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

Abstract

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 CO₃^2-/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 > CO₃^2-/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^-/CO₃^2-/HCO₃^- and UV-C/CO₃^2-/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^-/CO₃^2-/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^-/CO₃^2-/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/CO₃^2-/HCO₃^- reaction. Our results imply that the water components can change the degradation mechanism of BPA during UV photolysis.

Keywords: Bisphenol A; Carbonate/bicarbonate; Humic acid; LC/MS-MS; Nitrate; UV photolysis.