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

Abstract

The bicarbonate and carbonate ions (HCO3− &CO32−) will consume hydroxyl radical (HO•) to generate carbonate radical anion (CO3•−) in hydroxyl radical based advanced oxidation processes (HO•−AOPs) resulting in reduced oxidation efficiencies of the systems. However, despite the HO• quenching effect of carbonate species, the contribution of CO3•− to the degradation of bisphenol A (BPA) was observed in UV/sodium percarbonate (UV/SPC). In order to study the performance of UV/SPC for BPA degradation and the role of CO3•− 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•− and HO• contributed to the degradation of BPA in UV/SPC. Second order rate constant of CO3•− towards BPA (kCO3•−+BPA = 2.23 × 108 M−1 s−1) and steady state concentrations of CO3•− ([CO3•−]ss = 2.3 × 10−12 M) and HO• ([HO•]ss = 1.82 × 10−14 M) in UV/SPC were determined with competition kinetics at 1 mM SPC and pH 8.5. The high [CO3•−]ss observed in UV/SPC compensated for the smaller kCO3•−+BPA compared to kHO•+BPA and the consumption of HO• 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•−AOPs. This also demonstrates potential for CO3•− based water purification technologies.