Geochemical Switches

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Helz9980532Biogenic sulfide profoundly affects geochemical behavior of many elements. This study focuses on interactions between HS- and oxyanions of the elements Mo, Re, Ge, As and Sb. In some cases soluble thioanions are formed by replacement of O atoms by S atoms around the core element in the presence of HS-. A dramatic switch in the geochemical properties of the core element ensues. The best studied example, the transformation of MoO42- to MoS42-, occurs abruptly at H2S(aq) = 11 3 ?M under near-equilibrium conditions. The abrupt character of the transformation and marked difference in geochemical properties of the thioanion vs. the oxyanion have led us to describe this transformation as a geochemical switch (Helz et al. 1996). Sulfide-actuated switches affect important geochemical properties of core elements: mobility, bioavailability, and chemical interactions with other solutes. Because thioanions are multidentate, sulfur-donating ligands, they are expected to interact particularly strongly with soft metals, which include many hazardous metals. The environmental and biogeochemical implications of such interactions are still unknown. Calibrating the action points of switches will enhance the use of trace elements as paleoredox indicators. The chief goal is production of the kinds of fundamental data needed in geochemical models to predict behavior of elements in reducing environments.