Chemical Oxidation of Poly- and Perfluoroalkyl Contaminants: Implications for Remediation of AFFF-Impacted Groundwater
Poly- and perfluoroalkyl substances (PFASs) are persistent organic contaminants that have been detected in an increasing number of water supplies. In many instances, the contamination is associated with the use of PFAS-containing aqueous film-forming foam (AFFF) used for firefighting activities. Many established methods are ineffective for removing PFASs from groundwater, and there is a need for innovative remedial technologies capable of treating water at sites affected by this class of compounds. In situ chemical oxidation (ISCO) is one technology that has been proposed to fill this need. This research investigated the potential for remediating PFAS-contaminated groundwater with ISCO using heat-activated persulfate and Fenton’s reagent.
Experiments were performed to assess the potential for using heat-activated persulfate to remediate perfluoroalkyl acids (PFAAs), a subfamily of PFASs that contain a fully fluorinated carbon chain attached to an acid moiety. To gain insight into PFAA removal and transformation product generation, experiments were carried out under a variety of solution conditions designed to be representative of ISCO treatment. The effect of pH, chloride concentration and aquifer solids were examined using perfluorooctanoic acid (PFOA) and perfluorooctane sulfonic acid (PFOS) as model PFAAs. Solution pH had a strong influence on the removal of PFOA as it was transformed into shorter-chain perfluorocarboxylic acids at pH values below 3. The presence of chloride and aquifer sediments decreased the efficiency of the process by less than 25% under conditions likely to be encountered in drinking water aquifers. Heat-activated persulfate did not transform PFOS under any conditions.
Experiments were also performed using heat-activated persulfate to treat AFFF. Fluorotelomer- and perfluoroalkane sulfonamide-based polyfluorinated compounds were transformed to perfluorinated carboxylic acids, which underwent further degradation under acidic conditions. The presence of aquifer sediments decreased the efficiency of the remedial process but did not alter the transformation pathways. At high concentrations, the presence of organic solvents, such as those present in AFFF formulations, inhibited transformation of a representative perfluorinated compound, PFOA.
This research also examined the efficacy of Fenton’s reagent for transformation of PFOA. Experiments were performed to assess potential PFOA loss mechanisms occurring during treatment with high concentrations of H2O2 and Fe(III). Results of these experiments indicated that PFOA is not susceptible to transformation by this oxidant, but was instead removed from solution by sorption to insoluble iron species.
Despite challenges related to the creation of acidic conditions within the aquifers, the potential for generation of undesirable short-chain perfluorinated carboxylic acids, and the release of toxic metals, heat-activated persulfate may be a useful in situ treatment for sites contaminated with perfluorinated carboxylic acids and fluorotelomer-based compounds, including those used in aqueous film-forming foams. At sites where perfluorooctane sulfonic acids are also present, heat-activated persulfate could be used as part of a treatment-train approach to reduce the contaminant mass in source zones, but groundwater extraction and ex situ treatment by physical processes would still be required.