Silicon and calcium controls on iron and aluminum mobility in Arctic soils

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.

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.

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

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.

Antimony and arsenic speciation, redox-cycling and contrasting mobility in a mining-impacted river system

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 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) (~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.

Humic acid impacts antimony partitioning and speciation during iron(II)-induced ferrihydrite transformation

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 PO₄^3--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.

Chromium(VI) formation via heating of Cr(III)-Fe(III)-(oxy)hydroxides: A pathway for fire-induced soil pollution

Mixed Cr(III)-Fe(III) (oxy)hydroxides are important Cr-bearing phases in natural, unpolluted soil. Fires frequently affect large areas of land around the world, causing the temporary development of elevated soil temperatures. This study examines the hypothesis that heating Cr(III)-Fe(III) (oxy)hydroxides at temperatures which occur in surface soils during fires can drive rapid oxidation of Cr(III) to hazardous Cr(VI). To test this, poorly-ordered Cr(III)_x-Fe(III)_1-x (oxy)hydroxides, with x spanning 0.1 to 0.9, were heated at up to 800 °C for 2 h. Heating at 400-800 °C produced a highly crystalline hematite-eskolaite solid-solution (Fe^III_2-nCr^III_nO₃, where n ranges from 0 to 2). Chromium K-edge X-ray absorption spectroscopy showed that during heating up to ∼40% of the initial Cr(III) was oxidized to Cr(VI), with the greatest extent of Cr(VI) formation occurring at 200-400 °C. At these temperatures, a substantial proportion (17%-70%) of the newly-formed Cr(VI) was exchangeable (i.e. extracted by a pH 7.2, 10 mM PO₄^3- solution). This suggests that much of the Cr(VI) formed by heating of Cr(III)_x-Fe(III)_1-x (oxy)hydroxides at 200-400 °C is likely to be relatively mobile in fire-impacted soils. The results of this study provide new insights into a potentially-important pathway for the in-situ formation of Cr(VI) in soil.

Plant uptake and availability of antimony, lead, copper and zinc in oxic and reduced shooting range soil

Shooting ranges polluted by antimony (Sb), lead (Pb), copper (Cu) and zinc (Zn) are used for animal grazing, thus pose a risk of contaminants entering the food chain. Many of these sites are subject to waterlogging of poorly drained soils. Using field lysimeter experiments, we compared Sb, Pb, Cu and Zn uptake by four common pasture plant species (Lolium perenne, Trifolium repens, Plantago lanceolata and Rumex obtusifolius) growing on a calcareous shooting range soil under waterlogged and drained conditions. To monitor seasonal trends, the same plants were collected at three times over the growing season. Additionally, variations in soil solution concentrations were monitored at three depths over the experiment. Under reducing conditions, soluble Sb concentrations dropped from ∼50 μg L^-1 to ∼10 μg L^-1, which was attributed to the reduction of Sb(V) to Sb(III) and the higher retention of the trivalent species by the soil matrix. Shoot Sb concentrations differed by a factor of 60 between plant species, but remained at levels <0.3 μg g^-1. Despite the difference in soil solution concentrations between treatments, total Sb accumulation in shoots for plants collected on the waterlogged soil did not change, suggesting that Sb(III) was much more available for plant uptake than Sb(V), as only 10% of the total Sb was present as Sb(III). In contrast to Sb, Pb, Cu and Zn soil solution concentrations remained unaffected by waterlogging, and shoot concentrations were significantly higher in the drained treatment for many plant species. Although showing an increasing trend over the season, shoot metal concentrations generally remained below regulatory values for fodder plants (40 μg g^-1 Pb, 150 μg g^-1 Zn, 15-35 μg g^-1 Cu), indicating a low risk of contaminant transfer into the food chain under both oxic and anoxic conditions for the type of shooting range soil investigated in this study.

Antimony mobility during prolonged waterlogging and reoxidation of shooting range soil: A field experiment

Due to its increasing anthropogenic use, antimony (Sb) soil pollution is of growing concern. Many soils experience fluctuating hydrological conditions, yet very little is known about how this affects the mobility of this toxic element under field conditions. In this study, we performed an outdoor lysimeter experiment to compare Sb leaching from a calcareous shooting range soil under drained and prolonged waterlogged conditions (1.5-2.75years), followed by a 1.5-year period of soil reoxidation. Waterlogging reduced Sb leachate concentrations significantly compared to drained conditions and soil solution concentrations decreased with depth due to the increased reducing conditions. This was attributed to the reduction of Sb(V) to Sb(III) and the more effective sorption of the latter to metal (hydr)oxides. However, reductive dissolution of iron (hydr)oxides released Sb into solution, although Sb concentrations never exceeded those in the drained lysimeters. On reoxidation of the soil, Sb was remobilized, but even after 1.5years under reoxidised conditions, Sb leachate and soil solution concentrations still remained below those of the drained lysimeters. Our results demonstrate that prolonged waterlogging may have an irreversible effect on Sb leachate and soil solution concentrations.

Diffusive Gradients in Thin Films Reveals Differences in Antimony and Arsenic Mobility in a Contaminated Wetland Sediment during an Oxic-Anoxic Transition

Antimony (Sb) and arsenic (As) are priority environmental contaminants that often co-occur at mining-impacted sites. Despite their chemical similarities, Sb mobility in waterlogged sediments is poorly understood in comparison to As, particularly across the sediment-water interface (SWI) where changes can occur at the millimeter scale. Combined diffusive gradients in thin films (DGT) and diffusive equilibration in thin films (DET) techniques provided a high resolution, in situ comparison between Sb, As, and iron (Fe) speciation and mobility across the SWI in contaminated freshwater wetland sediment mesocosms under an oxic-anoxic-oxic transition. The shift to anoxic conditions released Fe(II), As(III), and As(V) from the sediment to the water column, consistent with As release being coupled to the reductive dissolution of iron(III) (hydr)oxides. Conversely, Sb(III) and Sb(V) effluxed to the water column under oxic conditions and fluxed into the sediment under anoxic conditions. Porewater DGT-DET depth profiles showed apparent decoupling between Fe(II) and Sb release, as Sb was primarily mobilized across the SWI under oxic conditions. Solid-phase X-ray absorption spectroscopy (XAS) revealed the presence of an Sb(III)-S phase in the sediment that increased in proportion with depth and the transition from oxic to anoxic conditions. The results of this study showed that Sb mobilization was decoupled from the Fe cycle and was, therefore, more likely linked to sulfur and/or organic carbon (e.g., most likely authigenic antimony sulfide formation or Sb(III) complexation by reduced organic sulfur functional groups).

Arsenic Mobilization from Historically Contaminated Mining Soils in a Continuously Operated Bioreactor: Implications for Risk Assessment

Concentrations of soil arsenic (As) in the vicinity of the former Złoty Stok gold mine (Lower Silesia, southwest Poland) exceed 1000 μg g(-1) in the area, posing an inherent threat to neighboring bodies of water. This study investigated continuous As mobilization under reducing conditions for more than 3 months. In particular, the capacity of autochthonic microflora that live on natural organic matter as the sole carbon/electron source for mobilizing As was assessed. A biphasic mobilization of As was observed. In the first two months, As mobilization was mainly conferred by Mn dissolution despite the prevalence of Fe (0.1 wt % vs 5.4 for Mn and Fe, respectively) as indicated by multiple regression analysis. Thereafter, the sudden increase in aqueous As[III] (up to 2400 μg L(-1)) was attributed to an almost quintupling of the autochthonic dissimilatory As-reducing community (quantitative polymerase chain reaction). The aqueous speciation influenced by microbial activity led to a reduction of solid phase As species (X-ray absorption fine structure spectroscopy) and a change in the elemental composition of As hotspots (micro X-ray fluorescence mapping). The depletion of most natural dissolved organic matter and the fact that an extensive mobilization of As[III] occurred after two months raises concerns about the long-term stability of historically As-contaminated sites.

Antimony retention and release from drained and waterlogged shooting range soil under field conditions

Many soils polluted by antimony (Sb) are subject to fluctuating waterlogging conditions; yet, little is known about how these affect the mobility of this toxic element under field conditions. Here, we compared Sb leaching from a calcareous shooting range soil under drained and waterlogged conditions using four large outdoor lysimeters. After monitoring the leachate samples taken at bi-weekly intervals for >1.5 years under drained conditions, two of the lysimeters were subjected to waterlogging with a water table fluctuating according to natural rainfall water infiltration. Antimony leachate concentrations under drained conditions showed a strong seasonal fluctuation between 110 μg L(-1) in summer and <40 μg L(-1) in winter, which closely correlated with fluctuations in dissolved organic carbon (DOC) concentrations. With the development of anaerobic conditions upon waterlogging, Sb in leachate decreased to 2-5 μg L(-1) Sb and remained stable at this level. Antimony speciation measurements in soil solution indicated that this decrease in Sb(V) concentrations was attributable to the reduction of Sb(V) to Sb(III) and the stronger sorption affinity of the latter to iron (Fe) (hydr)oxide phases. Our results demonstrate the importance of considering seasonal and waterlogging effects in the assessment of the risks from Sb-contaminated sites.

Effect of iron plaque on antimony uptake by rice (Oryza sativa L.)

Although iron (Fe) plaque has been shown to significantly affect the uptake of toxic antimony (Sb) by rice, knowledge about the influence of iron plaque on antimony (Sb) (amount, mechanisms, etc) is, however, limited. Here, the effect of Fe plaque on Sb(III) and Sb(V) (nominal oxidation states) uptake by rice (Oryza sativa L.) was investigated using hydroponic experiments and synchrotron-based techniques. The results showed that iron plaque immobilized Sb on the surface of rice roots. Although the binding capacity of iron plaque for Sb(III) was markedly greater than that for Sb(V), significantly more Sb(III) was taken up by roots and transported to shoots. In the presence of Fe plaque, Sb uptake into rice roots was significantly reduced, especially for Sb(III). However, this did not translate into decreasing Sb concentrations in rice shoots and even increased shoot Sb concentrations during high Fe-Sb(III) treatment.

Release of antimony from contaminated soil induced by redox changes

Soil contamination by toxic antimony (Sb) released from corroding ammunition has become an issue of public concern in various countries. Many of these soils are at least occasionally subject to waterlogging; yet mechanisms controlling Sb mobility under anaerobic conditions are still poorly understood. We investigated Sb concentration and speciation dynamics in a calcareous shooting range soil in terms of changing redox conditions using microcosm experiments. The transition to reducing conditions invoked by indigenous microbial activity at first led to the immobilization of Sb, as Sb(V) was converted to Sb(III), which binds more extensively to iron (hydr)oxides. When reducing conditions continued, the previously sorbed Sb(III) was gradually released into solution due to reductive dissolution of the iron (hydr)oxides. Speciation measurements in the solid phase by Sb K-edge XANES spectroscopy and in the soil solution by liquid chromatography ICP-MS provided the first evidence that Sb(III) predominated at low redox conditions (Eh <0.05 V) in both phases. The results show that Sb(V) is less stable in reducing environments than commonly assumed. Given that Sb(III) is generally more toxic than Sb(V), the mobilization of Sb(III) under Fe-reducing conditions may significantly increase (eco)toxicological risks arising from Sb-contaminated soils that are prone to flooding or waterlogging.

Changes in Sb speciation with waterlogging of shooting range soils and impacts on plant uptake

A pot experiment was conducted to investigate the solubility and redox species of antimony (Sb) in a relocated shooting range soil and its uptake by Lolium perenne L. and Holcus lanatus L. under different water regimes. After 1-week waterlogging, the total Sb concentration in soil solution decreased from ∼110 μg L(-1) to <20 μg L(-1), and slowly increased over the following 4 weeks, with the dissolution of Fe and Mn (hydr)oxides. In this process, half of the Sb in soil solution was reduced to Sb(III), which greatly affected the plant uptake of Sb. Waterlogging increased shoot Sb concentrations of L. perenne by ∼10 fold but decreased uptake in H. lanatus by 80%. Results indicate that Sb might primarily be taken up as Sb(III) by L. perenne and as Sb(V) by H. lanatus. Temporary waterlogging of soil may increase the risk of trace elements entering the food chain.

Accumulation of Sb, Pb, Cu, Zn and Cd by various plants species on two different relocated military shooting range soils

Annually, more than 400 t Pb and 10 t Sb enter Swiss soils at some 2000 military shooting ranges. After the decommission of military shooting ranges, heavily contaminated soils (>2000 mg kg(-1) Pb) are landfilled or processed by soil washing, whereas for soils with less contamination, alternate strategies are sought. Although the use of military shooting ranges for grazing in Switzerland is common practice, no assessment has been done about the uptake of Sb in plants and its subsequent potential intake by grazing animals. We determined the uptake of Sb, Pb, Cu, Zn and Cd in the aboveground biomass of nine plant species growing on a calcareous (Chur) and a weakly acidic (Losone) military shooting range soil in order to assess if grazing would be safe to employ on decommissioned military shooting ranges. The two soils did not differ in their total concentrations of Cu, Zn, Sb and Cd, they differed however in the total concentration of Pb. Additionally, their physical and chemical properties were significantly different. The accumulation of Zn, Cu, Cd and Pb in the shoots of all nine plant species remained below the Swiss tolerance values for fodder plants (150 mg kg(-1) Zn, 15-35 mg kg(-1) Cu, 40 mg kg(-1) Pb, and 1 mg kg(-1) Cd DW), with the only exception of Pb in Chenopodium album shoots which reached a concentration of 62 mg kg(-1) DW. Antimony concentrations were 1.5-2.6-fold higher in plants growing on the calcareous soil than on the weakly acidic soil. Considering Cu, Zn, Pb, Sb and Cd, all plants, with the exception C. album, would be suitable for grazing on similar shooting range soils.

Effects of three amendments on extractability and fractionation of Pb, Cu, Ni and Sb in two shooting range soils

Contamination of shooting range soils with toxic trace elements, in particular Pb and Sb, is of increasing environmental concern worldwide. We studied the extractability of Sb, and other metals in two shooting range soils: a calcareous soil (pH 8) with low organic carbon (0.5%) and a non-calcareous soil (pH 6.3) with elevated organic carbon content (5%). Both soils contained total concentrations of around 500 mg kg(-1) Pb, 65 mg kg(-1) Cu, 100 mg kg(-1) Zn and 20 mg kg(-1) Sb. We tested the effects of Ca(OH)(2), phosphate and sodium humate amendments on metals and Sb extractability. Extracts with H(2)O and NaNO(3) contained 0.02-0.05% of the total Zn and Pb; 0.1-0.5% of total Ni and Cu and approximately 1% of total Sb. Sequential extraction procedure of Zeien and Brümmer resulted in similar percentages for the sum of the two most labile fractions (F1+F2) in two soils: 10% Pb, and 15-20% Sb. Water and NaNO(3)-extractable Sb concentrations increased after phosphate addition, but were not affected by the addition of sodium humate. The results show that leaching of Sb from shooting ranges into ground and surface waters may generate a serious environmental risk under widely different soils conditions.