Comparison of Halide Impacts on the Efficiency of Contaminant Degradation by Sulfate and Hydroxyl Radical-Based Advanced Oxidation Processes (AOPs)

The effect of halides on organic contaminant destruction efficiency was compared for UV/H2O2 and UV/S2O8(2-) AOP treatments of saline waters; benzoic acid, 3-cyclohexene-1-carboxylic acid, and cyclohexanecarboxylic acid were used as models for aromatic, alkene, and alkane constituents of naphthenic acids in oil-field waters. In model freshwater, contaminant degradation was higher by UV/S2O8(2-) because of the higher quantum efficiency for S2O8(2-) than H2O2 photolysis. The conversion of (•)OH and SO4(•-) radicals to less reactive halogen radicals in the presence of seawater halides reduced the degradation efficiency of benzoic acid and cyclohexanecarboxylic acid. The UV/S2O8(2-) AOP was more affected by Cl(-) than the UV/H2O2 AOP because oxidation of Cl(-) is more favorable by SO4(•-) than (•)OH at pH 7. Degradation of 3-cyclohexene-1-carboxylic acid, was not affected by halides, likely because of the high reactivity of halogen radicals with alkenes. Despite its relatively low concentration in saline waters compared to Cl(-), Br(-) was particularly important. Br(-) promoted halogen radical formation for both AOPs resulting in ClBr(•-), Br2(•-), and CO3(•-) concentrations orders of magnitude higher than (•)OH and SO4(•-) concentrations and reducing differences in halide impacts between the two AOPs. Kinetic modeling of the UV/H2O2 AOP indicated a synergism between Br(-) and Cl(-), with Br(-) scavenging of (•)OH leading to BrOH(•-), and further reactions of Cl(-) with this and other brominated radicals promoting halogen radical concentrations. In contaminant mixtures, the conversion of (•)OH and SO4(•-) radicals to more selective CO3(•-) and halogen radicals favored attack on highly reactive reaction centers represented by the alkene group of 3-cyclohexene-1-carboxylic acid and the aromatic group of the model compound, 2,4-dihydroxybenzoic acid, at the expense of less reactive reaction centers such as aromatic rings and alkane groups represented in benzoic acid and cyclohexanecarboxylic acid. This effect was more pronounced for the UV/S2O8(2-) AOP.