Degradation and transformation of bisphenol A in UV/Sodium percarbonate: Dual role of carbonate radical anion

The bicarbonate and carbonate ions (HCO3- & CO32-) will consume hydroxyl radical (HO center dot) to generate carbonate radical anion (CO3 center dot-) in hydroxyl radical based advanced oxidation processes (HO center dot - AOPs) resulting in reduced oxidation efficiencies of the systems. However, despite the HO center dot quenching effect of carbonate species, the contribution of CO3 center dot- to the degradation of bisphenol A (BPA) was observed in UV/sodium percarbonate (IN/SPC). In order to study the performance of UV/SPC for BPA degradation and the role of CO3 center dot- in this process, the degradation kinetics and mechanisms of BPA in UV/SPC and in UV/hydrogen peroxide (UV/H2O2) were compared at equivalent concentration of H2O2. In this study, the observed degradation rates of BPA by UV/SPC and by UV/H2O2 in Milli-Q water were similar. Variation of the BPA degradation rates in the presence of radical quenchers, tert-butanol and phenol, suggested that both CO3 center dot- and HO center dot contributed to the degradation of BPA in IN/SPC. Second order rate constant of CO3 center dot- towards BPA (k(CO3 center dot-+BPA) = 2.23 x 10(8) M-1 s(-1)) and steady state concentrations of CO3 center dot-([CO3 center dot-](ss) = 2.3 x 10(-12) M) and HO center dot ([HO center dot](ss) = 1.82 x 10(-14) M) in UV/SPC were determined with competition kinetics at 1 mM SPC and pH 8.5. The high [CO3 center dot-](ss) observed in UV/SPC compensated for the smaller k(CO3 center dot-+BPA) compared to k(HO center dot+BPA) and the consumption of HO center dot making the degradation rate of BPA in UV/SPC comparable to that in UV/H2O2. Detailed studies on identification of transformation products (TPs) of BPA in UV/SPC revealed that phenol ring and isopropylidene bridge were the main reactive sites of BPA. Degradation pathways were proposed accordingly. The results of kinetic and mechanistic studies provide better fundamental understanding of the degradation of BPA in UV/SPC and HCO3- & CO32- impact on BPA degradation by HO center dot - AOPs. This also demonstrates potential for CO3 center dot- based water purification technologies. (C) 2019 Elsevier Ltd. All rights reserved.