise and changes in mineral surface charge and sorption capacity occur, resulting in changes in hydration as well as flux of intercalated species. Here we examine the sorption of water to the Fe-containing clay minerals nontronite NAu-2 (23 wt% Fe) and illite IMt-2 (7 wt% Fe) as a function of Fe oxidation state and exchangeable cations by means of water vapour volumetry and N-2 surface area analysis. The clay minerals were chemically reduced using sodium dithionite. Sorption isotherms of water vapour and nitrogen, controlled relative humidity diffractograms, and chemical analyses were recorded. The results show that, after reduction using sodium dithionite, increased amounts of water vapour and nitrogen were adsorbed to the high Fe content nontronite, despite decreased interlayer separation. Little change was observed for the non-swelling and low Fe content illite. Sodium from the reducing agent was found to exchange with calcium present in the starting clay minerals, and sodium balanced the additional mineral charge generated during reduction. The findings presented in this study deliver improved understanding of sorption at the surface of the reduced clay minerals, which aid constrain the role of clay mineral interfaces in subsurface environments.

Clay minerals are important redox buffers in natural and engineered environments and they determine the fate of nutrients, (trace) elements, and organic and inorganic contaminants. In clay minerals, redox active Fe can be present in both octahedral and tetrahedral sheets of the silicate structure and amount to up to 30 wt% of the mineral. In contrast to other important iron-bearing minerals such as iron (oxyhydr)oxides, reduction of mineral ferric to ferrous iron primarily leads to structural re-arrangements instead of reductive dissolution. This property of iron-bearing clay minerals is thought to be the reason for the observation that these minerals can undergo redox reactions over an unusually large range of reduction potentials ( ∆ E H of 600 mV). However, this hypothesis largely ignores the fact that many clay minerals also contain Fe in tetrahedral sheets and that the redox reactivity of tetrahedrally coordinated Fe may be considerably different than that of Fe atoms in the octahedral sheet. In this study, we aim to complement the understanding of the redox buffering by iron-containing minerals in soils and groundwaters by characterizing the role of iron in tetrahedral sheets of clay minerals. We synthesized five high Fe content clay minerals (nontronites) with a range of tetrahedral Fe content (0-44% of the total mineral Fe bound in tetrahedral sheets) and subjected them to two subsequent cycles of Fe reduction and re-oxidation. The redox properties of these samples were characterized with mediated electrochemistry and linked to structural parameters using a suite of spectroscopic methods. We used FTIR, XMCD, and Mössbauer spectroscopy to determine the extent and reversibility of electron transfer to and from tetrahedral Fe and monitored mineralogical changes with XRD and TEM. Our preliminary results suggest that tetrahedral Fe can reversibly undergo reduction and re-oxidation in nontronites with low tetrahedral Fe content. Conversely, reduction and re-oxidation of clay minerals with high tetrahedral Fe content appear to yield some distinct structural alterations of the clay mineral structure.

Over 60 years of nuclear activities have resulted in a global legacy of radioactive wastes, with uranium considered a key radionuclide in both disposal and contaminated land scenarios. With the understanding that U has been incorporated into a range of iron (oxyhydr)oxides, these minerals may be considered a secondary barrier to the migration of radionuclides in the environment. However, the long-term stability of U-incorporated iron (oxyhydr)oxides is largely unknown, with the end-fate of incorporated species potentially impacted by biogeochemical processes. In particular, studies show that significant electron transfer may occur between stable iron (oxyhydr)oxides such as goethite and adsorbed Fe(II). These interactions can also induce varying degrees of iron (oxyhydr)oxide recrystallization (<4% to >90%). Here, the fate of U(VI)-incorporated goethite during exposure to Fe(II) was investigated using geochemical analysis and X-ray absorption spectroscopy (XAS). Analysis of XAS spectra revealed that incorporated U(VI) was reduced to U(V) as the reaction with Fe(II) progressed, with minimal recrystallization (approximately 2%) of the goethite phase. These results therefore indicate that U may remain incorporated within goethite as U(V) even under iron-reducing conditions. This develops the concept of iron (oxyhydr)oxides acting as a secondary barrier to radionuclide migration in the environment.

Fe-bearing clay minerals are important redox-active components of the subsurface and engineered barriers. Their interfacial reactivity plays an essential role in environmental processes such as biogeochemical cycling of various elements and contaminants. A detailed mechanism of Fe(II) surface speciation and interfacial electron transfer (ET) to Fe(III) in the octahedral sheet is still under debate despite its well-established consequence as one of the most effective reductants in anoxic environments. Recent developments have shown that edge-bound Fe(II) adsorption complexes at different surface sites may coexist on different edge facets. It has also been shown that complexes at ferrinol FeO(H) edge sites are the most energetically favorable and ET at these sites is facile and coupled to proton exchange. However, ET from Fe(II) sorbed to the external basal surface to octahedral Fe(III) is predicted to be predominantly thermodynamically uphill. A major enduring uncertainty in experiments is the extent to which Fe(II) can displace interlayer cations through cation exchange and there become a more effective reductant for Fe(III) in the octahedral sheet. Herein, we apply density functional theory (DFT) calculations to provide atomistic insights into the valence-interchange ET energetics and kinetics between Fe(II) in the interlayer site and an Fe(III) in the octahedral sheet, which reveals a lower ET barrier due to desolvation of interlayer Fe(II).

Over 60 years of nuclear activities have resulted in a global legacy of radioactive waste and contaminated land. Higher activity wastes are destined for disposal in a deep underground geological facility (GDF), with iron (oxyhydr)oxide phases expected to be ubiquitous in and around the repository. Additionally, uranium will be a significant radionuclide in many of these wastes. Given that a range of uranium-incorporated iron (oxyhydr)oxides have been reported in the literature, these mineral phases may be considered a secondary barrier to the migration of uranium in the environment. However, the long-term stability of these phases under fluctuating geochemical conditions is unknown. Stable iron oxyhydroxides (e.g. goethite) have undergone extensive recrystallisation (>90%) during Fe-atom exchange, with incorporated species released and/or reduced during the recrystallisation process. Here, the stability and fate of uranium-incorporated goethite during Fe-atom exchange was investigated. A U(VI)-goethite species was hydrothermally synthesized and reacted with aqueous Fe(II). The system was monitored using geochemical analysis and X-ray absorption spectroscopy (XAS), with an aqueous 57 Fe(II) tracer used to track the extent of Fe-atom exchange. This revealed that only ~2% of structural Fe(III) was exchanged with aqueous Fe(II), with the retention of incorporated U confirmed by acid digestions. Despite this, M IV -edge HR XANES and L III -edge EXAFS revealed an ingress of near-surface U(V) as the Fe-atom

Reductive transformation of organic contaminants by FeS in anoxic environments has been documented previously, whereas the transformation in oxic environments remains poorly understood. Here we show that phenol can be efficiently oxidized in oxic FeS suspension at circumneutral pH value. We found that hydroxyl radicals (•OH) were the predominant reactive oxidant and that a higher O2 content accelerated phenol degradation. Phenol oxidation depended on •OH production and utilization efficiency, i.e., phenol degraded per •OH produced. Low FeS contents (≤1 g/L) produced less •OH but higher utilization efficiency, while high contents produced more •OH but lower utilization efficiency. Consequently, the most favorable conditions for phenol oxidation occurred during the long-term interaction between dissolved O2 and low levels of FeS (i.e., ≤1 g/L). Mössbauer spectroscopy suggests that FeS oxidation to lepidocrocite initially produced an intermediate Fe(II) phase that could be explained by the apparent preferential oxidation of structural S(-II) relative to Fe(II), rendering a higher initial •OH yield upon unit of Fe(II) oxidation. Trichloroethylene can be also oxidized under similar conditions. Our results demonstrate that oxidative degradation of organic contaminants during the oxygenation of FeS can be a significant but currently underestimated pathway in both natural and engineered systems.

For decades, there has been evidence that Fe-containing minerals might contribute to abiotic degradation of chlorinated ethene (CE) plumes. Here, we evaluated whether Fe(II) in clay minerals reduces tetrachloroethene (PCE) and trichloroethene (TCE). We found that structural Fe(II) in both low (SWy-2) and high (NAu-1) Fe clay minerals did not reduce PCE or TCE under anoxic conditions. There was also no reduction of PCE or TCE after adding 5 mM dissolved Fe(II) to the clay mineral suspensions. In the presence of high Fe(II) concentrations (20 mM), however, PCE and TCE reduction products were observed in the presence of low Fe-content clay mineral SWy-2. Mössbauer spectroscopy results indicate that a mixed-valent Fe(II)-Fe(III) precipitate formed in the reactive SWy-2 suspensions. In contrast, in suspensions containing 20 mM Fe(II) alone or Fe-free clay mineral (Syn-1), we observed a purely Fe(II)-containing precipitate (Fe(OH)2) and also PCE and TCE reduction products. Interestingly, the amount of CE products decreased in the order of Fe-free clay mineral Syn-1 > Fe(OH)2 > low Fe-content clay mineral SWy-2, suggesting that clay mineral Fe controlled the formation of the reactive mineral phase. Additional experiments with hexachloroethane (HCA) revealed that faster HCA reduction occurred with decreasing clay mineral Fe content. Kinetic modeling yielded invariable second-order rate constants and increasing concentrations of reactive Fe(II) as the Fe(II)/Fe(total) content of the precipitates increased. Our data suggest that clay mineral Fe(III) is a sink for electrons from added Fe(II) that otherwise might have reduced the CEs. Furthermore, our findings are consistent with the hypothesis that active precipitation of Fe(II)-containing reactive mineral intermediates (RMI) may be important to CE reduction and suggest that RMI formation depends on clay mineral presence and Fe content.

Our understanding of how Fe(II) reacts with Fe(III) oxides has evolved based on evidence for electron transfer at the oxide–water interface and Fe(II)-catalyzed recrystallization. There is, however...