Antimony (Sb) is a priority environmental pollutant, with mobility and fate that are tightly linked to sorption reactions with iron(III) (oxyhydr)oxide minerals. However, the impact of particle size and co-existing cations (e. g., Ni2+, a representative divalent cation commonly present in Sb-mining-impacted environments) on Sb(V) adsorption capacity and retention mechanisms remains poorly understood. In this study, we synthesized goethite and hematite of varying particle sizes, performed batch Sb(V) adsorption and Ni2+-Sb(V) co-sorption experiments, and characterized samples using micro X-ray diffraction (mu-XRD), scanning and transmission electron microscopy (SEM/TEM), 57Fe Mo & uml;ssbauer spectroscopy, and extended X-ray absorption fine structure (EXAFS) spectroscopy. Our findings reveal that (i) Sb(V) adsorption capacities ranged from 2.5 to 55.7 mg/g, with higher affinities observed for smaller particles and for goethite relative to hematite, even after accounting for surface area, (ii) the presence of Ni2+ markedly enhanced Sb(V) adsorption onto both goethite and hematite, with adsorption capacities increasing by 16-89% as particle size decreased, and (iii) the retention mechanism for Sb (V) adsorption changed from being dominated by edge-sharing inner-sphere complexes to enhanced electrostatic interactions and the likely formation of ternary surface complexes in the presence of Ni2+. These findings demonstrate that nanoparticulate goethite and hematite minerals possess even higher potential for Sb(V) removal in complex multi-pollutant systems than previously suggested. Moreover, the dominance of inner-sphere binding and the likely formation of ternary surface complexes suggest that the adsorbed Sb(V) is relatively stable, favoring long-term sequestration in Fe-rich environments.

Antimony (Sb) is a toxic environmental contaminant, whose environmental behaviour is tightly linked to sorption or co-precipitation reactions with iron(III) (oxyhydr)oxides. These nanoparticulate minerals are susceptible to reductive dissolution via the activity of dissimilatory Fe(III)-reducing microorganisms (DIRB), thereby potentially mobilizing associated Sb. However, the impacts of Sb (generally in the form of Sb(V)) associated with iron(III) (oxyhydr)oxides of different particle sizes on microbial Fe(III) reduction, as well as on subsequent Sb speciation and distribution, remain poorly understood. In this study, we synthesized Sb(V)-containing goethite and hematite of varying particle sizes and characterized them using micro X-ray diffraction (& micro;-XRD), scanning/transmission electron microscopy (S/TEM), Fe-57 M & ouml;ssbauer spectroscopy, extended X-ray absorption fine structure (EXAFS) spectroscopy, and wet chemical extractions. We then examined their reductive dissolution by Shewanella oneidensis MR-1. Our findings reveal that (i) Sb(V) was incorporated into nanoscale iron(III) (oxyhydr)oxides to varying extents (Sb: Fe ratios ranging from 2:100 to 8:100), with no clear correlation between the extent of coprecipitation and mineral type or particle size. (ii) The presence of Sb(V) significantly inhibited both the extent and kinetics of microbial Fe(III) reduction across all samples of varying particle sizes, with suppression extent ranging from 25 to 80%. (iii) Sb release was limited (<10% of total Sb for goethite and <4% of total Sb for hematite), exhibited incongruence with the reductive dissolution of Fe, and showed only negligible Sb(V) reduction to Sb(III) across all sample conditions. These collective results indicate that the incorporation of Sb(V) into goethite and hematite may enhance the stability of these iron(III) (oxyhydr)oxides and serve as an effective sink for Sb immobilization. This stabilization persists even under reducing conditions, highlighting the potential for long-term Sb sequestration in Fe-rich environments.

Sulfidized nano zero-valent iron (S-nZVI) particles are known to stimulate the reductive removal of various oxyanions due to enhanced electron selectivity and electron conductivity between the Fe(0) core and the target compound. Sulfidation creates a number of reactive sulfur species, the role of which has not yet been investigated in the context of S-nZVI. In this study, we investigated the contribution of reactive sulfur species to Se(VI) reduction by S-nZVI at different molar S/Fe ratios (0, 0.1 and 0.6) and Se(VI) concentrations (0, 5 and 50 mg L-1). In the presence of S-nZVI, the rate of reduction was accelerated by a factor of up to ten. X-ray Absorption Near-Edge Structure (XANES) spectroscopy and surface-sensitive X-ray photoelectron spectroscopy (XPS) identified Se(0) as the predominant reduction product (similar to 90%). The reduction reaction was accompanied by a loss of FeS and the formation of surface-bound Fe(II) polysulfide (FeSx) and S(0) species. Likewise, wet chemical extraction techniques suggested a direct involvement of acid volatile sulfide (AVS) species (surface-bound FeS) in the reduction of Se(IV) to Se(0) and formation of S(0). Mass balance estimates reveal that between 9 and 15% of the conversion of Se(0) originates from oxidation of FeS to FeSx. From these findings, we propose that surface-bound Fe sulfide species are important but previously overlooked reactants contributing to the reduction of oxyanions associated with S-nZVI particles, as well as in natural environments undergoing sulfidation reactions.

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.

Schwertmannite is a metastable sulfate-rich ferric iron Fe(III) (oxyhydr)oxide and a common mineral in acid mine drainage sites and acid sulfate soils. Schwertmannite is also used as a sorbent in various industrial applications, including phosphate removal in water treatment and environmental remediation. Phosphate sorption to schwertmannite, however, is complex and likely involves ligand exchange for inner- and outer-spherically coordinated sulfate groups, both on the surface and in the tunnel structure of the mineral. Here, we investigated phosphate sorption, concomitant sulfate release and their impact on the structure of schwertmannite as a function of pH and phosphate concentration. Kinetic and equilibrium batch experiments with synthetic schwertmannite were carried out at pH 3, 6, and 8, and the solid-phase was analyzed using a combination of microscopic, spectroscopic, and X-ray diffraction techniques. We found a strong correlation between phosphate sorption and sulfate release, with both following a two-step sorption model. The kinetics of phosphate sorption and sulfate release were faster at higher pH levels. Maximum phosphate sorption was found at pH 6 (1.7 mmol PO 4 3- g -1 ), which decreased to 1.5 and 1.2 mmol PO 43- g -1 at pH 3 and 8, respectively. Fourier transform infrared spectroscopy revealed a shift from inner- to outer-spherical coordination of sulfate with increasing pH. 57 Fe- Mo ssbauer analyses of schwertmannite demonstrated an initial transformation of schwertmannite at neutral to alkaline pH, characterized by a rise in a partially ordered sextet area. This change was interpreted as an increase in crystallinity resulting from the transition from Fe-SO 4 to Fe - O domains. The emerging phase differed from the original schwertmannite, but did not represent a complete change to a new crystalline phase, thus indicating a proto-transformation of schwertmannite. This pH-induced proto-transformation was inhibited in the presence of phosphate. We concluded that the phosphate sorption rate and maximum as well as the proto-transformation of schwertmannite were strongly affected by the mineral ' s affinity for sulfate. Sorption to schwertmannite should primarily be regarded as a competitive exchange reaction between the sorbing oxyanion and the bound sulfate. As a result, the highest phosphate sorption occurred at circumneutral pH, in stark contrast to non-sulfatecontaining Fe(III) (oxyhydr)oxides, where phosphate sorption is highest at acidic pH. Our findings are important for a fundamental understanding of the sorption properties of schwertmannite in phosphate-rich environments as they point towards the central role of sulfate coordination for phosphate immobilization.

Arctic soils are the largest pool of organic carbon compared with other soils globally and serve as a main source for greenhouse gases, especially in the course of the predicted future temperature increase. With increasing temperatures, substantial thawing of the permafrost layer of soils is expected, altering the availability of calcium in those soils, with an increase by ∼5 mg Ca g-1 DW predicted for Alaska. Here we show for two representative soils in Alaska (initially Ca-poor or Ca-rich) that this increase in Ca availability will lead to decreases in CO2 release by 50% and 57%. It is already well-known that the cation bridging of Ca ions to organic carbon renders this carbon unavailable for microbial respiration and that Ca is altering the transformation of Corg by microbes. Here we show that the decrease of the soil CO2 release may be also due to enhanced aragonite formation (by 300% for Ca-poor and 90-200% for Ca-rich soils), as revealed by synchrotron-based scanning transmission X-ray microscopy. We therefore call upon field experiments for validation of this process and inclusion of this process in global and local carbon budget models.

Schwertmannite is a nanocrystalline ferric oxyhydroxysulfate mineral with a variable chemical composition, simplified as Fe 8 O 8 (OH) 8-2x (SO 4 ) x (with 1≤ x ≤ 1.75).Although there is still some controversy with regard to its actual structure, schwertmannite is typically described as an akaganeite-like framework of iron(III) octahedra with a tunnel structure that hosts inner-and outer-spherical complexes of sulfate.Due to its high specific surface area and the ability to incorporate oxyanions into the tunnel cavities, schwertmannite has recently been proposed as a promising sorbent for environmental remediation technologies to immobilize phosphate.Phosphate sorption to schwertmannite is likely to occur via ligand exchange with surface and tunnel sulfate, resulting in the release of sulfate.The importance of phosphate interaction with sulfate groups and its consequences for the structure of schwertmannite are still poorly understood, hampering the use of schwertmannite as a sorbent in natural systems.Here, we investigated phosphate sorption, concomitant sulfate release and their impact on the structure of schwertmannite as a function of pH and phosphate loading.Kinetic and equilibrium batch experiments with synthetic schwertmannite were performed at pH 3, 6, and 8, and solid-phase samples of unreacted and phosphate-sorbed schwertmannite were analyzed using a combination of diffractometric, spectroscopic, and microscopic techniques.Phosphate sorption to schwertmannite resulted in the substitution of up to ~90% of all solid-phase sulfate.Alkaline conditions were more favorable for phosphate sorption despite an increase in electrostatic repulsion, which we attributed to sulfate groups being more loosely (outer-spherically) bound at higher pH.Diffractometric and scanning electron microscopic analyses suggest no transformation to other mineral phases and no morphological changes during equilibration with phosphate.However, infrared spectra showed a broadening of the sulfate vibrational bands with increasing pH, confirming a change in sulfate speciation with pH. 57 Mössbauer showed that the solidphase did not undergo major mineralogical changes, but in the presence of phosphate, the proportion of partially ordered schwertmannite decreased, indicating a loss in crystallinity.Our results are important for a fundamental understanding of the sorption properties of schwertmannite in phosphate-rich environments as they point towards the central role of sulfate coordination for phosphate immobilization by schwertmannite.

Arctic permafrost soils store large amounts of organic carbon and nutrients. With deepening of the perennial thawing upper active layer due to rising temperatures in the Arctic, not only the mobility of organic matter (OM), but also those of elements like silicon (Si) or calcium (Ca) may increase. It is known that major elements like Si and Ca can affect mineralization rates of OM, consequently influencing the carbon cycle. But only little is known about the interactions of Si and Ca with inorganic nutrients like iron (Fe) or potentially toxic elements like aluminum (Al) in Artic soils. In this study, we analyzed the effect of Si and Ca fertilization in laboratory incubation experiments with soil samples from several Arctic regions. Our results show a significant increase in Fe and Al mobility (Mehlich-3 extractable) after increasing Si. Using high resolution X-ray microscopy (STXM/NEXAFS), we show that Si promotes Fe(II) phases and by this increases Fe mobility. Al mobility was increased for acidic and neutral pH soils but decreased for alkaline soils after increasing Si. Furthermore, we show a decreased Al mobility after increasing Ca, independent on the original pH values and the OM content of the soils. These results demonstrate the importance of interactions between Si and Ca on one hand and Fe and Al mobility on the other hand for Arctic soils.

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.

The environmental mobility of antimony (Sb) is controlled by interactions with iron (Fe) oxides, such as ferrihydrite. Under near-neutral pH conditions, Fe(II) catalyzes the transformation of ferrihydrite to more stable phases, thereby potentially altering the partitioning and speciation of associated Sb. Although largely unexplored, Sb itself may also influence ferrihydrite transformation pathways. Here, we investigated the impact of Sb on the Fe(II)-induced transformation of ferrihydrite at pH 7 across a range of Sb(V) loadings (Sb:Fe(III) molar ratios of 0, 0.003, 0.016, and 0.08). At low and medium Sb loadings, Fe(II) induced rapid transformation of ferrihydrite to goethite, with some lepidocrocite forming as an intermediate phase. In contrast, the highest Sb:Fe(III) ratio inhibited lepidocrocite formation, decreased the extent of goethite formation, and instead resulted in substantial formation of feroxyhyte, a rarely reported FeOOH polymorph. At all Sb loadings, the transformation of ferrihydrite was paralleled by a decrease in aqueous and phosphate-extractable Sb concentrations. Extended X-ray absorption fine structure spectroscopy showed that this Sb immobilization was attributable to incorporation of Sb into Fe(III) octahedral sites of the neo-formed minerals. Our results suggest that Fe oxide transformation pathways in Sb-contaminated systems may strongly differ from the well-known pathways under Sb-free conditions.

The environmental mobility and bioavailability of antimony (Sb) are strongly influenced by sorption to Fe(III) oxide minerals, such as goethite (alpha FeOOH). Exposure to aqueous Fe(II), as occurs, for example, under reducing conditions in soils and sediments, catalyzes the recrystallization of goethite. Herein, we examine, for the first time, the effect of Fe(II)-catalyzed goethite recrystallization on the sorption of co-associated Sb(V). The use of an enriched aqueous Fe-57(II) tracer provided direct evidence of Fe(II)-catalyzed goethite recrystallization and revealed lower levels of recrystallization at higher Sb(V) loadings. Goethite recrystallization caused increases in the total amount of sorbed Sb along with decreases in the percentage of sorbed Sb that was surface-bound (based on PO(4)3extractions). Extended X-ray absorption fine structure (EXAFS) spectroscopy at the Sb K-edge shows that goethite recrystallization led to increases in the coordination number (CN) for both edge-sharing and double-corner-sharing linkages between Sb(V)-O and Fe(III)-O octahedra. The CN increases are consistent with partial structural Sb(V) incorporation into goethite, with the degree of incorporation increasing with the extent of recrystallization. Overall, the results indicate that Fe(II)-catalyzed recrystallization of goethite caused a shift from Sb(V) adsorption at the goethite surface to partial Sb(V) incorporation into the goethite structure [via heterovalent substitution for up to similar to 1.4 mol % Fe(III)]. These results necessitate a re-evaluation of current concepts regarding the sorption of Sb to goethite. In particular, goethite should be considered as a dynamic phase whose solid structure (not just the reactive surface) may be available for Sb(V) sorption via structural incorporation during Fe(II)-catalyzed recrystallization.

Iron (Fe) oxides are important host phases for antimony (Sb), a toxic metalloid of environmental concern. In wetland soils and sediments, poorly ordered Fe oxides such as ferrihydrite may undergo reductive dissolution and mineralogical transfor- mation upon reaction with dissolved sulfide (S(-II)). The consequences of these processes for the mobility of associated Sb have not been investigated to date. Here, we allowed Sb(V)-bearing ferrihydrite (molar ratio of Fe:Sb = 400) to react with varying S(-II) concentrations (Fe(III):S(-II) = 0.2, 0.5, and 1) at pH 6 and 8 over 32 days. Changes in speciation and concen- tration of Fe, S and Sb in the aqueous, colloidal and solid phase were examined through a combination of aqueous -phase analyses, X-ray diffraction and synchrotron X-ray absorption spectroscopy. Addition of S(-II) caused rapid reduction of Fe(III), thereby producing elemental S and Fe(II). X-ray diffractometry and Fe K -edge extended X-ray absorption fine struc- ture (EXAFS) spectroscopy revealed that S(-II) addition resulted in the precipitation of Fe(II) sulfides (mackinawite (FeS) and pyrite (FeS2 )) and formation of secondary Fe(III) oxides (goethite (FeOOH) and hematite (Fe2O3 )). The formation of mackinawite and pyrite was further confirmed by S K -edge X-ray absorption near -edge structure (XANES) spectroscopy, and was found to occur to the greatest extent in the high -sulfide treatments. The initial reductive dissolution of ferrihydrite was paralleled by a fast increase in dissolved Sb concentrations, with similar to 25% of total Sb being released in the high -sulfide treat- ments. The Sb release was followed by Sb immobilization within similar to 1-7 days. Since ion -chromatography ICP-MS revealed antimonate (Sb(OH)(6)(-) ) as the primary Sb aqueous phase species throughout the experiment, with only negligible concentra- tions of antimonite (Sb(OH)(3)) and only very minor amounts (<4% of total Sb) of tri- and tetrathioantimonate (HxSbS3Ox-3 / HxSbS4x-3 ), the decrease in Sb concentrations was attributed to surface -based sorption and structural incorporation of primar- ily Sb(V) by the secondary Fe oxides. In accordance with the dominance of aqueous Sb(V), Sb K -edge XANES spectroscopy showed that Sb(V) was also the dominant Sb species in the solid phase, comprising up to 90% of solid -phase Sb in the low - sulfide treatments. However, higher S(-II) addition and lower pH favored production of Sb(III) and resulted in up to 40% and 20% of solid -phase Sb(V) being reduced to Sb(III) at pH 6 and 8, respectively, with this Sb(III) comprising a mixture of O - and S -coordinated species. Around 15% of 0.45 - mu m Sb occurred in the colloidal (3 kDa) size fraction at pH 8 under med- ium and high S(-II) conditions, while no colloidal Sb was found in other treatments. Together, these results show that Fe oxide sulfidization can have opposing effects on Sb mobility. On the one hand, the initial sulfide -promoted Fe oxide dissolu- tion triggers Sb release into the aqueous phase. On the other hand, Sb can subsequently be immobilized via sorption to sec- ondary Fe oxides and newly -formed Fe sulfides during the later stages of sulfidization. Sulfidization reactions, and the complex opposing impacts on Sb mobility, should therefore be considered for the risk assessment and derivation of adequate management strategies at Sb-impacted sites which experience sulfidic conditions. (C) 2020 Elsevier Ltd. All rights reserved.

The Madeay River in eastern Australia is severely impacted by historic stibnite- and arsenopyrite-rich minetailings. We explore the partitioning, speciation, redox-cycling, mineral associations and mobility of antimony and arsenic along >70 km reach of the upper Macleay River. Elevated Sb/As occur throughout the active channel-zone and in floodplain pockets up to the regolith margin, indicating broad dispersal during floods. Sb concentrations in bulk-sediments decay exponentially downstream more efficiently than As, likely reflecting sediment dilution, hydraulic sorting and comparatively greater leaching of (more mobile) Sb(V) species. However, Sb in bulk-sediments becomes proportionally more bio-available downstream. Sb(V) and As(V) species dominate stream fine-grained (<180 mu m) bulk-sediments, reflecting oxidative weathering downstream. Increasing poorly-crystalline Fe(III) [Fe(III)(HCl)] in bulk-sediments also indicates progressive oxidative weathering of Fe (II)-bearing minerals downstream and significant (P < .05) correlations exist between PO4-3-exchangeable As and Sb fractions and Fe(III)(HCl). Accumulations of poorly-crystalline Fe(III) precipitates (mainly ferrihydrite/ feroxyhyte) occur intermittently in hyporheic-zone seeps and are enriched in As relative to Sb and contain some As(III) and Sb(III) (similar to 30-40%). There is dynamic in-stream redox-cycling of both Sb and As, with localised S-coordinated As and Sb species re-forming in organic-rich, hyporheic sediments subject to contemporary sulfidogenesis. Sb [mainly Sb(V)] is comparatively more mobile in hyporheic and surface waters under oxic conditions, whereas As [mainly As(III)] is more mobile in hyporheic porewaters subject to reducing/sulfidogenic conditions. Repeat water-leaching of bulk-sediments confirms that Sb is proportionally more mobile than As. Mean concentrations of Sb in river water 168 km downstream from the mine are significantly (P < .05) higher than As, while K-d data indicate Sb is more strongly partitioned to the aqueous phase than As. Although the (mainly) oxic flow path of this river favours aqueous Sb mobility compared to As, localised redox-driven shifts in spedation of both elements strongly influence their respective mobility and partitioning. (C) 2020 Elsevier B.V. All rights reserved.