An Investigation of Factors Affecting the Adsorption of Per- and Polyfluoroalkyl Substances (PFAS) on Colloidal Activated Carbon (CAC): Implications for In-situ Immobilization of PFAS

Abstract

The immobilization of per- and polyfluoroalkyl substances (PFAS) by colloidal activated carbon (CAC) barriers has been proposed as a potential in-situ method to mitigate the transport of plumes of PFAS in the subsurface. However, if PFAS are continuously released from a source zone, adsorptive sites on CAC will eventually become saturated, upon which point breakthrough of PFAS from a barrier will occur. To predict the long-term effectiveness of CAC barriers, it is important to investigate the factors that may affect the adsorption of PFAS on CAC. The objective of this research is to investigate some of these factors by answering the following questions: (1) How do co-contaminants, aquifer materials, and typical groundwater constituents affect the adsorption of PFAS by CAC?; and (2) How does reducing the particle size of activated carbons (ACs) affect their physico-chemical properties and ability to adsorb PFAS? To address the first research question, the adsorption of seven anionic PFAS on a polymer-stabilized CAC (i.e., PlumeStop®) and a polymer-free CAC was investigated using batch experiments (Chapter 3). The research employed synthetic solutions consisting of one PFAS, 1 mM of sodium bicarbonate (NaHCO3), and inorganic and organic solutes, including Na+, Cl-, Ca2+, dissolved organic carbon (DOC), diethylene glycol butyl ether (DGBE), trichloroethylene (TCE), benzene, 1,4-dioxane, and ethanol. It was observed that the affinity of PFAS to CACs was in the following order: PFOS > 6:2 FTS > PFHxS > PFOA > PFBS > PFPeA > PFBA. This result indicates that hydrophobic interaction was the predominant adsorption mechanism and that hydrophilic compounds such as PFBA and PFPeA will breakthrough CAC barriers first. The partition coefficient Kd for the adsorption of PFAS on the polymer-stabilized CAC was 1.3–3.5 times smaller than the Kd for the adsorption of PFAS on the polymer-free CAC, suggesting that the polymers decreased the adsorption, presumably due to competition. Thus, the PFAS adsorption capacity of PlumeStop CAC barriers could increase once the polymers are biodegraded and/or desorbed. The affinity of PFOS and PFOA to CAC increased when the ionic strength of the solution increased from 1 to 100 mM, or when the concentration of Ca2+ increased from 0 to 2 mM. In contrast, less mass of PFOS and PFOA was adsorbed in the presence of 1–20 mgC/L Suwannee River fulvic acid, which represented dissolved organic carbon, or in the presence of 10–100 mg/L diethylene glycol butyl ether (DGBE), which is a major component in some aqueous film-forming foam (AFFF) formulations. Therefore, information on the occurrence of DGBE and other glycol ethers in AFFF-impacted groundwater is needed to assess if the effect of these species on CAC barrier performance is appreciable. The presence of 0.5–4.8 mg/L benzene or 0.5–8 mg/L TCE, the co-contaminants that may comingle with PFAS at AFFF-impacted sites, diminished PFOS adsorption but had no effect or slightly enhanced PFOA adsorption. When the initial concentration of TCE was 8 mg/L, the Kd (514 ± 240 L/g) for the adsorption of PFOS was approximately 20 times lower than that in the TCE-free system (Kd = 9,579 ± 829 L/g). Therefore, the effect of TCE and benzene may depend on the type of PFAS. To gain insight into the effect of aquifer materials and water chemistry, the adsorption of PFOS, PFOA, and PFBS on CAC was investigated in the presence of six aquifer materials. Further, the removal of five PFAS (PFOS, PFOA, PFHxA, PFHxS, and 6:2 FTS) from six actual groundwater samples was studied (Chapter 4). Under the experimental conditions employed, the mass of PFBS, PFOA, and PFOS removed from the solution in the presence of CAC and aquifer materials was 2 to 4 orders of magnitude greater than the mass removed when only aquifer materials were present. It was also observed that the presence of aquifer materials did not appreciably affect the adsorption of PFBS, PFOA, and PFOS on CAC. In the experiments with six actual groundwater samples, the affinity of the studied PFAS to CAC was in the following order: PFOS > 6:2 FTS > PFOA ~ PFHxS > PFHxA, except for two instances of 6:2 FTS being the compound removed to the greatest extent. The adsorption affinity trend among the studied PFAS is consistent with the adsorption being driven by the hydrophobic effect. Principal component analyses (PCA) of the results obtained from the experiments with aquifer materials demonstrated that the correlation between the partition coefficient Kd for each PFAS and Ca2+ and DOC was the opposite of the correlations observed in Chapter 3. In the groundwater experiments, the correlation between Kd for each PFAS and ionic strength and Ca2+ was also the opposite of the correlations observed in Chapter 3. These opposite effects were hypothesized to be due to a complex interplay among various parameters affecting the adsorption of PFAS on CAC, which may confound the effect of each parameter. The results of this study indicate that laboratory experiments designed to evaluate the retention of PFAS in a CAC barrier should employ site-specific groundwater and aquifer materials. To address the second research question, four commercial ACs (three granular and one powdered) were pulverized by grinding and micromilling to create powdered activated carbons (PACs) and CACs, and the adsorption of PFBS, PFOA, and PFOS on these adsorbents (11 in total) was investigated (Chapter 5). All three PFAS were adsorbed less by CACs (d50 = 1.2–2.5 m) than by their parents PACs (d50 = 12–107 m). A detailed characterization of the properties (surface area, micropore, and mesopore volumes, pHpzc, and surface elemental composition) of these adsorbents suggests that the reduced adsorption capacity of CACs was likely the result of AC oxidation during milling, which decreased surface hydrophobicity. Granular activated carbons (GACs, 425–1,700 m) adsorbed PFAS less than PACs and CACs, partly due to the slow rate of adsorption. Of all ACs, the materials made from wood possessed the greatest surface area and porosity but adsorbed PFAS the least. The repulsion between the negatively charged surface of these wood-based ACs (pHpzc = 5.1) and the negatively charged headgroup of PFBS, PFOA, and PFOS molecules was identified to be the dominant factor that inhibited adsorption. The results of this study suggest that the adsorption kinetic advantage of CACs may be achieved at the expense of reduced adsorption affinity and that the role of electrostatic interaction between PFAS and AC should be considered when selecting AC for PFAS treatment applications.