Photo-reduction of hexavalent chromium in aqueous solution in the presence of zinc oxide as semiconductor catalyst

Common sources of chromium in wastewater are electroplating and leather industries. Hexavalent chromium is more toxic and carcinogenic compared to its trivalent counterpart. Conversion of Cr(VI) to Cr(III) in an aqueous medium by photocatalytic reduction using UV radiation and ZnO semiconductor catalyst has been investigated using potassium dichromate as the model compound. Effects of the process parameters such as ZnO loading (0–3 g/L) and intensity of UV radiation (0–125 W medium pressure Hg vapour lamp) on photocatalytic reduction were investigated. Initial concentration of substrate solution was varied from 40 to 120 mg/L. A comparison of the performance of the ZnO photocatalyst achieved in this study with that reported in the literature for photocatalytic and conventional processes is presented. The initial rate of the photochemical method was found to be independent of the concentration of potassium dichromate. Methanol was added as a hole scavenger for enhancement of the photo-reduction. Initial rate of photo-reduction was found to be independent of the concentration of methanol above a particular value. A probable mechanism and the corresponding kinetic model have been proposed for the photo-reduction and tested by experimental results. An alternative rate equation based on the LHHW model compares well with the mechanistic rate equation.