Efficient defluorination of perfluorooctanoic acid enabled by single-atom Cu/reduced graphene oxide electrocatalytic anode and peroxymonosulfate activation

Abstract

In this study, an electrocatalytic system using reduced graphene oxide supported single-atom copper (SA-Cu/rGO) anode was constructed for perfluorooctanoic acid (PFOA) defluorination. The successful doping of SA-Cu elevated the oxygen evolution potential and electrochemical active surface area of the anode and provided high electrocatalytic properties. Under optimum initial conditions with a peroxymonosulfate (PMS) concentration of 10 mM, a current density of 15 mA·cm−2, and an unadjusted pH of 7.75, the SA-Cu/rGO system achieved 98.8% removal ratio, 92.5% defluorination, and 94.4% mineralization of 20 mg·L-1 PFOA in 120 min. Kinetic studies indicate PFOA degradation following a pseudo-first-order model, with a rate constant of 4.7 × 10−2 min−1. The presence of PMS in the electrocatalytic system significantly improved the degradation rate and defluorination ratio. Radical trapping and quenching experiments confirmed that both ·OH and ·SO4- radicals contributed to PFOA degradation, with ·SO4- playing a more important role in defluorination. The system show high flexibility in initial PFOA concentrations (0.2–20 mg·L−1) and environmental pH values (3.0–10.0), as well as good durability. The electrocatalytic degradation mechanism of PFOA was proposed through determination of intermediates and density functional theory (DFT) calculation. The total short-chain perfluorocarboxylic acids (PFCAs) intermediates generated throughout the degradation were far from sufficient recovery of the PFOA loss, indicating that apart from the ·OH mediated decarboxylation-hydroxylation-elimination-hydrolysis (DHEH) pathway, the ·SO4- mediated decarboxylation-hydroxylation-oxidation-decarbonyl fluoride (DHOD) pathway may play a significant role in the near-complete defluorination and mineralization of PFOA. Moreover, DFT calculations suggest that the DHOD pathway is energetically favorable over DHEH. This study provides a further understanding of the defluorination mechanism of PFOA in electrocatalytic systems and demonstrates that electrochemical degradation with single-atom copper catalyst is an promising method for PFOA remediation in aqueous environment.