Roles of bromine radicals, HOBr and Br2 in the transformation of flumequine by the UV/chlorine process in the presence of bromide

Abstract

In the presence of bromide (Br−), new reactive species, such as bromine and reactive bromine species (RBS), are formed by the UV/chlorine process. In this study, the effects of Br− on the kinetics, products, and pathways of flumequine (FLU) transformation by the UV/chlorine process were investigated. The pseudo first-order rate constant (k′) for FLU degradation in the UV/chlorine process was enhanced by 2.5 times by 50 μM Br− at pH 7, as HOBr, Br2, RBS and HO• were formed, and they contributed 29.2%, 7.8%, 57.4% and 4.8% to k′, respectively. The second-order rate constants (k) of FLU reacting with HOBr, OBr−, Br2 and Br• were determined to be 115.6 M−1 s−1, negligible, 1.5 × 106 M−1 s−1 and 2.4 × 108 M−1 s−1, respectively. The contribution of Br2 was enhanced at a higher Br− level and a lower pH, which accounted for 48.2% of k′ at pH 6. The overall k′ value decreased by 97.9% from pH 6 to 9, during which k′ by HOBr, Br2 and RBS decreased significantly. The formation of brominated products was mainly attributed to HOBr and Br2, while RBS and HO• primarily promoted hydroxylation and ketonization during UV/chlorine treatment with Br−. The formation of total organic bromine (TOBr) by the UV/chlorine process with Br− was comparable to that during dark chlorination with Br−, while the yield of brominated disinfection byproducts in the former was much higher, further indicating that RBS and HO• promoted the formation of small-molecule products. In addition, the acute toxicity of FLU was significantly reduced during the UV/chlorine process with Br−. This is the first study to reveal the significant roles of Br2 and Br• in the degradation of micropollutants by the UV/chlorine process in Br−-containing water.