AN INVESTIGATION OF TIO2 PHOTOCATALYSIS FOR THE TREATMENT OF WATER CONTAMINATED WITH METALS AND ORGANIC-CHEMICALS

Laboratory experiments were performed to investigate TiO2 photocatalys is for treating water contaminated with dissolved metals (Ag, Au, Cd, Cr, Cu, Hg, Ni, and Pt) and a variety of organics (e.g., methanol, for mic acid, salicylic acid, EDTA, phenol,and nitrobenzene). It was found that only those metals with half-reaction standard reduction potentia ls more positive than 0.3 V (vs normal hydrogen electrode) can be trea ted using TiO2 as the photocatalyst. Kinetic data illustrating the syn ergism between oxidation and reduction are presented. Experiments usin g singly substituted benzenes as electron donors show that the rate of reduction of Cr(VI) is correlated with Hammett sigma constants. Photo efficiencies approaching 85% were measured for the conversion of Cr(VI ) to Cr(III) using citric acid as the reductant. In contrast, photoeff iciency was only 4% when oxidizing salicylic acid using O2 as the oxid ant. It is concluded that efficient designs of photocatalytic systems for wastewater treatment must take into account both oxidation and red uction processes.