Practical Considerations for the Optimization of In Situ Mineralization of Perfluorocarboxylic Acids and Polyfluoroalkyl Substances using Persulfate Oxidation

Abstract

Military bases and airports are often contaminated by per- and polyfluoroalkyl substances (PFAS) due to the repeated use of aqueous film forming foams (AFFFs) from decades of training exercises, equipment testing, and extinguishing of fuel- and solvent-based fires. Pump-and-treat systems combined with sorption processes are common ex situ remediation strategies; however, they can be expensive and may require decades of operation, particularly at sites where long-term diffusion and desorption of contaminants are the primary release processes. Alternatively, in situ chemical oxidation is an effective remediation strategy in which oxidants (e.g., persulfate, hydrogen peroxide) are injected into an aquifer to react with contaminants on site, and is competitive with alternative remediation techniques, such as pump-and-treat and ex situ treatment options. Specifically, heat-activated persulfate oxidation (HAPO) creates highly reactive sulfate radicals that under sufficiently acid conditions can mineralize perfluoroalkyl carboxylic acids (PFCAs) and many of the polyfluoroalkyl substances in AFFF. Sulfate radicals, however, can be scavenged by solutes present in groundwater, reducing the efficiency of PFCA transformation. To assess the application of HAPO, we conducted experiments under conditions typical of source zones where remediation is likely to be employed. We found that repeated treatment of aquifer solids with modest amounts of persulfate (50-300 mM) at low temperature activation (40 °C) could reduce the concentrations of precursors and PFCAs with chain lengths greater than three carbons by over 95%. Following treatment, addition of strong base (i.e., NaOH) was needed to neutralize acidity and convert dissolved metals back into less mobile forms.