This study was aimed at developing a novel treatment train that combines electrocoagulation (EC) with electrochemical oxidation (ECO) treatment to remove and degrade per- and polyfluoroalkyl substances (PFAS) including perfluoroalkyl acids (PFAAs) and their organic co-chemicals of concern in impacted groundwater. PFAS are extremely persistent because of their unique molecular structures, and currently, there is no cost-effective technology that is applicable for on-site PFAA destruction. This study is of great societal and environmental significance by providing a technology potentially practical for elimination and destruction of PFAS in groundwater. Although PFAS were evaluated as the target chemicals of concern in this study, the EC and ECO technologies can be used individually or in combination to address a wide range of different co-chemicals of concern.
In this study, PFAS and co-chemicals of concern were sorbed and concentrated on the flocs formed through EC. The flocs were then dissolved in a low volume of acidic solution releasing PFAS into the same acidic solution. PFAS were then destroyed effectively with the ECO process.
The study contained two tasks. Task 1 was a laboratory bench study to verify and optimize the performance of EC and separation of PFAS and co-chemicals of concern from flocs. Task 2 was a laboratory bench study to combine the individual treatment processes into an integral train and evaluate its performance. Both the spiked water system and the Department of Defense (DoD) site groundwater system were evaluated in Task 1 and Task 2.
Under the optimized EC treatment conditions, the removal rates for most long-chain PFAS (carbon number 6-10) in spiked water, such as perfluorononanoic acid (PFNA), perfluorooctanoic acid (PFOA), perfluorooctanesulfonic acid (PFOS), perfluorohexanesulfonic acid (PFHxS), fluorotelomer sulfonic acid 8:2 (8:2 FtS), fluorotelomer sulfonic acid 6:2 (6:2 FtS), and perfluorinated sulfonamide (FOSA) were above 90%. The PFAS removal efficiency followed the order of FOSA≈ 8:2 FtS≈PFNA > PFOS > PFOA > 6:2 FtS > PFHxS > PFHpA > PFHxA ≈ perfluorobutanesulfonic acid (PFBS). For groundwater samples, under a high current density (5 milliampere per square centimeter [mA/cm2]), the removal for PFOA, PFNA, PFHxS, PFOS, 6:2 FtS, and 8:2 FtS was above 90%. It was observed that EC-derived foam was generated when a relatively high current density (> 1 mA/cm2) was applied to a relatively high PFAS concentration (> 0.1 micromolar [µM]) during EC. The floc and foam could be completely dissolved by a small amount of H2SO4, and the recovery rate was about 100%. Preliminary ECO tests indicated that PFAS in all three EC-derived solutions were removed efficiently during EO. Except for PFBS, PFHxA, and fluorotelomer sulfonic acid 4:2, the concentrations of all other PFAS were close to completely removed.
This research supports the mission to reduce the environmental liabilities by developing sustainable, cost-effective technologies for expedited site cleanup and closure by proposing a treatment train that can remove and degrade PFAS and co-chemicals of concern in groundwater. This coupling approach allows for the treatment of impacted concentrations from parts per trillion to parts per million levels and requires low energy consumption with no sorbent regeneration and PFAA waste generation. The project demonstrates the treatment effectiveness of the individual processes and the coupled processes. It is also important to note that this treatment train involves ECO destruction on site, thereby avoiding off-site transportation and disposal of PFAS-laden wastes. (Project Completion - 2022)
Shi, H., S.Y.D. Chiang, Y. Wang, Y. Wang, S. Liang, J. Zhou, R. Fontanez, S. Gao, and Q. Huang. 2021. An Electrocoagulation and Electrooxidation Treatment Train to Remove and Degrade Per-and Polyfluoroalkyl Substances in Aqueous Solution. Science of The Total Environment, 788:147723.