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Antimony, as the Sb(V) species, often occurs in oxic soils and sediments as coprecipitates with poorly-crystalline Fe(III)-bearing minerals. It is common for these Sb(V)-Fe(III) coprecipitates to also contain varying quantities of co-occurring humic acid (HA). When exposed to reducing conditions, the production of Fe(II) may cause the initial metastable HA-Sb(V)-Fe(III) phases to undergo rapid transformations to more stable phases, thereby potentially influencing the geochemical behavior of coprecipitated Sb(V). However, little is known about the impacts of this transformation on the mobility and speciation of Sb. In this study, we reacted synthetic HA-Sb(V)-Fe(III) coprecipitates (Fe:Sb ratio = 4, and C:Fe molar ratios = 0, 0.3, 0.8 and 1.3) with 0, 1 or 10 mM Fe(II) under O 2 -free conditions at pH 7.0 for 15 days. Fe K-edge EXAFS spectroscopy revealed that solid-phase Fe(III) in the initial coprecipitates contained a mixture of ∼4/5 ferrihydrite (Fe 10 O 14 (OH) 2 ) and ∼1/5 tripuhyite (FeSbO 4 ), regardless of the corresponding amount of coprecipitated HA. Tripuhyite persisted throughout the full experiment duration, while ferrihydrite was partially replaced by goethite (FeOOH) when either 1 or 10 mM Fe(II) aq was added to the coprecipitates. The greatest level of goethite formation (∼55% of solid-phase Fe) was observed in the HA-free/10 mM Fe(II) aq treatment, with ferrihydrite transformation being partially attenuated at higher levels of HA. Mobilisation of aqueous Sb was the greatest for 1 mM Fe(II) treatments at high HA:Fe ratios. Sb K-edge XANES spectroscopy showed that the largest reduction of Sb(V) to Sb(III) (∼37%) and the greatest repartitioning of Sb to the mineral surface (∼7.9-9.8%) occurred in the coprecipitates with the highest HA contents in the presence of 10 mM Fe(II). The results indicate that the amount of HA in HA-Sb(V)-Fe(III) coprecipitates can greatly influence mobility and speciation of Sb in Fe(II)-rich conditions. The results of this study provide new insights into alterations in Sb mobility and retention in response to Fe cycling under organic matter-rich reducing conditions., (Copyright © 2019 Elsevier Ltd. All rights reserved.)

Jarosite [KFe 3 (SO 4 ) 2 (OH) 6 ] is considered a potent scavenger for arsenic (As) and antimony (Sb) under oxidizing conditions. Fluctuations in water levels in re-flooded acid sulfate soils (ASS) can lead to high Fe 2+ (aq) concentrations (∼10-20 mM) in the soil solution under acidic to circumneutral pH conditions. This may create favorable conditions for the Fe 2+ -induced transformation of jarosite. In this study, synthetic arsenate [As(V)]/antimonate [Sb(V)]-bearing jarosite was subjected to Fe 2+ (aq) (20 mM) at pH 4.0 and 5.5 for 24 h to simulate the pH and Fe 2+ (aq) conditions of re-flooded freshwater ASS/acid mine drainage (AMD)-affected environments at early and mid-stages of remediation, respectively. The addition of Fe 2+ at pH 5.5 resulted in the formation of a metastable green rust sulfate (GR- SO 4 ) phase within ∼60 min, which was replaced by goethite within 24 h. In contrast, at pH 4.0, jarosite underwent no significant mineralogical transformation. Although the addition of Fe 2+ (aq) induced the dissolution/transformation of jarosite at pH 5.5 and increased the mobility of Sb during the initial stages of the experiment (Sb (aq) = ∼0.05 μmol L -1 ), formation of metastable green rust (GR-SO 4 ) and subsequent transformation to goethite effectively sequestered dissolved Sb. Aqueous concentrations of As remained negligible in both pH treatments, with As being mostly repartitioned to the labile (∼10%) and poorly crystalline Fe(III)-associated phases (∼10-30%). The results imply that, under moderately acidic conditions (i.e. pH 5.5), reaction of Fe 2+ (aq) with jarosite can drive the dissolution of jarosite and increase Sb mobility prior to the formation of GR-SO 4 and goethite. In addition, repartitioning of As to the labile fractions at pH 5.5 may enhance the risk of its mobilisation during future mineral transformation processes in Fe 2+ -rich systems., (Copyright © 2017 Elsevier Ltd. All rights reserved.)

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 SbVO 6 and FeIIIO 6 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 PO 4 3-. 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. [Display omitted] • Sb(V) sorption by schwertmannite was investigated under acid sulfate conditions. • Sorption involved incorporation of Sb(V) into the schwertmannite structure. • Sb(V)-sorbed schwertmannite showed resistance to desorption and dissolution. • Schwertmannite may be a secure host-phase for Sb(V) in acid sulfate environments. [ABSTRACT FROM AUTHOR]

Arsenic (As) and antimony (Sb) speciation and mobility in river systems can be influenced by the redox conditions of benthic and hyporheic zone sediments. However, our understanding of how temperature fluctuations influence riverine As and Sb mobility over diel and seasonal time frames, via moderation of sediment redox conditions, is poorly constrained. Here we compare diel and seasonal water quality data from a river system contaminated with As and Sb with results from sediment‐water incubations conducted at temperatures spanning 8°C to 32°C under low‐dissolved oxygen conditions. Higher incubation temperatures caused faster microbial respiration, lower redox potential, and greater Asaq concentrations, coincident with reduction of As(V), Mn (III/IV), Fe (III), and SO42−. Initial rates of As3+aq formation increased exponentially with temperature (Q10 = 3.3 ± 0.5) and were positively correlated with microbial respiration (r2 = 0.99, P < 0.01). In contrast, Sbaq displayed an initially negative and comparatively weak overall temperature dependence during incubations. In river waters, diel mobilization of reduced species (As3+aq and Fe2+aq) and contrasting attenuation of Sb was coincident with lower redox potential during nightly respiration cycles. Distinct seasonal oscillations in river water Asaq were exponentially correlated with temperature (r2 = 0.68, P < 0.01), whereas Sbaq was not. These characteristics reflect the fundamental role of anaerobic metabolism (as influenced by temperature) within benthic and hyporheic zone sediments as a key driver of contrasting riverine As and Sb mobility. The findings imply that riverine As mobility may be enhanced, while Sb mobility possibly attenuated, by eutrophication or by elevated stream temperatures due to a warming climate. Key Points: Riverine arsenic mobility increases at warmer temperatures due to enhanced anaerobic metabolism in benthic sedimentsTemperature is not a key driver of riverine Sb mobilityA warming climate and higher seasonal temperature extremes may enhance riverine As mobility [ABSTRACT FROM AUTHOR]

The Fe(II)-induced transformation of ferrihydrite, a potent scavenger for antimony (Sb), can considerably influence Sb mobility in reducing soils, sediments and groundwater systems. In these environments, humic acids (HA) are prevalent, yet their influence on Sb behaviour during ferrihydrite transformation is poorly understood. In this study, we investigated the effect of HA on (1) Sb partitioning between solid, colloidal and dissolved phases and (2) Sb redox speciation during the Fe(II)-induced transformation of Sb(V)-bearing ferrihydrite at pH 6.0 and 8.0 and Fe(II) concentrations of 0, 1 and 10 mM. The results show that, at pH 8.0 and in the presence of 10 mM Fe(II), ferrihydrite was replaced by goethite, lepidocrocite and magnetite across a wide range of HA concentrations. At pH 6.0 in the 10 mM Fe(II) treatments, ferrihydrite transformed to mainly lepidocrocite and goethite in both HA-free and low HA treatments. In contrast, high HA concentrations retarded the rate and extent of ferrihydrite transformation at both pH 6.0 and 8.0 in the 1 mM Fe(II) treatments. Antimony K-edge XANES spectroscopy revealed up to 60% reduction of solid-phase Sb(V) to Sb(III), which corresponded with an increase in the PO 4 3−-extractable fraction of solid-phase Sb in HA- and Fe(II)-rich conditions at pH 8.0. In contrast to the observations at pH 8.0, minimal reduction of solid-phase Sb(V) was observed in the pH 6.0 treatments with the highest HA content, yet some reduction of Sb(V) occurred (~30–40%) at intermediate HA concentrations. Humic acid-rich conditions were also found to promote the formation of substantial amounts of colloidal Sb in the <0.45 μm to 3 kDa size range at both pH 6.0 and 8.0. Our results demonstrate that HA can exert an important control on the partitioning, mobility and speciation of Sb during Fe(II)-induced transformation of ferrihydrite in sub-surface environments. Unlabelled Image • The effect of HA on Sb behaviour during Fe(II)-induced ferrihydrite transformation was investigated • Ferrihydrite underwent fast transformation in Fe(II)- and HA-rich treatments at pH 6.0 and 8.0. • In low Fe(II) or Fe(II)-free treatments HA hindered the transformation of ferrihydrite. • In HA-rich treatments transformation of Sb(V)-ferrihydrite generated substantial Sb(III). • HA promoted the formation of substantial amounts of colloidal Sb. [ABSTRACT FROM AUTHOR]

Jarosite can be an important scavenger for arsenic (As) and antimony (Sb) in acid mine drainage (AMD) and acid sulfate soil (ASS) environments. When subjected to reducing conditions, jarosite may undergo reductive dissolution, thereby releasing As, Sb, and Fe2+ coincident with a rise in pH. These conditions can also trigger the Fe2+-induced transformation of jarosite to more stable Fe(III) minerals, such as goethite. However, the consequences of this transformation process for As and Sb are yet to be methodically examined. We explore the effects of abiotic Fe2+-induced transformation of jarosite on the mobility, speciation, and partitioning of associated As(V) and Sb(V) under anoxic conditions at pH 7. High concentrations of Fe2+ (10 and 20 mM) rapidly (<10 min) transformed jarosite to a green rust intermediary, prior to the subsequent precipitation of goethite within 24 h. In contrast, lower concentrations of Fe2+ (1 and 5 mM) led to the formation of lepidocrocite. As K-edge XANES spectroscopy revealed some reduction of As(V) to As(III) at higher concentrations of Fe2+, while Sb L1-edge XANES spectroscopy indicated no reduction of Sb(V). The transformation processes enhanced Sb mobilization into the aqueous phase, while As was instead repartitioned to a surface-bound exchangeable phase. The results imply that Fe2+-induced transformation of As/Sb-jarosite can increase Sb mobility and exert major influences on As partitioning and speciation. [ABSTRACT FROM AUTHOR]

Manganese (Mn) oxide minerals, such as birnessite, are thought to play an important role in affecting the mobility and fate of antimony (Sb) in the environment. In this study, we investigate Sb partitioning and speciation during anoxic incubation of Sb(V)-coprecipitated birnessite in the presence and absence of Mn(II) aq at pH 5.5 and 7.5. Antimony K-edge XANES spectroscopy revealed that Sb(V) persisted as the only solid-phase Sb species for all experimental treatments. Manganese K-edge EXAFS and XRD results showed that, in the absence of Mn(II), the Sb(V)-bearing birnessite underwent no detectable mineralogical transformation during 7 days. In contrast, the addition of 10 mM Mn(II) at pH 7.5 induced relatively rapid (within 24 h) transformation of birnessite to manganite (~93%) and hausmannite (~7%). Importantly, no detectable Sb was measured in the aqueous phase for this treatment (compared with up to ∼90 μmol L−1 Sb in the corresponding Mn(II)-free treatment). At pH 5.5 , birnessite reacted with 10 mM Mn(II) aq displayed no detectable mineralogical transformation, yet had substantially increased Sb retention in the solid phase, relative to the corresponding Mn(II)-free treatment. These findings suggest that the Mn(II)-induced transformation and recrystallization of birnessite can exert an important control on the mobility of co-associated Sb. ga1 • Sb(V)-birnessite underwent no detectable transformation in the absence of Mn(II). • Mn(II) caused transformation of Sb(V)-birnessite to manganite and hausmannite at pH 7.5. • Transformation of birnessite immobilised Sb at pH 5.5 and 7.5. • Sb(V) was the only detectable solid-phase Sb species for all experimental treatments. [ABSTRACT FROM AUTHOR]

Abstract Antimony is an environmental contaminant, whose mobility in soils, sediments and groundwater systems is strongly influenced by interactions with Fe(III) oxide minerals. When exposed to reducing conditions, these minerals can undergo reductive dissolution via the activity of Fe(III)-reducing microorganisms, thereby potentially liberating previously retained Sb. In addition, microbial Fe(III)-reduction and the consequent production of Fe(II) can induce the formation of secondary Fe(III)- and Fe(II)-bearing minerals, which may alter the speciation and partitioning of Sb. In this study, we examined Sb behaviour during (1) the microbially-mediated reduction and transformation of Sb(V)-bearing ferrihydrite by the dissimilatory Fe(III)-reducing bacterium, Shewanella putrefaciens (strain CN32), and (2) during the associated abiotic Fe(II)-catalyzed transformation of Sb(V)-bearing ferrihydrite. Antimony K-edge XANES spectroscopy showed negligible reduction of Sb(V) to Sb(III) in both experiments, reflecting the redox stability of Sb(V) under these conditions. X-ray diffraction and Fe K-edge EXAFS spectroscopy revealed that both microbially-mediated Fe(II) production as well as the experimental addition of aqueous Fe(II) under abiotic conditions triggered rapid transformation of the initial ferrihydrite to feroxyhyte (δ′-FeOOH) and goethite (α-FeOOH). Goethite has been widely observed as a product of the Fe(II)-catalyzed transformation of ferrihydrite. However, the present study is the first to document the formation of feroxyhyte via this pathway, with feroxyhyte formation appearing to be favored by the presence of Sb(V). The formation of these secondary Fe(III) oxides was associated with substantial decreases in aqueous Sb concentrations and in the amount of surface-bound Sb (as defined via extractions with 1 M PO 4 3−). This is consistent with the incorporation of Sb(V) into the newly formed feroxyhyte and goethite via substitution for Fe(III). The results of this study provide new perspectives on coupling between Sb geochemistry and Fe mineralogy by showing that microbial Fe(III)-reduction and associated secondary Fe(III)-oxide formation can help to immobilize Sb(V) in reducing environments. [ABSTRACT FROM AUTHOR]

The Macleay River in eastern Australia is severely impacted by historic stibnite- and arsenopyrite-rich mine-tailings. 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 μm) 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 PO 4 −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) (~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 speciation of both elements strongly influence their respective mobility and partitioning. Unlabelled Image • We explore the fate and behaviour of Sb and As in a tailings contaminated river. • Sb(V) and As(V) species dominate most sediments. • Fe(III) precipitates in hyporheic-zone seeps are enriched in As relative to Sb. • Sb(V) and As(III) are more mobile in oxic and reducing hyporheic-zones respectively. • Aqueous Sb mobility is favoured compared to As along the (mainly) oxic flow path. [ABSTRACT FROM AUTHOR]