Still bottoms are wastes produced from regeneration of saturated ion exchange resin (IXR) by a solution containing high salt content and methanol followed by distillation to remove the methanol. Still bottoms contain high concentrations of per- and polyfluoroalkyl substances (PFAS), and thus their disposal can be a factor limiting the widespread application of IXR technology on PFAS-impacted sites. The goal of this proof-of-concept study was to investigate the treatment of perfluoroalkyl acids (PFAAs) in still bottoms during electrochemical oxidation using Magnéli phase titanium suboxide (Ti₄O₇) as the anode.

This study examined the feasibility of using Ti₄O₇-based EO to remove PFAS from still bottoms recovered from regenerated IXR media. The still bottoms were generated by a unique solvent/non-chloride brine regeneration of PFAS-loaded IXR resin. Various solvent/salt regenerant solutions were evaluated. The efficiency of IXR regeneration solutions was compared in terms of PFAS elution. The fate of chloride (Cl^-) was monitored and factors impacting the formation of chlorate (ClO₃^-) and perchlorate (ClO₄^-) during EO treatment were assessed.

All 10 monitored PFAAs were effectively degraded by electrochemical oxidation (EO) on a Magnéli phase Ti₄O₇ anode, and the removal was 61.1% after a 40-hour (h) EO treatment of a still bottoms sample that was produced from the use of a typical regenerant containing 2% sodium chloride (NaCl) and 80% methanol by mass. Total organic carbon (TOC) destruction exceeded 79% in this treatment. Chlorate (ClO₃^-) and perchlorate (ClO₄^-) were formed after 8 h and reached a maximum of 2.3 mM and 2.2 mM, respectively, after 40 h. To minimize perchlorate generation by EO, five other salts were also tested instead of NaCl to regenerate IXR that had treated groundwater containing PFAAs. The regenerant containing NaI exhibited the greatest PFAA elution, comparable to that of NaCl. EO treatment of the still bottoms produced from the NaI regenerant for 200 h led to a total of 98.1% removal of all 10 monitored PFAAs. No ClO₃^- or ClO₄^- was detected after 100 h of EO treatment. It was found that the methanol residue in still bottoms effectively suppressed the formation of ClO₃^- and ClO₄^- during electrooxidation by scavenging chlorine radicals. Persulfate pretreatment did not appear to enhance PFAA degradation during EO treatment.

The results of this study demonstrate the promise of coupling regenerable IXR technology and EO for removing PFAAs from water and destroying them onsite.

Shi, H., Y. Wang, C. Li, R. Pierce, S. Gao, and Q. Huang. 2019. Degradation of Perfluorooctanesulfonate by Reactive Electrochemical Membrane Composed of Magneli Phase Titanium Suboxide. Environmental Science & Technology, 53(24):14528-14537.

Wang, L., J. Lu, L. Li, Y. Wang, and Q. Huang. 2020. Effects of Chloride on Electrochemical Degradation of Perfluorooctanesulfonate by Magnéli Phase Ti₄O₇ and Boron Doped Diamond Anodes. Water Research, 170:115254.

Wang, Y., H. Shi, C. Li, and Q. Huang. 2020. Electrochemical Degradation of Perfluoroalkyl Acids by Titanium Suboxide Anodes. Environmental Science: Water Research & Technology, 6(1):144-152.