Theme: 07. Per- and Polyfluorinated Alkyl Substances (PFAS)

Topic Area: 7f. In Situ PFAS Treatment Approaches

Formatted Title:

Efficient Sorption of Short-Chain and Ether PFAS on a Modified Clay

Background/Objectives:

Water contamination by per- and polyfluoroalkyl substances (PFAS) impacts many countries and leads to regulatory actions to reduce human exposure to these persistent chemicals. Implementing stringent guidelines results in costly water treatment processes because existing adsorbents that remove PFAS from water are often less effective than other water contaminants. Activated carbon shows limited efficacy for removing short-chain PFAS (n < 6) because of their high solubility imparted by charged functional groups. Alternatively, ion exchange resins are utilized to remove anionic PFAS. An affordable solution involves modifying smectite clays by integrating quaternary ammonium surfactants into the interlayer space through ion exchange with cations. The first-generation surface modified smectite clays (SMC), commercially known as FLUORO-SORB® Adsorbent, have shown highly effective in removing many different PFAS compounds. In particular, SMCs show high affinity for long-chain PFAS compounds. SMCs are increasingly becoming integral components in treating PFAS-contaminated water and remediating soil. A notable feature of SMCs is their resistance to fouling with high organic matter background content, thus making them suitable materials for in-situ treatments, such as in permeable reactive barriers. However, the demand for treating short-chain PFAS and emerging PFAS such as polyfluoroalkyl ether carboxylic acids (PFECAs) necessitates further enhancement of sorbent performance to address these challenges.

In this study, we conducted a comprehensive assessment of the second generation of clay products (FLUORO-SORB® FLEX Adsorbent) with the following objectives. 1. To evaluate the performance of the new SMC for PFAS, particularly for short-chain PFAS and PFECAs; 2. To compare the performance of FLEX with a first-generation SMC (Flouro-Sorb) and activated carbon and ion exchange resins; 3. To investigate the impact of water chemistry (e.g., pH, anions, cations, and organic matter) and organic co-contaminants; 4. To elucidate the sorption mechanism via mineralogy characterization and molecular simulations.

Approach/Activities:

The treatment performance was evaluated via batch adsorption experiments that compared FLUORO-SORB® 200, FLEX, GAC (F400), ion exchange resin, and biochar. The effect of co-contaminants and water chemistry was conducted through the addition of diesel, NH4Cl, CaCl2, NaCl, Na2SO4, NaNO3, Na3PO4, and IHSS organic matter into groundwater samples, respectively. The concentrations of PFAS were determined by a liquid chromography coupled with Q-Exactive Orbitrap mass spectrometer (LC-Orbitrap-HRMS) operated in Full Scan MS mode. The amount of PFAS taken up by the adsorbent was calculated by the PFAS mass difference between the blank samples and equilibrated samples. All experiments were run in triplicates.

Results/Lessons Learned:

The maximum adsorption capacity of FLEX for PFBS, PFOS, PFBA, PFOA, PFMPA, HFPO-DA and HFPO-TA was determined to be 22.2 mg/g, 71.1 mg/g, 15.15 mg/g, 41.1 mg/g, 21.8 mg/g, 54.2 mg/g and 61 mg/g, respectively. The adsorption capacity of FLEX for PFAS remained high in a wide pH range. FLEX's performance for long-chain PFAAs was comparable to that of the first-generation Fluoro-Sorb, but FLEX outperformed FLUORO-SORB 200, granular activated carbon and ion exchange resin for short-chain perfluoroalkyl carboxylates and PFECAs. Organic co-contaminants such as diesel and dissolved organic matter, as well as varying levels of common cations and anions at the levels typically observed in groundwater, do not affect the sorptive removal. Only when organic matter is unusually high (over 100 mg/L), the adsorption capacity of FLEX for PFAS decreased by 25 to 30%. Material simulation results match the experimentally determined basal spacing via X-ray diffraction patterns of the modified clay loaded with PFAS. The combined experimental and molecular simulation studies indicated that hydrophobic interactions and electrostatic interactions play a crucial role in the adsorption of PFAS on FLEX.

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Bei Yan