Sequential Process Combination of Photocatalytic Oxidation and Dark Reduction for the Removal of Organic Pollutants and Cr(VI) using Ag/TiO_2

摘要

We investigated a sequential photocatalysis-dark reaction, wherein organic pollutants were degraded on Ag/TiO_2 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_2 which were then utilized for reducing Cr(Vl) in the postirradiation period. The dark reduction efficiency of Cr(VI) was much higher with Ag/TiO_2 (87%), compared with bare TiO_2 (27%) and Pt/TiO_2 (22%). The Cr(Vl) removal by Ag/TiO_2 (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_2 continued for hours, which is consistent with the observation that a residual potential persisted on the Ag/ TiO_2 electrode in the dark whereas little residual potential was observed on bare TiO_2 and Pt/TiO_2 electrodes. The stored electrons in Ag/TiO_2 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_2 reacted with O_2 with showing a sign of low-level OH radical generation in the dark period.