Enhanced photocatalytic reduction of chromium (VI) by Cu-doped TiO2 under UV-A irradiation

This paper describes the photocatalytic reduction of aqueous hexavalent chromium (Cr(VI)), a potential human carcinogen and genotoxin, by copper doped TiO2 (Cu-TiO2) prepared by the simple-evaporation and calcination of CuCl2 and TiO2, under UV-A irradiation in the absence of hole scavengers. The Cu-TiO2 prepared at a Cu to TiO2 mass ratio of 1.5:100 and a calcination temperature of 300 degrees C showed the fastest photocatalytic reduction of Cr(VI). The reduction by the Cu-TiO2 followed pseudo-first order reaction kinetics, and its rate was twice as fast as that of unmodified TiO2 at pH 3. The Cu-TiO2 reached its maximum Cr(VI) reduction rate at pH 5, producing steady state hydrated electron concentrations ([e(-)](ss)) 3.3 times higher than those by unmodified TiO2. The optimum working pH of the Cu-TiO2 is higher than that of many other photocatalysts. The XPS, XRD, TEM and SEM spectra all confirmed the incorporation of Cu into TiO2, and the BET analysis showed the similar surface areas of the Cu-TiO2 and unmodified TiO2. The enhanced reduction rate by the Cu-TiO2 was thus attributed to the reduced electron-hole recombination rate. More importantly, the Cr(VI) reduction rate by the CuTiO2 was only reduced by 20% with increasing alkalinity from 0 to 200 mg/L at pH 7, and it was not affected by the presence of NOM at concentrations up to 1 ppm. Lastly, the Cr(VI) reduction rate by the Cu-TiO2 was faster in tap water than in double deionized water. The wider working pH (optimum at pH 5), without addition of hole scavengers, and the faster reduction in tap water suggest the superior performance of the Cu-TiO2 photocatalyst in real water matrices.