Humic Acid-Sensitized Photoreduction of Cr(VI) on ZnO Particles

The possibility that humic acids act as sensitizers in the photoinduced reduction of Cr(VI) to Cr(III) in aqueous suspensions containing ZnO particles was verified. Equilibrium sorption studies showed that Cr(III) is totally adsorbed on ZnO, while Cr(VI) adsorption on ZnO is very low. Solar irradiation of the ZnO suspensions induced the photoreduction of Cr(VI), leading to a marked increase of the amount of chromium adsorbed on the mineral oxide. The yield of the reaction is enhanced both in the presence of humic acids and under anoxic conditions. Humic acids adsorbed onto the oxide surface sensitize Cr(VI) photoreduction and the competitive photoreduction of molecular oxygen leading to hydrogen peroxide.