Historical use of per and polyfluoroalkyl substances (PFAS) results in their widespread occurrence in natural waters. Separation technologies, such as ion exchange resin (IXR) adsorption and foam fraction (FF), have been shown to be effective for removing PFAS from water and are currently used to manage PFAS-impacted waters. These separation technologies concentrate PFAS but do not destroy them; therefore it is beneficial to couple them with a destructive treatment such as electrochemical oxidation (EO), which has been shown to effectively degrade PFAS via direct electron transfer and indirect free radical reactions.

The objective of Phase I of this project was to demonstrate the degradation of perfluoroalky acids (PFAA) by EO with titanium suboxide anodes (TSO). EO is primarily an interfacial process with reactions occurring at the anode surface, and thus its efficiency is dependent on PFAS concentrations. Therefore, coupling EO in a treatment train with a separation/concentration technology allows for the destruction of PFAS in a highly concentrated stream of reduced volume.

The objective of Phase II of this project is to build upon Phase I results to provide a basis for the design and optimization of IXR-EO and FF-EO treatment trains for PFAS. Phase II will investigate fluorine mass balance, the reuse of EO-treated still bottoms for IXR regeneration, the optimized coupling between EO and FF, and on-site methods of chlorate and perchlorate management.

In Phase I of this project, the feasibility of an IXR-EO treatment train was demonstrated by TSO-based EO treatment for destroying PFAS in the concentrated wastes, also known as still bottoms, resulting from regeneration of spent IXR used for PFAS-impacted waters. The energy efficiency of the IXR-EO treatment train is favorable because the EO process treats very concentrated solutions of high PFAS concentrations and reduced volumes. Such favorable energy efficiency is also applicable to other concentration-destruction treatment trains, such as FF-EO.

In Phase II of this project, a comprehensive study is planned to further evaluate the performance of TSO-based EO in degrading a wide range of PFAS in still bottoms and FF concentrates under various operational conditions. Task 1 will further explore the fate of PFAS during EO treatment of still bottoms, particularly examining the mass balance of fluorine in the EO treatment system. Task 2 will involve a set of laboratory experiments to examine the feasibility of recycling EO-treated still bottoms to regenerate PFAS-loaded IXR columns. Task 3 will examine the use of TSO-based EO treatment to destroy PFAS in the concentrates resulting from FF treatment of groundwater and wastewater. Task 4 will evaluate methods that can be used on-site to manage chlorate and perchlorate that may be formed during EO treatment.

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.

Figure 1. PFAA concentrations over 200 hours of EO treatment of still bottoms samples at 10 mA/cm2 (left)

Figure 2. Still bottoms sample at 0 hr treatment (left) and 200 hr treatment (right)

A treatment train coupling a separation/concentration technology and a destruction technology provides a complete solution to manage PFAS-impacted waters and can offer improved energy efficiency. The results of this project are expected to provide critical information needed for the design and optimization of an IXR-EO or FF-EO treatment train for removal and degradation of PFAS in water. These novel treatment trains will provide useful methods to manage PFAS at Department of Defense sites and may be used as alternative technologies to treat PFAS-containing wastewater. (Anticipated Phase II Completion - 2027)

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.