Integrated flow anodic oxidation and ultrafiltration system for continuous defluorination of perfluorooctanoic acid...

While flow anodic oxidation systems can efficiently generate hydroxyl radicals (·OH) and significantly enhance direct electron transfer (DET) processes that result in the oxidation of target contaminants via the charge percolating network of flow anode particles, challenges remain in constructing a flow anodic oxidation system that can be operated continuously with stable performance. Here we incorporate an ultrafiltration (UF) membrane module into the flow anodic oxidation system and achieve the continuous defluorination of perfluorooctanoic acid (PFOA) for 12 days with high efficiency (94.1%) and reasonable energy consumption (38.1 Wh mg^-1) compared to other advanced oxidation processes by using a mixture of conducting Ti_xO_2x-1 and Pd/CNT particles as the flow anode. The results indicate that DET, ·OH mediated oxidation and adsorption processes play critical roles in the degradation of PFOA during the flow anodic oxidation processes. The synergistic effect of the Ti_xO_2x-1 and Pd/CNT particles enhances the defluorination efficiency by 3.2 times at 4.5 V vs Ag/AgCl compared to the control experiment (no flow anode particles present) and promotes the release of F^- into solution while other intermediate products remain adsorbed to the surface of the Pd/CNT particles. Although the Pd/CNT particles were oxidized after the long-term operation, no obvious Pd ion leakage into solution was observed. Results of this study support the feasibility of continuous operation of a flow anode/UF system with stable performance and pave the way for the translation of this advanced oxidation technology to practical application.