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

Publisher Website
Google Scholar
Abstract
The availability and toxicity of trace metals in fresh water are known to be regulated by the complexation of free metal ions with dissolved organic matter1,2,3. The potential role of inorganic sulphides in binding trace metals 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 Maryland4,5. Here we report dissolved (< 0.2 µm particle diameter) sulphide concentrations 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 dissolved Fe and Zn and 45% of the total dissolved Cu. Fourier transform mass spectrometry reveals that these complexes are not simple M(HS)+ protonated species6,7 but are higher-order unprotonated clusters (M3S3, M4S 6, M2S4), similar to those found in laboratory solutions8,9,10 and bio-inorganic molecules11. These extended structures have high stability constants8,10 and are resistant to oxidation and dissociation10,12, which may help control the toxicity of these and other less abundant, but more toxic, trace metals, such as silver, cadmium and mercury.