Zero-valent sulfur and metal speciation in sediment porewaters of freshwater lakes.

We measured the aqueous solubility of rhombic sulfur and used this information to incorporate, in speciation codes, the thermodynamic constants reported in the literature for the formation of polysulfide complexes. Using the values of pH and total concentrations of dissolved zerovalent sulfur, sulfide, humic substances, trace metals (Ag, Cd, Cu, Hg, methylmercury, Pb, Zn) and major ions measured in anoxic porewaters of nine oligotrophic Canadian lakes as input to these speciation codes, we show that the porewaters of these lakes are undersaturated or close to saturation with respect to rhombic sulfur and that sulfide and polysulfide ligands play a dominant role in controlling metal speciation in freshwater anoxic environments when they are present at micromolar levels. The study also highlights the need for further research on the formation constants of metal complexes with sulfide and polysulfides.