Eliminating the need for organic hole scavenger in the photoreduction of selenite and selenate from aqueous phase

Abstract

This research work investigated the photoreduction of selenite [Se(IV)] and selenate [Se(VI)] species from the aqueous phase without the use of organic hole (h+) scavengers. The use of organic hole scavengers to aid in photocatalytic removal of selenium species leads to the discharge of partially treated organics to water bodies which may, in turn, pose a high organic loading, leading to the utilization of the oxygen meant for aquatic lives. To achieve the said objective, ZnO/TiO2 nano-composite was synthesized via the sol–gel method. ZnO was immobilized on TiO2 to increase the lifespan of excitons enough for the reduction process to complete, in the end, no organic hole scavenger is needed. The morphology, functional group, and crystal structure of the photo-catalyst were confirmed via characterization with SEM/EDS, FTIR, and XRD respectively. Photocatalytic removal of 20 ppm Se(IV)/Se(VI) were investigated at pH 4 under UV irradiation using 1 g/L ZnO/TiO2 photo-catalyst dosage. Results obtained after 22 h irradiation show a 100% reduction of Se(VI) to the non-toxic and non-mobile form of elemental selenium. On the other hand, results obtained for the removal of 20 ppm Se(IV) show 100% oxidation of Se(IV) to Se(VI) followed by subse-quent conversion of Se(VI) to elemental selenium with 79% efficiency. The results presented in this study show that complete selenite and selenate removal from the aqueous phase can be achieved through ZnO/TiO2 photocatalysis without the use of h+ scavengers. The photoreduction reaction kinetics were found best fitted to zeroth and first order for selenate and selenite respectively.