Performance, mechanism, and ecotoxicity assessment of sulfamethoxazole degradation by coupling visible-light photocatalysis with persulphate activation over Fe3O4/ZnO composites

Abstract

ABSTRACT To probe the efficient and ecofriendly treatment of emerging pollutants, a magnetically separable Fe3O4/ZnO composite was facilely synthesised and utilised to establish a combined oxidation process by coupling photocatalysis with persulphate activation (Fe3O4/ZnO-PS). As proved by various characterisations, the Fe3O4/ZnO composite was constituted of highly dispersed Fe3O4 nanoparticles with ZnO nanorods as the support, and it showed the increased visible-light harvesting and facilitated charge transfer. The Fe3O4/ZnO-PS process significantly enhanced sulfamethoxazole (SMX) degradation under visible light irradiation, and the degradation rate constant and synergy coefficient approached 0.222 h−1 and 1.98, respectively. The enhancement mechanism of Fe3O4/ZnO-PS process for SMX degradation was resulted from the promotion of reactive oxygen species formation. Electron spin resonance and fluorescence measurements indicated that •OH and SO4•‒ majorly contribute to the SMX degradation. In addition, the effects of catalyst dosage, PS dosage, solution pH and inorganic anions were systematically investigated. Based on liquid chromatography – mass spectrometry (LC/MS) identification, there were totally 12 products of SMX degradation found in the Fe3O4/ZnO-PS process, the degradation pathways of which involve hydroxylation, S‒N bond cleavage, and SO2 abstraction. The ecotoxicities of these degradation products were predicted by quantitative structure-activity relationship software and were verified by actual experiments using Vibrio fischeri. The ecotoxicity assessment results indicated that although the Fe3O4/ZnO-PS process degraded SMX effectively, more toxic intermediates appeared and accordingly increased the ecological risk, which should be deeply concerned.