Novel Pd-decorated graphite electrodes for perfluorooctanoic acid degradation in aqueous phase

Perfluorinated alkyl substances, such as perfluorooctanoic acid (PFOA) have become a major environmental threat due to their high persistence in the environment, and their chronic toxicity. Electrochemical degradation has great potential for PFOA degradation, due to the combination of oxidation reactions over the anode, which initiate the degradation through elimination of the terminal carboxylic group, and reduction reactions on the cathode, responsible for the molecule defluorination. A previous work demonstrated the key role of the cathode in PFOA defluorination. Hence, this study investigated novel Pd-decorated cathodes for PFOA transformation. These Pd/C cathodes, with nominal palladium loadings ranging from 0.25 to 1 mg, were synthesized by wetness impregnation of commercial graphite sheets (C), followed by calcination and reduction. Pd acted as electrocatalyst, enhancing the reduction reactions and therefore PFOA defluorination. Two primary reaction mechanisms were identified in PFOA degradation: (i) polymerization of PFOA and its intermediates on the cathode surface, followed by defluorination catalyzed by Pd clusters; and (ii) sequential oxidation-reduction steps, each resulting in the loss of two fluor and one carbon from the molecule treated in each step. Under the optimal operating conditions (PFOA 0 : 100 mg/L, Na 2 SO 4 : 0.5 g/L, j: 5 mA/cm 2 , T: 25ºC and pH 0 : 4), 91.04 % TOC removal and 76.1 % F - release were reached after 120 min using the highest Pd load. Pd-1 cathode was stable over 10 reaction cycles, demonstrating the feasibility of this material for PFOA degradation in aqueous phase. • Novel Pd cathodes were synthesized by PdCl 2 wetness impregnation. • Pd acts as electrocatalyst, enhancing PFOA defluorination. • PFOA degradation undergoes two different reaction mechanisms. • The synthesized Pd cathode (Pd-1) remained stable over 10 reaction cycles. • High mineralization and defluorination rates were reached (X TOC : 91.04 %, X F- : 76.1 %).