The historical use of aqueous film forming foam (AFFF) formulations containing per- and polyfluoroalkyl substances (PFASs) for firefighting and training activities at DoD sites has led to concern over the potential for contamination of groundwater. Because PFASs have been detected at concentrations exceeding regulatory guidance, cost-effective in situ groundwater treatment approaches that consider their unique chemical properties (high solubility, surface-active behavior, recalcitrance, and presence as mixtures) are needed. The objective of this work was to develop a predictable and low cost in situ horizontal reactive media well treatment train (technically named in situ chemical oxidation of sorbed contaminants (ISCO-SC)) for remediating PFAS contaminated groundwater. This research evaluated the feasibility, effectiveness, and sustainability of ISCO-SC, where granular activated carbon (GAC) was used to sorb and concentrate PFASs and AFFF derived co-contaminants in situ, followed by contaminant destruction and GAC regeneration in situ using activated persulfate oxidation.
The research involved four major components. The first of these was not specifically funded by SERDP; however,it relates directly to the ISCO-SC concept and is thus reported herein. A flexible operational model of the horizontal well that adequately represents field application and allows for data collection across a wide range of test conditions was designed and constructed. This physical model was used to determine the expected capture zone of a horizontal well under a range of hydraulic conductivity differentials.
In the second component of the research, six types of coal-based GAC sources, as well as one coconut-based source, were characterized. Characterization included physico-chemical properties as well as sorption capacity for PFASs. Two GAC sources, F400 (coal-based) and CBC (coconut-based) from Calgon Corporation, were chosen for further evaluation based on their sorption behavior. Langmuir and Freundlichs isotherms were developed for PFOA and PFOS both individually and when present in mixture, in the presence of co-contaminants, and under varied geochemical conditions. A parallel study was conducted to increase the uptake of PFASs through surface property modifications of GAC by chemical treatment using hydrochloric acid, sodium hydroxide base, heat activated persulfate and catalyzed hydrogen peroxide.
The third research component evaluated the effectiveness of heat-activated persulfate oxidation for degradation of PFASs sorbed onto the GAC through time-series batch tests and two-dimensional flow-through column tests. Column tests employed two different spent GAC sources from a field-scale pump-and-treat site. Here, the persulfate dose efficiency, PFASs oxidation byproducts and intermediates, the possibility for in situ regeneration of PFASs sorbed onto GAC, and GAC reusability with treatment were explored broadly.
The fourth component involved characterization of the biotransformation pathways for persistent PFOS using Meta-PC degradation simulation software.
Research results demonstrated the feasibility of horizontal well capture of contaminated groundwater. Results can be used to upscaling design and predicting field-scale performance. The ratio of hydraulic conductivity within the well to outside of the well is a key design parameter.
Sorption results indicates that GAC sorption capacity for individual compounds is greater than when PFASs are present in mixture. Of the GAC sources evaluated, Calgon F400 GAC showed the highest sorption capacity. The presence of co-contaminants (kerosene, trichloroethene, and ethanol), and variations in groundwater conditions (pH, the presence of sulfate anions, natural organic carbon, and iron oxides) demonstrated limited impact on the sorption behavior of PFASs under all experimental conditions tested. The extent of sorption of tested PFASs increased with hydrochloric acid treatment likely due to increasing the positive charge density. Treatment with sodium hydroxide did not affect sorption capacity, and sorption decreased under oxidative treatment. All treated GAC had lower active surface area compared to untreated GAC, which is the main physical property deemed responsible for reduced sorption.
In persulfate treatment evaluations, PFOS exhibited no transformation even with increased activated persulfate oxidant dose. PFOA underwent degradation yielding shorter chain compounds, however, only when suspended in aqueous phase, avoiding oxidant attack when sorbed onto GAC. In the aqueous phase, the greatest extent of PFOA removal was observed with more frequent low concentration oxidant doses. Temperature, aqueous pH, and GAC surface polarity, which were altered through persulfate treatment, were found to control the PFAS interaction with the GAC surface significantly. Persulfate treatment also altered the partitioning behavior of PFASs sorbed onto GAC. Sorption was actually enhanced by persulfate treatment, making contaminants even less amenable to oxidative treatment. Evaluations of the degradability of recalcitrant PFOS using predictive software indicate that degradation is possible only under high energy photo-degradation and certain enzyme-catalyzed metabolism reactions.
Results of this research has enhanced knowledge and understanding toward the design and application of GAC treatment systems for removal of PFASs. Data collected can be used to design and predict treatment at pilot and full scale. Furthermore, results guide the development of new PFAS sorption materials through improved understanding of sorption mechanisms and approaches for enhancing sorption. While persulfate treatment is ineffective for PFOS, the conditions resulting from its use, including generation of low pH and excess sulfate, can impact sorption of PFASs. Results can be used to develop more effective sorption techniques and to extend the life of GAC.