Elsevier

Chemical Geology

Volume 262, Issues 3–4, 30 May 2009, Pages 179-185
Chemical Geology

Behaviour of Sb(V) in the presence of dissolved sulfide under controlled anoxic aqueous conditions

https://doi.org/10.1016/j.chemgeo.2009.01.008Get rights and content

Abstract

Antimony (Sb) exists in natural waters in two oxidation states, Sb(V) and Sb(III), with the reduced form generally leading to more toxic behaviour. Factors leading to the appearance of Sb(III) are therefore important from toxicity, bioavailability and ultimately water quality standpoints, particularly under reducing conditions found in eutrophic lakes and sediment porewaters. In this study, kinetics of the reduction of aqueous Sb(V) to the more toxic Sb(III) by dissolved sulfide were explored under controlled anoxic conditions and at environmentally relevant concentrations of the reagents. Hydride Generation Atomic Fluorescence Spectrometry (HG-AFS) was employed for redox speciation analysis of Sb and visible spectrophotometry was used for both dissolved sulfide and elemental sulfur analyses. At room temperature, reduction of Sb(V) initially present as Sb(OH)6, by dissolved sulfide was extensive and far more rapid at rmore acidic pH values. The reaction was found to be approximately first order with respect to all three of [Sb(OH)6], [H2S] and [H+]. Metastibnite, Sb2S3(s), precipitated after reduction of Sb(V) in working experimental samples at buffered pH of 5.0 and 6.0. Stable Sb(III)–S complexes were postulated as intermediates in the redox reaction between Sb(OH)6 and dissolved sulfide under anoxic conditions based on acquired data and previous literature. The oxidation product of dissolved sulfide was identified as elemental sulfur. This study has demonstrated the ability of dissolved sulfide to reduce Sb(OH)6 under a variety of environmentally relevant concentrations and conditions.

Introduction

Until recently, antimony (Sb) had largely been overlooked as an element of environmental concern and its study with respect to environmental conditions has been neglected. In the last five years however, an increasing number of publications have appeared on the presence and behaviour of Sb in environmental systems. As such, compounds of this element are now considered pollutants of priority interest by the United States Environmental Protection Agency (USEPA, 1979) and the European Union (Council of European Communities, 1976), although there is still insufficient knowledge of the redox transformation, transport, biogeochemistry and resulting availability of these compounds in the natural environment (Filella et al., 2002a). This lack of fundamental understanding of Sb fate in the aquatic environment needs to be addressed in order for further insight to be obtained. Sb occurs in a variety of oxidation states (− III, 0, III, V), however, it is mainly found in the III and V oxidation states in environmental, biological and geochemical systems and the element is not considered essential for plants or animals (Filella et al., 2007). In nature, Sb is a strong chalcophile element, occurring mainly as Sb2S3 (stibnite, antimonite), Sb2O3 (valentinite) and sulfosalts. Sb(V) is usually the predominant oxidation state of antimony in oxic waters likely present as Sb(OH)6 at pH conditions of natural systems (see Filella et al., 2002b and references wherein). However, the presence of Sb(III) has been reported (Bertine and Lee, 1983, Andreae and Froelich, 1984, Cutter, 1991) in oxic systems. Sb(III) and Sb(V) can also interact with chloride, organic low molecular weight ligands, natural organic matter and sulfide (Filella et al., 2002b). In a literature review of laboratory data, Spycher and Reed (1989) came to the conclusion that the main aqueous Sb complexes in equilibrium with stibnite were H2Sb2S4, HSb2S4 and Sb2S42− but only HSb2S4 would be stable within the pH range of typical geochemical systems. They also indicated that Sb(V) sulfide complexes do not need to be considered in reduced sulfide solutions except at very high sulfide concentrations. However, the presence of SbS43− in alkaline solutions containing stibnite and molar levels of NaHS was proposed by Mosselmans et al. (2000) and that of Sb(HS)4+ in alkaline sulfide and sulfide–chloride hydrothermal solutions at different temperatures by Sherman et al. (2000), the results of both studies being supported by EXAFS data. The formation of the mixed Sb(III–V), HSb2S5 and the Sb(V), Sb2S62− complexes was observed by Helz et al. (2002) when stibnite and orthorhombic sulfur react in alkaline sulfidic solutions. Their calculated stability fields of dissolved Sb species as a function of the activities of sulfur and sulfide indicate that the mixed valence complex could predominate in the Black Sea. The existence of those Sb(III–V) and Sb(V) sulfide complexes is also supported by the thermodynamic calculations of Tossell (2003). Even if they have not yet been clearly observed, the existence of thioantimony (V) complexes under neutral or slightly acidic solutions cannot be excluded.

The geochemical behaviour of Sb was studied in the sediments of two lakes distinctively different at their respective sediment–water interface (Chen et al., 2003). Sb(III) was present under reducing conditions, possibly as SbS2 (or Sb2S42−) according to their thermodynamic calculations. They also found that Sb(V) was present in porewaters under mildly reducing conditions and they attributed this presence to the oxidizing effect of iron and manganese oxyhydroxides or the slow kinetics of reduction or possible complexation by dissolved sulfide. These could explain the presence of Sb(V) in anoxic waters if it is coupled to a slow reduction rate of Sb(V) species under such conditions. It was also suggested that iron sulfides could play a role in controlling the solubility of Sb in reducing sediments.

Laboratory studies have shown that Sb(III) could be oxidized by O2 and H2O2 (Leuz and Johnson, 2005) or by amorphous iron and manganese oxyhydroxides (Belzile et al., 2001) but much less work has been done on the reduction of Sb(V) species into Sb(III). Recently, the abiotic reduction of Sb(V) by green rust has been reported by Mitsunobu (2008).

Reduced sulfur compounds are ubiquitous components of anaerobic sediments. The presence of sulfides usually indicates characteristic biologic processes in anoxic sediments and marine waters, which oxidize organic matter through bacterial reduction of sulfate and production of H2S(g) (Stumm and Morgan, 1996). Solubility and protonation of H2S(g) depends on pH, and is negatively correlated to both salinity and temperature. Dissolved sulfide is known to be a strong reducing agent (Morse et al., 1987 and references therein) and is a likely candidate to participate in redox reactions with trace elements such as Sb in sediments. The role of dissolved sulfide in controlling the redox speciation of Sb has not yet been addressed. The specific objective of this research was to study redox transformations of Sb that are likely occurring in the anoxic porewaters of lake sediments. Reduction of Sb(V) to Sb(III) by dissolved sulfide was studied at environmentally relevant concentrations and pH levels, under anoxic conditions in a controlled atmosphere glove box. Hydride generation atomic fluorescence spectrometry (HG-AFS) was employed to study antimony redox speciation and UV–visible spectrophotometry was employed to study the evolution of dissolved sulfide and appearance of elemental sulfur in working experimental samples.

Section snippets

Sample preparation

All sample preparation and experimentation prior to dilution of samples for analysis was performed in a Vacuum Atmospheres Co. OMNI-Lab inert atmosphere laboratory system at oxygen levels ≤ 30 ppmv. This controlled atmosphere glove box (CAGB) was capable of maintaining oxygen concentrations of ≈ 1 ppmv for the duration of experimental timeframes. Buffer solutions were prepared in autoclaved, double deionized water (DDW). Degassing of water was carried out inside the CAGB with stirring (N2, Air

Reduction of Sb(OH)6 by dissolved sulfide at pH 7.0

At pH 7.0, relatively little reduction of Sb(OH)6 was observed in the working experimental samples that initially contained 0.020 and 0.20 mM TDS. When the TDS concentration was kept at 0.020 mM, the percentage of reduced Sb(OH)6 was only 3–5% after 162 h of reaction time; when the concentration of total dissolved sulfide was increased to 0.20 mM, 11 to 16% of Sb(OH)6 was reduced to Sb(III), possibly as Sb2S42− and HSb2S4− (Krupp, 1988), over the same period of time. At a TDS concentration

Conclusions

Under anoxic conditions, Sb(OH)6 can be reduced to Sb(III) by dissolved sulfide under a wide range of environmentally relevant concentrations and pH values. The products of the redox reaction between Sb(OH)6 and H2S include elemental sulfur and various inferred aqueous complexes of Sb(III)(aq) which depend on pH. Under acidic conditions, metastibnite was found to precipitate after the reduction of Sb(OH)6 by dissolved sulfide. Pseudo first-order kinetics as well as the initial rate method

Acknowledgements

This study received the financial support from the Natural Sciences and Engineering Research Council of Canada. Critical comments from Dr. Joy Gray-Munro are sincerely acknowledged. The manuscript greatly benefited from the judicious comments of anonymous reviewers.

References (32)

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