Photocatalytic generation of sulphate and hydroxyl radicals using zinc oxide under low-power UV to oxidise phenolic contaminants in wastewater

Photochemical and photocatalytic reaction and the combination of both for the oxidation of phenol in aqueous solution were investigated. Photochemical reaction was carried out using three different types of oxidants in the presence of UV radiation while photocatalytic reaction was carried out on zinc oxide under UV radiation. Three different oxidants viz peroxymonosulphate (PMS), peroxydisulphate (PDS) and hydrogen peroxide (H2O2) were employed to generate active sulphate/hydroxyl radicals for oxidation. The synergetic effect of combination of UV/oxidant/ZnO was investigated for the three different oxidant systems. It was found that homogeneous photochemical oxidation of phenol using PMS, PDS or H2O2 exhibited much better performance than heterogeneous photocatalytic oxidation using UV/ZnO. The combination of photochemical and photocatalytic oxidation by combining UV/oxidant with ZnO at low UV radiation showed different degradation trends depending on the oxidant. Scattering of UV radiation in UV/PDS/ZnO and UV/H2O2/ZnO systems showed a slower rate of phenol degradation as compared to the photochemical oxidation under UV/PDS and UV/H2O2 systems. However, UV/PMS/ZnO showed higher rate of phenol oxidation as compared to the photochemical oxidation under UV/PMS, which was mainly attributed to the self-chemical reaction between ZnO and PMS. The phenol degradation in UV/PMS/ZnO followed the first-order kinetics while the phenol degradation resulted from the self-chemical reaction of ZnO/PMS followed zero-order kinetics.