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

Abstract

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 (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 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^•.

Keywords: Advanced oxidation process; Bromide; Hydroxyl radicals; Micropollutants; Sulfate radicals; Water treatment.