Radical chemistry and PPCP degradation in the UV/peroxydisulfate process in the presence of chloride at freshwater levels

Abstract

Chloride (Cl−) affects the sulfate radical (SO4•−)-based advanced oxidation processes for application. This study investigated the effects of Cl− at freshwater levels (5–1000 μM) on radical chemistry and pharmaceuticals and personal care products (PPCPs) degradation in a UV/peroxydisulfate (UV/PDS) process. By using radical probes and a kinetic model, SO4•− was converted to chlorine radical (Cl•), and Cl• was subsequently transformed to hydroxyl radical (HO•). The SO4•− concentration monotonically decreased by 56.8–91.7% at 5–1000 μM Cl− at pH 7, and that of HO• increased by 127–419%. Cl•/Cl2•− formation was less prevalent at ≤ 100 μM Cl−, but Cl2•− was notable at 1000 μM Cl−. Increasing pH promoted the transformation from SO4•− to HO• and drove the conversion from Cl•/Cl2•− to HO•, resulting in less Cl•/Cl2•− at pH 10. Increasing Cl− from 10 to 1000 μM decreased the pseudo-first-order rate constants (kʹs) of 18 PPCPs from 1.6% to 93.9% but increased the kʹs of 5 other PPCPs by 2.5–33.4%, compared to those without Cl−. Cl− (1000 μM) primarily transformed SO4•− to HO• and Cl• with the conversion ratios of 89.8% and 0.7%, respectively. Therefore, the contributions of HO• increased from 16–65.9% to 83.6–99.5%, and those of SO4•− decreased from 10.2–62.6% to < 8.6%, as Cl− rose from 10 to 1000 μM. This study demonstrated that at freshwater levels, Cl− primarily drove the conversion from SO4•− to HO•, emphasizing the importance of HO• in PPCP degradation in the UV/PDS process.