INDIRECT PHOTOREDUCTION OF AQUEOUS CHROMIUM(VI)

Field studies in a shallow estuary have revealed daytime decreases in the dissolved Cr(VI)/Cr(III) ratio that correlate temporally with increases in the H2O2 concentration. Photolyzing water samples from the field site in silica flasks produces Cr(VI) loss and Cr(III) gain in essentially equimolar amounts. Reduction occurs neither in dark controls nor in prefiltered samples, indicating that both light and particles are essential. Field and laboratory results are consistent with reduction initiated by Fe(II), which has been generated photolytically from particulate ferric oxyhydroxides plus organic matter. Alternative mechanisms are discussed. At neutral pH, micromolar Fe(II) reduces nanomolar Cr(VI) in air-saturated solutions, despite O2 competition for the Fe(II). Because photoreduction of Fe appears to be common in nature and because the reaction of Fe(II) with Cr(VI) is extremely rapid, this reaction should be of general importance in surface waters. This natural process should be harnessable for treatment of Cr wastes.