Article pubs.acs.org/est
Bisulfide Reaction with Natural Organic Matter Enhances Arsenite Sorption: Insights from X‑ray Absorption Spectroscopy Martin Hoffmann, Christian Mikutta,* and Ruben Kretzschmar Institute of Biogeochemistry and Pollutant Dynamics, Department of Environmental Systems Science, ETH Zurich, CH-8092 Zurich, Switzerland S Supporting Information *
ABSTRACT: 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 K-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.
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INTRODUCTION Arsenic is a toxic trace element which occurs naturally in aquatic and terrestrial ecosystems, causing serious health concerns in many regions throughout the world.1 The toxicity of As depends on its speciation and hence on the prevailing redox conditions.2 Among inorganic As species, the more toxic arsenite (As(III)) predominates under anoxic conditions, whereas arsenate (As(V)) is the major As species in oxic environments.1 While As sorption to mineral phases has been extensively studied in the past,1,3,4 much less is known about As binding to natural organic matter (NOM). Terrestrial ecosystems rich in NOM, for example, peat- and wetlands, are increasingly recognized as biogeochemical sinks for As,5−7 but the governing mechanisms and their relative importance are still elusive. Ternary complex formation between As oxyanions and ferric iron (Fe(III)) complexes of NOM is the association mechanism most frequently invoked to explain As binding to NOM.8−11 Recently, this inner-sphere complexation was verified by X-ray absorption spectroscopy (XAS) for As(V) reacted with Fe(III)-humate and -fulvate complexes.9 In addition to ternary complex formation, Buschmann et al.12 proposed a ligand exchange reaction of As(III) and As(V) with phenolate groups of NOM and the formation of an anionic adduct between As(III) and a carboxylate group, stabilized by an internal hydrogen bond. Besides inner-sphere complexation, outer-sphere binding of As(III) and As(V) to strongly basic amino groups of NOM was suggested by Thanabalasingam and Pickering13 as another As © 2012 American Chemical Society
sequestration pathway. To the best of our knowledge, none of the above-mentioned mechanisms has yet been identified in field studies. In a recent study, Langner et al.14 investigated the speciation of As in a minerotrophic, slightly acidic peatland naturally enriched in As (
