UV/sodium percarbonate (UV/SPC) is an emerging technology not yet been fully studied for water and wastewater treatment. The application of UV/SPC is debatable because that the Na2CO3 in SPC reacts with hydroxyl radical (HO*) to generate carbonate radical anion (CO3*-)) may undermine the oxidation efficiency of HO* based technologies. However, CO3*- has high reactivity with organic compounds of electron rich moieties. The steady state concentration of CO3*- ([CO3*-]ss) in nature waters is orders higher than that of HO* ([HO*]ss). The cost performance, stability, and safety of SPC are higher than those of liquid H2O2. Bisphenol A (BPA), a well-known endocrine disruptor with two electron rich rings, is a suitable compound for investigation of contaminant destruction in UV/SPC. Experiments on BPA transformation in UV/SPC and UV/H2O2 were conducted and compared in this study to better understand the kinetics and mechanism underlying UV/SPC, the environmental compatibility of UV/SPC treated effluents, and the potential of UV/SPC for water and wastewater treatment.
Results of this study showed comparable efficiency of UV/SPC and UV/H2O2 for BPA degradation in Milli-Q water at equivalent concentration of H2O2. The actual degradation efficiency of UV/SPC for BPA removal in various field water samples demonstrated the potential of UV/SPC for treatment of effluents from membrane processes in water reuse. The impact patterns of initial concentration of oxidant, pH, common ions, and different types of natural organic matter on BPA removal in UV/SPC were similar to those in UV/H2O2 though less interferences were observed in UV/SPC.
In addition to HO*, CO3*- was another reactive species contributing to the degradation of BPA in UV/SPC with a second order rate constant of 2.23× 10^8 M-1 s-1. The [CO3*-]ss and [HO*]ss in the solution of 1 mM SPC activated by 0.093 mW cm-2 UV were 2.3× 10^-12 M and 1.82 × 10^-14 M, respectively. The two orders of magnitude higher [CO3*-]ss than [HO*]ss compensated the consumption of HO* for CO3*- generation and the relatively small k(CO3*-+ BPA). BPA degradation in UV/SPC was realized through ring modification, isopropylidene bridge modification, double-ring split, and single-ring cleavage. Electron transfer, demethylation, and ipso attack initiated CO3*- transformation of BPA. In comparison with HO* transformation of BPA, CO3*- transformation of BPA mainly relied on electron transfer.
Quantitative structure-activity relationship analysis on identified transformation products (TPs, excluding benzoquinone TPs) indicated increased biotoxicities of matrices containing multi-hydroxylated TPs and reduced bioaccumulation of TPs after BPA treatment by UV/H2O2 and UV/SPC. In vivo assays with Escherichia coli (E. coli) showed inhibited cell growth, arrested cell cycle, and increased cell death in mixture of 2 µM BPA treated with 1mM H2O2 and 1470 mJ cm-2 UV which containing benzoquinone TPs. Metabolomic analysis identified BPA impact on alanine, aspartate and glutamate metabolism, starch and sucrose metabolism, pentose phosphate pathway, and lysine degradation which were mitigated after both treatments. UV/SPC exhibited an advantage of UV attenuated impact on citrate cycle for BPA treatment compared to UV/H2O2.