Electrochemical oxidation of PFOA and PFOS in concentrated waste streams

The electrochemical oxidation (EO) of environmentally persistent perfluorooctanoic acid (PFOA) and perfluorooctanesulfonic acid (PFOS) with a Magnéli phase Ti4O7 electrode was investigated in this study. After 3 hours (hr) of electrolysis, 96.0 percent of PFOA (10 milligrams per liter [mg/L] in 100 milliliters [mL] 100 millimolar [mM] Na2SO4 solution) was removed following pseudo first‐order kinetics (k = 0.0226 per minute [min]) with the degradation half‐life of 30.7 min. Under the same treatment conditions, PFOS (10 mg/L in 100 mL 100 mM Na2SO4 solution) removal reached 98.9 percent with a pseudo first‐order degradation rate constant of 0.0491/min and the half‐life of 14.1 min. Although, the degradation of PFOA was slower than PFOS, when subjected to EO treatment in separate solutions, PFOA appeared to degrade faster than PFOS when both are present in the same solution, indicating possible competition between PFOA and PFOS during Ti4O7 anode‐based EO treatment with PFOA having the competitive advantage. Moreover, the EO treatment was applied to degrade highly concentrated PFOA (100.5 mg/L) and PFOS (68.6 mg/L) in ion‐exchange resin regenerant (still bottom) with high organic carbon content (15,800 mg/L). After 17‐hr electrolysis, the total removal of PFOA and PFOS was 77.2 and 96.5 percent, respectively, and the fluoride concentration increased from 0.84 mg/L to 836 mg/L. Also, the dark brown color of the original solution gradually faded during EO treatment. In another test using still bottom samples with lower total organic carbon (9,880 mg/L), the PFOA (15.5 mg/L) and PFOS (25.5 mg/L) concentrations were reduced to levels below the limits of quantification after 16‐hr treatment. In addition, the performance of EO treatment using different batch reactor setups was compared in this study, including one‐sided (one anode:one cathode) and two‐sided (one anode:two cathodes) setups. The two‐sided reactor configuration significantly enhanced the degradation efficiency, likely due to the larger anode area available for reactions.