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Monothioarsenate transformation kinetics determines arsenic sequestration by sulfhydryl groups of peat Johannes Besold, Ashis Biswas, Elke Suess, Andreas C Scheinost, Andre Rossberg, Christian Mikutta, Ruben Kretzschmar, Jon Petter Gustafsson, and Britta Planer-Friedrich Environ. Sci. Technol., Just Accepted Manuscript • Publication Date (Web): 31 May 2018 Downloaded from http://pubs.acs.org on May 31, 2018
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Monothioarsenate transformation kinetics determines
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arsenic sequestration by sulfhydryl groups of peat
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Johannes Besold1, Ashis Biswas1,2, Elke Suess3, Andreas C. Scheinost4, André Rossberg4,
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Christian Mikutta5, Ruben Kretzschmar6, Jon Petter Gustafsson7 and Britta Planer-Friedrich*1
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1
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Research (BAYCEER), Bayreuth University, 95440 Bayreuth, Germany
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2
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Research (IISER) Bhopal, Bhopal Bypass Road, Bhauri 462066, MP, India
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Department of Environmental Geochemistry, Bayreuth Center for Ecology and Environmental
Department of Earth and Environmental Sciences, Indian Institute of Science Education and
Eawag, Swiss Federal Institute of Aquatic Science and Technology, 8600 Dübendorf,
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Switzerland
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4
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Dresden-Rossendorf (HZDR), Institute of Resource Ecology, Bautzner Landstraße 400, 01328
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Dresden, Germany
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Callinstr. 3, 30167 Hannover, Germany
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Science, ETH Zurich, CHN, CH-8092 Zurich, Switzerland
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750 07, Uppsala, Sweden
The Rossendorf Beamline (ROBL) at ESRF, 38043 Grenoble, France and Helmholtz-Zentrum
Soil Mineralogy, Institute of Mineralogy, Gottfried Wilhelm Leibniz Universität Hannover,
Institute of Biogeochemistry and Pollutant Dynamics, Department of Environmental Systems
Department of Soil and Environment, Swedish University of Agricultural Sciences, Box 7014,
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ABSTRACT
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In peatlands, arsenite was reported to be effectively sequestered by sulfhydryl groups of natural
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organic matter. To which extent porewater arsenite can react with reduced sulfur to form
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thioarsenates and how this affects arsenic sequestration in peatlands, is unknown. Here, we show
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that in the naturally arsenic-enriched peatland Gola di Lago, Switzerland, up to 93% of all arsenic
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species in surface and porewaters were thioarsenates. The dominant species, monothioarsenate,
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likely formed from arsenite and zero-valent sulfur-containing species. Laboratory incubations
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with sulfide-reacted, purified model peat showed for both, monothioarsenate and arsenite,
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increasing total arsenic sorption with decreasing pH from 8.5 to 4.5. However, X-ray absorption
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spectroscopy revealed no binding of monothioarsenate via sulfhydryl groups. The sorption
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observed at pH 4.5 was acid-catalyzed dissociation of monothioarsenate, forming arsenite. The
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lower the pH and the more sulfhydryl sites, the more arsenite sorbed which in turn shifted
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equilibrium towards further dissociation of monothioarsenate. At pH 8.5, monothioarsenate was
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stable over 41 days. In conclusion, arsenic can be effectively sequestered by sulfhydryl groups in
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anoxic, slightly acidic environments where arsenite is the only arsenic species. At neutral to
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slightly alkaline pH, monothioarsenate can form and its slow transformation into arsenite and low
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affinity to sulfhydryl groups suggest that this species is mobile in such environments.
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INTRODUCTION
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Arsenic (As) is a toxic metalloid whose speciation and thus mobility is strongly affected by
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(microbially triggered) redox transformations.1-3 In most terrestrial environments, inorganic
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arsenite (HxAsIIIO33-x, x=1-3) and arsenate (HxAsVO43-x, x=1-3) dominate aqueous As speciation,1
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hence their sorption behavior to many environmentally relevant mineral phases has been
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extensively studied.4 In particular, natural organic matter (NOM), for example in peatlands has
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been recognized as an important sink for As in recent years.5-8 Contrary, complexation to
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dissolved or colloidal organic matter can lead to (re-)mobilization of As in organic rich surface-,
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pore- and groundwaters.9-11
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Several inner-sphere complexation mechanisms of As to NOM have been shown by X-ray
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absorption spectroscopy (XAS). Ternary complex formation between As oxoacids and iron (Fe,
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(III))-NOM complexes9,12,13 has been revealed by XAS for both, arsenite14 and arsenate.15
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Moreover, direct coordination with phenolate groups of NOM has at first been postulated for
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arsenite and arsenate by Buschmann et al.16 and has recently been verified for arsenite by
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Hoffmann et al.14 Additionally, coordination of trivalent As with sulfhydryl groups of NOM,
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presumably provided by organic sulfur (S) from plant debris, was identified as prevailing
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sequestration mechanism in deep layers of the minerotrophic peatland Gola di Lago,
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Switzerland17,18 and in a laboratory study using XAS.19
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Although arsenite generally dominates aqueous As speciation under reducing conditions,
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multiple studies showed that thioarsenates (HxAsVS-IInO4-n3-x; n=1-4; x=1-3) can form and
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dominate in sulfidic20-25 environments and even in presence of high ferrous iron concentrations.26
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Thioarsenates are proposed to form in two steps from arsenite: First, at conditions of excess SH4 ACS Paragon Plus Environment
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over OH-, ligand exchange leads to formation of thioarsenites (HxAsIIIS-IInO3-n3-x; n=1-3; x=1-3)
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as unstable intermediates.27,28 Second, addition of zero-valent sulfur transforms thioarsenites to
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thioarsenates.29 Only formation of monothioarsenate (HxAsVS-IIO33-x; x=1-3; MTAs(V)) does not
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require excess sulfide, because it forms directly from arsenite and zero-valent sulfur or zero-
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valent sulfur containing species (thereafter abbreviated as S(0)-species) as e.g. colloidal
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elemental sulfur or polysulfides29 at neutral to alkaline pH. At acidic pH (
