Understanding the Importance of Labile Fe(III) during Fe(II)-Catalyzed Transformation of Metastable Iron Oxyhydroxides

Ferrihydrite sulfidation transformation and coupled As(V) and Cd(II) mobilization under anoxic conditions

Ferrihydrite sulfidation is an important process influencing the environmental behavior of co-existent arsenate (As(V)) and cadmium (Cd(II)) pollutants in mining-impacted environments. However, the mineral evolution of ferrihydrite and the coupled mobilization behavior of co-existent As(V) and Cd(II) remain unclear. In this study, we have investigated As(V)-Cd(II)-bearing ferrihydrite conversion behavior induced by environmentally relevant concentrations of S(-II) (1 and 5 mM). PXRD, HR-TEM, and XAS results demonstrate that the co-existent As(V) and Cd(II) inhibit the conversion of ferrihydrite to secondary lepidocrocite (γ-FeO(OH)) and subsequently to goethite (α-FeO(OH)) at different S(-II) concentrations. Elevated As(V) and Cd(II) levels promote the formation of amorphous mackinawite (FeS) and pyrite (FeS2). Lepidocrocite and greenockite (CdS) are the predominant secondary phases at 1 mM S(-II) but lepidocrocite and pyrite are dominant at 5 mM S(-II) when the As(V) and Cd(II) levels are low. These sulfidation transformation pathways reduce the mobilization of the co-existent As(V) and Cd(II). Cs-TEM and chemical extraction results reveal that substantial portions of Cd(II) and As(V) are incorporated into secondary pyrite and lepidocrocite, in addition to surface adsorption and greenockite precipitation. These findings not only enhance our understanding of the geochemical cycling of Fe(III), As(V), and Cd(II) in natural anoxic sulfidic environments but also may provide guidelines for developing effective remediation methods for As-Cd co-contaminated settings.

Physical Contact between Bacteria and Carbonaceous Materials: The Key Switch Triggering Activated Carbon and Biochar to Promote Microbial Iron Reduction

Carbonaceous materials, including activated carbon and pyrolytic carbon, have been recognized for about over a decade as effective electron shuttles or conductive materials in promoting microbial Fe(III) mineral reduction. However, recent studies reveal inhibitory effects, sparking debates about their overall impact. We hypothesized that the physical contact between bacteria and carbon is an overlooked yet critical factor in determining whether carbon promotes or inhibits microbial Fe(III) reduction. Using systems containing Shewanella oneidensis MR-1, activated carbon, and ferrihydrite, we investigated how carbon-iron oxide aggregate structure affects Fe(III) reduction kinetics. At low activated carbon-to-iron oxide ratios (C/Fe = 5:7 by mass), ferrihydrite aggregated with carbon, forming carbon-encapsulated particles that suppressed Fe(III) reduction rates. Conversely, at higher ratios (C/Fe = 100:7), the ferrihydrite dispersed on the carbon surface, enhancing both the rate and extent of Fe(III) reduction. Tests with 11 different carbonaceous materials (activated carbon and biochar) all confirmed that the microstructure of iron oxides─whether encapsulating or dispersed─on carbon surfaces is critical for determining Fe(III) reduction rates. This insight resolves the debate on whether carbonaceous materials promote or inhibit Fe(III) mineral reduction and enhances our understanding of their roles in biogeochemical processes and environmental remediation.

Sunlight-Driven Transformation of Ferrihydrite via Ligand-to-Metal Charge Transfer: The Critical Factors and Arsenic Repartitioning

Ferrihydrite, a poorly ordered metastable iron oxide, is closely associated with dissolved organic matter (DOM) in soils and sediments. Although sunlight-induced photoreductive dissolution of ferrihydrite via ligand-to-metal charge transfer (LMCT) has been extensively studied, its potential impacts on mineralogical transformation and environmental behaviors of coexisting contaminants remain largely unknown. Here, we systematically investigated the effects of environmental parameters (e.g., solution pH, pO2 level, arsenic speciation, and content) on ferrihydrite transformation with the representative DOM-oxalate under simulated solar irradiation. Results showed that the oxalate-mediated LMCT process synchronously initiated Fe(II) production and proton consumption, the latter of which facilitated interfacial electron transfer and atom exchange (IET-AEFh-Fe2+) processes among ferrihydrite and newly formed Fe(II). At pH 5.0-8.0, ferrihydrite was prone to transform into goethite due to sufficient Fe(II) (approximately 80-2700 μM) from LMCToxa and enough affinity of Fe(II) with mineral to trigger IET-AEFh-Fe2+, while it only underwent reductive dissolution at pH 3.0-5.0 or kept a quasi-steady state over pH 8.0. Increasing the pO2 level and arsenic content hampered the recrystallization of ferrihydrite by reducing Fe(II) duration or altering the surface property of ferrihydrite, whereas the presence of As(III/V) also led to the formation of lepidocrocite with As(V) being more prominent. Additionally, chemical extraction and As K-edge EXAFS spectroscopy revealed that As was consecutively incorporated into the structures of goethite and lepidocrocite in the form of As(V) regardless of primary As speciation. These findings shed novel insights into low-crystalline iron oxide transformation and element migration driven by sunlight in natural environments.

Interplay of Structural Properties and Redox Behavior in CeO2 Nanoparticles: Impact on Reactivity and Bioavailability

The environmental redox transformation of CeO2 is crucial for evaluating its ecological risk and understanding the geochemical cycling of cerium (Ce). In this study, we examined the effects of crystallinity on CeO2 dissolution and monitored the structural evolution during redox transformations. The reductive dissolution and reoxidation behavior of CeO2 (100 mg/L) was examined in the presence of 200 μM citrate. Our findings indicate that ligand-induced dissolution is more pronounced in CeO2 with lower crystallinity under both dark and light conditions. This dependence is related to the intensive ligand complexation at oxygen vacancy sites, resulting in a higher complexation of Ce(III) and more efficient photoelectron generation for Ce(IV) reduction. During cyclic dissolution-reprecipitation, CeO2 notably transformed into an amorphous phase, progressively decreasing the crystallinity of the nanoparticles. Consequently, the dissolution fraction of well-crystallized CeO2 increased significantly from 1.2% in the first cycle to 11.4% in the third cycle, suggesting a transition to structures with higher interfacial reactivity. Similar transformation and dissolution behavior was observed in redox oscillations in a soil environment. Additionally, hydroponic exposure experiments with Arabidopsis thaliana, treated with 100 mg/L CeO2 for 7 days, demonstrated increased Ce uptake by roots post-transformation, with a higher proportion of CePO4 detected within the plants. This comprehensive study not only provides vital mechanistic insights into the transformation processes of CeO2 but also aids in assessing the ecological risks associated with engineered CeO2 nanomaterials.

pH induced incongruent-dissolution impacts Al-ferrihydrite transformations and As mobilization

This study investigated the role of Al and As fate during the transformation process of ferrihydrite influenced by different pH values under oxic conditions. The results indicate that the Al doping greatly enhanced the transformation of ferrihydrite (Fh) to Al-substituted goethite at all acidic or alkaline pH values under oxic conditions by promoting the incongruent dissolution and reprecipitation reactions of Al-substituted ferrihydrite (AlFh). Under acidic conditions, the preferential dissolution of structural Fe (4.73 mg/L) from AlFh occurs, whereas under alkaline conditions, the preferential dissolution of structural Al (1.25 mg/L) takes place. In contrast, under neutral conditions, the low solubility of Fh and AlFh induces the significant particle assembly, with Fe/Al minerals primarily transforming into goethite through oriented aggregation. As predominantly remains in an adsorbed state at all pH values during the transformation of Fh and AlFh, with the highest proportion of adsorbed As (86.9-96.7%) observed under neutral conditions. During the aging process, the adsorbed As gradually transforms into non-extractable As, and the changes in As speciation within Fe/Al minerals are closely coupled with the transformation of AlFh and Fh. Under alkaline and acidic conditions, the proportion of non-extractable As in the transformation products of Fh and AlFh increases by 14.02-19.72% and 12.27-16.28%, respectively, while under neutral conditions, it increases only by 12-13.02%. Therefore, regulating soil pH can partially modify As speciation and mitigate its environmental impact by altering the mineral transformation process. The results of this study facilitate better understanding of the role of Al substitution in the transformation of Fh and the cycling of As in the environment.

Molecular Insights into the Synergistic Inhibition of Microplastics-Derived Dissolved Organic Matter and Anions on the Transformation of Ferrihydrite

Ferrihydrite (Fh), as a ubiquitous iron (oxyhydr)oxide, plays an essential role in nutrient cycling and pollutant transformation due to its high surface area and diversified reaction sites. In the natural environment, Fh transformation could be easily influenced by coexisting components (particularly dissolved organic matter (DOM) and anions). As a new and important carbon source, microplastic-derived DOM (MP-DOM) directly or indirectly affects the morphology and fate of Fh, but limited knowledge exists about the combined effect of MP-DOM and anions on Fh transformation. Herein, this study elucidates the joint effects of polystyrene DOM (PS-DOM) and anions (such as Cl-, SO42-, and PO43-) on Fh transformation. Single anions (especially PO43-) were shown to inhibit the transformation of Fh to hematite (Hm) by hindering the dissolution and recrystallization of Fe(III). However, the inhibitory effect was strongly enhanced when PS-DOM and anions coexisted, which is attributed to their synergetic effects on inhibiting dissolution/recrystallization by occupying more active sites and hindering electron transfer. Furthermore, Fh transformation was predominantly controlled by PS-DOM, especially those containing high-unsaturation, high-oxidation-state, and O-rich phenolic compounds. These findings provide a new perspective on the significance of considering the joint effects of DOM and anions in evaluating the transformation of iron minerals.

Nano-Scale Insights into Clay Minerals Regulating the Fe(II)-Catalyzed Ferrihydrite Transformation under Anoxic Conditions

Metastable ferrihydrite nanoparticles and clay minerals always coexist as heteroaggregates in nature due to their abundance, opposite charge, and large interface energy. However, the impact of clay minerals on the transformation of ferrihydrite under anoxic conditions remains elusive. This study systematically investigated the effect of distinct clay minerals on the Fe(II)-catalyzed transformation of ferrihydrite and clarifying the underlying nanoscale mechanisms for the first time. Our results demonstrated that clay minerals could affect the production and recrystallization of labile Fe(III) (an active Fe(III) intermediate species formed by oxidation of Fe(II) at the ferrihydrite surface) by dispersing ferrihydrite aggregates. This modulation led to different transformation rates, higher crystallinity of formed lepidocrocite, and enhanced goethite formation in the heteroaggregates. Importantly, montmorillonite can accommodate Fe(II) and labile Fe(III) within its interlayer spaces, which further led to the inhibited crystallization of Fe(II) to magnetite and long-term preservation of labile Fe(III). Additionally, clay minerals served as templates for forming dendritic goethite and hexagonal magnetite nanoplates. Our findings provide new insights into the complicated roles of clay minerals in controlling the ferrihydrite transformation and other iron (oxyhydr)oxides formation, which is significant for predicting the bioavailability of iron and the fate of other coexisting contaminants.

Effects of dimethylarsenate coprecipitation with ferrihydrite on Fe(II)-induced mineral transformation and the release of dimethylarsenate

Organoarsenicals are toxic pollutants of global concern, and their environmental geochemical behavior might be greatly controlled by iron (Fe) (hydr)oxides through coprecipitation, which is rarely investigated. Here, the effects of the incorporation of dimethylarsenate (DMAs(V)), a typical organoarsenical, into the ferrihydrite (Fh) structure on the mineral physicochemical properties and Fe(II)-induced phase transformation of DMAs(V)-Fh coprecipitates with As/Fe molar ratios up to 0.0876 ± 0.0036 under anoxic conditions and the accompanying DMAs(V) release were investigated. The presence of DMAs(V) during Fh formation gradually decreases the mineral crystallinity. With increasing DMAs(V) content, the specific surface areas of the coprecipitates are decreased owing to particle aggregation, while the micropore sizes are negligible changed. Fourier transformed infrared (FTIR) and As K-edge X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopy show that, part of DMAs(V) binds to Fh surfaces in the coprecipitates by forming bidentate binuclear inner-sphere complexes through As-O-Fe bonds. During the reaction of the coprecipitate with 1 mM Fe(II) for 336 h, DMAs(V) inhibits the Fh transformation to goethite. No goethite forms at pH 4; at pH 7 low content of DMAs(V) hinders the further conversion of lepidocrocite to goethite, while high content of DMAs(V) completely inhibits goethite formation. DMAs(V) in the coprecipitate is continuously released into the solution, with the released proportion being generally increased with the increase of DMAs(V) content, pH and Fe(II) addition, probably owing to the desorption of weak inner- and outer-sphere DMAs(V) complexes bound on the Fh surfaces upon the Fh aging and transformation to lepidocrocite and goethite. These results provide deep insights into the fate and mobility of organoarsenical pollutants mediated by Fe (hydr)oxides in natural environments, and help design effective and ecofriendly remediation strategies for As polluted soils and sediments.

Effects of coexisting goethite or lepidocrocite on Fe(II)-induced ferrihydrite transformation pathways and Cd speciation

The efficacy of ferrihydrite in remediating Cd-contaminated soil is tightly regulated by Fe(II)-induced mineralogical transformations. Despite the common coexistence of iron minerals such as goethite and lepidocrocite, which can act as templates for secondary mineral formation, the impact of these minerals on Fe(II)-induced ferrihydrite transformation and the associated Cd fate have yet to be elucidated. Herein, we investigated the simultaneous evolution of secondary minerals and Cd speciation during Fe(II)-induced ferrihydrite transformation in the presence of goethite versus lepidocrocite. The presence of goethite resulted in a more pronounced ferrihydrite transformation than lepidocrocite because goethite facilitates electron transfer. Coexisting goethite promoted the production of secondary goethite with different morphology by triggering template-directed nucleation and growth of labile Fe(III) derived from ferrihydrite and intermediate lepidocrocite, respectively. However, coexisting lepidocrocite impeded goethite formation from ferrihydrite and acted as the template to facilitate secondary lepidocrocite production. Furthermore, variations in the crystallinity of coexisting lepidocrocite influenced the particle size and crystallinity of the secondary lepidocrocite, reflecting different dominant mechanisms in secondary lepidocrocite formation. Despite partial Cd mobilization into the solution due to Fe(II)-induced ferrihydrite transformation, secondary goethite and lepidocrocite re-sequestered Cd through lattice Fe(III) substitution, indicated by an increased structural Cd proportion, expanded lattice spacing, and reduced hyperfine field intensity. Additionally, secondary goethite was more effective than secondary lepidocrocite in sequestering Cd. Coexisting goethite increased the structural Cd proportion by 3.5-fold compared to coexisting lepidocrocite, demonstrating the superior ability of coexisting goethite in enhancing Cd stability during Fe(II)-induced ferrihydrite transformation in natural soils. These findings highlight the impact of template-driven mineralogical transformation on Cd fate in polluted soils and provide crucial implications for toxic metal remediation using mineral amendments.

Triplet-Excited Riboflavin Promotes Labile Fe(III) Accumulation and Changes Mineralization Pathways in Fe(II)-Catalyzed Ferrihydrite Transformation

Flavins are well-known endogenous electron shuttles that facilitate long-distance extracellular electron transfer in dissimilatory iron reduction (DIR), but the effects of their photosensitivity on DIR and the transformation of metastable iron (oxyhydr)oxides like ferrihydrite (Fh) remain underexplored. This study compared the kinetics, pathways, and products of Fh transformation catalyzed by aqueous Fe(II) (Fe(II)aq) in the presence of oxidized riboflavin (RFox) at pH 7 under both dark and light conditions. While RFox has a negligible impact on Fe(II)-catalyzed Fh transformation in the dark, its photoexcited triplet state (3RF*) can significantly accelerate interfacial electron transfer (IET) from Fe(II)aq to Fh, increasing the reductive dissolution rate of Fh and boosting the accumulation rate of the key intermediate labile Fe(III) (Fe(III)labile) from 14.2 μM·h-1 to 35.6 μM·h-1. The 3RF*-promoted Fe(II)-Fh IET favors the oxolation of Fe(III)labile to lepidocrocite (Lp) over goethite (Gt) formation during Fh transformation and promotes the subsequent conversion of Lp to magnetite (Mt), altering the mineral products from sole Gt to a mixture of Lp (24.1%), Gt (45.4%), and Mt (30.5%). These findings highlight the notable effects of riboflavin as a photosensitizer on Fh biotransformation, with implications for microbial respiration and elemental cycling in natural environments.

Time-dependent redistribution of soil arsenic induced by transformation of iron species during zero-valent iron biochar composites amendment: Effects on the bioaccessibility of As in soils

Zero-valent iron biochar composites (ZVI/BC) are considered as effective amendments for arsenic (As)-contaminated soils. However, the mechanisms of transformation of various soil As species during ZVI/BC amendments remain unclear. This study investigated As transformation in four soils (namely, GX, ZJ, HB, and HN) treated with ZVI/BC for 65 days under two soil moisture conditions, unsaturated and oversaturated. Results showed that the 65-day treatment was divided into two stages based on the variation of labile As content. Within 2 days (stage 1), ZVI/BC addition quickly reduced labile As content by 5.91-90.3 % in soils under unsaturated conditions. During days 2-65 (stage 2), labile As ultimately decreased by 0.06-0.31 mg/kg in GX, ZJ, and HB while increasing by 22.1 mg/kg in HN soil, due to its lower pH value and Fe content. The variations of labile As were attributed to changes in multiple Fe minerals and associated As species. In stage 1, the corrosion of ZVI/BC generated amorphous Fe oxides to immobilize labile As, resulting in the accumulation of meta-stable As. In stage 2, amorphous Fe oxides were transformed into crystalline Fe oxides, resulting in the release and re-precipitation of As along with transformation, thus redistributing immobilized As into labile and stable As, which was evidenced by multiple methods, including chemical extraction, XRD, and TEM-EDX. The elevated soil moisture condition would enhance the corrosion of ZVI/BC in stage 1, further forming a reductive environment to facilitate the transformation of Fe minerals in stage 2. Besides, As bioaccessibility in soils was reduced by 10.8-38.7 % after ZVI/BC treatment in in-vitro gastrointestinal simulations. Overall, our study revealed the time-dependent transformation mechanism of soil As species and associated Fe minerals under different soil moisture with ZVI/BC treatments, and highlighted the effectiveness of ZVI/BC as a long-term amendment for As-contaminated soils.

The Mobility of Mo during Microbially Mediated Ferrihydrite Phase Transformation

Molybdenum (Mo) is an essential nutrient for almost all organisms. However, at high concentrations, it can be toxic to animals and plants. This study investigated the interactions of Mo(VI) with iron oxyhydroxides during ferrihydrite bioreduction in the presence of Fe(III)-reducing Geobacter sulfurreducens. Here, we showed that Mo concentration controlled ferrihydrite phase transformation, leading to Mo release. With the biotic reduction of ferrihydrite and Fe(II) production, Mo(VI) reduction and Mo(IV)O2 formation were observed for the first time, which further immobilized Mo after surface adsorption of Mo(VI). At low Mo levels (Mo/Fe molar ratios of 1-2%), sufficient Fe(II) adsorption onto ferrihydrite resulted in its transformation into magnetite nanoparticles (>80%, ∼25 nm), which catalyzed the reduction of Mo(VI) to form Mo(IV)O2 and immobilized Mo. Contrastingly, at high Mo concentrations (Mo/Fe molar ratios of 5-10%), Mo(VI)O42- adsorption onto ferrihydrite limited Fe(II) adsorption; subsequently, less magnetite (<8-12%) formed while more goethite (∼30-50%, width and length >15 and 100 nm, respectively) and siderite (∼20-30%, width and length >100 and 200 nm, respectively) with larger particle sizes formed instead, causing Mo(VI) release due to lower Mo adsorption. This study provides a comprehensive understanding of the interaction mechanisms among Geobacter sulfurreducens, Mo(VI), and iron oxyhydroxides, enabling predictions and controls of long-term Mo mobility and Fe mineral transformation under a variety of biogeochemical scenarios.

Remediation of biochar-supported effective microorganisms and microplastics on multiple forms of nitrogenous and phosphorous in eutrophic lake

Lots of studies on eutrophication, but there is a lack of comprehensive research on the repair of multiple forms of nitrogen and phosphorus under combined heavy metals (HMs) pollution. This work investigated the various forms of nitrogen and phosphorus in the water-sediment systems of eutrophic lakes with the application of biochar, Effective Microorganisms (EMs) and microplastics, aiming to deliberate the repair behavior of multiple forms of nitrogen/phosphorus and the integrated repairment of these nutrients and HMs in different remediations. For amended-groups, the application of biochar-supported EMs (BE) achieved the most desirable remediation for removing nitrogen, phosphorus and HMs in water and improved their stability in sediment due to the improved microbial activity and the developed biofilm system created by biochar. The addition of aging microplastics (MP) obviously reduced the systematic levels of nitrogen, phosphorus and HMs due to the stimulation of microbial activity and the adsorption of biofilm/EPS, but its high movability also increased the Fe(II) and S(-II) levels and the pollutants' ecological risks in sediment. The co-application of BE and MP (MBE) destroyed the ecosystem and decreased the removal of nitrogen and phosphorus, while greatly removing HMs by the superfluous biofilms/EPS. The application of biochar (BC) preferentially adsorbed and degraded dissolved nitrogen and phosphorus, releasing HMs into water. From these amended-groups, it's also knew that the removal of nitrogen and phosphorus mainly came from the degradation/assimilation of NH3-N, SRP and dissolved matters, particularly those molecular weight below 3 kDa; the higher removal of phosphorus than nitrogen was attributed to the coprecipitation of Fe-S-P hydroxides and the adsorption of particulates; however, the colloidal (3-100 kDa) nitrogen and phosphorus had low accessibility and bioavailability, and it also showed the competitive adsorption with colloidal HMs, causing their relatively low removal in water. This study provides insight into the comprehensive repair of nitrogen, phosphorus and HMs in various forms by biochar-immobilized microbes and the influence of microplastics on nutrients and HMs in eutrophic lakes.

The reduction of nitrobenzene by Fe(II)-goethite-hematite heterogeneous systems: Insight from thermodynamic parameters of reduction potential

Determining the contaminants reduction rate by dissolved ferrous iron (Fe(II)aq) bound to iron oxides is curial for evaluating the abiotic attenuation of contaminants in aquifers. However, few studies have assessed the contaminants reduction rate controlled by thermodynamic parameters in heterogeneous systems with different iron oxides. In this study, a linear free energy relationship (LFER) was established between the nitrobenzene reduction rate and the thermodynamic driving force (reduction potential (EH) and pH) in Fe(II)aq-goethite-hematite co-existing systems. Results showed that the reduction rate of nitrobenzene correlated with the EH of the heterogeneous system. The standard reduction potential (EH0mix) of the mixed iron oxides could be obtained by a proportionate linear combination of the single iron oxide system EH0. Based on this, the EH of the heterogeneous systems could be calculated theoretically by combining EH0mix and the Nernst equation. Furthermore, a parallel LFER with the slope of 1 was established to associate the nitrobenzene reduction rate with EH and pH. The intercept term was related to the adsorption capacity of different iron oxides towards Fe(II)aq. The Fe(II)aq saturation adsorption capacity of hematite was 1.5 times higher than that of goethite. After normalizing the nitrobenzene reduction rate to the Fe(II)aq saturation adsorption capacity, the maximum difference in intercept terms was reduced from 37% to 15%. These findings would provide an important and feasible methodological support for the quantitative evaluation of abiotic attenuation of contaminants in groundwater.

The Influence of Seawater on Fe(II)-Catalyzed Ferrihydrite Transformation and Its Subsequent Consequences for C Dynamics

Short-range-ordered minerals like ferrihydrite often bind substantial organic carbon (OC), which can be altered if the minerals transform. Such mineral transformations can be catalyzed by aqueous Fe(II) (Fe(II)aq) in redox-dynamic environments like coastal wetlands, which are inundated with seawater during storm surges or tidal events associated with sea-level rise. Yet, it is unknown how seawater salinity will impact Fe(II)-catalyzed ferrihydrite transformation or the fate of bound OC. We reacted ferrihydrite with Fe(II)aq under anoxic conditions in the absence and presence of dissolved organic matter (DOM). We compared treatments with no salts (DI water), NaCl-KCl salts, and artificial seawater mixes (containing Ca and Mg ions) with or without SO42-/HCO3-. Both XRD and Mössbauer showed that NaCl-KCl favored lepidocrocite formation, whereas Ca2+/Mg2+/SO42-/HCO3- ions in seawater overrode the effects of NaCl-KCl and facilitated goethite formation. We found that the highly unsaturated and phenolic compounds (HuPh) of DOM selectively bound to Fe minerals, promoting nanogoethite formation in seawater treatments. Regardless of salt presence, only 5-9% of Fe-bound OC was released during ferrihydrite transformation, enriching HuPh relative to aliphatics in solution. This study offers new insights into the occurrence of (nano)goethite and the role of Fe minerals in OC protection in coastal wetlands.

Magnetite enhances As immobilization during nitrate reduction and Fe(II) oxidation by Acidovorax sp. strain BoFeN1

Arsenic (As) cycling in groundwater is commonly coupled to the biogeochemical cycling of iron (Fe) and the associated transformation of Fe minerals present. Numerous laboratory studies suggested that Fe minerals can act as nucleation sites for further crystal growth and as catalysts for abiotic Fe(II) oxidation. In view of the widespread existence of magnetite in anoxic environments where As is often dissolved, we firstly exploited magnetite to enhance As immobilization during nitrate-reducing Fe(II) oxidation (NRFO) induced by Acidovorax sp. strain BoFeN1, a mixotrophic nitrate-reducing Fe(II)-oxidizing bacterium that can oxidize Fe(II) through both enzymatic and abiotic pathways. Subsequently, we investigated how magnetite affects NRFO and As immobilization. Results demonstrated a significant increase in As(III) removal efficiency from 75.4 % to 97.2 % with magnetite, attributed to the higher amount of NRFO and As(III) oxidation promoted by magnetite. It was found that magnetite stimulated the production of extracellular polymeric substances (EPS), which could decrease the diffusion of nitrate in the periplasm of bacteria and shield them against encrustation, resulting in a more rapid reduction of nitrate in the system with magnetite than that without magnetite. Meanwhile, Fe(II) was almost completely oxidized in the presence of magnetite during the whole 72 h experiment, while in the absence of magnetite, 47.7 % of Fe(II) remained, indicating that magnetite could obviously accelerate the chemical oxidation of Fe(II) with nitrite (the intermediates of nitrate bioreduction). Furthermore, the formation of labile Fe(III), an intermediate product of electron transfer between Fe(II) and magnetite, was reasonably deduced to be vital for anoxic As(III) oxidation. Additionally, the XPS analysis of the solid phase confirmed the oxidation of 43.8 % of As(III) to As(V). This study helps to understand the biogeochemical cycling of Fe and As in the environment, and provides a cost-effective and environmentally friendly option for in situ remediation of As-contaminated groundwater.

Enhanced Cr(VI) removal and stabilization from bioleached wastewater by zero-valent iron coupled with hetero and autotrophic bacteria

Zero-valent iron (Fe0) usually suffers from organic acid complexation and ferrochrome layer passivation in Cr(VI) removal from bioleached wastewater of Cr slag. In this work, a synergetic system combined Fe0 and mixed hetero/autotrophic bacteria was established to reduce and stabilize Cr(VI) from bioleached wastewater. Due to bacterial consumption of organic acid and hydrogen, severe iron corrosion and structured-Fe(II) mineral generation (e.g., magnetite and green rust) occurred on biotic Fe0 surface in terms of solid-phase characterization, which was crucial for Cr(VI) adsorption and reduction. Therefore, compared with the abiotic Fe0 system, this integrated system exhibited a 6.1-fold increase in Cr(VI) removal, with heterotrophic reduction contributing 3.4-fold and abiotic part promoted by hydrogen-autotrophic bacteria enhancing 2.7-fold. After reaction, the Cr valence distribution and X-ray photoelectron spectroscopy indicated that most Cr(VI) was converted into immobilized products such as FexCr1-x(OH)3, Cr2O3, and FeCr2O4 by biotic Fe0. Reoxidation experiment revealed that these products exhibited superior stability to the immobilized products generated by Fe0 or bacteria. Additionally, organic acid concentration and Fe0 dosage showed significantly positive correlation with Cr(VI) removal within the range of biological adaptation, which emphasized that heterotrophic and autotrophic bacteria acted essential roles in Cr(VI) removal. This work highlighted the enhanced effect of heterotrophic and autotrophic activities on Cr(VI) reduction and stabilization by Fe0 and offered a promising approach for bioleached wastewater treatment.

Iron (hydr)oxides-induced activation of sulfite for contaminants degradation: The critical role of structural Fe(III)

Iron-based sulfite (S(IV)) activation has emerged as a novel strategy to generate sulfate radicals (SO4•-) for contaminants degradation. However, numerous studies focused on dissolved iron-induced homogeneous activation processes while the potential of structural Fe(III) remains unclear. In this study, five iron (hydr)oxide soil minerals (FeOx) including ferrihydrite, schwertmannite, lepidocrocite, goethite and hematite, were successfully employed as sources of structural Fe(III) for S(IV) activation. Results showed that the catalytical ability of structural Fe(III) primarily depended on the crystallinity of FeOx instead of their specific surface area and particle size, with ferrihydrite and schwertmannite being the most active. Furthermore, in-situ ATR-FTIR spectroscopy and 2D-COS analysis revealed that HSO3- was initially adsorbed on FeO6 octahedrons of FeOx via monodentate inner-sphere complexation, ultimately oxidized into SO42- which was then re-adsorbed via outer-sphere complexation. During this process, strong oxidizing SO4•- and •OH were formed for pollutants degradation, confirmed by radical quenching experiments and electron spin resonance. Moreover, FeOx/S(IV) system exhibited superior applicability with respect to recycling test, real waters and twenty-six pollutants degradation. Eventually, plausible degradation pathways of three typical pollutants were proposed. This study highlights the feasibility of structural Fe(III)-containing soil minerals for S(IV) activation in wastewater treatment.

Iron Oxyhydroxide Transformation in a Flooded Rice Paddy Field and the Effect of Adsorbed Phosphate

The mobility and bioavailability of phosphate in paddy soils are closely coupled to redox-driven Fe-mineral dynamics. However, the role of phosphate during Fe-mineral dissolution and transformations in soils remains unclear. Here, we investigated the transformations of ferrihydrite and lepidocrocite and the effects of phosphate pre-adsorbed to ferrihydrite during a 16-week field incubation in a flooded sandy rice paddy soil in Thailand. For the deployment of the synthetic Fe-minerals in the soil, the minerals were contained in mesh bags either in pure form or after mixing with soil material. In the latter case, the Fe-minerals were labeled with 57Fe to allow the tracing of minerals in the soil matrix with 57Fe Mössbauer spectroscopy. Porewater geochemical conditions were monitored, and changes in the Fe-mineral composition were analyzed using 57Fe Mössbauer spectroscopy and/or X-ray diffraction analysis. Reductive dissolution of ferrihydrite and lepidocrocite played a minor role in the pure mineral mesh bags, while in the 57Fe-mineral-soil mixes more than half of the minerals was dissolved. The pure ferrihydrite was transformed largely to goethite (82-85%), while ferrihydrite mixed with soil only resulted in 32% of all remaining 57Fe present as goethite after 16 weeks. In contrast, lepidocrocite was only transformed to 12% goethite when not mixed with soil, but 31% of all remaining 57Fe was found in goethite when it was mixed with soil. Adsorbed phosphate strongly hindered ferrihydrite transformation to other minerals, regardless of whether it was mixed with soil. Our results clearly demonstrate the influence of the complex soil matrix on Fe-mineral transformations in soils under field conditions and how phosphate can impact Fe oxyhydroxide dynamics under Fe reducing soil conditions.

Incorporation of Shewanella oneidensis MR-1 and goethite stimulates anaerobic Sb(III) oxidation by the generation of labile Fe(III) intermediate

Dissimilatory iron-reducing bacteria (DIRB) affect the geochemical cycling of redox-sensitive pollutants in anaerobic environments by controlling the transformation of Fe morphology. The anaerobic oxidation of antimonite (Sb(III)) driven by DIRB and Fe(III) oxyhydroxides interactions has been previously reported. However, the oxidative species and mechanisms involved remain unclear. In this study, both biotic phenomenon and abiotic verification experiments were conducted to explore the formed oxidative intermediates and related processes that lead to anaerobic Sb(III) oxidation accompanied during dissimilatory iron reduction. Sb(V) up to 2.59 μmol L-1 combined with total Fe(II) increased to 188.79 μmol L-1 when both Shewanella oneidensis MR-1 and goethite were present. In contrast, no Sb(III) oxidation or Fe(III) reduction occurred in the presence of MR-1 or goethite alone. Negative open circuit potential (OCP) shifts further demonstrated the generation of interfacial electron transfer (ET) between biogenic Fe(II) and goethite. Based on spectrophotometry, electron spin resonance (ESR) test and quenching experiments, the active ET production labile Fe(III) was confirmed to oxidize 94.12% of the Sb(III), while the contribution of other radicals was elucidated. Accordingly, we proposed that labile Fe(III) was the main oxidative species during anaerobic Sb(III) oxidation in the presence of DIRB and that the toxicity of antimony (Sb) in the environment was reduced. Considering the prevalence of DIRB and Fe(III) oxyhydroxides in natural environments, our findings provide a new perspective on the transformation of redox sensitive substances and build an eco-friendly bioremediation strategy for treating toxic metalloid pollution.

Divergent redistribution behavior of divalent metal cations associated with Fe(II)-mediated jarosite phase transformation

The Fe(II)/Fe(III) cycle is an important driving force for dissolution and transformation of jarosite. Divalent heavy metals usually coexist with jarosite; however, their effects on Fe(II)-induced jarosite transformation and different repartitioning behavior during mineral dissolution-recrystallization are still unclear. Here, we investigated Fe(II)-induced (1 mM Fe(II)) jarosite conversion in the presence of Cd(II), Mn(II), Co(II), Ni(II) and Pb(II) (denoted as Me(II), 1 mM), respectively, under anaerobic condition at neutral pH. The results showed that all co-existing Me(II) retarded Fe(II)-induced jarosite dissolution. In the Fe(II)-only system, jarosite first rapidly transformed to lepidocrocite (an intermediate product) and then slowly to goethite; lepidocrocite was the main product. In Fe(II)-Cd(II), -Mn(II), and -Pb(II) systems, coexisting Cd(II), Mn(II) and Pb(II) retarded the above process and lepidocrocite was still the dominant conversion product. In Fe(II)-Co(II) system, coexisting Co(II) promoted lepidocrocite transformation into goethite. In Fe(II)-Ni(II) system, jarosite appeared to be directly converted into goethite, although small amounts of lepidocrocite were detected in the final product. In all treatments, the appearance or accumulation of lepidocrocite may be also related to the re-adsorption of released sulfate. By the end of reaction, 6.0 %, 4.0 %, 76.0 % 11.3 % and 19.2 % of total Cd(II), Mn(II), Pb(II) Co(II) and Ni(II) were adsorbed on the surface of solid products. Up to 49.6 %, 44.3 %, and 21.6 % of Co(II), Ni(II), and Pb(II) incorporated into solid product, with the reaction indicating that the dynamic process of Fe(II) interaction with goethite may promote the continuous incorporation of Co(II), Ni(II), and Pb(II).

Antimony mobility in soil near historical waste rock at the world's largest Sb mine, Central China

Soil near waste rock often contains high concentrations of antimony (Sb), but the mechanisms that mobilize Sb in a soil closely impacted by the waste rock piles are not well understood. We investigated these mobility mechanisms in soils near historical waste rock at the world's largest Sb mine. The sequential extraction (BCR) of soil reveal that over 95 % Sb is present in the residual fraction. The leached Sb concentration is related to the surface protonation and deprotonation of soil minerals. SEM-EDS shows Sb in the soil is associated with Fe and Ca. Moreover, X-ray absorption spectroscopy (XAS) results show Sb is predominantly present as Sb(V) and is associated with Fe in the form of tripuhyite (FeSbO4) as well as edge- and corner-sharing complexes on ferrihydrite and goethite. Thus, Fe in soils is important in controlling the mobility of Sb via surface complexation and co-precipitation of Sb by Fe oxides. The initially surface-adsorbed Sb(V) or co-precipitation is likely to undergo a phase transformation as the Fe oxides age. In addition, Sb mobility may be controlled by small amounts of calcium antimonate. These results further the understanding of the effect of secondary minerals in soils on the fate of Sb from waste rock weathering and inform source treatment for Sb-contaminated soils.

Cooperative roles of phosphate and dissolved organic matter in inhibiting ferrihydrite transformation and their distinct fates

Phosphate and dissolved organic matter (DOM) mediate the crystalline transformation of ferrihydrite catalyzed by Fe(II) in subsurface environments such as soils and groundwater. However, the cooperative mechanisms underlying the mediation of phosphate and DOM in crystalline transformation of ferrihydrite and the feedback effects on their own distribution and speciation remain unresolved. In this study, solid characterization indicates that phosphate and DOM can collectively inhibit the crystalline transformation of ferrihydrite to lepidocrocite and thus goethite, via synergetic effects of inhibiting recrystallization and electron transfer. Phosphate can be retained on the surface or transformed to a nonextractable form within Fe oxyhydroxides; DOM is either released into the solution or preserved in an extractable form, while it is not incorporated or retained in the interior. Element distribution and DOM composition analysis on Fe oxyhydroxides reveals even distribution of phosphate on the newly formed Fe oxyhydroxides, while the distribution of DOM depends on its specific species. Electrochemical and dynamic force spectroscopic results provide molecular-scale thermodynamic evidence explaining the inhibition of electron transfer between Fe(II) and ferrihydrite by phosphate and DOM, thus influecing the crystalline transformation of ferrihydrite and the distribution of phosphate and DOM. This study provides new insights into the coupled biogeological cycle of Fe with phosphate and DOM in aquatic and terrestrial environments.

Contact with soil impacts ferrihydrite and lepidocrocite transformations during redox cycling in a paddy soil

Iron (Fe) oxyhydroxides can be reductively dissolved or transformed under Fe reducing conditions, affecting mineral crystallinity and the sorption capacity for other elements. However, the pathways and rates at which these processes occur under natural soil conditions are still poorly understood. Here, we studied Fe oxyhydroxide transformations during reduction-oxidation cycles by incubating mesh bags containing ferrihydrite or lepidocrocite in paddy soil mesocosms for up to 12 weeks. To investigate the influence of close contact with the soil matrix, mesh bags were either filled with pure Fe minerals or with soil mixed with 57Fe-labeled Fe minerals. Three cycles of flooding (3 weeks) and drainage (1 week) were applied to induce soil redox cycles. The Fe mineral composition was analyzed with Fe K-edge X-ray absorption fine structure spectroscopy, X-ray diffraction analysis and/or 57Fe Mössbauer spectroscopy. Ferrihydrite and lepidocrocite in mesh bags without soil transformed to magnetite and/or goethite, likely catalyzed by Fe(II) released to the pore water by microbial Fe reduction in the surrounding soil. In contrast, 57Fe-ferrihydrite in mineral-soil mixes transformed to a highly disordered mixed-valence Fe(II)-Fe(III) phase, suggesting hindered transformation to crystalline Fe minerals. The 57Fe-lepidocrocite transformed to goethite and small amounts of the highly disordered Fe phase. The extent of reductive dissolution of minerals in 57Fe-mineral-soil mixes during anoxic periods increased with every redox cycle, while ferrihydrite and lepidocrocite precipitated during oxic periods. The results demonstrate that the soil matrix strongly impacts Fe oxyhydroxide transformations when minerals are in close spatial association or direct contact with other soil components. This can lead to highly disordered and reactive Fe phases from ferrihydrite rather than crystalline mineral products and promoted goethite formation from lepidocrocite.

Organic binding iron formation and its mitigation in cation exchange resin assisted anaerobic digestion of chemically enhanced primary sedimentation sludge

Fe based chemically enhanced primary sedimentation (CEPS) is an effective method of capturing the colloidal particles and inorganic phosphorous (P) from wastewater but also produces Fe-CEPS sludge. Anaerobic digestion is recommended to treat the sludge for energy and phosphorus recovery. However, the aggregated sludge flocs caused by the coagulation limited sludge hydrolysis and P release during anaerobic digestion process. In this study, cation exchange resin (CER) was employed during anaerobic digestion of Fe-CEPS sludge with aims of prompting P release and carbon recovery. CER addition effectively dispersed the sludge flocs. However, the greater dispersion of sludge flocs could not translate to higher sludge hydrolysis. The maximum hydrolysis and acidification achieved at lower CER dosage of 0.5 g CER/g TS. It was observed that the extents of sludge hydrolysis and acidification had a strongly negative correlation with the organic binding iron (OBI) concentration. The presence of CER during anaerobic digestion favored Fe(III) reduction to Fe(II), and then further induced iron phase transformation, leading to the OBI formation from the released organic matters. Meanwhile, higher CER dosage resulted in higher P release efficiency and the maximum efficiency at 4 g CER/g TS was four times than that of the control. The reduction of BD-P, NaOH-P and HCl-P in solid phase contributed most P release into the supernatant. A new two-stage treatment process was further developed to immigrate the OBI formation and improve the carbon recovery efficiency. Through this process, approximately 45% of P was released, and 63% of carbon was recovered as methane from Fe-CEPS sludge via CER pretreatment.

Labile Fe(III) phase mediates the electron transfer from Fe(II,III) (oxyhydr)oxides to carbon tetrachloride

Labile Fe(III) phase (includes Fe(III)aq, Fe(III)ads, or Fe(III)s species) is an important intermediate during the interaction between Fe(II) and Fe(III) (oxyhydr)oxides, but how does labile Fe(III) influence the electron transfer from Fe(II) to oxidant environmental pollutant during this Fe(II)-Fe(III) interaction is unclear. In this work, the dynamic change of Fe(II,III) (oxyhydr)oxides at the same time scale is simulated by synthesizing Fe(III)-Fe(II)-I (Fe(III)+NaOH+Fe(II)+NaOH) with different Fe(II)/Fe(III) ratios. CCl4 is used as a convenient probe to test the reduction kinetics of mixed valence Fe(II,III)(oxyhydr)oxides with different Fe(II):Fe(III) ratios. The Mössbauer spectra results reveal the Fe(III)labile in the solid phase is in octahedral coordination. The electron-donating capability of Fe(II) was improved with increasing Fe(III) content, but suppressed when [Fe(III)] ≥ 30 mM. The reductive dechlorination of CT by Fe(III)-Fe(II)-I decreased gradually with the increase of Fe(III) content, because more amount Fe(III)labile in solid phase is accumulated. This shows that the electron transfer from Fe(II) to Fe(III)labile rather than to CT is enhanced with increasing Fe(III) content. FTIR data shows that the hydroxylation of Fe(II) with Fe(OH)3 occurs preferentially in the non-hydrogen bonded hydroxyl group, causing the decrease of its reductive reactivity. The presence of [Fe(III)-O-Fe(II)]+ in Fe(III)-Fe(II)-I can stabilize the dichlorocarbene anion (:CCl2-), favouring the conversion of CT to CH4 (13.1%). The aging experiment shows that Fe(III)labile surface may maintain the reductive reactivity of Fe(II) during aging when [Fe(III)] = 5-20 mM. This study deepens our understanding of the mass transfer pathway of iron oxyhydroxides induced by Fe(II) and its impact on the reductive dechlorination of CT.

Unveiling the crucial role of iron mineral phase transformation in antimony(V) elimination from natural water

Antimony (Sb) in natural water has long-term effects on both the ecological environment and human health. Iron mineral phase transformation (IMPT) is a prominent process for removing Sb(V) from natural water. However, the importance of IMPT in eliminating Sb remains uncertain. This study examined the various Sb-Fe binding mechanisms found in different IMPT pathways in natural water, shedding light on the underlying mechanisms. The study revealed that the presence of goethite (Goe), hematite (Hem), and magnetite (Mag) significantly affected the concentration of Sb(V) in natural water. Elevated pH levels facilitated higher Fe content in iron solids but impeded the process of removing Sb(V). To further our understanding, polluted natural water samples were collected from various locations surrounding Sb smelter sites. Results confirmed that converting ferrihydrite (Fhy) to Goe significantly reduced Sb levels (<5 μg/L) in natural water. The emergence of secondary iron phases resulted in greater electrostatic attraction and stabilized surface complexes, which was the most likely cause of the decline of Sb concentration in natural water. The comprehensive findings offer new insights into the factors governing IMPT as well as the Sb(V) behavior control.

The key roles of Fe oxyhydroxides and humic substances during the transformation of exogenous arsenic in a redox-alternating acidic paddy soil

Arsenic (As) from mine wastewater is a significant source for acidic paddy soil pollution, and its mobility can be influenced by alternating redox conditions. However, mechanistic and quantitative insights into the biogeochemical cycles of exogenous As in paddy soil are still lacking. Herein, the variations of As species in paddy soil spiking with As(III) or As(V) were investigated in the process of 40 d of flooding followed 20 d of drainage. During flooding process, available As was immobilized in paddy soil spiking As(III) and the immobilized As was activated in paddy soil spiking As(V) owing to deprotonation. The contributions of Fe oxyhydroxides and humic substances (HS) to As immobilization in paddy soil spiking As(III) were 80.16% and 18.64%, respectively. Whereas the contributions of Fe oxyhydroxides and HS to As activation in paddy soil spiking As(V) were 47.9% and 52.1%, respectively. After entering drainage, available As was mainly immobilized by Fe oxyhydroxides and HS and adsorbed As(III) was oxidized. The contribution of Fe oxyhydroxides to As fixation in paddy soil spiking As(III) and As(V) was 88.82% and 90.26%, respectively, and of HS to As fixation in paddy soil spiking As(III) and As(V) was 11.12% and 8.95%, respectively. Based on the model fitting results, the activation of Fe oxyhydroxides and HS bound As followed with available As(V) reduction were key processes during flooding. This may be because the dispersion of soil particles and release of soil colloids activated the adsorbed As. Immobilization of available As(III) by amorphous Fe oxyhydroxides followed with adsorbed As(III) oxidation were key processes during drainage. This may be ascribe to the occurrence of coprecipitation and As(III) oxidation mediated by reactive oxygen species from Fe(II) oxidation. The results are beneficial for a deeper understanding of As species transformation at the interface of paddy soil-water as well as an estimation pathway for the impacts of key biogeochemical cycles on exogenous As species under a redox-alternating condition.

Electron Transfer, Atom Exchange, and Transformation of Iron Minerals in Soils: The Influence of Soil Organic Matter

Despite substantial experimental evidence of electron transfer, atom exchange, and mineralogical transformation during the reaction of Fe(II)_aq with synthetic Fe(III) minerals, these processes are rarely investigated in natural soils. Here, we used an enriched Fe isotope approach and Mössbauer spectroscopy to evaluate how soil organic matter (OM) influences Fe(II)/Fe(III) electron transfer and atom exchange in surface soils collected from Luquillo and Calhoun Experimental Forests and how this reaction might affect Fe mineral composition. Following the reaction of ⁵⁷Fe-enriched Fe(II)_aq with soils for 33 days, Mössbauer spectra demonstrated marked electron transfer between sorbed Fe(II) and the underlying Fe(III) oxides in soils. Comparing the untreated and OM-removed soils indicates that soil OM largely attenuated Fe(II)/Fe(III) electron transfer in goethite, whereas electron transfer to ferrihydrite was unaffected. Soil OM also reduced the extent of Fe atom exchange. Following reaction with Fe(II)_aq for 33 days, no measurable mineralogical changes were found for the Calhoun soils enriched with high-crystallinity goethite, while Fe(II) did drive an increase in Fe oxide crystallinity in OM-removed LCZO soils having low-crystallinity ferrihydrite and goethite. However, the presence of soil OM largely inhibited Fe(II)-catalyzed increases in Fe mineral crystallinity in the LCZO soil. Fe atom exchange appears to be commonplace in soils exposed to anoxic conditions, but its resulting Fe(II)-induced recrystallization and mineral transformation depend strongly on soil OM content and the existing soil Fe phases.

A New Approach for Investigating Iron Mineral Transformations in Soils and Sediments Using 57Fe-Labeled Minerals and 57Fe Mössbauer Spectroscopy

Iron minerals in soils and sediments play important roles in many biogeochemical processes and therefore influence the cycling of major and trace elements and the fate of pollutants in the environment. However, the kinetics and pathways of Fe mineral recrystallization and transformation processes under environmentally relevant conditions are still elusive. Here, we present a novel approach enabling us to follow the transformations of Fe minerals added to soils or sediments in close spatial association with complex solid matrices including other minerals, organic matter, and microorganisms. Minerals enriched with the stable isotope ⁵⁷Fe are mixed with soil or sediment, and changes in Fe speciation are subsequently studied by ⁵⁷Fe Mössbauer spectroscopy, which exclusively detects ⁵⁷Fe. In this study, ⁵⁷Fe-labeled ferrihydrite was synthesized, mixed with four soils differing in chemical and physical properties, and incubated for 12+ weeks under anoxic conditions. Our results reveal that the formation of crystalline Fe(III)(oxyhydr)oxides such as lepidocrocite and goethite was strongly suppressed, and instead formation of a green rust-like phase was observed in all soils. These results contrast those from Fe(II)-catalyzed ferrihydrite transformation experiments, where formation of lepidocrocite, goethite, and/or magnetite often occurs. The presented approach allows control over the composition and crystallinity of the initial Fe mineral, and it can be easily adapted to other experimental setups or Fe minerals. It thus offers great potential for future investigations of Fe mineral transformations in situ under environmentally relevant conditions, in both the laboratory and the field.

Cooperative effect of slow-release ferrous and phosphate for simultaneous stabilization of As, Cd and Pb in soil

The different chemical behavior of anionic As and cationic Cd and Pb makes the simultaneous stabilization of soils contaminated with arsenic (As), cadmium (Cd), and lead (Pb) challenging. The use of soluble, insoluble phosphate materials and iron compounds cannot simultaneously stabilize As, Cd, and Pb in soil effectively due to the easy re-activation of heavy metals and poor migration. Herein, we propose a new strategy of "cooperatively stabilizing Cd, Pb, and As with slow-release ferrous and phosphate". To very this theory, we developed ferrous and phosphate slow-release materials to simultaneously stabilize As, Cd, and Pb in soil. The stabilization efficiency of water-soluble As, Cd and Pb reached 99% within 7d, and the stabilization efficiencies of NaHCO₃-extractable As, DTPA-extractable Cd and Pb reached 92.60%, 57.79% and 62.81%, respectively. The chemical speciation analysis revealed that soil As, Cd and Pb were transformed into more stable states with the reaction time. The proportion of residual fraction of As, Cd, and Pb increased from 58.01% to 93.82%, 25.69 to 47.86%, 5.58 to 48.54% after 56 d, respectively. Using ferrihydrite as a representative soil component, the beneficial interactions of phosphate and slow-release ferrous material in stabilizing Pb, Cd, and As were demonstrated. The slow-release ferrous and phosphate material reacted with As and Cd/Pb to form stable ferrous arsenic and Cd/Pb phosphate. Furthermore, the slow-release phosphate converted the adsorbed As into dissolved As, then the dissolved As reacted with released ferrous to form a more stable form. Concurrently, As, Cd and Pb were structurally incorporated into the crystalline iron oxides during the ferrous ions-catalyzed transformation of amorphous iron (hydrogen) oxides. The results demonstrates that the use of slow-release ferrous and phosphate materials can aid in the simultaneous stabilization of As, Cd, and Pb in soil.

Antimony Isotope Fractionation Revealed from EXAFS during Adsorption on Fe (Oxyhydr)oxides

A lack of knowledge about antimony (Sb) isotope fractionation mechanisms in key geochemical processes has limited its environmental applications as a tracer. Naturally widespread iron (Fe) (oxyhydr)oxides play a key role in Sb migration due to strong adsorption, but the behavior and mechanisms of Sb isotopic fractionation on Fe (oxyhydr)oxides are still unclear. Here, we investigate the adsorption mechanisms of Sb on ferrihydrite (Fh), goethite (Goe), and hematite (Hem) using extended X-ray absorption fine structure (EXAFS) and show that inner-sphere complexation of Sb species with Fe (oxyhydr)oxides occurs independent of pH and surface coverage. Lighter Sb isotopes are preferentially enriched on Fe (oxyhydr)oxides due to isotopic equilibrium fractionation, with neither surface coverage nor pH influencing the degree of fractionation (Δ¹²³Sb_{aqueous-adsorbed}). Limited Fe atoms are present in the second shell of Hem and Goe, resulting in weaker surface complexes and leading to greater Sb isotopic fractionation than with Fh (Δ¹²³Sb_{aqueous-adsorbed} of 0.49 ± 0.004, 1.12 ± 0.006, and 1.14 ± 0.05‰ for Fh, Hem, and Goe, respectively). These results improve the understanding of the mechanism of Sb adsorption by Fe (oxyhydr)oxides and further clarify the Sb isotope fractionation mechanism, providing an essential basis for future application of Sb isotopes in source and process tracing.

Effects of Fe oxides and their redox cycling on Cd activity in paddy soils: A review

Cadmium (Cd) contamination of soils is a global problem, particularly in paddy soils. Fe oxides, as a key fraction of paddy soils, can significantly affect the environmental behavior of Cd, which is controlled by complicated environmental factors. Therefore, it is necessary to systematically collect and generalize relevant knowledge, which can provide more insight into the migration mechanism of Cd and a theoretical basis for future remediation of Cd contaminated paddy soils. This paper summarized that (1) Fe oxides influence Cd activity through adsorption, complexation, and coprecipitation during transformation; (2) compared with the flooded period, the activity of Cd during the drainage period is stronger in paddy soils, and the affinity of different Fe components for Cd was distinct; (3) Fe plaque reduced Cd activity but was associated with plant Fe²⁺ nutritional status; (4) the physicochemical properties of paddy soils have the greatest impact on the interaction between Fe oxides and Cd, especially with pH and water fluctuations.

Pyrogenic Carbon Improves Cd Retention during Microbial Transformation of Ferrihydrite under Varying Redox Conditions

Fe(III) (oxyhydr)oxides are ubiquitous in paddy soils and play a key role in Cd retention. Recent studies report that pyrogenic carbon (PC) may largely affect the microbial transformation processes of Fe(III) (oxyhydr)oxides, yet the impact of PC on the fate of Fe(III) (oxyhydr)oxide-associated Cd during redox fluctuations remains unclear. Here, we investigated the effects of PC on Cd retention during microbial (Shewanella oneidensis MR-1) transformation of Cd(II)-bearing ferrihydrite under varying redox conditions. The results showed that in the absence of PC, microbial reduction of ferrihydrite resulted in Cd release under anoxic conditions and Fe(II) oxidation by oxygen resulted in Cd retention under subsequent oxic conditions. The presence of PC facilitated microbial ferrihydrite reductive dissolution under anoxic conditions, promoted Fe(II) oxidative precipitation under oxic conditions, and inhibited Cd release under both anoxic and oxic conditions. The presence of PC and frequent shifts in redox conditions (i.e., redox cycling) inhibited the transformation of ferrihydrite to highly crystalline goethite and magnetite that exhibited less Cd adsorption. As a result, PC enhanced Cd retention by 41-59% and 55-77% after the redox shift and redox cycling, respectively, while in the absence of PC, Cd retention decreased by 5% after the redox shift and increased by 11% after redox cycling. Sequential extraction analysis revealed that 63-78% of Cd was associated with Fe minerals, while 3-12% of Cd was bound to PC, indicating that PC promoted Cd retention mainly through inhibiting ferrihydrite transformation. Our results demonstrate the great impacts of PC on improving Cd retention under dynamic redox conditions, which is essential for applying PC in remediating Cd-contaminated paddy soils.

Fe(II)-Catalyzed Transformation of Ferrihydrite with Different Degrees of Crystallinity

Natural occurring ferrihydrite (Fh) nanoparticles have varying degrees of crystallinity, but how Fh crystallinity affects its transformation behavior remains elusive. Here, we investigated the Fe(II)-catalyzed transformation of Fh with different degrees of crystallinity (i.e., Fh-2h, Fh-12h, and Fh-85C). X-ray diffraction patterns of Fh-2h, Fh-12h, and Fh-85C exhibited two, five, and six diffraction peaks, respectively, indicating the order of crystallinity: Fh-2h < Fh-12h < Fh-85C. Fh with the lower crystallinity has a higher redox potential, corresponding to the faster Fe(II)-Fh interfacial electron transfer and Fe(III)_labile production. With the increase of initial Fe(II) concentration ([Fe(II)_aq]_int.) from 0.2 to 5.0 mM, the transformation pathways of Fh-2h and Fh-12h change from Fh → lepidocrocite (Lp) → goethite (Gt) to Fh → Gt, but that of Fh-85C switches from Fh → Gt to Fh → magnetite (Mt). The changes are rationalized using a computational model that quantitatively describes the relationship between the free energies of formation for starting Fh and nucleation barriers of competing product phases. Gt particles from the Fh-2h transformation exhibit a broader width distribution than those from Fh-12h and Fh-85C. Uncommon hexagonal Mt nanoplates are formed from the Fh-85C transformation at [Fe(II)_aq]_int.= 5.0 mM. The findings are crucial to comprehensively understand the environmental behavior of Fh and other associated elements.

Zinc Stable Isotope Fractionation Mechanisms during Adsorption on and Substitution in Iron (Hydr)oxides

The Zn isotope fingerprint is widely used as a proxy of various environmental geochemical processes, so it is crucial to determine which are the mechanisms responsible for isotopic fractionation. Iron (Fe) (hydr)oxides greatly control the cycling and fate and thus isotope fractionation factors of Zn in terrestrial environments. Here, Zn isotope fractionation and related mechanisms during adsorption on and substitution in three FeOOH polymorphs are explored. Results demonstrate that heavy Zn isotopes are preferentially enriched onto solids, with almost similar isotopic offsets (Δ^66/64Zn_{solid-solution} = 0.25-0.36‰) for goethite, lepidocrocite, and feroxyhyte. This is consistent with the same average Zn-O bond lengths for adsorbed Zn on these solids as revealed by Zn K-edge X-ray absorption fine structure spectroscopy. In contrast, at an initial Zn/Fe molar ratio of 0.02, incorporation of Zn into goethite and lepidocrocite by substituting for lattice Fe preferentially sequesters light Zn isotopes with Δ^66/64Zn_{substituted-stock solution} of -1.52 ± 0.09‰ and -1.18 ± 0.15‰, while Zn-substituted feroxyhyte (0.06 ± 0.11‰) indicates almost no isotope fractionation. This is closely related to the different crystal nucleation and growth rates during the Zn-doped FeOOH formation processes. These results provide direct experimental evidence of incorporation of isotopically light Zn into Fe (hydr)oxides and improve our understanding of Zn isotope fractionation mechanisms during mineral-solution interface processes.

Humic acid controls cadmium stabilization during Fe(II)-induced lepidocrocite transformation

Abiotic reduction of iron (oxyhydr)oxides by aqueous Fe(II) is one of the key processes affecting the Fe cycle in soil. Lepidocrocite (Lep) occurs naturally in anaerobic, clayey, non-calcareous soils in cooler and temperate regions; however, little is known about the impacts of co-precipitated humic acid (HA) on Fe(II)-induced Lep transformation and its consequences for heavy metal immobilization. In this study, the Fe(II)-induced phase transformation of Lep-HA co-precipitates was analyzed as a function of the C/Fe ratio, and its implications for subsequent Cd(II) concentration dynamic in dissolved and solid form was further investigated. The results revealed that secondary Fe(II)-bearing magnetite commonly formed during the Fe(II)-induced transformation of Lep, which further changed the mobility and distribution of Cd(II). The co-precipitated HA resulted in a decrease in the Fe solid phase transformation as the C/Fe ratios increased. Magnetite was found to be a secondary mineral in the 0.3C/Fe ratio Lep-HA co-precipitate, while only Lep was observed at a C/Fe ratio of 1.2 using X-ray diffraction (XRD) and Mössbauer spectroscopy. Based on XRD, scanning electron microscopy (SEM), Mössbauer, X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared spectroscopy (FTIR) results, newly formed magnetite may immobilize Cd(II) through surface complexes, incorporation, or structural substitution. The presence of HA was beneficial for binding Cd(II) and affected the mineralogical transformation of Lep into magnetite, which further induced the distribution of Cd(II) into the newly formed secondary minerals. These results provide insights into the behavior of Cd(II) in response to reaction between humic matter and iron (oxyhydr)oxides in anaerobic environments.

Arsenic mobilization in nZVI residue by Alkaliphilus sp. IMB: Comparison between static and flowing incubation

Arsenate reducing bacteria (AsRB) enhance arsenic (As) release via reducing As(V) to As(III), and As mobility is usually controlled by As(III) re-uptake on in-situ formed secondary iron minerals. The re-uptake of As(III) under groundwater flow conditions significantly impacts the fate and transport of As. Herein, a novel As(V)-reducing bacterium Alkaliphilus IMB was isolated in an As-contaminated soil. Scanning transmission X-ray microscopy showed that dissolved As(V) was mainly bound to the cell walls whereas dissolved As(III) was homogeneously distributed around IMB, indicating that As(V) reduction occurs outside the cell membrane. To explore the effect of IMB on As mobility, IMB was incubated with As-loaded nanoscale zero-valent iron (nZVI) residues under static and flowing conditions. IMB reduced 100% dissolved As(V) to As(III) even in a short contact time (∼1 h) during flowing incubation. The formation of As(III) did not influence As mobility under static condition as evidenced by the comparable concentrations of released As in the presence of IMB (8.5% to total As) and the abiotic control (10% to total As). Biogenic As(III) was re-adsorbed on the solids as shown by the higher ratio of solid-bound As(III) to total As in the presence of IMB (54%) than that in the abiotic control (12%). By contrast, the degree of As(III) re-adsorption was inhibited in the flowing environment, as suggested by the lower As(III) ratio in the solid (31%). This inhibition can be ascribed to the relatively slow adsorption of As(III) compared with the quick reduction of As(V) (∼1 h). Thus, IMB significantly enhanced As release during flowing incubation as shown that 9.8% As was released in the presence of IMB while 2.1% As in the abiotic control. This study found the contrary effect of AsRB on As mobility in static and flowing environments, highlighting the importance of re-adsorption rate of As(III).

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.

Ferrihydrite transformations in flooded paddy soils: rates, pathways, and product spatial distributions

Complex interactions between redox-driven element cycles in soils influence iron mineral transformation processes. The rates and pathways of iron mineral transformation processes have been studied intensely in model systems such as mixed suspensions, but transformation in complex heterogeneous porous media is not well understood. Here, mesh bags containing 0.5 g of ferrihydrite were incubated in five water-saturated paddy soils with contrasting microbial iron-reduction potential for up to twelve weeks. Using X-ray diffraction analysis, we show near-complete transformation of the ferrihydrite to lepidocrocite and goethite within six weeks in the soil with the highest iron(II) release, and slower transformation with higher ratios of goethite to lepidocrocite in soils with lower iron(II) release. In the least reduced soil, no mineral transformations were observed. In soils where ferrihydrite transformation occurred, the transformation rate was one to three orders of magnitude slower than transformation in comparable mixed-suspension studies. To interpret the spatial distribution of ferrihydrite and its transformation products, we developed a novel application of confocal micro-Raman spectroscopy in which we identified and mapped minerals on selected cross sections of mesh bag contents. After two weeks of flooded incubation, ferrihydrite was still abundant in the core of some mesh bags, and as a rim at the mineral-soil interface. The reacted outer core contained unevenly mixed ferrihydrite, goethite and lepidocrocite on the micrometre scale. The slower rate of transformation and uneven distribution of product minerals highlight the influence of biogeochemically complex matrices and diffusion processes on the transformation of minerals, and the importance of studying iron mineral transformation in environmental media.

Spatiotemporal Mineral Phase Evolution and Arsenic Retention in Microfluidic Models of Zerovalent Iron-Based Water Treatment

Arsenic (As) is a toxic element, and elevated levels of geogenic As in drinking water pose a threat to the health of several hundred million people worldwide. In this study, we used microfluidics in combination with optical microscopy and X-ray spectroscopy to investigate zerovalent iron (ZVI) corrosion, secondary iron (Fe) phase formation, and As retention processes at the pore scale in ZVI-based water treatment filters. Two 250 μm thick microchannels filled with single ZVI and quartz grain layers were operated intermittently (12 h flow/12 h no-flow) with synthetic groundwater (pH 7.5; 570 μg/L As(III)) over 13 and 49 days. Initially, lepidocrocite (Lp) and carbonate green rust (GRC) were the dominant secondary Fe-phases and underwent cyclic transformation. During no-flow, lepidocrocite partially transformed into GRC and small fractions of magnetite, kinetically limited by Fe(II) diffusion or by decreasing corrosion rates. When flow resumed, GRC rapidly and nearly completely transformed back into lepidocrocite. Longer filter operation combined with a prolonged no-flow period accelerated magnetite formation. Phosphate adsorption onto Fe-phases allowed for downstream calcium carbonate precipitation and, consequently, accelerated anoxic ZVI corrosion. Arsenic was retained on Fe-coated quartz grains and in zones of cyclic Lp-GRC transformation. Our results suggest that intermittent filter operation leads to denser secondary Fe-solids and thereby ensures prolonged filter performance.

Coexisting Goethite Promotes Fe(II)-Catalyzed Transformation of Ferrihydrite to Goethite

In redox-affected soil environments, electron transfer between aqueous Fe(II) and solid-phase Fe(III) catalyzes mineral transformation and recrystallization processes. While these processes have been studied extensively as independent systems, the coexistence of iron minerals is common in nature. Yet it remains unclear how coexisting goethite influences ferrihydrite transformation. Here, we reacted ferrihydrite and goethite mixtures with Fe(II) for 24 h. Our results demonstrate that with more goethite initially present in the mixture more ferrihydrite turned into goethite. We further used stable Fe isotopes to label different Fe pools and probed ferrihydrite transformation in the presence of goethite using 57Fe Mössbauer spectroscopy and changes in the isotopic composition of solid and aqueous phases. When ferrihydrite alone underwent Fe(II)-catalyzed transformation, Fe atoms initially in the aqueous phase mostly formed lepidocrocite, while those from ferrihydrite mostly formed goethite. When goethite was initially present, more goethite was formed from atoms initially in the aqueous phase, and nanogoethite formed from atoms initially in ferrihydrite. Our results suggest that coexisting goethite promotes formation of more goethite via Fe(II)-goethite electron transfer and template-directed nucleation and growth. We further hypothesize that electron transfer onto goethite followed by electron hopping onto ferrihydrite is another possible pathway to goethite formation. Our findings demonstrate that mineral transformation is strongly influenced by the composition of soil solid phases.

Mechanistic and modeling insights into the immobilization of Cd and organic carbon during abiotic transformation of ferrihydrite induced by Fe(II)

Iron (Fe) oxides and fulvic acid (FA) are the key components affecting the fate of cadmium (Cd) in soil. The presence of FA influences Fe mineral transformation, and FA may complicate phase transformation and dynamic behavior of Cd. How varying Fe minerals and FA affect Cd immobilization during the ferrihydrite transformation induced by various Fe(II) concentrations, however, is still lack of quantitative understanding. In this study, we built a model for Cd species quantification during phase transformation based on mechanistic insights obtained from batch experiments. Spectroscopic analysis showed that Fe(II) concentrations affected secondary Fe minerals formation under the condition of co-existence of Cd and FA, and ultimately changed the distribution of Cd and FA. Microscopic analysis revealed that besides surface adsorption, part of Cd was sequestrated by magnetite, whereas FA was able to diffuse into lepidocrocite defects. The model revealed that adsorbed Cd was mainly controlled by FA and ferrihydrite, and direct complexation of Cd by FA had a strong impact on the continuous change in Cd at lower Fe(II) concentration. The results contribute to an in-depth understanding of the mobility of Cd in the environment and provide a method for quantifying the dynamic behavior of heavy metals in multi-reactant systems.

Stabilization of Ferrihydrite and Lepidocrocite by Silicate during Fe(II)-Catalyzed Mineral Transformation: Impact on Particle Morphology and Silicate Distribution

Interactions between aqueous ferrous iron (Fe(II)) and secondary Fe oxyhydroxides catalyze mineral recrystallization and/or transformation processes in anoxic soils and sediments, where oxyanions, such as silicate, are abundant. However, the effect and the fate of silicate during Fe mineral recrystallization and transformation are not entirely understood and especially remain unclear for lepidocrocite. In this study, we reacted (Si-)ferrihydrite (Si/Fe = 0, 0.05, and 0.18) and (Si-)lepidocrocite (Si/Fe = 0 and 0.08) with isotopically labeled ⁵⁷Fe(II) (Fe(II)/Fe(III) = 0.02 and 0.2) at pH 7 for up to 4 weeks. We followed Fe mineral transformations with X-ray diffraction and tracked Fe atom exchange by measuring aqueous and solid phase Fe isotope fractions. Our results show that the extent of ferrihydrite transformation in the presence of Fe(II) was strongly influenced by the solid phase Si/Fe ratio, while increasing the Fe(II)/Fe(III) ratio (from 0.02 to 0.2) had only a minor effect. The presence of silicate increased the thickness of newly formed lepidocrocite crystallites, and elemental distribution maps of Fe(II)-reacted Si-ferrihydrites revealed that much more Si was associated with the remaining ferrihydrite than with the newly formed lepidocrocite. Pure lepidocrocite underwent recrystallization in the low Fe(II) treatment and transformed to magnetite at the high Fe(II)/Fe(III) ratio. Adsorbed silicate inactivated the lepidocrocite surfaces, which strongly reduced Fe atom exchange and inhibited mineral transformation. Collectively, the results of this study demonstrate that Fe(II)-catalyzed Si-ferrihydrite transformation leads to the redistribution of silicate in the solid phase and the formation of thicker lepidocrocite platelets, while lepidocrocite transformation can be completely inhibited by adsorbed silicate. Therefore, silicate is an important factor to include when considering Fe mineral dynamics in soils under reducing conditions.