The overall goal of this research was to develop and assess oxidative electrocatalytic and reductive catalytic approaches for treatment of per- and polyfluoroalkyl substances (PFAS) associated with aqueous film-forming foams (AFFFs) in groundwater. Initial testing of the reductive approach indicated that treatment with supported metal catalysts was ineffective for defluorination of the perfluoroalkyl acids (PFAAs), thus the reductive approach was modified to focus on reduction via (1) application of vitamin B12 and related cobalt complexes as homogeneous catalysts, and (2) application of ultra violet (UV)-sulfite for generation of hydrated electrons (eaq-).
Electrochemical (EC) experiments were performed using boron-doped diamond (BDD) anodes to assess the oxidation of both individual PFAAs and the wide range of potential PFAA precursors commonly found in AFFF. Experiments focused on identifying the extent of defluorination in both “fresh” AFFF solutions, and in groundwater collected from historically AFFF-impacted sites. Both target and suspect (including several classes of potential PFAA precursors) analyses, as well as fluoride generation, were used to evaluate results.
Reductive transformation of PFAS via cobalt complexes and hydrated electrons generated via the UV-sulfite process were performed in a series of batch experiments. Similar to the approach used for the EC experiments, treatment of individual PFAAs in simple electrolytes as well as treatment of a wider range of PFAS in AFFF-spiked solutions and AFFF-impacted groundwater were evaluated with respect to parent chemical degradation and extent of defluorination. The influences of PFAS structure and geochemical conditions on reaction rates was examined.
Results of the EC experiments showed that defluorination of PFAAs occurred in AFFF-impacted waters. EC treatment of PFAA precursors showed an oxidation pathway that in some cases resulted in formation of PFAAs, but in all waters studied resulted in substantial defluorination. The generation of perchlorate, though treatable with subsequent biological reduction, was a notable unwanted reaction product. Results from Vitamin B12-catalyzed experiments showed that both a biogenic cobalt-corrin complex (B12) and an artificial cobalt-porphyrin complex (Co-PP) catalytically defluorinate multiple C−F bonds in branched PFAS, but were unreactive with linear PFAS structures. Results of the UV-sulfite experiments demonstrated that a wide range of PFAS in AFFF were transformed upon generation of hydrated electrons, but that apparent reaction rates were heavily dependent upon PFAS structure. Analysis indicates that up to 50 percent of the organic fluorine in AFFF was liberated as fluoride ion during treatment. Finally, experiments demonstrated that the same PFAS present in impacted groundwater obtained from Department of Defense (DoD) facilities can also be treated by UV-sulfite, with similar structure-reactivity trends observed among PFAS.
Results from these studies have shown that both EC, B12, and UV-sulfite treatment can defluorinate a wide range of PFAS, and offer potential solutions to the DoD for AFFF-impacted waters. Additional longer-term and larger scale testing is recommended to further evaluate these technologies and demonstrate their potential effectiveness under field conditions.
Liu, J., D.J. Van Hoomissen, T. Liu, A. Maizel, X. Huo, S.R. Fernández, C. Ren, X. Xiao, Y. Fang, C. Schaefer, C.P. Higgins, S. Vyas, and T.J. Strathmann. 2018. Reductive Defluorination of Branched Per-and Polyfluoroalkyl Substances with Cobalt Complex Catalysts. Environmental Science & Technology Letters. doi:10.1021/acs.estlett.8b00122
Schaefer, C.E., C. Andaya, A. Burant, C.W. Condee, A. Urtiaga, T.J. Strathmann, and C.P. Higgins. 2017. Electrochemical Treatment of Perfluorooctanoic Acid and Perfluorooctane Sulfonate: Insights into Mechanisms and Application to Groundwater Treatment. Chemical Engineering Journal, 317:424-432.
Schaefer, C.E., S. Choyke, P.L. Ferguson, C. Andaya, A. Burant, A. Maizel, T.J. Strathmann, and C.P. Higgins. 2018. Electrochemical Transformations of Perfluoroalkyl Acid (PFAA) Precursors and PFAAs in Groundwater Impacted with Aqueous Film Forming Foams. Environmental Science & Technology, 52(18):10689-10697.
Schaefer, C.E., C. Andaya, A. Maizel, and C.P. Higgins. 2019. Assessing Continued Electrochemical Treatment of Groundwater Impacted by Aqueous Film-Forming Foams. Journal of Environmental Engineering, 145(12):06019007.