Photocatalytic Degradation of Diclofenac using Different Mixed Oxide Catalysts

Abstract

Elimination of pharmaceutical compounds and their metabolites from the aquatic systems has been a tedious process. The various photocatalytic process using TiO2 as semiconductor photocatalysts has enormous potential to cope with the challenges in the removal of pharmaceutical compounds. However, the TiO2-mediated photocatalytic water treatment suffers from the faster rate of recombination and wide bandgap energy corresponding to UV light. Coupling with other semiconductor oxides has reportedly reduced the recombination rate and drawbacks of the short excitation range. In the present work, the degradation of diclofenac in the photocatalytic system is carried out by using different mixed oxide catalysts prepared by hydrothermal method. The heterojunction mixed oxide catalysts can improve the photocatalytic activity by reducing the recombination rate of charge carriers and enhanced the excitation ability of the coupled catalysts in the visible light region. A series of mixed oxide catalysts were prepared with different molar concentrations of TiO2-SnO2, TiO2-WO3, ZnO-WO3 and TiO2-CdS and were characterized by XRD, TEM, BET surface area and UV spectrophotometric analyses. Initially, the performance of a series of TiO2-SnO2 mixed oxide catalysts was studied. The photocatalytic efficiency was analyzed in the degradation of diclofenac and the degradation kinetics were extensively investigated. The influence of various parameters such as initial drug concentration, pH and catalyst loading was also studied. The TiO2-WO3 mixed oxide catalysts were tested for its photocatalytic efficiency and the results suggested that the series of prepared mixed oxide catalysts exhibited better catalytic activity than the pure TiO2 under visible light irradiation. The diclofenac degradation using ZnO-WO3 heterojunction catalysts under visible light irradiation were evaluated and the synthesized ZnO-WO3 mixed oxide catalyst produced better performance than the individual components of the photocatalyst. Degradation of diclofenac using TiO2-CdS mixed oxides shows that the presence of the optimum amount of CdS in the coupled heterostructure exhibited higher photocatalytic efficiency under visible light. Comparing the performance of all the mixed oxide catalysts, TiO2-WO3 mixed oxide catalysts displayed the best catalytic activity among others under optimum operating conditions. Degradation experiment data of all the mixed oxide catalysts well fitted to pseudo-first-order reaction and the rate constants were determined. The photocatalytic degradation of diclofenac was greatly affected by initial pH, catalyst dosage and initial diclofenac concentration. The degradation was highly effective under the acidic condition for all the prepared coupled photocatalysts and the surface charge property of photocatalysts played an important role in the adsorption of drug diclofenac onto the catalyst surface. The degradation reaction mechanisms of the mixed oxide catalysts were studied and it must be noted that the hydroxyl radicals and photogenerated holes were the main active species in the diclofenac degradation process. The charge transfer between heterostructure photocatalysts has been confirmed by the photoluminescence studies. LCMS was used to analyze the various degradation products formed during the irradiation and it is revealed that several MS peaks corresponding to partially degraded products were observed during the course of photocatalytic degradation of diclofenac. Mainly these observed degradation products were preceded by the attack of •OH radicals and hydroxylation reactions which was further followed by decarboxylation, dechlorination and C-N cleavage reactions.