Synergistic degradation of GenX (hexafluoropropylene oxide dimer acid) by pairing graphene-coated Ni-foam and boron doped diamond electrodes

Abstract

Perfluoroalkyl substances (PFAS) are well-recognized water pollutants. Even though most studies have traditionally focused on long-chain PFAS, their progressive replacement by short-chain PFAS has made the latter a new motive of concern. For example, short-chain hexafluoropropylene oxide dimer acid (known as GenX) was introduced as a supposedly sustainable substitute for toxic perfluorooctanic acid, but is now being detected in several aquatic environments around the world, with various reports evidencing its harmful effects. In this work, we investigated the degradation/mineralization of GenX by electro-Fenton (EF) using a graphene-Ni-foam (Gr-Ni-foam) cathode paired with a boron doped diamond (BDD) anode. It was found that homogeneous •OH radicals formed by EF showed limited reactivity with the GenX molecule itself. Instead, GenX degradation was initiated by direct electron transfer at the BDD surface, and then homogeneous •OH continued and enhanced the degradation/mineralization process. In contrast, Pt and fluorine doped tin oxide (FTO) anodes did not favor GenX degradation. A synergy between EF and BDD oxidation achieved 92.2 ± 1.0% of GenX mineralization after 6 h of treatment at 16 mA cm−2, vs. only 9.2 ± 0.1% by EF alone and 73.6 ± 6.2% with BDD alone. Based on the degradation by-products detected by high performance liquid chromatography-mass spectrometry (HPLC-MS), a mineralization pathway for GenX is proposed, involving two different routes initiated by electron transfer at the carboxylic and ether groups of the GenX molecule.