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Abstract
Per- and polyfluoroalkyl substances (PFASs) are a class of synthetic chemicals that raises public concern due to their global distribution, environmental persistence and potential carcinogenicity and toxicity. Perfluorooctanoic acid (PFOA) and perfluorooctane sulfonate (PFOS) are two representative PFASs that have been classified as contaminant of emerging concern by the U.S. Environmental Protection Agency.Electrooxidation (EO) is one of the most promising technologies to treat PFAS, since it combines both direct electron transfer and reactions mediated by hydroxyl radicals (·OH) to degrade recalcitrant organic compounds on the anode surface. It was observed in this study that perfluoroalkyl acids (PFAAs) can be effectively degraded on Magnéli phase titanium suboxide (TSO) anode, and the degradation rates appear to correlate to their molecular polarizability.
The effect of pore structure and composition of TSO anodes on electrooxidation was also investigated in this study with PFOS as a model PFAS. The anode materials of different primary compositions (Ti4O7 or Ti9O17), with different porous structures, were investigated. It reveals that the greater fraction of Ti3+ on the Ti4O7 anode enhanced the adsorption affinity of PFOS leading to higher PFOS degradation efficiency. The results also suggest that the pores with diameter smaller than 1.03 μm may not contribute to the effective electroactive sites because of restrictive electrolyte transport.
This study also indicated that surface fluorination of the TSO anode significantly eliminated chlorate and perchlorate formation during EO treatment of PFASs. Chlorate and perchlorate formation was fully inhibited in the permeate flow on surface fluorinated Ti4O7 in reactive electrochemical membrane (REM) mode at a proper anodic potential range (< 3.0 V vs SHE). It results from a combination of much reduced adsorption of Cl- and less ·OH generation on fluorinated Ti4O7 anode than on pristine Ti4O7 anode, slower intermediate reaction steps and shorter resident time in REM mode.
The data collected in this study showed an environmentally friendly EO process for PFAS treatment on TSO anodes at ambient condition. The results of this study can provide a basis for design and optimization a TSO-based electrochemical treatment of PFAS-contaminated waters.