Electrochemical degradation of GenX using boron-doped diamond anodes

Abstract

The presence of harmful pollutants and toxic pathogens in water is a risk to both living beings and the environment. Water treatment plays a crucial role in the removal of these contaminants through different stages of filtration. Among the existing pollutants, a family of per-and polyfluoroalkyl substances (PFAS) escapes from all treatment methods and ends up in our food, water and, finally, in our blood. Current treatment methods are not effective due to their inability to break the strong C-F bonds in PFAS. Perfluorooctanoic acid (PFOA) and perfluorooctanesulfonate (PFOS) are the most widely studied PFAS due to their widespread contamination of various environmental and biological matrices. Due to the global ban of PFOA, a short-chain fluorinated compound named GenX (the ammonium salt of hexafluoropropylene oxide dimer acid) is currently used as an alternative. However, recent studies have shown that GenX has higher toxicity compared to PFOA and is more easily soluble in water, thus making it more difficult for removal. Hence, this research surveys the potential of using boron-doped diamond (BDD) anodes, which are known to have the largest potential window and high stability over time, for GenX degradation. During the electrochemical advanced oxidation process (EAOP), the highly reactive hydroxyl radicals (OH•) produced at the BDD surface break the C-F bonds to form fluoride (Fˉ) and CO2 products. Till date, very limited research is reported on the GenX degradation and they present a contradiction on the effect of sulfate radicals (SO4•ˉ), considered for their high redox potential, in the GenX degradation. In the present study, we investigate the degradation and defluorination efficiency of GenX using boron-doped diamond anodes in EAOP. This study aims to elucidate the first step in the degradation mechanism of GenX and to clarify the contradictions previously reported on the role of sulfate radicals. Experiments are performed separately with sodium sulfate and sodium perchlorate to assess the effect of SO4•ˉ. The results demonstrate that sulfate radicals are ineffective in GenX degradation due to the steric hindrance by the -CF3 branch which blocks the trajectory of SO4•ˉ for electron transfer reaction. The effects of electrolyte concentration, current density, and chloride radicals on the degradation and defluorination are investigated for the first time to provide in-depth understanding of the degradation mechanism. A possible degradation pathway is proposed by determination of the intermediate products using mass spectrometry. From the proposed pathway, it is inferred that GenX completely mineralizes to CO2 and Fˉ via formation of three intermediates. By comparing the electrochemical degradation of GenX with that of PFOA, it is observed that the presence of the -CF3 branch increases the complexity of electron transfer in the GenX degradation even though the mineralization rate is faster for GenX than for PFOA due to lesser number of intermediates. Hence, the direct electron transfer from GenX to the BDD anode is observed to be the rate-determining step in the GenX degradation. Additionally, by comparing different BDD anodes based on their material properties and surface morphology, it is observed that the presence of sp2 regions which act as active sites for effective electron transfer is necessary to initiate the GenX degradation mechanism. Electrochemical degradation of GenX using the BDD anodes has resulted in the complete mineralization to CO2 and Fˉ which supports EAOP using BDD anodes as a promising approach towards effective PFAS degradation.