Antimony speciation in alkaline sulfide solutions: role of zerovalent sulfur

Antimony is subject to a lower drinking water standard than arsenic, its notorious group 15 cohort in the periodic table. Both elements often co-vary in nature and are fairly soluble under reducing, alkaline conditions. Of the two, much less is known about the environmental chemistry of Sb. Measurements of Sb solubility in sulfidic solutions equilibrated with stibnite (Sb2S3) and orthorhombic sulfur reveal the existence of two new complexes that may control Sb behavior in many reducing environments. Formation reactions and stability constants (23 +/- 2 degrees C) are HS- + S(s) + Sb2S3(s) <==> HSb2S5-, log K = -1.47 +/- 0.17; and HS- + 2S(s) + Sb2S3(s) <==> Sb2S6(2-) + H+, log K = -9.55 +/- 0.07. The first complex is a mixed-valence Sb(III,V) complex; the second is an Sb(V) complex. Their stability in sulfidic solutions may explain previously puzzling evidence of Sb(V) in natural anoxic environments. Owing to these complexes, zerovalent S can enhance stibnite solubility up to 3 orders of magnitude. In neutral-to-alkaline, reducing environments, less than 7 microM HS- will transform O-coordinated, electrically neutral Sb(OH)3o to predominantly anionic S-coordinated complexes. This transformation could diminish the adsorption of Sb to negatively charged mineral surfaces, lowering retardation factors in anoxic aquifers.