Indirect reduction of hexavalent chromium by copper in the presence of superoxide

Indirect photoreduction of chromium(VI) was investigated in the presence of superoxide radical (O2−) produced by continuous radiolysis of water. Although there is no direct reaction between superoxide and chromium(VI), the presence of nanomolar concentrations of copper can result in Cr(VI) reduction through copper(I) produced during superoxide redox cycling. The reduction rate is a function of metal speciation and concentrations. Cr(VI) reduction rates increase with increasing copper concentrations. At chloride concentrations typical of estuaries and oceans (i.e., >0.01 M), reduction rates decrease because CuCl2− is much less reactive towards chromium(VI) than Cu+. At pH>6.5, Cr(VI) reduction rates decrease because CrO42− is much less reactive than HCrO4−. Experiments performed in natural water samples suggest that photo-initiated reduction of chromium(VI) by copper and superoxide is the most important mechanism of homogeneous, abiotic chromium(VI) reduction in atmospheric and surface waters with low ionic strength and pH. The reaction is relatively insignificant in marine or estuarine waters.