Evaluation of chlorate/perchlorate formation during electrochemical oxidation of PFAS: The roles of free chlorine and hydroxyl radical

Abstract

Per- and polyfluoroalkyl substances (PFAS) have received significant attention due to their bioaccumulation, toxicity, and persistence. Electrochemical oxidation (EOX) has been reported to be a promising destructive technology for PFAS. However, potential formation of toxic by-products such as chlorate and perchlorate in the presence of chloride are problematic. In our work, we used the boron-doped diamond (BDD) electrodes for the degradation of PFAS and evaluated formation of chlorate/perchlorate. In this study, effect of operating conditions (applied current density and initial pH) and water matrix (initial chloride concentration, background chemical oxygen demand (COD), and carbonate species) were investigated for their formation and PFAS degradation. Alkaline pH and high applied current density seem to favor the formation of chlorate/perchlorate. Higher chloride concentrations led to increase in chlorate formation, however, the conversion of chlorate to perchlorate was lower with higher chloride concentrations. Background COD significantly increased their formation. The presence of carbonate species reduced the formation of chlorate/perchlorate. The roles of free chlorine and hydroxyl radicals was investigated to explain the mechanism of chlorate/perchlorate formation, using scavengers such as hydrogen peroxide (EOX-HP), sodium bicarbonate (EOX-NaHCO3), methanol (EOX-MeOH), and sodium bisulfite (EOX-NaHSO3). By the comparison of EOX, EOX-HP, EOX-NaHCO3, EOX-MeOH, and EOX-NaHSO3 processes, it was confirmed that free chlorine plays the dominant role compared to hydroxyl radical on the formation of chlorate/perchlorate. The formation of chlorate/perchlorate can be largely inhibited without affecting PFAS degradation by adding hydrogen peroxide (≥50 mM). Similarly, sodium bisulfite considerably inhibited their formation while PFAS degradation was not affected.