Oxidation of Organosulfur-Coordinated Arsenic and Realgar in Peat: Implications for the Fate of Arsenic

Organosulfur-coordinated As­(III) and realgar (α-As₄S₄) have been identified as the dominant As species in the naturally As-enriched minerotrophic peatland <i>Gola di Lago</i>, Switzerland. In this study, we explored their oxidation kinetics in peat exposed to atmospheric O₂ for up to 180 days under sterile and nonsterile conditions (25 °C, ∼100% relative humidity). Anoxic peat samples were collected from a near-surface (0–38 cm) and a deep peat layer (200–250 cm) and studied by bulk As, Fe, and S <i>K</i>-edge X-ray absorption spectroscopy as well as selective extractions as a function of time. Over 180 days, only up to 33% of organosulfur-coordinated As­(III) and 44% of realgar were oxidized, corresponding to half-life times, <i>t</i>_1/2, of 312 and 215 days, respectively. The oxidation of both As species was mainly controlled by abiotic processes. Realgar was oxidized orders of magnitude slower than predicted from published mixed-flow reactor experiments, indicating that mass-transfer processes were rate-limiting. Most of the As released (>97%) was sequestered by Fe­(III)–(hydr)­oxides. However, water-extractable As reached concentrations of 0.7–19 μmol As L^–1, exceeding the WHO drinking water limit by up to 145 times. Only a fraction (20–36%) of reduced S­(-II to I) was sensitive to oxidation and was oxidized faster (<i>t</i>_1/2 = 50–173 days) than organosulfur-coordinated As­(III) and realgar, suggesting a rapid loss of reactive As-sequestering S species following a drop in the water table. Our results imply that wetlands like <i>Gola di Lago</i> can serve as long-term sources for As under prolonged oxidizing conditions. The maintenance of reducing conditions is thus regarded as the primary strategy in the management of this and other As-rich peatlands

Bisulfide Reaction with Natural Organic Matter Enhances Arsenite Sorption: Insights from X‑ray Absorption Spectroscopy

Terrestrial ecosystems rich in natural organic matter (NOM) can act as a sink for As. Recently, the complexation of trivalent As by sulfhydryl groups of NOM was proposed as the main mechanism for As–NOM interactions in anoxic S- and NOM-rich environments. Here we tested the molecular-scale interaction of bisulfide (S­(-II)) with NOM and its consequences for arsenite (As­(III)) binding. We reacted 0.2 mol C/L peat and humic acid (HA) with up to 5.8 mM S­(-II) at pH 7 and 5, respectively, and subsequently equilibrated the reaction products with 55 μM As­(III) under anoxic conditions. The speciation of S and the local coordination environment of As in the solid phase were studied by X-ray absorption spectroscopy. Our results document a rapid reaction of S­(-II) with peat and HA and the concomitant formation of reduced organic S species. These species were highly reactive toward As­(III). Shell fits of As <i>K</i>-edge extended X-ray absorption fine structure spectra revealed that the coordination environment of trivalent As was progressively occupied by S atoms. Fitted As–S distances of 2.24–2.34 Å were consistent with sulfhydryl-bound As­(III). Besides As­(III) complexation by organic monosulfides, our data suggests the formation of nanocrystalline As sulfide phases in HA samples and an As sorption process for both organic sorbents in which As­(III) retained its first-shell oxygens. In conclusion, this study documents that S­(-II) reaction with NOM can greatly enhance the ability of NOM to bind As in anoxic environments

Effects of Manganese Oxide on Arsenic Reduction and Leaching from Contaminated Floodplain Soil

Reductive release of the potentially toxic metalloid As from Fe­(III) (oxyhydr)­oxides has been identified as an important process leading to elevated As porewater concentrations in soils and sediments. Despite the ubiquitous presence of Mn oxides in soils and their oxidizing power toward As­(III), their impact on interrelated As, Fe, and Mn speciation under microbially reducing conditions remains largely unknown. For this reason, we employed a column setup and X-ray absorption spectroscopy to investigate the influence of increasing birnessite concentrations (molar soil Fe-to-Mn ratios: 4.8, 10.2, and 24.7) on As speciation and release from an As-contaminated floodplain soil (214 mg As/kg) under anoxic conditions. Our results show that birnessite additions significantly decreased As leaching. The reduction of both As and Fe was delayed, and As­(III) accumulated in birnessite-rich column parts, indicating the passivation of birnessite and its transformation products toward As­(III) oxidation and the precipitation of Fe­(III)­(oxyhydr)­oxides. Microbial Mn reduction resulted in elevated soil pH values, which in turn lowered the microbial activity in the birnessite-enriched soil. We conclude that in Mn-oxide-rich soil environments undergoing redox fluctuations, the enhanced As adsorption to newly formed Fe­(III) (oxyhydr)­oxides under reducing conditions leads to a transient stabilization of As

Impact of Birnessite on Arsenic and Iron Speciation during Microbial Reduction of Arsenic-Bearing Ferrihydrite

Elevated solution concentrations of As in anoxic natural systems are usually accompanied by microbially mediated As­(V), Mn­(III/IV), and Fe­(III) reduction. The microbially mediated reductive dissolution of Fe­(III)-(oxyhydr)­oxides mainly liberates sorbed As­(V) which is subsequently reduced to As­(III). Manganese oxides have been shown to rapidly oxidize As­(III) and Fe­(II) under oxic conditions, but their net effect on the microbially mediated reductive release of As and Fe is still poorly understood. Here, we investigated the microbial reduction of As­(V)-bearing ferrihydrite (molar As/Fe: 0.05; Fe_tot: 32.1 mM) by <i>Shewanella</i> sp. ANA-3 (10⁸ cells/mL) in the presence of different concentrations of birnessite (Mn_tot: 0, 0.9, 3.1 mM) at circumneutral pH over 397 h using wet-chemical analyses and X-ray absorption spectroscopy. Additional abiotic experiments were performed to explore the reactivity of birnessite toward As­(III) and Fe­(II) in the presence of Mn­(II), Fe­(II), ferrihydrite, or deactivated bacterial cells. Compared to the birnessite-free control, the highest birnessite concentration resulted in 78% less Fe and 47% less As reduction at the end of the biotic experiment. The abiotic oxidation of As­(III) by birnessite (<i>k</i>_initial = 0.68 ± 0.31/h) was inhibited by Mn­(II) and ferrihydrite, and lowered by Fe­(II) and bacterial cell material. In contrast, the oxidation of Fe­(II) by birnessite proceeded equally fast under all conditions (<i>k</i>_initial = 493 ± 2/h) and was significantly faster than the oxidation of As­(III). We conclude that in the presence of birnessite, microbially produced Fe­(II) is rapidly reoxidized and precipitates as As-sequestering ferrihydrite. Our findings imply that the ability of Mn-oxides to oxidize As­(III) in water-logged soils and sediments is limited by the formation of ferrihydrite and surface passivation processes

New Clues to the Local Atomic Structure of Short-Range Ordered Ferric Arsenate from Extended X‑ray Absorption Fine Structure Spectroscopy

Short-range ordered ferric arsenate (FeAsO₄·<i>x</i>H₂O) is a secondary As precipitate frequently encountered in acid mine waste environments. Two distinct structural models have recently been proposed for this phase. The first model is based on the structure of scorodite (FeAsO₄·2H₂O) where isolated FeO₆ octahedra share corners with four adjacent arsenate (AsO₄) tetrahedra in a three-dimensional framework (framework model). The second model consists of single chains of corner-sharing FeO₆ octahedra being bridged by AsO₄ bound in a monodentate binuclear ²C complex (chain model). In order to rigorously test the accuracy of both structural models, we synthesized ferric arsenates and analyzed their local (<6 Å) structure by As and Fe K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy. We found that both As and Fe K-edge EXAFS spectra were most compatible with isolated FeO₆ octahedra being bridged by AsO₄ tetrahedra (<i>R</i>_Fe–As = 3.33 ± 0.01 Å). Our shell-fit results further indicated a lack of evidence for single corner-sharing FeO₆ linkages in ferric arsenate. Wavelet-transform analyses of the Fe K-edge EXAFS spectra of ferric arsenates complemented by shell fitting confirmed Fe atoms at an average distance of ∼5.3 Å, consistent with crystallographic data of scorodite and in disagreement with the chain model. A scorodite-type local structure of short-range ordered ferric arsenates provides a plausible explanation for their rapid transformation into scorodite in acid mining environments

Arsenite Binding to Sulfhydryl Groups in the Absence and Presence of Ferrihydrite: A Model Study

Binding of arsenite (As­(III)) to sulfhydryl groups (S_org(-II)) plays a key role in As detoxification mechanisms of plants and microorganisms, As remediation techniques, and reduced environmental systems rich in natural organic matter. Here, we studied the formation of S_org(-II)–As­(III) complexes on a sulfhydryl model adsorbent (Ambersep GT74 resin) in the absence and presence of ferrihydrite as a competing mineral adsorbent under reducing conditions and tested their stability against oxidation in air. Adsorption of As­(III) onto the resin was studied in the pH range 4.0–9.0. On the basis of As X-ray absorption spectroscopy (XAS) results, a surface complexation model describing the pH dependence of As­(III) binding to the organic adsorbent was developed. Stability constants (log <i>K</i>) determined for dithio ((AmbS)₂AsO^–) and trithio ((AmbS)₃As) surface complexes were 8.4 and 7.3, respectively. The ability of sulfhydryl ligands to compete with ferrihydrite for As­(III) was tested in various anoxic mixtures of both adsorbents at pH 7.0. At a 1:1 ratio of their reactive binding sites, <i>R</i>–SH and ≡FeOH, both adsorbents possessed nearly identical affinities for As­(III). The oxidation of S_org(-II)–As­(III) complexes in water vapor saturated air over 80 days, monitored by As and S XAS, revealed that the complexed As­(III) is stabilized against oxidation (<i>t</i>_1/2 = 318 days). Our results thus document that sulfhydryl ligands are highly competitive As­(III) complexing agents that can stabilize As in its reduced oxidation state even under prolonged oxidizing conditions. These findings are particularly relevant for organic S-rich semiterrestrial environments subject to periodic redox potential changes such as peatlands, marshes, and estuaries

Arsenite Binding to Natural Organic Matter: Spectroscopic Evidence for Ligand Exchange and Ternary Complex Formation

The speciation of As in wetlands is often controlled by natural organic matter (NOM), which can form strong complexes with Fe­(III). Here, we elucidated the molecular-scale interaction of arsenite (As­(III)) with Fe­(III)–NOM complexes under reducing conditions. We reacted peat (40–250 μm size fraction, 1.0 g Fe/kg) with 0–15 g Fe/kg at pH <2, removed nonreacted Fe, and subsequently equilibrated the Fe­(III) complexes formed with 900 mg As/kg peat at pH 7.0, 8.4, and 8.8. The solid-phase speciation of Fe and As was studied by electron paramagnetic resonance (Fe) and X-ray absorption spectroscopy (As, Fe). Our results show that the majority of Fe in the peat was present as mononuclear Fe­(III) species (<i>R</i>_Fe–C = 2.82–2.88 Å), probably accompanied by small Fe­(III) clusters of low nuclearity (<i>R</i>_Fe–Fe = 3.25–3.46 Å) at high pH and elevated Fe contents. The amount of As­(III) retained by the original peat was 161 mg As/kg, which increased by up to 250% at pH 8.8 and an Fe loading of 7.3 g/kg. With increasing Fe content of peat, As­(III) increasingly formed bidentate mononuclear (<i>R</i>_As–Fe = 2.88–2.94 Å) and monodentate binuclear (<i>R</i>_As–Fe = 3.35–3.41 Å) complexes with Fe, thus yielding direct evidence of ternary complex formation. The ternary complex formation went along with a ligand exchange reaction between As­(III) and hydroxylic/phenolic groups of the peat (<i>R</i>_As–C = 2.70–2.77 Å). Our findings thus provide spectroscopic evidence for two yet unconfirmed As­(III)–NOM interaction mechanisms, which may play a vital role in the cycling of As in sub- and anoxic NOM-rich environments such as peatlands, peaty sediments, swamps, or rice paddies

Mineralogical Controls on the Bioaccessibility of Arsenic in Fe(III)–As(V) Coprecipitates

X-ray amorphous Fe­(III)–As­(V) coprecipitates are common initial products of oxidative As- and Fe-bearing sulfide weathering, and often control As solubility in mine wastes or mining-impacted soils. The formation conditions of these solids may exert a major control on their mineralogical composition and, hence, As release in the gastric tract of humans after incidental ingestion of As-contaminated soil. Here, we synthesized a set of 35 Fe­(III)–As­(V) coprecipitates as a function of pH (1.5–8) and initial molar Fe/As ratio (0.8–8.0). The solids were characterized by synchrotron X-ray diffraction, FT-IR spectroscopy, and electrophoretic mobility measurements, and their As bioaccessibility (BA_As) was evaluated using the gastric-phase Solubility/Bioavailability Research Consortium in vitro assay (SBRC-G). The coprecipitates contained 1.01–4.51 mol kg^–1 As (molar Fe/As_solid: 1.00–8.29) and comprised varying proportions of X-ray amorphous hydrous ferric arsenates (HFA_am) and As­(V)-adsorbed ferrihydrite. HFA_am was detected up to pH 6 and its fraction decreased with increasing pH and molar Fe/As ratio. Bioaccessible As ranged from 2.9 to 7.3% of total As (<i>x̅</i> = 4.8%). The BA_As of coprecipitates formed at pH ≤ 4 was highest at formation pH 3 and 4 and controlled by the intrinsically high solubility of the HFA_am component, possibly enhanced by sorbed sulfate. In contrast, the BA_As of coprecipitates dominated by As­(V)-adsorbed ferrihydrite was much lower and controlled by As readsorption and/or surface precipitation in the gastric fluid. Bioaccessible As increased up to 95% with increasing liquid-to-solid ratio, indicating an enhanced solubility of these solids due to interactions between Fe and the glycine buffer. We conclude (i) that natural Fe­(III)–As­(V) coprecipitates exhibit a particularly high solubility in the human gastric tract when formed at pH ∼ 3–4 in the presence of sulfate, and (ii) that the in vitro bioaccessibility of As in Fe­(III)–As­(V) coprecipitates as assessed by tbe SBRC-G assay depends critically on their solid-phase concentration in As-contaminated soil and mine-waste materials

Tetra- and Hexavalent Uranium Forms Bidentate-Mononuclear Complexes with Particulate Organic Matter in a Naturally Uranium-Enriched Peatland

Peatlands frequently serve as efficient biogeochemical traps for U. Mechanisms of U immobilization in these organic matter-dominated environments may encompass the precipitation of U-bearing mineral­(oid)­s and the complexation of U by a vast range of (in)­organic surfaces. The objective of this work was to investigate the spatial distribution and molecular binding mechanisms of U in soils of an alpine minerotrophic peatland (pH 4.7–6.6, E_h = −127 to 463 mV) using microfocused X-ray fluorescence spectrometry and bulk and microfocused U L₃-edge X-ray absorption spectroscopy. The soils contained 2.3–47.4 wt % organic C, 4.1–58.6 g/kg Fe, and up to 335 mg/kg geogenic U. Uranium was found to be heterogeneously distributed at the micrometer scale and enriched as both U­(IV) and U­(VI) on fibrous and woody plant debris (48 ± 10% U­(IV), <i>x̅</i> ± σ, <i>n</i> = 22). Bulk U X-ray absorption near edge structure (XANES) spectroscopy revealed that in all samples U­(IV) comprised 35–68% of total U (<i>x̅</i> = 50%, <i>n</i> = 15). Shell-fit analyses of bulk U L₃-edge extended X-ray absorption fine structure (EXAFS) spectra showed that U was coordinated to 1.3 ± 0.2 C atoms at a distance of 2.91 ± 0.01 Å (<i>x̅</i> ± σ), which implies the formation of bidentate-mononuclear U­(IV/VI) complexes with carboxyl groups. We neither found evidence for U shells at ∼3.9 Å, indicative of mineral-associated U or multinuclear U­(IV) species, nor for a substantial P/Fe coordination of U. Our data indicates that U­(IV/VI) complexation by natural organic matter prevents the precipitation of U minerals as well as U complexation by Fe/Mn phases at our field site, and suggests that organically complexed U­(IV) is formed via reduction of organic matter-bound U­(VI)

Iron(II)-Catalyzed Iron Atom Exchange and Mineralogical Changes in Iron-rich Organic Freshwater Flocs: An Iron Isotope Tracer Study

In freshwater wetlands, organic flocs are often found enriched in trace metal­(loid)­s associated with poorly crystalline Fe­(III)-(oxyhydr)­oxides. Under reducing conditions, flocs may become exposed to aqueous Fe­(II), triggering Fe­(II)-catalyzed mineral transformations and trace metal­(loid) release. In this study, pure ferrihydrite, a synthetic ferrihydrite-polygalacturonic acid coprecipitate (16.7 wt % C), and As- (1280 and 1230 mg/kg) and organic matter (OM)-rich (18.1 and 21.8 wt % C) freshwater flocs dominated by ferrihydrite and nanocrystalline lepidocrocite were reacted with an isotopically enriched ⁵⁷Fe­(II) solution (0.1 or 1.0 mM Fe­(II)) at pH 5.5 and 7. Using a combination of wet chemistry, Fe isotope analysis, X-ray absorption spectroscopy (XAS), ⁵⁷Fe Mössbauer spectroscopy and X-ray diffraction, we followed the Fe atom exchange kinetics and secondary mineral formation over 1 week. When reacted with Fe­(II) at pH 7, pure ferrihydrite exhibited rapid Fe atom exchange at both Fe­(II) concentrations, reaching 76 and 89% atom exchange in experiments with 0.1 and 1 mM Fe­(II), respectively. XAS data revealed that it transformed into goethite (21%) at the lower Fe­(II) concentration and into lepidocrocite (73%) and goethite (27%) at the higher Fe­(II) concentration. Despite smaller Fe mineral particles in the coprecipitate and flocs as compared to pure ferrihydrite (inferred from Mössbauer-derived blocking temperatures), these samples showed reduced Fe atom exchange (9–30% at pH 7) and inhibited secondary mineral formation. No release of As was recorded for Fe­(II)-reacted flocs. Our findings indicate that carbohydrate-rich OM in flocs stabilizes poorly crystalline Fe minerals against Fe­(II)-catalyzed transformation by surface-site blockage and/or organic Fe­(II) complexation. This hinders the extent of Fe atom exchange at mineral surfaces and secondary mineral formation, which may consequently impair Fe­(II)-activated trace metal­(loid) release. Thus, under short-term Fe­(III)-reducing conditions facilitating the fast attainment of solid-solution equilibria (e.g., in stagnant waters), Fe-rich freshwater flocs are expected to remain an effective sink for trace elements