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

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.