Insights into the effects of bromide at fresh water levels on the radical chemistry in the UV/peroxydisulfate process.

Bromide (Br) is a typical scavenger to sulfate radical (SO) and hydroxyl radical (HO), which simultaneously forms secondary reactive bromine species (RBS) such as Br and Br. This study investigated the effects of Br at fresh water levels (~μM) on the radical chemistry in the UV/peroxydisulfate (UV/PDS) process by combining the degradation kinetics of probe compounds (nitrobenzene, metronidazole, and benzoate) with kinetic model. Br at 1 - 50 μM promoted the conversion from SO to HO and RBS in the UV/PDS process. At pH 7, the concentration of SO monotonically decreased by 31.5 - 94.8% at 1 - 50 μM Br, while that of HO showed an increasing and then decreasing pattern, with a maximum increase by 171.7% at 5 μM Br. The concentrations of Br and Br (10 - 10 M) were 2 - 3 orders of magnitude higher than SO and HO. Alkaline condition promoted the conversion from SO to HO, and drove the transformation from RBS to HO, resulting in much lower concentrations of RBS at pH 10. Br at 1 μM and 5 μM decreased the pseudo-first-order reaction rates (k's) of 15 pharmaceuticals and personal care products (PPCPs) by 15.2 - 73.9%, but increased k's of naproxen and ibuprofen by 13.7 - 57.3% at pH 7. The co-existence of 10 - 1000 μM Cl with 5 μM Br further promoted the conversion from SO to HO compared to Br alone. Bicarbonate consumed SO and HO but slightly affected RBS, while natural organic matter (NOM) exerted scavenging effects on HO and RBS more significantly than SO. This study demonstrated that Br at fresh water levels significantly altered the radical chemistry of the UV/PDS process, especially for promoting the formation of HO.