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  • 1
    Language: English
    In: Environmental science & technology, 18 February 2014, Vol.48(4), pp.2281-9
    Description: Organosulfur-coordinated As(III) and realgar (α-As4S4) have been identified as the dominant As species in the naturally As-enriched minerotrophic peatland Gola di Lago, 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 K-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, t1/2, 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 (t1/2 = 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 Gola di Lago 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.
    Keywords: Arsenic -- Chemistry ; Arsenicals -- Chemistry ; Soil -- Chemistry ; Sulfides -- Chemistry ; Sulfur Compounds -- Chemistry
    ISSN: 0013936X
    E-ISSN: 1520-5851
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  • 2
    Language: English
    In: Environmental Science and Technology, 04 October 2016, Vol.50(19)
    Description: 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, Eh = -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 plus or minus 10% U(IV), x plus or minus s, n = 22). Bulk U X-ray absorption near edge structure (XANES) spectroscopy revealed that in all samples U(IV) comprised 35-68% of total U (x = 50%, n = 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 plus or minus 0.2 C atoms at a distance of 2.91 plus or minus 0.01 A (x plus or minus s), which implies the formation of bidentate-mononuclear U(IV/VI) complexes with carboxyl groups. We neither found evidence for U shells at ~3.9 A, 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).
    Keywords: Engineering ; Environmental Sciences
    ISSN: 0013-936X
    E-ISSN: 1520-5851
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  • 3
    Language: English
    In: Nature Geoscience, 1/2012, Vol.5(1), pp.66-73
    Description: Wetlands cover more than 6% of the global ice-free land area, and have been recognized as important sinks for arsenic. Wetland soils and sediments are subject to frequent changes in redox conditions, driven by fluctuations in the water table and shifts in biological activity. Under oxic conditions, natural organic matter promotes arsenic release from metal-(hydr)oxides, thereby enhancing arsenic mobility. Under strongly reducing conditions, however, organic matter triggers the formation of arsenic-sequestering sulphides, leading to a reduction in arsenic mobility. Furthermore, the sorption of arsenic to natural organic matter is increasingly thought to suppress arsenic mobility, but the binding mechanisms have remained elusive. Here we use X-ray absorption spectroscopy to analyse the speciation of solid-phase arsenic in peat samples collected from a naturally arsenic-enriched peatland in Switzerland. We show that natural organic matter can completely sequester arsenic through the formation of covalent bonds between trivalent arsenic and organic sulphur groups, which have an average arsenic–sulphur bond distance of 2.26 Å. We suggest that by binding arsenic in this way, natural organic matter plays an active role in arsenic immobilization in sulphur-enriched, anoxic wetlands.
    Keywords: Geology;
    ISSN: 1752-0894
    E-ISSN: 1752-0908
    Source: Nature Publishing Group (via CrossRef)
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  • 4
    Language: English
    In: CHIMIA International Journal for Chemistry, 11/26/2012, Vol.66(11), pp.877-877
    ISSN: 00094293
    Source: CrossRef
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  • 5
    Language: English
    In: Environmental Science and Technology, 03 September 2013, Vol.47(17)
    Description: Arsenic binding by sulfhydryl groups of natural organic matter (NOM) was recently identified as an important As sequestration pathway in the naturally As-enriched minerotrophic peatland Gola di Lago, Switzerland. Here, we explore the microscale distribution, elemental correlations, and chemical speciation of As in the Gola di Lago peat. Thin sections of undisturbed peat samples from 0-37 cm and 200-249 cm depth were analyzed by synchrotron microfocused X-ray fluorescence (...-XRF) spectrometry and X-ray absorption spectroscopy (...-XAS). Additionally, peat samples were studied by bulk As, Fe, and S K-edge XAS. Micro-XRF analyses showed that As in the near-surface peat was mainly concentrated in 10-50 ...m sized hotspots, identified by ...-XAS as realgar ( alpha -...). In the deep peat layer samples, however, As was more diffusely distributed and mostly associated with particulate NOM of varying decomposition stages. The NOM-associated As was present as trivalent As bound by sulfhydryl groups. Arsenopyrite (FeAsS) and arsenian pyrite (FeAs...) of 〈25 ...m size, which have escaped detection by bulk As and Fe K-edge XAS, were found as minor As species in the peat. Bulk S K-edge XAS revealed that the deep peat layers were significantly enriched in reduced organic S species. Our findings suggest an authigenic formation of realgar and arsenopyrite in strongly reducing microenvironments of the peat and indicate that As(III)-NOM complexes are formed by the passive sorption of As(III) to NOM. This reaction appears to be favored by a combination of abundant reduced organic S and comparatively low As solution concentrations preventing the formation of secondary As-bearing sulfides. (ProQuest: ... denotes formulae/symbols omitted.)
    Keywords: Env ; Engineering ; Environmental Sciences
    ISSN: 0013-936X
    E-ISSN: 1520-5851
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  • 6
    Language: English
    In: Nat. Geosci. 5: 66-73, 2012, 29 September 2014
    Keywords: Chem
    Source: SciTech Connect (U.S. Dept. of Energy - Office of Scientific and Technical Information)
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