Impact of Antimony(V) on Iron(II)-Catalyzed Ferrihydrite Transformation Pathways: A Novel Mineral Switch for Feroxyhyte Formation

Ferrous iron oxidation in the presence of antimonate at neutral pH: Mutual effects on iron mineral products and antimony sequestration

Antimony is a priority pollutant, whose mobility in redox-dynamic environments may be controlled by interactions with Fe(III) hydroxide minerals that form via Fe(II) oxidation. In this study, we examined the Fe(III) hydroxide precipitates and associated mechanisms of Sb(V) sequestration that result from Fe(II) oxidation in the presence of Sb(V) under neutral pH conditions. To achieve this aim, oxidation experiments were carried out in O2-saturated, Fe(II)-bearing solutions (buffered at pH 7) over a range of environmentally relevant Sb(V) concentrations (equivalent to Sb(V):Fe(II) molar ratios of 0, 0.01, 0.04, 0.1 and 0.25). Under these experimental conditions, Fe(II) oxidation occurred rapidly (within 20 minutes) causing associated removal of Sb(V) from solution via coprecipitation with the resulting Fe(III) hydroxides. At low Sb(V):Fe(II) ratios (< 0.1), lepidocrocite was the only Fe(III) mineral product of Fe(II) oxidation, whereas higher ratios resulted in formation of feroxyhyte. Both lepidocrocite and feroxyhyte retained Sb(V) within their crystal structure via Sb(V)-for-Fe(III) substitution. This mechanism of Sb(V) retention largely protected the solid-phase Sb(V) from release processes. Collectively, these results highlight the coupled role that interactions between Sb geochemistry and the Earth's near-surface Fe cycle can play in controlling both Fe(III) hydroxide mineralogy and Sb mobility.

Adsorption of As(III) to schwertmannite: impact factors and phase transformation

The phase transformation of Schwertmannite (SCH) can significantly affect the interface behavior and toxic effect of As(III). Previous studies have predominantly focused on pollutants adsorption by SCH on a long-time scale (such as 30 days), without paying attention to the adsorption characteristics within shorter time frames (e.g., within 24 h). This work compared the adsorption characteristics of As(III) on three synthesized SCHs under various environmental conditions. The adsorption of As(III) by M-SCH (SCH synthesized by KMnO4 oxidation method) and Y-SCH (SCH synthesized by ethanol modification method) were governed by physical adsorption, while that of H-SCH (SCH synthesized by H2O2 oxidation method) was dominated by chemisorption. Acidity, selected ions (NO3-, Ca2+, CO32-, and PO43-), fulvic acid (FA), and Sb(III) adversely impacted the As(III) adsorption by SCHs. Furthermore, alkalinity, selected ions and FA induced the phase transformation of SCHs to iron hydroxide, while no transformation was observed in the presence of Sb(III) and Cr(VI). The iron hydroxide observed in this study is presumed to represent an intermediate stage in the transformation of SCH into goethite. These findings enhance our comprehension of the transformation process of SCH to goethite and provide scientific insights for SCH utilization in immobilizing As(III) in water.

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.

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.

Antimony-bearing schwertmannite transformation to goethite: A driver of antimony mobilization in acid mine drainage

Antimony(V) mobility in acid mine drainage (AMD) is often controlled by sorption and coprecipitation with schwertmannite - a poorly-ordered Fe(III) oxyhydroxysulfate mineral. However, due to its metastable nature, schwertmannite transforms over time to more thermodynamically stable Fe(III) phases, such as goethite. This study examines how transformation of Sb(V)-bearing schwertmannite to goethite impacts Sb(V) mobility, while also assessing the role that Sb(V) may play in stabilizing schwertmannite against such transformation. To address these aims, Sb(V)-free, Sb(V)-sorbed and Sb(V)-coprecipitated schwertmannite were allowed to undergo partial transformation to goethite under acid sulfate conditions. Iron K-edge EXAFS spectroscopy revealed that sorbed and coprecipitated Sb(V) partly stabilized schwertmannite against transformation. The onset of schwertmannite transformation to goethite was found to drive clear mobilization of Sb(V) into solution, regardless of the Sb(V) loading or whether Sb(V) was initially sorbed or coprecipitated with the precursor schwertmannite. This initial phase of Sb(V) mobilization was followed by subsequent solid-phase recapture of the released Sb(V), with Sb K-edge EXAFS spectroscopy revealing that this process involved Sb(V) incorporation into the newly-formed goethite. Our findings show that, although schwertmannite transformation to goethite is partially inhibited by co-existing Sb(V), the initial stage of this transformation process drives significant Sb(V) mobilization in AMD systems.

Antimony(V) sorption and coprecipitation with ferrihydrite: An examination of retention mechanisms and the selectivity of commonly-applied extraction procedures

We investigated the mechanisms that control Sb(V) sorption and coprecipitation with ferrihydrite across a range of Sb(V) loadings, and examined the associated effects on Sb(V) extractability during the commonly-applied 1 M HCl extraction scheme and the BCR and Wenzel sequential extraction schemes. EXAFS spectroscopy reveals that Sb(V) sorption and coprecipitation mainly involved Sb(V) incorporation into the ferrihydrite structure via edge sharing and double-corner sharing between SbO6 and FeO6 octahedra. Large amounts of these linkages partially stabilized ferrihydrite against extraction with 1 M HCl. Negligible (< 0.5 %) ferrihydrite-bound Sb(V) was recovered in the "acid extractable" and "reducible" fractions of the BCR scheme, while 1-16 % was recovered in the "oxidizable" fraction. As such, the BCR scheme risks ferrihydrite-bound Sb(V) being misidentified as Sb residing mainly in "residual" phases. In contrast, in the Wenzel scheme, almost all sorbed- and coprecipitated-Sb(V) was recovered in the "amorphous hydrous oxide-bound" fraction, with only 0.6-3.3 % in the "specifically-bound" fraction (consistent with our finding of Sb(V) retention via incorporation into ferrihydrite, as opposed to adsorption by the ferrihydrite surface). Collectively, the results provide new insights into the retention mechanisms and extraction behaviour of ferrihydrite-bound Sb(V), enhancing our ability to assess Sb contamination in soils, sediments and geogenic wastes.

Antimony sorption to schwertmannite in acid sulfate environments

Schwertmannite is a poorly-crystalline Fe(III) oxyhydroxysulfate mineral that may control Sb(V) mobility in acid sulfate environments, including acid mine drainage and acid sulfate soils. However, the mechanisms that govern uptake of aqueous Sb(V) by schwertmannite in such environments are poorly understood. To address this issue, we examined Sb(V) sorption to schwertmannite across a range of environmentally-relevant Sb(V) loadings at pH 3 in sulfate-rich solutions. Antimony K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy revealed that Sb(V) sorption (at all loadings) involved edge and double-corner sharing linkages between SbVO6 and FeIIIO6 octahedra. The coordination numbers for these linkages indicate that sorption occurred by Sb(V) incorporation into the schwertmannite structure via heterovalent Sb(V)-for-Fe(III) substitution. As such, Sb(V) sorption to schwertmannite was not limited by the abundance of surface complexation sites and was strongly resistant to desorption when exposed to 0.1 M PO43-. Sorption of Sb(V) also conferred increased stability to schwertmannite, based on changes in the schwertmannite dissolution rate during extraction with an acidic ammonium oxalate solution. This study provides new insights into Sb(V) sorption to schwertmannite in acid sulfate environments, and highlights the role that schwertmannite can play in immobilizing Sb(V) within its crystal structure.

Antimony(V) Adsorption and Partitioning by Humic Acid-Modified Ferrihydrite: Insights into Environmental Remediation and Transformation Processes

Antimony (Sb) migration in soil and water systems is predominantly governed by its adsorption onto ferrihydrite (FH), a process strongly influenced by natural organic matter. This study investigates the adsorption behavior, stability, and mechanism of FH and FH-humic acid (FH-HA) complexes on Sb(V), along with the fate of adsorbed Sb(V) during FH aging. Batch adsorption experiments reveal that initial pH and concentration significantly influence Sb(V) sorption. Lower pH levels decrease adsorption, while higher concentrations enhance it. Sb(V) adsorption increases with prolonged contact time, with FH exhibiting a higher adsorption capacity than FH-HA complexes. Incorporating HA onto FH surfaces reduces reactive adsorption sites, decreasing Sb(V) adsorption. Adsorbed FH-HA complexes exhibit a higher specific surface area than co-precipitated FH-HA, demonstrating stronger Sb(V) adsorption capacity under various conditions. X-ray photoelectron spectroscopy (XPS) confirms that Sb(V) adsorption primarily occurs through ligand exchange, forming Fe-O-Sb complexes. HA inhibits the migration of Sb(V), thereby enhancing its retention within the FH and FH-HA complexes. During FH transformation, a portion of Sb(V) may replace Fe(III) within converted iron minerals. However, the combination of relatively high adsorption capacity and significantly lower desorption rates makes adsorbed FH-HA complexes promising candidates for sustained Sb adsorption over extended periods. These findings enhance our understanding of Sb(V) behavior and offer insights for effective remediation strategies in complex environmental systems.

Unveiling the Influence of Antimony Substitution on the Surface Properties and Adsorption Behavior of Ferrihydrite: From Molecular Mechanisms to Environmental Implications

Antimony(V) substitution is common in secondary ferrihydrite, especially in mining areas and tailings. However, its impact on the adsorption behavior of ferrihydrite is still unclear. Therefore, this study investigated the influential mechanisms of Sb(V) substitution on the lattice structure and surface properties of Sb-substituted ferrihydrite (SbFh), and its adsorption of coexisting Sb(OH)6-. Antimony(V) is substituted at Fe1 sites and is primarily distributed on the surface. Substitution has opposing effects on the outer- and inner-sphere complexation of Sb(OH)6-. On one hand, substituted-Sb(V) transfers more positive charges to ≡FeOH, reducing the number of H bonds. Subsequently, the charge saturation of ≡FeOH decreases, surface charge increases, and outer-sphere complexation is promoted. On the other hand, the elevated bond valence of Sb-O increases charge saturation of ≡FeOH, reducing the charge capacity that ≡FeOH can accommodate from inner-sphere complexes. Thus, inner-sphere complexation is inhibited. Inner-sphere complexation plays a more important role, and Sb(OH)6- adsorption is inhibited. Additionally, the primary complexation modes of Sb(OH)6- transform from bidentate to monodentate complexation. This research has important implications for understanding the environmental behavior of ferrihydrite, as well as the fate and bioavailability of antimony in mining areas and tailings.

Novel Insights into Sb(III) Oxidation and Immobilization during Ferrous Iron Oxygenation: The Overlooked Roles of Singlet Oxygen and Fe (oxyhydr)oxides Formation

Reactive oxygen species (ROS) produced from the oxygenation of reactive Fe(II) species significantly affect the transformation of metalloids such as Sb at anoxic-oxic redox interfaces. However, the main ROS involved in Sb(III) oxidation and Fe (oxyhydr)oxides formation during co-oxidation of Sb(III) and Fe(II) are still poorly understood. Herein, this study comprehensively investigated the Sb(III) oxidation and immobilization process and mechanism during Fe(II) oxygenation. The results indicated that Sb(III) was oxidized to Sb(V) by the ROS produced in the aqueous and solid phases and then immobilized by formed Fe (oxyhydr)oxides via adsorption and coprecipitation. In addition, chemical analysis and extended X-ray absorption fine structure (EXAFS) characterization demonstrated that Sb(V) could be incorporated into the lattice structure of Fe (oxyhydr)oxides via isomorphous substitution, which greatly inhibited the formation of lepidocrocite (γ-FeOOH) and decreased its crystallinity. Notably, goethite (α-FeOOH) formation was favored at pH 6 due to the greater amount of incorporated Sb(V). Moreover, singlet oxygen (1O2) was identified as the dominant ROS responsible for Sb(III) oxidation, followed by surface-adsorbed ·OHads, ·OH, and Fe(IV). Our findings highlight the overlooked roles of 1O2 and Fe (oxyhydr)oxide formation in Sb(III) oxidation and immobilization during Fe(II) oxygenation and shed light on understanding the geochemical cycling of Sb coupled with Fe in redox-fluctuating environments.

Factors driving antimony accumulation in soil-pakchoi and wheat agroecosystems: Insights and predictive models

The accumulation of antimony (Sb) in plants and its potential effects on human health are of increasing concern. Nevertheless, only a few countries or regions have established soil Sb thresholds for agricultural purposes, and soil properties have not been taken into account. This study investigated the accumulation of Sb in the edible parts of pakchoi and wheat grain by adding exogenous Sb to 21 soils with varying properties. The results revealed a positive correlation between bioavailable Sb (Sbava, extracted by 0.1 M K2HPO4) in soil and Sb in the edible parts of pakchoi (R2 = 0.77, p < 0.05) and wheat grain (R2 = 0.54, p < 0.05). Both machine learning and traditional multiple regression analysis indicated Sbava was the most critical feature and the main soil properties that contributed to Sb uptake by pakchoi and wheat were CaCO3 and clay, respectively. The advisory food limits for Sb in pakchoi and wheat were estimated based on health risk assessment, and used to derive soil thresholds for safe pakchoi and wheat production based on Sbtot and Sbava, respectively. These findings hold potential for predicting Sb uptake by crops with different soil properties and informing safe production management strategies.

Antimony release and volatilization from organic-rich and iron-rich submerged soils

Antimony (Sb) is an poorly understood, increasingly common pollutant, especially in soils susceptible to waterlogging. We investigated the impact of waterlogging on Sb release, methylation, and volatilization from an organic-rich wetland soil and an iron (Fe)-rich floodplain soil in a 27-day microcosm experiment. The release of Sb into the porewaters of the organic-rich soil was environmentally relevant and immediate with waterlogging (3.2 to 3.5 mg L-1), and likely associated with a complex interplay of sulfide precipitation, sorption with organic matter and manganese (Mn) (oxyhydr)oxides in the soil. The release of Sb from the Fe-rich soil was likely associated with Fe-(oxyhydr)oxide reduction and immobilized due to co-precipitation with Fe-sulfides or as Sb-sulfides. Volatile Sb was produced from the soils after waterlogging. The organic-rich soil produced more volatile Sb (409 to 835 ng kgsoil-1), but the Fe-rich soil volatilized Sb more efficiently. The negligible association of Sb volatilization with soil parameters indicates a more complex underlying, potentially microbial, mechanism and that antimony volatilization could be ubiquitous and not dependent on specific soil properties. Future works should investigate the microbial and physiochemical drivers of Sb volatilization in soils as it may be an environmentally relevant part of the biogeochemical cycle.

Organic Matter Counteracts the Enhancement of Cr(III) Extractability during the Fe(II)-Catalyzed Ferrihydrite Transformation: A Nanoscale- and Molecular-Level Investigation

Phase transformation of ferrihydrite to more stable Fe (oxyhydr)oxides, catalyzed by iron(II) [Fe(II)], significantly influences the mobility of heavy metals [e.g., chromium (Cr)] associated with ferrihydrite. However, the impact of organic matter (OM) on the behavior of Cr(III) in the Fe(II)-catalyzed transformation of ferrihydrite and the underlying mechanisms are unclear. Here, the Fe(II)-catalyzed transformation of the coprecipitates of Fe(III), Cr(III), or rice straw-derived OM was studied at the nanoscale and molecular levels using Fe and Cr K-edge X-ray absorption spectroscopy and spherical aberration corrected scanning transmission electron microscopy (Cs-STEM). Batch extraction results suggested that the OM counteracted the enhancement of Cr(III) extractability during the Fe(II)-catalyzed transformation. Cs-STEM and XAS analysis suggested that Cr(III) could be incorporated into the goethite formed by Fe(II)-catalyzed ferrihydrite transformation, which, however, was inhibited by the OM. Furthermore, Cs-STEM analysis also provided direct nanoscale level evidence that residual ferrihydrite could re-immobilize the released Cr(III) during the Fe(II)-catalyzed transformation process. These results highlighted that the decreased extractability of Cr(III) mainly resulted from the inhibition of OM on the Fe(II)-catalyzed transformation of ferrihydrite to secondary Fe (oxyhydr)oxides, which facilitates insightful understanding and prediction of the geochemical cycling of Cr in soils with active redox dynamics.

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.

Assessment of industrial by-products as amendments to stabilize antimony mine wastes

The spread of antimony from mine wastes to the environment represents a matter of great concern due to its adverse effects on impacted ecosystems. There is an urgent need for developing and adopting sustainable and inexpensive measures to deal with this type of wastes. In this study the Sb leaching behavior of mine waste rocks and mine tailings derived from the exploitation of Sb ore deposits was characterized using standard batch leaching tests (TCLP and EN-12457-4) and column leaching essays. Accordingly, these mine wastes were characterized as toxic (>0.6 mg Sb L^-1) and not acceptable at hazardous waste landfills (>5 mg Sb kg^-1), showing also an ongoing Sb release under prolonged leaching conditions. Two industrial by-products were evaluated as amendments to stabilize them, namely deferrisation sludge (DFS) and a by-product derived from the treatment of aluminum salt slags (BP-Al). Mine wastes were amended with different doses (0-25%) of DFS or BP-Al and the performance of these treatments was evaluated employing also batch and column leaching procedures. The effectiveness of DFS to immobilize Sb was much higher than that exhibited by BP-Al. Thus, treatments with 25% BP-Al showed Sb immobilization levels of approximately 33-53%, whereas treatments with 5 and 25% DFS already attained Sb immobilization levels up to approximately 80-90 and 90-99%, respectively. Mine tailings amended with 5% DFS and mine waste rocks amended with 25% DFS decreased their leachable Sb contents below the limit for non-hazardous waste landfill acceptance (<0.7 mg Sb kg^-1). Likewise, these DFS treatments were able to revert their toxic characterization. Moreover, the 25% DFS treatment showed to be a long-lasting stabilizing system, efficient at least during a leaching period equivalent to 10-year rainfall with a great Sb leaching reduction (close to 98%). After this long-term leaching process, DFS-treated mine wastes kept their non-hazardous and non-toxic characterization. The amorphous Fe (oxyhydr)oxides composing DFS were responsible for the important Sb removal capacity showed by this by-product. Thus, when DFS was applied to mine wastes mobile Sb was importantly fixed as non-desorbable Sb, showing also a considerable Sb removal capacity in presence of strong competing anions such as phosphate. The application of DFS as amendment presents a great potential to be used as a sustainable long-term stabilizing system of Sb mine wastes.

Reductive Sequestration of Cr(VI) and Immobilization of C during the Microbially Mediated Transformation of Ferrihydrite-Cr(VI)-Fulvic Acid Coprecipitates

Cr(VI) detoxification and organic matter (OM) stabilization are usually influenced by the biological transformation of iron (Fe) minerals; however, the underlying mechanisms of metal-reducing bacteria on the coupled kinetics of Fe minerals, Cr, and OM remain unclear. Here, the reductive sequestration of Cr(VI) and immobilization of fulvic acid (FA) during the microbially mediated phase transformation of ferrihydrite with varying Cr/Fe ratios were investigated. No phase transformation occurred until Cr(VI) was completely reduced, and the ferrihydrite transformation rate decreased as the Cr/Fe ratio increased. Microscopic analysis was uncovered, which revealed that the resulting Cr(III) was incorporated into the lattice structure of magnetite and goethite, whereas OM was mainly adsorbed on goethite and magnetite surfaces and located within pore spaces. Fine line scan profiles showed that OM adsorbed on the Fe mineral surface had a lower oxidation state than that within nanopores, and C adsorbed on the magnetite surface had the highest oxidation state. During reductive transformation, the immobilization of FA by Fe minerals was predominantly via surface complexation, and OM with highly aromatic and unsaturated structures and low H/C ratios was easily adsorbed by Fe minerals or decomposed by bacteria, whereas Cr/Fe ratios had little effect on the binding of Fe minerals and OM and the variations in OM components. Owing to the inhibition of crystalline Fe minerals and nanopore formation in the presence of Cr, Cr sequestration and C immobilization can be synchronously favored at low Cr/Fe ratios. These findings provide a profound theoretical basis for Cr detoxification and synchronous sequestration of Cr and C in anoxic soils and sediments.

Direct Atomic-Scale Insight into the Precipitation Formation at the Lanthanum Hydroxide Nanoparticle/Solution Interface

Understanding precipitation formation at lanthanum hydroxide (La(OH)₃) nanoparticle-solution interfaces plays a crucial role in catalysis, adsorption, and electrochemical energy storage applications. Liquid-phase transmission electron microscopy enables powerful visualization with high resolution. However, direct atomic-scale imaging of the interfacial metal (hydro)oxide nanostructure in solutions has been a major challenge due to their beam-driven dissolution. Combining focused ion beam and aberration-corrected high-angle annular dark-field scanning transmission electron microscopy, we present an atomic-scale study of precipitation formation at La(OH)₃ nanoparticle interfaces after reaction with phosphate. The structure transformation is observed to occur at high- and low-crystalline La(OH)₃ nanoparticle surfaces. Low-crystalline La(OH)₃ mostly transformed and high-crystalline ones partly converted to LaPO₄ precipitations on the outer surface. The long-term structure evolution shows the low transformation of high-crystalline La(OH)₃ nanoparticles to LaPO₄ precipitation. Because precipitation at solid-solution interfaces is common in nature and industry, these results could provide valuable references for their atomic-scale observation.

Mechanistic insights into the detoxification of Cr(VI) and immobilization of Cr and C during the biotransformation of ferrihydrite-polygalacturonic acid-Cr coprecipitates

Coupled reactions among chromium (Cr), organic matter (OM), and iron (Fe) minerals play significant roles in Cr and carbon (C) cycling in Cr-contaminated soils. Although the inhibitory effects of Cr or polysaccharides acid (PGA) on ferrihydrite transformation have been widely studied, mechanistic insights into detoxification of Cr(VI) and immobilization of Cr and C during the microbially mediated reductive transformation of ferrihydrite remain unclear. In this study, underlying sequestration mechanisms of Cr and C during dissimilatory Fe reduction at various Cr/Fe ratios were investigated. Solid-phase analysis showed that reductive transformation rates of ferrihydrite were impeded by high Cr/Fe ratio and more magnetite was found at low Cr loadings. Microscopic analysis showed that formed Cr(III) was immobilized by magnetite and goethite through isomorphous substitution, whereas PGA was adsorbed on the crystalline Fe mineral surface. Spectroscopic results uncovered that binding of Fe minerals and PGA was achieved by surface complexation of structural Fe with carboxyl functional groups, and that the adhesion order of PGA functional groups and Fe minerals was influenced by the Cr/Fe ratios. These findings have significant implications for remediating Cr contaminants, realizing C fixation, and developing a quantitative model for Cr and C cycling by coupling reductive transformation in Cr-contaminated environments.

Synchronous stabilization of As, Cd, and Pb in soil by sustained-release of iron-phosphate

Anionic arsenic (As) exhibits geochemical behavior opposite to those of cationic cadmium (Cd), and lead (Pb), which makes the synchronous remediation of As, Cd, and Pb challenging. The synchronous stabilization of As, Cd, and Pb to form Cd/Pb-phosphate and iron‑arsenic precipitates is a promising strategy. However, the effectiveness of soluble phosphate or iron-based materials is limited by the activation of Cd, Pb, or As, while low mobility hinders insoluble particles. In this study, we developed an amorphous structure that releases iron and phosphate at a sustained rate. Thus, the stabilization efficiencies of NaHCO₃-extractable As, DTPA-extractable Cd and Pb reached 44.6 %, 40.8 %, and 48.1 %, respectively. The proportion of residual fraction of As, Cd, and Pb increased by 12.1 %, 14.5 %, and 36.4 %, respectively, after 28 d. Ferrihydrite was chosen as the soil component to monitor the chemical behavior and speciation transformation of As, Cd, and Pb in the reaction. During the process, the released iron directly reacted with dissolved As to form iron‑arsenic precipitation and phosphate directly reacted with Cd/Pb to form Cd/Pb-phosphate precipitation. Simultaneously, phosphate replaced the adsorbed As and transformed into a dissolved state, which could be re-precipitated with the released iron ions. Thus, this study provides a reliable strategy for the remediation of As, Cd, and Pb combined pollution in soil.

Microbial Reduction of Antimony(V)-Bearing Ferrihydrite by Geobacter sulfurreducens

The reduction of Sb(V)-bearing ferrihydrite by Geobacter sulfurreducens was studied to determine the fate of the metalloid in Fe-rich systems undergoing redox transformations. Sb(V) added at a range of concentrations adsorbed readily to ferrihydrite, and the loadings had a pronounced impact on the rate and extent of Fe(III) reduction and the products formed. Magnetite dominated at low (0.5 and 1 mol%) Sb(V) concentrations, with crystallite sizes decreasing at higher Sb loadings: 37-, 25-, and 17-nm particles for no-Sb, 0.5% Sb, and 1% Sb samples, respectively. In contrast, goethite was the dominant end product for samples with higher antimony loadings (2 and 5 mol%), with increased goethite grain size in the 5% Sb sample. Inductively coupled mass spectrometry (ICP-MS) analysis confirmed that Sb was not released to solution during the bioreduction process, and X-ray photoelectron spectroscopy (XPS) analyses showed that no Sb(III) was formed throughout the experiments, confirming that the Fe(III)-reducing bacterium Geobacter sulfurreducens cannot reduce Sb(V) enzymatically or via biogenic Fe(II). These findings suggest that Fe (bio)minerals have a potential role in limiting antimony pollution in the environment, even when undergoing redox transformations. IMPORTANCE Antimony is an emerging contaminant that shares chemical characteristics with arsenic. Metal-reducing bacteria (such as Geobacter sulfurreducens) can cause the mobilization of arsenic from Fe(III) minerals under anaerobic conditions, causing widespread contamination of aquifers worldwide. This research explores whether metal-reducing bacteria can drive the mobilization of antimony under similar conditions. In this study, we show that G. sulfurreducens cannot reduce Sb(V) directly or cause Sb release during the bioreduction of the Fe(III) mineral ferrihydrite [although the sorbed Sb(V) did alter the Fe(II) mineral end products formed]. Overall, this study highlights the tight associations between Fe and Sb in environmental systems, suggesting that the microbial reduction of Fe(III)/Sb mineral assemblages may not lead to Sb release (in stark contrast to the mobilization of As in iron-rich systems) and offers potential Fe-based remediation options for Sb-contaminated environments.

Mechanisms of Arsenic and Antimony Co-sorption onto Jarosite: An X-ray Absorption Spectroscopic Study

Jarosite, a common mineral in acidic sulfur-rich environments, can strongly sorb both As(V) and Sb(V). However, little is known regarding the mechanisms that control simultaneous co-sorption of As(V) and Sb(V) to jarosite. We investigated the mechanisms controlling As(V) and Sb(V) sorption to jarosite at pH 3 (in dual and single metalloid treatments). Jarosite was found to sorb Sb(V) to a greater extent than As(V) in both single and dual metalloid treatments. Relative to single metalloid treatments, the dual presence of both As(V) and Sb(V) decreased the sorption of both metalloids by almost 50%. Antimony K-edge EXAFS spectroscopy revealed that surface precipitation of an Sb(V) oxide species was the predominant sorption mechanism for Sb(V). In contrast, As K-edge EXAFS spectroscopy showed that As(V) sorption occurred via bidentate corner-sharing complexes on the jarosite surface when Sb(V) was absent or present at low loadings or by formation of similar complexes on the Sb(V) oxide surface precipitate when Sb(V) was present at high loadings. These results point to a novel mechanism by which Sb(V) impacts the co-sorption of As(V). Overall, these findings highlight a strong contrast in the sorption mechanisms of Sb(V) versus As(V) to jarosite under acidic environmental conditions.

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.

Fate of antimony contamination generated by road traffic - A focus on Sb geochemistry and speciation in stormwater ponds

Although antimony (Sb) contamination has been documented in urban areas, knowledge gaps remain concerning the contributions of the different sources to the Sb urban biogeochemical cycle, including non-exhaust road traffic emissions, urban materials leaching/erosion and waste incineration. Additionally, details are lacking about Sb chemical forms involved in urban soils, sediments and water bodies. Here, with the aim to document the fate of metallic contaminants emitted through non-exhaust traffic emissions in urban aquatic systems, we studied trace element contamination, with a particular focus on Sb geochemistry, in three highway stormwater pond systems, standing as models of surface environments receiving road-water runoff. In all systems, differentiated on the basis of lead isotopic signatures, Sb shows the higher enrichment factor with respect to the geochemical background, up to 130, compared to other traffic-related inorganic contaminants (Co, Cr, Ni, Cu, Zn, Cd, Pb). Measurements of Sb isotopic composition (δ¹²³Sb) performed on solid samples, including air-exposed dusts and underwater sediments, show an average signature of 0.07 ± 0.05‰ (n = 25, all sites), close to the δ¹²³Sb value measured previously in certified reference material of road dust (BCR 723, δ¹²³Sb = 0.03 ± 0.05‰). Moreover, a fractionation of Sb isotopes is observed between solid and dissolved phases in one sample, which might result from Sb (bio)reduction and/or adsorption processes. SEM-EDXS investigations show the presence of discrete submicrometric particles concentrating Sb in all the systems, interpreted as friction residues of Sb-containing brake pads. Sb solid speciation determined by linear combination fitting of X-Ray Absorption Near Edge Structure (XANES) spectra at the Sb K-edge shows an important spatial variability in the ponds, with Sb chemical forms likely driven by local redox conditions: "dry" samples exposed to air exhibited contributions from Sb(V)-O (52% to 100%) and Sb(III)-O (<10% to 48%) species whereas only underwater samples, representative of suboxic/anoxic conditions, showed an additional contribution from Sb(III)-S (41% to 80%) species. Altogether, these results confirm the traffic emission as a specific source of Sb emission in surface environments. The spatial variations of Sb speciation observed along the road-to-pond continuum likely reflect a high geochemical reactivity, which could have important implications on Sb transfer properties in (sub)surface hydrosystems.

Antimony(V) Incorporation into Schwertmannite: Critical Insights on Antimony Retention in Acidic Environments

This study examines incorporation of Sb(V) into schwertmannite─an Fe(III) oxyhydroxysulfate mineral that can be an important Sb host phase in acidic environments. Schwertmannite was synthesized from solutions containing a range of Sb(V)/Fe(III) ratios, and the resulting solids were investigated using geochemical analysis, powder X-ray diffraction (XRD), dissolution kinetic experiments, and extended X-ray absorption fine structure (EXAFS) spectroscopy. Shell-fitting and wavelet transform analyses of Sb K-edge EXAFS data, together with congruent Sb and Fe release during schwertmannite dissolution, indicate that schwertmannite incorporates Sb(V) via heterovalent substitution for Fe(III). Elemental analysis combined with XRD and Fe K-edge EXAFS spectroscopy shows that schwertmannite can incorporate Sb(V) via this mechanism at up to about 8 mol % substitution when formed from solutions having Sb/Fe ratios ≤0.04 (higher ratios inhibit schwertmannite formation). Incorporation of Sb(V) into schwertmannite involves formation of edge and double-corner sharing linkages between Sb^VO₆ and Fe^III(O,OH)₆ octahedra which strongly stabilize schwertmannite against dissolution. This implies that Sb(V)-coprecipitated schwertmannite may represent a potential long-term sink for Sb in acidic environments.

Antimony release and volatilization from rice paddy soils: Field and microcosm study

The fate of antimony (Sb) in submerged soils and the impact of common agricultural practices (e.g., manuring) on Sb release and volatilization is understudied. We investigated porewater Sb release and volatilization in the field and laboratory for three rice paddy soils. In the field study, the porewater Sb concentration (up to 107.1 μg L^-1) was associated with iron (Fe) at two sites, and with pH, Fe, manganese (Mn), and sulfate (SO₄^2-) at one site. The surface water Sb concentrations (up to 495.3 ± 113.7 μg L^-1) were up to 99 times higher than the regulatory values indicating a potential risk to aquaculture and rice agriculture. For the first time, volatile Sb was detected in rice paddy fields using a validated quantitative method (18.1 ± 5.2 to 217.9 ± 160.7 mg ha^-1 y^-1). We also investigated the influence of two common rice agriculture practices (flooding and manuring) on Sb release and volatilization in a 56-day microcosm experiment using the same soils from the field campaign. Flooding induced an immediate, but temporary, Sb release into the porewater that declined with SO₄^2-, indicating that SO₄^2- reduction may reduce porewater Sb concentrations. A secondary Sb release, corresponding to Fe reduction in the porewater, was observed in some of the microcosms. Our results suggest flooding-induced Sb release into rice paddy porewaters is temporary but relevant. Manuring the soils did not impact the porewater Sb concentration but did enhance Sb volatilization. Volatile Sb (159.6 ± 108.4 to 2237.5 ± 679.7 ng kg^-1 y^-1) was detected in most of the treatments and was correlated with the surface water Sb concentration. Our study indicates that Sb volatilization could be occurring at the soil-water interface or directly in the surface water and highlights that future works should investigate this potentially relevant mechanism.

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.

Tooeleite Transformation and Coupled As(III) Mobilization Are Induced by Fe(II) under Anoxic, Circumneutral Conditions

Tooeleite [Fe^III₆(As^IIIO₃)₄SO₄(OH)₄.4H₂O] is an important As(III) host phase in diverse mining-impacted environments. Tooeleite has also received attention as a target phase for immobilizing As(III) in environmental and engineered settings. However, little is known regarding tooeleite's environmental stability, with no previous research examining the possible role of Fe(II) in inducing tooeleite transformation (as occurs for Fe(III) oxide minerals). We investigated shifts in solid-phase Fe and As speciation and associated As mobilization into the aqueous phase during exposure of tooeleite to aqueous Fe(II) under anoxic conditions at pH 4 to 8. Our results demonstrate that environmentally relevant concentrations of aqueous Fe(II) (i.e., 1 to 10 mM) induce significant mobilization of As(III) from tooeleite under near-neutral pH conditions, with greater As(III) mobilization occurring at higher pH. Extended X-ray absorption fine structure spectroscopy at both the As and Fe K-edge reveals that the observed As(III) mobilization was coupled with partial Fe(II)-induced transformation of tooeleite to As(III)-bearing ferrihydrite at pH 6 to 8. These results provide new insights into the environmental stability of tooeleite and demonstrate a novel pathway for As(III) mobilization in tooeleite-bearing systems.

Antimony isotope fractionation during adsorption on aluminum oxides

The environmental fate of antimony (Sb) is often strongly affected by adsorption, and the Sb isotope fractionation mechanism during adsorption has not been reported. Four batch experiments (kinetic, isothermal, effect of pH, and effect of coexisting anions) were conducted to evaluate the mechanism of Sb(V) adsorption to γ-Al₂O₃ and the fractionation of Sb isotopes. Extended X-ray absorption fine structure (EXAFS) analyses show Sb(V) adsorption on γ-Al₂O₃ occurs via outer-sphere surface complexation. The triple-layer model (TLM) effectively predicted the theoretical Sb adsorption amount under different pH conditions. The Sb isotope fractionation in the adsorption process can be divided into an initial kinetic stage (Rayleigh model, α_{adsorbed-aqueous} = 0.99975 ± 0.00003) and subsequent isotopic equilibrium stage due to isotope exchange; however, no significant equilibrium isotope fractionation (Δ¹²³Sb_{aqueous-adsorbed} = ~0 ± 0.08‰) was evident by the end of the experiments. We propose the lack of significant equilibrium isotope fractionation in the effect of pH and isothermal experiments is due to Sb forming an outer-sphere complex on γ-Al₂O_3. This study reveals Sb equilibrium isotope fractionation does not occur during Sb(V) adsorption onto γ-Al₂O₃, providing a reference for the future study of Sb isotopes and furthering understanding of the Sb isotope fractionation mechanism.

Reduction of antimony mobility from Sb-rich smelting slag by Shewanella oneidensis: Integrated biosorption and precipitation

The dissimilatory Fe(III)-reducing bacteria play a significant role in the mobility of antimony (Sb) under reducing environment. Sb-rich smelting slag is iron (Fe)-containing antimonic mine waste, which is one of the main sources of antimony pollution. In this study, the soluble antimony reacted with Fe(III) by S. oneidensis (Shewanella oneidensis strain MR-1) was performed in reduction condition, then the dissolution behavior of the Sb-rich smelting slag with S. oneidensis was investigated. The results showed that the released Sb was immobilized by S. oneidensis and the strain adsorbed Sb(III) preferentially. Sb(V) can be reduced by S. oneidensis without aqueous Fe. In the presence of Fe(III), S. oneidensis mediated Sb bio-adsorption and the chemical redox of Sb-Fe occurred simultaneously. Sb was co-precipitated with Fe to form the Sb(V)-O-Fe(III) secondary mineral, which was identified as the bidentate mononuclear edge-sharing structure by extended X-ray absorption fine structure (EXAFS) analysis. These results suggest that S. oneidensis has a positive effect on the immobilization and minimizing toxicity of antimony in anoxic soil and groundwater, which provides a theoretical basis for the treatment of antimony contamination.

Mechanistic Study for Antimony Adsorption and Precipitation on Hematite Facets

Heterogeneous reactions at the mineral-water interface are of paramount importance in controlling the transport of contaminants. Herein, antimony (Sb) adsorption and subsequent precipitation on Fe₂O₃ facets were explored to understand its partitioning mechanisms by multiple complementary techniques. Our extended X-ray absorption fine structure spectroscopy and density functional theory results provided a consensus on the local coordination environment of Sb(III) and Sb(V) on Fe₂O₃ facets. We observed that Sb adsorption and the following precipitation are associated with both Sb concentrations and Fe₂O₃ facets, and a change in the Sb surface-binding mode from edge-sharing to corner-sharing is preferred in precipitation. Fe₂O₃ facets determine Sb binding structures, resulting in a facet-dependent transformation of adsorption to precipitation. The preferred corner-sharing complexes on the {001} facet facilitated the formation of Sb₂O₃ and NaSb(OH)₆ precipitates at a lower Sb concentration compared with other two {110} and {214} facets. In addition, the facet-specific binding configuration renders a heterogeneous epitaxial growth of Sb₂O₃. Our study provides a molecular understanding of facet effects on Sb adsorption and precipitation on minerals.

Removal of Antimony(V) from Drinking Water Using nZVI/AC: Optimization of Batch and Fix Bed Conditions

Antimony (Sb) traces in water pose a serious threat to human health due to their negative effects. In this work, nanoscale zero-valent iron (Fe0) supported on activated carbon (nZVI) was employed for eliminating Sb(V) from the drinking water. To better understand the overall process, the effects of several experimental variables, including pH, dissolved oxygen (DO), coexisting ions, and adsorption kinetics on the removal of Sb(V) from the SW were investigated by employing fixed-bed column runs or batch-adsorption methods. A pH of 4.5 and 72 h of equilibrium time were found to be the ideal conditions for drinking water. The presence of phosphate (PO43-), silicate (SiO42-), chromate (CrO42-) and arsenate (AsO43-) significantly decreased the rate of Sb(V) removal, while humic acid and other anions exhibited a negligible effect. The capacity for Sb(V) uptake decreased from 6.665 to 2.433 mg when the flow rate was increased from 5 to 10 mL·min-1. The dynamic adsorption penetration curves of Sb(V) were 116.4% and 144.1% with the weak magnetic field (WMF) in fixed-bed column runs. Considering the removal rate of Sb(V), reusability, operability, no release of Sb(V) after being incorporated into the iron (hydr)oxides structure, it can be concluded that WMF coupled with ZVI would be an effective Sb(V) immobilization technology for drinking water.

Antimonate Controls Manganese(II)-Induced Transformation of Birnessite at a Circumneutral pH

Manganese (Mn) oxides, such as birnessite (δ-MnO₂), are ubiquitous mineral phases in soils and sediments that can interact strongly with antimony (Sb). The reaction between birnessite and aqueous Mn(II) can induce the formation of secondary Mn oxides. Here, we studied to what extent different loadings of antimonate (herein termed Sb(V)) sorbed to birnessite determine the products formed during Mn(II)-induced transformation (at pH 7.5) and corresponding changes in Sb behavior. In the presence of 10 mM Mn(II)_aq, low Sb(V)_aq (10 μmol L^-1) triggered the transformation of birnessite to a feitknechtite (β-Mn(III)OOH) intermediary phase within 1 day, which further transformed into manganite (γ-Mn(III)OOH) over 30 days. Medium and high concentrations of Sb(V)_aq (200 and 600 μmol L^-1, respectively) led to the formation of manganite, hausmannite (Mn(II)Mn(III)₂O₄), and groutite (αMn(III)OOH). The reaction of Mn(II) with birnessite enhanced Sb(V)_aq removal compared to Mn(II)-free treatments. Antimony K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy revealed that heterovalent substitution of Sb(V) for Mn(III) occurred within the secondary Mn oxides, which formed via the Mn(II)-induced transformation of Sb(V)-sorbed birnessite. Overall, Sb(V) strongly influenced the products of the Mn(II)-induced transformation of birnessite, which in turn attenuated Sb mobility via incorporation of Sb(V) within the secondary Mn oxide phases.