Kinetics and byproduct formation of benzoic acid degradation by the UV/chlorine process

The kinetics of degradation of benzoic acid in the UV/chlorine process was investigatedin this research study. The UV/chlorine process has been recently considered as an efficient and energy-saving advanced oxidation process to generate hydroxyl radicals (HO•) to degrade emerging micropollutants. However, the photolysis of chlorine also forms the chlorine atom (Cl•), which is a strong oxidant with standard reduction potential comparable with that of HO•. The specific roles of HO• and Cl• on micropollutant degradation in the UV/chlorine process at different chlorine dosages, micropullutant concentrations, pH and chloride concentrations were thus investigated and modeled in this study. The steady-state kinetic modeling developed in this study fitted well with the experimental data and showed that the relative contributions of HO• and Cl• to BA degradation were strongly pH-dependent and were 36% and 64% at pH 6, 17% and 83% at pH 7.5, 2% and 98% at pH 9, respectively, in the UV/chlorine system under the experimental conditions evaluated. The relative contributions of HO• and Cl• changed slightly with the variations of chlorine dosages, BA concentrations and chloride concentrations. This study firstly demonstrated quantitatively the significant contribution of Cl• in the UV/chlorine system for BA degradation. The formation of byproducts including chloroform (TCM), chloral hydrate (CH), dichloropropanone (DCP), trichloropropanone (TCP), trichloroacetonitrile (TCAN), dichloroacetonitrile (DCAN) and trichloronitromethane (TCNM) in BA degradation by the UV/chlorine process was also studied. The byproduct formation from phenol, NOM and fulvic acid by the UV/chlorine process was also investigated for comparison. In most cases, statically significant increases in the byproduct formation from chlorination due to the additional UV irradiation are commonly observed. The impacts from UV exposure were found to be most significant in TCM formation among the five tested common disinfection byproducts. BA was more sensitive to UV addition than was the phenol, NOM and fulvic acid. This difference could be explained by the reactivity with chlorine. The TCM, CH, DCP, TCP and DCAN formation in the NOM water is higher than that in the fulvic acid water, due to their dissimilar compositions