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Environmental Processes
Antimonite complexation with thiol and carboxyl/phenol groups of peat organic matter Johannes Besold, Naresh Kumar, Andreas C Scheinost, Juan S. Lezama-Pacheco, Scott Fendorf, and Britta Planer-Friedrich Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.9b00495 • Publication Date (Web): 11 Apr 2019 Downloaded from http://pubs.acs.org on April 12, 2019
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“Antimonite complexation with thiol and
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carboxyl/phenol groups of peat organic matter”
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Johannes Besold1, Naresh Kumar2,3, Andreas C. Scheinost4, Juan Lezama Pacheco5, Scott
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Fendorf5 and Britta Planer-Friedrich*1
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Department of Environmental Geochemistry, Bayreuth Center for Ecology and Environmental Research (BAYCEER), Bayreuth University, 95440 Bayreuth, Germany 2
Department of Geological Sciences, School of Earth, Energy, and Environmental Sciences, Stanford University, Stanford, CA 94305, USA. 3
Department of Environmental Geosciences, University of Vienna, 1090 Vienna, Austria
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The Rossendorf Beamline (ROBL) at ESRF, 38043 Grenoble, France and Helmholtz-Zentrum Dresden-Rossendorf (HZDR), Institute of Resource Ecology, 01328 Dresden, Germany 5
Department of Earth System Science, School of Earth, Energy, and Environmental Sciences, Stanford University, Stanford, CA 94305, USA
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ABSTRACT
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Peatlands and other wetlands with abundant natural organic matter (NOM) are important sinks for
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antimony (Sb). While formation of Sb(III) sulfide phases or Sb(III) binding to NOM are discussed
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to decrease Sb mobility, the exact binding mechanisms remain elusive. Here, we reacted increasing
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sulfide concentrations with purified model peat at pH 6, forming reduced organic sulfur species,
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and subsequently equilibrated the reaction products with 50 µM antimonite under anoxic
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conditions. Sulfur solid-phase speciation and the local binding environment of Sb were analyzed
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using X-ray absorption spectroscopy. We found that 85% of antimonite was sorbed by untreated
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peat. Sulfide-reacted peat increased sorption to 98%. Shell-by-shell fitting of Sb K-edge X-ray
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absorption fine structure spectra revealed Sb in untreated peat bound to carboxyl or phenol groups
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with average Sb-carbon distances of ~2.90 Å. With increasing content of reduced organic sulfur,
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Sb was progressively coordinated to S atoms at distances of ~2.45 Å and Sb-carbon distances of
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~3.33 Å, suggesting increasing Sb-thiol binding. Iterative target factor analysis allowed exclusion
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of reduced inorganic Sb-sulfur phases with similar Sb-sulfur distances. In conclusion, even when
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free sulfide concentrations are too low for formation of Sb-sulfur precipitates, peat NOM can
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sequester Sb in anoxic, sulfur-enriched environments.
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Sorbed Sb(III) [%]
100
Sb C
S
R 90
Sb O CC
R
80
41 42 43 44 45
Organic S
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INTRODUCTION
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Antimony (Sb) is a potentially toxic1-3 metalloid, whose mobility and speciation is strongly
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influenced by its redox transformations.4,5 Under oxic to slightly reducing conditions, the
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pentavalent antimonate is the thermodynamically favored species, while the trivalent antimonite is
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predicted to dominate under anoxic conditions. Antimonate thereby prevails as negatively charged
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Sb(OH)6− whereas antimonite occurs as neutral oxoacid Sb(OH)3 at environmentally relevant pH
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values.4
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In contrast to its group-five neighbor arsenic (As), the biogeochemistry of Sb is relatively poorly
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studied,6,7 despite pollution due to increased mining activities in the past years8,9 as well as broad
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industrial (flame retardants, catalyst in plastics synthesis etc.)1,10 and military (ammunition)11 use.
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The majority of research has been conducted on oxic environments11-14 with special focus on the
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role of iron and manganese (oxyhydr)oxides as well as clay minerals for the mobility of Sb.5
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However, there is little information on Sb aqueous- and solid-phase speciation under anoxic, partly
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sulfidic conditions with high abundances of solid natural organic matter (NOM). Such conditions
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prevail in peatlands and other wetlands.5,15 While natural antimony background concentrations
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including values from pristine peatlands for comparison are commonly low (0.008-0.3 mg kg-
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1 16,17
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received from mining activities, principally gold17 and Sb12,18-20 mining (with resulting Sb
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concentrations up to 22,000 mg kg-1).18
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, high Sb contents have been reported from atmospheric deposition16 and from waste waters
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Several studies have demonstrated in recent years, particularly for As17,21-23 but also for other
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(radioactive) toxic trace metal(loid)s,24-28 that natural and engineered peatlands and wetlands can
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act as important contaminant sinks due to covalent binding to solid NOM. On the other hand,
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complexation with dissolved or colloidal organic matter could lead to (re)mobilization of 4 ACS Paragon Plus Environment
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metal(loids) in organic-rich surface-, pore-, and groundwaters.29-32 On the basis of current findings,
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although limited in number, Sb sequestration appears more important in organic carbon-rich
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environments compared to mobilization because complexation of antimonite with dissolved
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organic ligands was generally observed to be limited (< 30% of total Sb).33-35
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In peatlands, iron and manganese (oxyhydr)oxides undergo reductive dissolution with increasing
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depth due to increasing water content—and eventual saturation36,37; additionally, (microbially
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triggered) sulfate-reduction leads to formation of dissolved sulfide with depth.38 Released
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metal(oid)s such as Sb or As can then react with sulfide, forming authigenic amorphous As/Sb-S
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precipitates at low to neutral pH values.5,39 Despite generally sulfidic conditions, however, free
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sulfide concentrations in peatlands are commonly low40-43 since the produced sulfide can be
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effectively incorporated into NOM as organic thiols40,44, often the dominant sulfur species in such
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systems.45 Precipitation of Sb(III) sulfide phases may therefore play a minor role in peatlands
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compared to complexation by reduced organic sulfur (thiol) functional groups.
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The association of Sb with solid NOM has so far only been shown in a few field studies, including
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studies from peat bogs in Switzerland16 and the United Kindom46-48 where a strong association of
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atmospheric anthropogenic Sb depositions with NOM was observed. Further, Fawcett et al.20
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revealed the association of Sb with solid NOM, and the presence of Sb(III)-S phases in aquatic
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sediments residing adjacent to a former mining site. In a recent study, Arsic et al.49 also reported
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the formation of an Sb(III)-S phase in contaminated wetland soil mesocosm experiments. They
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speculated the formation of authigenic Sb(III) sulfide or Sb(III) complexation with thiol functional
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groups, but were unable to distinguish between them. With organic-rich sediments from the same
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wetland, Bennett et al.50 showed with bulk extended X-ray absorption fine-structure (EXAFS)
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spectroscopy that up to 44% of total Sb was 3-fold coordinated to sulfur at a distance of 2.46 Å,
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indicative for Sb-thiol coordination. Since such a coordination environment is common in both 5 ACS Paragon Plus Environment
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phases, Sb-thiol complexes as well as in disordered SbS3 formed via reaction of antimonite with
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mackinawite (FeS)51, unequivocal attribution to one of these two Sb species could not be made.
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The aims of the present work were hence twofold. First, to determine the extent of antimonite
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sorption and its local binding environment (
