Degradation of bisphenol A, bisphenol S, and bisphenol AF in the UV-LED/chlorine reaction: Effect of pH on the kinetics, transformation products, and degradation pathway

We investigated the UV-LED/chlorine reaction for the degradation of bisphenol A (BPA) and its alternatives, bisphenol S (BPS) and bisphenol AF (BPAF) in water. Three bisphenols (BPs) were removed by the pseudo-first-order kinetics. BPA showed the highest removal compared to BPS and BPAF at pH 7 due to the electron-donating isopropyl group in BPA compared to electron-withdrawing sulfonyl and hexafluoro-isopropyl groups in BPS and BPAF, respectively. While the degradation of BPs during the UV-LED/chlorine reaction was contributed mainly by reactive chlorine species (RCS) at pH 6 to 8, and mainly by chlorination reaction at pH 9 due to the different functional groups and pKa values of BPs, and the status of chlorine depending on pHs. While humic acid suppressed the degradation of BPs, HCO3− ions generally increased the removal of BPs. Chlorinated BPs (ClxBPs) were identified as major transformation products (TPs) in the UV-LED/chlorine reaction using LC-QTOF/MS analysis, and these ClxBPs were effectively removed by further reaction. In addition, >15 minor TPs and three mineralization products (acetate, formate, and chloroform) were identified for each BP degradation. Defluorination (37% in 4 hr) was observed an as the mineralization pathway for BPAF, indicating that UV-LED/chlorine reaction can break the strong C-F bond on BPAF. We proposed the degradation pathways of three BPs based on the identified TPs and theoretical reactive sites calculated using the condensed Fukui function. The major pathways were sequential chlorine substitution, coupling reaction, and hydroxylation reaction. The ECOSAR assessment showed that the UV-LED/chlorine process is effective in lowering the ecotoxicities of BPs and their TPs.