Sequential Process Combination of Photocatalytic Oxidation and Dark Reduction for the Removal of Organic Pollutants and Cr(VI) using Ag/TiO.

We investigated a sequential photocatalysis-dark reaction, wherein organic pollutants were degraded on Ag/TiO under UV irradiation and the dark reduction of hexavalent chromium (Cr(VI)) was subsequently followed. The photocatalytic oxidation of 4-chlorophenol (4-CP), a test organic substrate, induced the generation of degradation intermediates and the storage of electrons in Ag/TiO which were then utilized for reducing Cr(VI) in the postirradiation period. The dark reduction efficiency of Cr(VI) was much higher with Ag/TiO (87%), compared with bare TiO (27%) and Pt/TiO (22%). The Cr(VI) removal by Ag/TiO (87%) was contributed by adsorption (31%), chemical reduction by intermediates of 4-CP degradation (26%), and reduction by electrons stored in Ag (30%). When formic acid, humic acid or ethanol was used as an alternative organic substrate, the electron storage effect was also observed. The postirradiation removal of Cr(VI) on Ag/TiO continued for hours, which is consistent with the observation that a residual potential persisted on the Ag/TiO electrode in the dark whereas little residual potential was observed on bare TiO and Pt/TiO electrodes. The stored electrons in Ag/TiO and their transfer to Cr(VI) were also indicated by the UV-visible absorption spectral change. Moreover, the electrons stored in the preirradiated Ag/TiO reacted with O with showing a sign of low-level OH radical generation in the dark period.