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

•Br− converts SO4•− to HO• and bromine radicals and also scavenges HO• in UV/PDS.•Bromine radicals are much higher than SO4•− and HO• at pH 7 but lower at pH 10.•Cl− and Br− converts SO4•− to HO• but chlorine radicals are negligible.•Br− at several μM levels promote some micropollutant degradation in UV/PDS. Bromide (Br−) is a typical scavenger to sulfate radical (SO4•−) and hydroxyl radical (HO•), which simultaneously forms secondary reactive bromine species (RBS) such as Br• and Br2•−. 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 SO4•− to HO• and RBS in the UV/PDS process. At pH 7, the concentration of SO4•− 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 Br2•− (10−12 − 10−10 M) were 2 – 3 orders of magnitude higher than SO4•− and HO•. Alkaline condition promoted the conversion from SO4•− 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 (kʹs) of 15 pharmaceuticals and personal care products (PPCPs) by 15.2 – 73.9%, but increased kʹs 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 SO4•− to HO• compared to Br− alone. Bicarbonate consumed SO4•− and HO• but slightly affected RBS, while natural organic matter (NOM) exerted scavenging effects on HO• and RBS more significantly than SO4•−. 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•. [Display omitted]