Kinetic Model for Predicting Perfluoroalkyl Acid Degradation During UV-Sulfite Treatment

Hydrated electron (e_aq^-) treatment processes show great potential in remediating recalcitrant water contaminants, including perfluoroalkyl and polyfluoroalkyl substances (PFAS). However, treatment efficacy depends upon many factors relating to source water composition, UV light source characteristics, and contaminant reactivity. Here, we provide critical insights into the complex roles of solution parameters on contaminant abatement through application of a UV-sulfite kinetic model that incorporates first-principles information on e_aq^- photogeneration and reactivity. The model accurately predicts decay profiles of short-chain perfluoroalkyl acids (PFAAs) during UV-sulfite treatment and facilitates quantitative interpretation of the effects of changing solution composition on PFAS degradation rates. Model results also confirm that the enhanced degradation of PFAAs observed under highly alkaline pH conditions results from changes in speciation of nontarget e_aq^- scavengers. Reverse application of the model to UV-sulfite data collected for longer chain PFAAs enabled estimation of bimolecular rate constants (k₂, M^-1 s^-1), providing an alternative to laser flash photolysis (LFP) measurements that are not feasible due to the water solubility limitations of these compounds. The proposed model links the disparate means of investigating e_aq^- processes, namely, UV photolysis and LFP, and provides a framework to estimate UV-sulfite treatment efficacy of PFAS in diverse water sources.