Formatted Title
Co-Occurring Compounds at AFFF-Impacted Sites Decrease Perfluoroalkyl Acid (PFAA) Adsorption to Colloidal Activated Carbon and Impact Performance for In Situ Remediation
Background/Objectives
Subsurface injection of colloidal activated carbon (CAC) is a rapidly growing in situ remediation strategy for per- and polyfluorinated alkyl substances (PFAS) in groundwater, particularly at aqueous film forming foam (AFFF)-impacted sites. When injected into an aquifer, CAC slows PFAS plume migration and decreases downgradient mass flux, but long-term barrier performance remains uncertain for short-chain PFAS and under varying water chemistry scenarios. AFFF-impacted sites are complex, and in addition to PFAS there are often other co-occurring compounds. Current and historical fire-training exercises have led to both PFAS and diesel fuels co-existing at many AFFF sites. As more in situ CAC barriers are installed at these sites, it is necessary to quantify how site-specific co-occurring compounds and water chemistry will impact adsorption performance and affect barrier longevity. To accurately model CAC barrier lifetimes, adsorption parameters are needed for both short- and long-chain PFAS in water matrices that are representative of AFFF sites. Therefore, the primary objectives of this work are the following: (1) quantify the impact of the water soluble fraction (WSF) of diesel fuels commonly present at AFFF-impacted sites on PFAA adsorption to CAC, (2) evaluate CAC adsorption performance in a real groundwater matrix collected at AFFF-sites, and (3) elucidated the driving factors of CAC adsorption performance at AFFF sites.
Approach/Activities
The adsorption of six short- and long-chain perfluoroalkyl acids (PFAA) to a commercially available CAC was evaluated at the laboratory scale. The WSF of diesel was used as model co-occurring compounds due to expected presence at AFFF-impacted sites. Both fresh and synthetically weathered diesel was assessed, and synthetic weathering was defined as 50% mass loss under a gentle stream of nitrogen. WSF of each diesel fuel was prepared by equilibrating with water and then removing the aqueous phase which was then used in batch adsorption tests. Adsorption tests were also completed in real groundwater collected from two AFFF-impacted coastal sites with varying water chemistries to better understand how laboratory generated water matrices compare to adsorption in real groundwater.
Results/Lessons Learned
Preliminary results indicate that the WSF of unweathered diesel decreases the adsorption of both long- and short-chain PFAA. The magnitude of the of the decrease depended on PFAA chain length. The value of log Kd was reduced by about 50% for PFOA, 25% for PFPeA, and no adsorption was observed for PFBA, the shortest-chain PFAA. Total petroleum hydrocarbons (TPH) in each WSF of diesel will be quantified by established techniques to better interpret mechanisms of effect. Additional adsorption experiments will also be completed at lower WSF concentrations to assess the concentration-dependence of the effect observed and using weathered fuels.
Adsorption of selected PFAA was also evaluated in a real groundwater matrix collected from a site with known hydrocarbon contamination and historical fire-training activates. Adsorption of PFOA, PFPeA, and PFBS was significantly hindered in the real groundwater matrix compared to that determined in 1 mM NaHCO3- at pH = 7.5. Future data analysis aims to elucidate what components of AFFF-groundwater have the most significant impact of PFAA adsorption to CAC and in turn are the most important to consider for in situ CAC barrier design.