Investigation of the Efficacy of in situ Degradation Methods for Perfluorooctanoic Acid (PFOA) and Perfluorooctane Sulfonic Acid (PFOS) in Groundwater

Abstract

Per- and polyfluoroalkyl substances (PFASs) are a group of emerging contaminants that include perfluorooctanoic acid (PFOA) and perfluorooctane sulfonic acid (PFOS). PFASs can be released into groundwater through the application of fire-fighting foam, or effluent from industrial locations, wastewater facilities, and landfill leachate. Human exposure to PFASs should be limited due to the potential for human health implications; PFOA and PFOS are linked to liver, gastrointestinal, and thyroid toxic effects. Removal of PFASs from aqueous solutions can occur through capture, oxidation, reduction, or thermolysis. One of the most popular recent methods for the removal of aqueous PFOA in groundwater is thermally-activated persulfate. The goals of this thesis were to (1) evaluate the performance of a fluoride-selective electrode (FSE) in different matrix combinations that were representative of in situ groundwater remediation activities (2) investigate the removal of PFOA and PFOS with the addition of permanganate to thermally-activated or ambient persulfate, and (3) compare the removal of PFOA by thermally-activated or ambient persulfate in different sediment-slurry experiments.

A systematic investigation of the impacts of oxidant-based reagents and a quenching agent, aqueous geochemistry, and the presence of sediments was conducted for the FSE, in order to provide guidance on the use of this analytical tool. The hypothesis was that the quantification of fluoride (F-) using an ultrapure water calibration curve would be inaccurate in some of the combinations tested. Using matrix spike recovery and electrode slope measured in the various matrices as indicators, permanganate, ascorbic acid, and sediments were flagged as components of concern. While either a matrix-matched calibration curve or the standard addition method could be used for samples containing permanganate, the presence of sediments or ascorbic acid should be avoided for F- quantification with the FSE. Matrix spike recovery was within the acceptable bounds defined by the USEPA, and the electrode slopes were consistent with the slope of the calibration curve in the presence of persulfate and in different geochemical aqueous phases.

The impact of adding permanganate to both thermally-activated (60 °C) and ambient (20 °C) persulfate treatment systems for the removal of PFOA and PFOS was investigated using a 1:100 molar ratio of permanganate: persulfate. It was hypothesized that permanganate, or the manganese dioxide produced from permanganate in the presence of water, might be able to activate persulfate. Sacrificial, aqueous batch reactors prepared in the laboratory were used in this experiment. Analysis was conducted for pH using a pH probe, aqueous F- using the FSE, and aqueous PFASs using solid-phase extraction preparation and liquid-chromatography tandem mass spectrometry. PFASs with carbon chain lengths from four to eight were quantified. PFOA was successfully removed (> 99 %) in the thermally-activated persulfate with permanganate (dual-oxidant) and thermally-activated persulfate systems in both ultrapure and sodium bicarbonate simulated groundwater after seven days. Both short-chain PFCAs and aqueous F- were generated and indicated that PFOA was degraded in these experiments. The removal of PFOA was not evident in the ambient dual-oxidant and heated permanganate systems. The mass balance calculations for the PFOA systems accounted for nearly all of the initial PFOA (81 – 142 %). There was no indication of removal of PFOS by any combination of oxidants, and no degradation products were generated. Removal of PFOA or PFOS was not improved in the thermally-activated or ambient persulfate systems with the addition of permanganate at the tested ratio.

The challenges for the implementation of thermally-activated persulfate for the removal of PFOA in groundwater settings include the interaction of persulfate and PFOA with the aquifer sediments. The hypothesis of this experiment was that PFOA would be removed and converted into PFCAs and F- with thermally-activated persulfate treatment, even in the presence of sediments. The removal of PFOA by thermally-activated (60 °C) persulfate (50 mM, 9.6 g L-1) was compared using three different sediments in sacrificial sediment-slurry batch reactors. For each reactor, pH, aqueous F-, and aqueous PFCAs were determined, similar to the dual-oxidant experiment. In addition, liquid-solid extraction was used to quantify the sorbed PFASs in the solid phase. At least 60 % of the initial PFOA was removed after seven days in all three sediment slurries using thermally-activated persulfate. Removal of PFOA in all slurry reactors was lower than in aqueous reactors (99 % after 7 days). The detection of degradation products (short-chain PFCAs and F-) was also altered in sediment slurries compared to aqueous reactors. Short-chain PFCAs were retained within the systems longer when sediments were present. The decreased amount of PFCA removal led to the production of less F-. Furthermore, less F- could be measured in the sediments with high carbonate or organic carbon content. PFOA was extracted at higher concentrations from the sediment with the highest organic carbon content under acidic pH conditions. No removal of PFOA was measured under ambient persulfate treatment conditions. Thermally-activated persulfate was still effective for the removal of PFOA from soil-slurry reactors, but at decreased removal efficiency.

Thermally-activated persulfate can be considered as a potential remediation method for use in the removal of PFOA in groundwater settings. At the ratio tested, permanganate did not improve the effectiveness of persulfate under thermally-activated or ambient conditions. However, the investigation of a wider range of persulfate to permanganate ratios could provide further information. PFOS removal was not observed in thermally-activated or ambient persulfate treatment conditions. The quantification of degradation products, such as short-chain PFASs and F-, should be included in the analytical suite for any PFAS degradation project. The FSE is a valuable tool for the measurement of aqueous F- concentrations.