The overall goal of this project was to evaluate the effectiveness and sustainability of resin-based treatment systems for treatment of groundwater impacted by per- and polyfluoroalkyl substances (PFAS). Since ion exchange (IX) resins have emerged as an economically viable alternative to granular activated carbon (GAC) adsorption treatment systems, this project undertook an in-depth analysis of the performance, regenerability, cost, and sustainability of IX and other resins. Furthermore, the efficacy of regenerable IX resins was compared to emerging “PFAS-selective” single-use resins. This work evaluated the feasibility of widespread adoption of these technologies at sites impacted by aqueous film-forming foam (AFFF).

A series of experiments and modeling analyses were performed to meet the project objectives, including the following: 

• Evaluating and comparing the effectiveness of commercially available ion exchange and non-ionic resins for removal of the full range of PFAS in AFFF-impacted water. 
• Comparing regenerable anion exchange resins (AER) with emerging single-use “non-regenerable” AER. 
• Identifying resin characteristics associated with increased adsorption of PFAS. 
• Assessing the effect of important non-target groundwater constituents on PFAS adsorption. 
• Evaluating the links between PFAS adsorption observed in batch experiments and continuous-flow adsorption studies. 
• Characterizing the effectiveness of different salt brines, organic co-solvent, and other amendments for regenerating PFAS-loaded resins. 
• Quantifying PFAS destruction in waste ion exchange regenerant solutions using electrochemical oxidation and hydrothermal alkaline treatment technologies. 
• Comparing the life cycle environmental impacts and treatment costs for different regenerable ion exchange treatment system design options, and identifying system variables that offer the greatest potential for improving system sustainability. 

Experiments were conducted using both batch and continuous-flow experimental systems under both laboratory and field conditions.

PFAS adsorbed much more strongly to AER than to nonionic resins, cationic resins, and GAC. The extent of adsorption to AER varied with both PFAS and resin structure: longer-chain and sulfonic acid-based PFAS adsorbed more strongly than shorter-chain and carboxylic acid-based structures due to a combination of electrostatic and van der Waals interactions. Adsorption was greatest with polystyrene-based PFAS-selective AER possessing more hydrophobic functional groups. PFAS adsorption was relatively insensitive to most mobile counterions pre-loaded onto the resin (e.g., chloride versus sulfate).

Regeneration of PFAS-loaded AER, both strong- and weak-base forms, required regenerant mixtures containing both salt brine and alcohol co-solvent; the shortest chain perfluorocarboxylic acids were the only PFAS appreciably desorbed from AER using aqueous salt-only regenerants. Substitution of methanol with higher molecular weight alcohols improved regeneration efficiencies at comparable co-solvent levels. “Single-use” AER could be regenerated as well, although typically this required higher co-solvent percentages. The regenerant wastes were effectively treated using both electrochemical oxidation (ECO) and hydrothermal alkaline treatment (HALT) technologies. Rates of ECO were superior in sulfate- and bicarbonate-based synthetic waste brines compared to chloride-based brines. HALT destroyed and defluorinated the full suite of PFAS detected in a waste IX still bottoms obtained from a DoD site, with destruction rates for individual PFAS in the mixture being consistent with results reported previously for other liquid solutions (i.e. AFFF and AFFF-impacted groundwater).

Comparative analysis under a baseline scenario indicated that AER systems employing single-use “PFAS-selective” resins have lower environmental impacts and costs than systems using regenerable resins or GAC adsorbents (either single-use or thermally reactivated processes) for nearly all impact categories. Use of GAC operated as a single-use adsorbent led to the highest impacts for most categories, as well as the highest treatment costs. Thermally reactivated GAC proved to be less impactful than regenerable AER treatment. Generally, impacts of the adsorbent remediation systems were most sensitive to media usage rates or media regeneration frequency, which are highly dependent upon the PFAS breakthrough criteria used to determine when media replacement or regeneration are required. Environmental impacts of regenerable IX systems were most dependent upon the mass of residuals disposed of or incinerated off-site, and practices that reduce the mass of these residuals such as maximizing the fraction of co-solvent and brine that can be recycled on-site.

Results from this project support the continued deployment of IX treatment technologies for groundwater remediation at DoD sites impacted by AFFF. Results from both laboratory and field demonstration tests confirmed the successful removal of the broad suite of PFAS identified at AFFF-impacted sites from groundwater. Both single-use and regenerable IX systems can be successfully applied, and critical factors to the sustainable and cost-effective deployment of these technologies were identified. Results support future efforts aimed at demonstrating IX treatment of groundwater with diverse geochemical conditions and PFAS mixture composition. (Project Completion - 2023)

Boyer, T.H., A. Ellis, Y. Fang, C.E. Schaefer, C.P. Higgins, and T.J. Strathmann. 2021. Life Cycle Environmental Impacts of Regeneration Options for Anion Exchange Resin Remediation of PFAS Impacted Groundwater. Water Research, 207:117798. doi.org/10.1016/j.watres.2021.117798.

del Moral, L.L., Y.J. Choi, and T.H. Boyer. 2020. Comparative Removal of Suwanee River Natural Organic Matter and Perfluoroalkyl Acids by Anion Exchange: Impact of Polymer Composition and Mobile Counterion. Water Research, 178:115846. doi.org/10.1016/j.watres.2020.115846.

Dietz R., C. Kassar, and T.H. Boyer. 2021. Regeneration Efficiency of Strong-base Anion Exchange Resin for PFAS. AWWA Water Science, 3(6):e1259. doi.org/10.1002/aws2.1259. 

Ellis, A., C.J. Liu, Y. Fang, C. Bellona, T.H. Boyer, C.E. Schaefer, C.P. Higgins, and T.J. Strathmann. 2022. A Pilot Study Comparison of Regenerable and Emerging Single-use Anion exchange Resins for Treatment of Groundwater Contaminated by Per- and Polyfluoroalkyl Substances (PFASs). Water Research, 223:119010. doi.org/10.1016/j.watres.2022.119019.

Fang Y., A. Ellis, Y. Choi, T. Boyer, C.P. Higgins, C.E. Schaefer, and T.J. Strathmann. 2021. Removal of Poly- and Perfluoroalkyl Substance (PFAS) in Aqueous Film-forming Foam (AFFF) Impacted Water Using Ion Exchange and Non-ionic Resins. Environmental Science and Technology, 55(8):5001-5011. doi.org/10.1021/acs.est.1c00769.

Fang, Y., P. Meng, C. Schaefer, and D. Knappe. 2023. Removal and Destruction of Perfluoroalkyl Ether Carboxylic Acids (PFECAs) in an Anion Exchange Resin and Electrochemical Oxidation Treatment Train. Water Research, 230:119522. doi.org/10.1016/j.watres.2022.119522.

Kassar C., C. Graham, and T.H.Boyer. 2022. Removal of Perfluoroalkyl Acids and Common Drinking Water Contaminants by Weak-base Anion Exchange Resins: Impacts of Solution pH and Resin Properties. Water Research X, 17:100159. doi.org/10.1016/j.wroa.2022.100159.

Schaefer, C.E., D. Tran, Y. Fang, Y. Choi, C.P. Higgins, and T.J. Strathmann. 2020. Electrochemical Treatment of Poly- and Perfluoroalkyl Substances in Brines. Environmental Science: Water Research and Technology, 6:2704-2712. doi.org/10.1039/d0ew00377h.