Fate of elemental Selenium in Sulfur dominated environments

Selenium (Se) is an essential yet toxic trace element with one of the narrowest nutritional optimums of all elements. Se speciation plays a crucial role in its mobility, bioavailability, bioaccumulation, and toxicity. The current perception of Se environmental cycling encompasses a linear series of successive, bi-directional redox processes. Elemental Se is seen as a central species thermodynamically favored in redox conditions found in most environments. Most studies on Se environmental transformations focused on systems characterized by high Se concentrations. In nature though, sulfur (S) concentrations are in general orders of magnitude higher than those of Se. This work investigated elemental selenium reactivity in sulfur dominated environments. A set of laboratory experiments were conducted to determine the reaction rates of elemental selenium with sulfur in various environmental conditions. Our data clearly indicates that an abiotic reaction was occurring between elemental Se and S at neutral to alkaline conditions under anaerobic conditions, solubilizing elemental Se. At neutral pH (pH = 7), the reaction rates were low, whereas at high pH (pH = 12), the reaction was fast and all elemental Se was consumed by the reaction within 12 h. We present for the first time the detailed kinetics of reaction at various environmental conditions and discuss the control exerted by sulfur on selenium cycling.