Metal-thiometalate transport of biologically active trace elements in sulfidic environments. 1. Experimental evidence for copper thioarsenite complexing

Pore waters in metal-contaminated sediments and buried waste deposits often are sulfidic owing to biological sulfate reduction. Sulfide transforms certain oxyanions to thiometalates. Those thiometalates which can donate two or more S ligands are expected to be powerful complexing agents for soft cations. This concept is explored by determining the solubility of the assemblage, CuS(covellite)Cu1.8S(digenite)-Cu3AsS4 (cubic equivalent of enargite or luzonite) versus pH and total sulfide at 25 degrees C. A provisional free energy of formation of -200.2 kJ/mol is found for cubic Cu3AsS4. Copper in the solid assemblage suppresses As solubility by lowering the activity of the As2S3 component compared to solutions equilibrated with orpiment (As2S3). At the same time, copper concentrations exceed substantially those expected from the solubility of CuS+Cu1.8S alone, implying existence of a ternary complex: Cu+ + H2AsOS2- reversible arrow CuH2AsOS20 log K = 19.82 +/- 0.17. Via this uncharged complex, Cu and As can synergistically mobilize each other when sulfide is present and might promote mutual bioavailability, This is the first of what may be a number of examples of strong interactions between biologically active metals and thiometalates. Such interactions are not yet accounted for in standard geochemical models used to predict mobility and bioavailability of hazardous materials in the environment.