Effect of nitrate, carbonate/bicarbonate, humic acid, and H2O2 on the kinetics and degradation mechanism of Bisphenol-A during UV photolysis.

In this study, the effects of natural water components (nitrate, carbonate/bicarbonate, and humic acid) on the kinetics and degradation mechanisms of bisphenol A (BPA) during UV-C photolysis and UV/H2O2 reaction were examined. The presence of NO3- (0.04-0.4 mM) and CO32-/HCO3- (0.4-4 mM) ions increased BPA degradation during UV photolysis. Humic acid less than 3 mg/L promoted BPA degradation, but greater than 3 mg/L of humic acid inhibited BPA degradation. During the UV/H2O2 reaction, all water matrix components acted as radical scavengers in the order of humic acid > CO32-/HCO3- > NO3-. All of the degradation reactions agreed with the pseudo-first-order kinetics. While eight byproducts (m/z = 122, 136, 139, 164, 181, 244, 273, 289) were identified in UV-C/NO3- photolysis reaction, four (m/z = 122, 136, 164, 244) and three byproducts (m/z = 122, 136, 164) were observed during UV-C/NO3-/CO32-/HCO3- and UV-C/CO32-/HCO3- reactions. Nitrogenated and hydrogenated byproducts were first observed during the UV-C/NO3- photolysis, but only hydrogenated byproducts as adducts were detected during the UV-C/NO3-/CO32-/HCO3- photolysis. Nitrogenated and hydrogenated byproducts were formed in the early stage of degradation by OH or NO2 radicals, and these byproducts were subsequently degraded into smaller compounds with further reaction during UV-C/NO3- and UV-C/NO3-/CO32-/HCO3- reactions. In contrast, BPA was directly degraded into smaller compounds by β-scission of the isopropyl group by CO3-/HCO3 radicals during UV-C/CO32-/HCO3- reaction. Our results imply that the water components can change the degradation mechanism of BPA during UV photolysis.