Metal-sulfide species in oxic waters

Several analytical techniques were employed to characterize oxidation-resistant metal-sulfide species that have recently been suggested to be present in oxic waters. Sulfide in synthetic Cd-, Zn-, Pb-, Cu-sulfide solutions was found to persist in oxic waters for a prolonged period of time (2 to more than 10 weeks) whereas it was oxidized rapidly in Fe-, Mn- and Ni-sulfide solutions. Multiple lines of evidence did not support the presence of soluble molecular metal-sulfide clusters in these solutions as suggested previously. Instead, the metal-sulfide species resistant to oxidation in oxic waters were more likely a mixture of truly dissolved metal-sulfide complexes and dynamic metal-sulfide colloids. The morphology and size of the colloids, as monitored by transmission electron microscopy (TEM), changed significantly with time. Nanomolar to sub-micromolar levels of acid-volatile sulfide (AVS) and chromous-volatile sulfide (CVS) were measured in oxic surface waters of six lakes located on the Canadian Shield and in the nearby areas. Thermodynamic calculations indicated that at the AVS levels measured, the dissolved metal-sulfide complexes play a minor role in the speciation of Class B metal ions such as Pb, Cd, Cu, Hg. The relative importance of metal-sulfide colloids in these natural oxic waters remains unknown.