Photocatalytic activity of ZnO-WO3 for diclofenac degradation under visible light irradiation

Abstract

Diclofenac is known to be a persistent pharmaceutical compound, and their effective removal from water sources has been a rising concern. This study reports the visible light irradiated photocatalytic degradation of diclofenac using ZnO-WO3 mixed oxide catalysts, prepared by the hydrothermal method with variation in molar ratios of tungsten precursor. The prepared catalysts were characterized using a different technique, and the photocatalytic activity has been tested under visible light irradiation. Adsorption isotherm studies were performed to elucidate the preferential adsorption nature of the mixed oxide catalyst. The interaction of diclofenac and the prepared mixed oxides is based on the positively charged ZnO-WO3 surface and anionic diclofenac at solution pH 6. The adsorption and photodegradation kinetics is best expressed by the Langmuir isotherm model and pseudo-first-order kinetic model, respectively. Results indicated that all the prepared catalysts exhibited better catalytic activity than the bare ZnO under the visible light irradiation. The catalyst prepared with a molar ratio of 10:1 is proven to be an efficient catalyst and achieved 76%mineralization of diclofenac during the irradiation. The effect of operating variables like pH, initial diclofenac concentration, and catalyst loading was investigated and reported. The ZnO-WO3 mixed oxide catalysts were showed better stability, and the results revealed that the photocatalytic efficiency was retained up to 80% over the repeated reaction cycles. Several intermediate compounds formed during the photocatalytic reaction have been analyzed using LC–MS, and their degradation pathways have been found to primarily follows hydroxylation, dechlorination and decarboxylation reactions.