Per- and polyfluoroalkyl substances (PFAS) such as perfluorooctanesulfonate (PFOS) are persistent, acutely toxic synthetic organic compounds that are widespread in environmental waters. Advanced reduction processes which generate solvated electron reducing species are capable of attacking PFAS C–F bonds to facilitate degradation in water; however, the rate and extent of PFAS mineralization is hindered by solvated electron scavenging. Heterogeneous catalysts may enhance reductive defluorination processes and solvated electron efficacy; however, there are limited substrates available. In this study, we explore the use of vanadium carbide (V2C) MXene nanosheet and hydrogen peroxide (H2O2, a common water disinfectant) reactions to catalyze PFOS degradation in water. MXenes are a new family of 2-dimensional, conductive nanomaterials that have been used in energy storage applications. The rapid electron transfer properties of V2C MXenes may facilitate attack of nearby PFAS in water. Within 4 h, 0.15 mg/mL V2C nanosheets in the presence of 14.7 mM H2O2 were able to decrease 50 µg/L PFOS by 95% under aerobic conditions compared to reaction systems containing only V2C nanosheets (63% removal) or H2O2 (24% removal). Trifluoroacetic acid formation was identified in the V2C+H2O2 system which suggests defluorination occurred, and fluoride ion concentrations measured by ion chromatography confirm that complete defluorination was achieved. Characterization of the V2C nanosheet evolution before and after introduction to H2O2 and PFOS suggest that initial V(V) phases were reduced to V(III) phases within the V2C. Furthermore, electron paramagnetic resonance analysis detected the formation of singlet oxygen and solvated electron reactive species in solution. We hypothesize that PFOS first adsorbs to the V2C nanosheets which serve as a catalytic reactive center. Introducing H2O2 facilitates two-electron transfer within the V2C vanadium layers and promotes formation of solvated electrons which attack nearby PFOS. This MXene application could extend to treatment of other recalcitrant, halogenated aquatic contaminants.

Vanadium carbide (V2C) nanocatalysts for reductive defluorination of perfluorooctanesulfonate (PFOS) in the presence of hydrogen peroxide

ACS Fall 2021

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