In Situ Photochemical Activation of Sulfate for Enhanced Degradation of Organic Pollutants in Water

The advanced oxidation process (AOP) based on SO4•- radicals has been receiving growing attention in water and wastewater treatment. Producing SO4•- radicals by activation of peroxymonosulfate or persulfate faces the challenges of high operational cost and potential secondary pollution. In this study, we report the in situ photochemical activation of sulfate (i-PCAS) to produce SO4•- radicals with bismuth phosphate (BPO) serving as photocatalyst. The prepared BPO rod-like material could achieve remarkably enhanced degradation of 2,4-dichlorophenol (2,4-DCP) in the presence of sulfate, indicated by the first-order kinetic constant (k = 0.0402 min-1) being approximately 2.1 times that in the absence (k = 0.019 min-1) at pH-neutral condition. This presented a marked contrast with commercial TiO2 (P25), the performance of which was always inhibited by sulfate. The impact of radical scavenger and electrolyte, combined with electron spin resonance (ESR) measurement, verified the formation of •OH and SO4•- radicals during i-PCAS process. According to theoretical calculations, BPO has a sufficiently high valence band potential making it thermodynamically favorable for sulfate oxidation, and weaker interaction with SO4•- radicals resulting in higher reactivity toward target organic pollutant. The concept of i-PCAS appears to be attractive for creating new photochemical systems where in situ production of SO4•- radicals can be realized by using sulfate originally existing in aqueous environment. This eliminates the need for extrinsic chemicals and pH adjustment, which makes water treatment much easier, more economical, and more sustainable.