Treatment of perfluoroalkyl acids in concentrated wastes from regeneration of spent ion exchange resin by electrochemical oxidation using Magnéli phase Ti4O7 anode

Abstract

This study investigated the treatment of perfluoroalkyl acids (PFAAs) in still bottoms during electrochemical oxidation using Magnéli phase titanium suboxide (Ti4O7) as the anode. 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. All 10 monitored PFAAs were effectively degraded by electrochemical oxidation (EO) on a Magnéli phase Ti4O7 anode, and the removal was 61.1% after 40 h EO treatment of a still bottoms sample that was produced from the use of a typical regenerant containing 2% 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 PFAAs 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. The results of this study demonstrate the promise of coupling regenerable IXR technology and EO for removing PFAAs from water and destroying them onsite.

Keywords

• Electrooxidation
• PFAAs
• Still bottoms
• Perchlorate
• Groundwater