Transformation of Bisphenol AF during Aqueous Chlorination: Kinetics, Mechanisms, and Influence of pH

Abstract

In this study, the reaction kinetics and degradation mechanism of bisphenol AF (BPAF) with hypochlorous acid (HOCl) were investigated for the first time. Our results showed that BPAF could be effectively degraded by chlorine over a wide pH range from 5.3 to 11.7, with apparent second-order rate constants (kapp) of 1.0–175.5 M–1 s–1 at temperature 25 ± 1.8 °C. By kinetic modeling, kapp of un-ionized and ionized BPAF (i.e., BPAF, BPAF–, and BPAF2–) were calculated to be 2.9 ± 1.1, (9.0 ± 0.4) × 103, and (1.4 ± 0.2) × 104 M–1 s–1, respectively. Five chlorinated BPAF (Cl-BPAF) were identified by a gas chromatography–mass spectrometry method, and confirmed by authentic materials synthesized in laboratory. By comparing the concentration evolution of BPAF and Cl-BPAF under each designated pH level, we found that (1) higher pH favored the electrophilic substitution mechanism and lower pH favored the electron transfer mechanism. (2) the electron transfer mechanism of BPAF with free chlorine may be attributed to the redox reaction of HOCl. (3) Aqueous pH dominated the formation ratio of Cl2–BPAF isomers due to the pH-dependence of the dissociation forms of precursor 3-Cl1BPAF. Considering the structural similarity of bisphenol analogues, similar relationship between pH and degradation kinetics or mechanism could be applied for other bisphenol analogues during chlorination.