PFOS destruction in a continuous supercritical water oxidation reactor

Abstract

Effective end-of-life technologies are needed for remediation of PFAS-containing byproducts and wastewater effluent streams. We describe a continuous flow supercritical water oxidation (SCWO) reactor operated in an autogenic regime achieved by introducing pilot fuel and compressed air; this system can achieve sufficient temperatures to destroy-one of the most recalcitrant PFAS -- perfluorooctane sulfonic acid (PFOS). We parametrically investigate PFOS and perfluorooctanoic acid (PFOA) destruction at temperatures between 410 °C and 650 °C. Targeted LC-MS/MS analysis suggests that oxidative CS bond cleavage initiates PFOS degradation, followed by decarboxylation to facilitate sequential -CF2 elimination and chain shortening. Trifluoroacetic acid (TFA) was found to be the most recalcitrant intermediate product in the effluent, persisting at T = 600 °C and residence time of ∼ 32 s, emphasizing the need for TFA monitoring in SCWO treatment of PFAS-impacted matrices. Analysis of gaseous products shows the presence of the volatile organofluorine (VOF) such as 1H-perfluoroalkane at T = 420–530 °C; the GS/MS spectra of gaseous reaction products suggest that other VOFs may be present. Free fluoride measurement by ion chromatography shows complete PFOS defluorination at T = 650 °C. Overall, this work establishes the operational envelope for SCWO treatment of PFAS-impacted liquids and suggests the chemical routes for PFAS destruction. Additional characterization of intermediate species in gas and liquid effluent is required to fully elucidate a chemical reaction mechanism for PFAS destruction in SCWO environments.