Effective photocatalytic removal of selected pharmaceuticals and personal care products by elsmoreite/tungsten oxide@ZnS photocatalyst

In this study, elsmoreite/tungsten oxide is used to form a heterojunction with ZnS-containing industrial waste. The effect of the elsmoreite/tungsten oxide content on photocatalytic activity of ZnS using the different ratios of ZnS:Na₂WO₄ in the synthesis solution is estimated. The initial ZnS:Na₂WO₄ ratio leads to the formation of hexagonal WO₃∙0.33H₂O on the surface of ZnS. A further increase in the ZnS:Na₂WO₄ ratio results in the domination of cubic WO₃∙0.5H₂O over hexagonal WO₃. The ultraviolet–visible (UV–Vis) diffuse reflectance spectra of elsmoreite/tungsten oxide@ZnS composite photocatalysts show that the absorption onset shifts monotonously towards lower wavelengths from 450 nm to 400 nm. The microrods of hexagonal WO₃ and {111}-truncated submicron-sized crystals of WO₃∙0.5H₂O are grown on the ZnS surface. The transmission electron microscopy (TEM) results confirm the formation of a heterojunction between elsmoreite/tungsten oxide and ZnS. The photocatalytic activities of elsmoreite/tungsten oxide@ZnS composite photocatalysts are evaluated for the degradation of selected pharmaceuticals and personal care products (PPCPs): metoprolol - Mt, triclosan - TCS, and caffeine – CAF both in single and in mixture solutions. The elsmoreite/tungsten oxide@ZnS photocatalysts degrade 50% of Mt, 70% TCS, and 60% CAF in single solution and 35% of Mt, 20% of CAF, and 20% of TCS in mixture solution. Hydrated Mt and TCS are preferably adsorbed on the surface of WO₃∙0.5H₂O (111), and CAF has better affinity to the surface of WO₃. The elsmoreite/tungsten oxide@ZnS photocatalysts show a good reusability. Hydroxyl radicals (^•OH) and photogenerated holes (h⁺) are involved in the photocatalytic removal of Mt, while only h⁺ is involved in the photocatalytic removal of TCS. Interestingly, none of the above-mentioned species is involved in the photocatalytic removal of CAF. Also, nontoxic CAF is mainly degraded into intermediates with higher toxicity. The toxicity of the photocatalytically treated model wastewater in the mixture solution, tested with Vibrio fischeri, is much lower than that in the single solution.