Evidence for iron, copper and zinc complexation as multinuclear sulphide clusters in oxic rivers

The availability and toxicity of trace metals in fresh water are known to b e regulated by the complexation of free metal ions with dissolved organic m atter(1-3). The potential role of inorganic sulphides in binding trace meta ls has been largely ignored because of the reduced persistence of sulphides in these oxic waters. However, nanomolar concentrations of copper and zinc sulphides have been observed in four rivers in Connecticut and Maryland(4, 5). Here we report dissolved (< 0.2 mu m particle diameter) sulphide concen trations ranging up to 600 nM, with more than 90% being complexed by copper , iron and zinc. These complexes account for up to 20% of the total dissolv ed Fe and Zn and 45% of the total dissolved Cu. Fourier transform mass spec trometry reveals that these complexes are not simple M(HS)(+) protonated sp ecies(6,7) but are higher-order unprotonated clusters (M3S3, M4S6, M2S4), s imilar to those found in laboratory solutions(8-10) and bio-inorganic molec ules(11). These extended structures have high stability constants(8,10) and are resistant to oxidation and dissociation(10,12), which may help control the toxicity of these and other less abundant, but more toxic, trace metal s, such as silver, cadmium and mercury.