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

Topic Area: 7f. In Situ PFAS Treatment Approaches

Formatted Title:

PANI: A Polymer Adsorbent for In Situ Groundwater PFAS Treatment

Background/Objectives:

Per- and poly-fluoroalkyl substances (PFAS) are of emerging concern due to their ability to persist in groundwater and accumulate in humans and animals. PFAS compounds have become ubiquitous in water supplies and are often toxic or carcinogenic, requiring impacted water to be treated before use. Injectable sorbents, primarily based on activated carbon, are used to control the migration of these compounds in situ. Organic polymers can also be used to adsorb PFAS and have a number of advantages for in situ remediation, including ease of injection, improved adhesion to geo-media surfaces, selectivity for PFAS and higher matrix loading potential than activated carbon, making them potentially superior materials for in situ sequestration of PFAS.

This presentation will include the results of laboratory studies of the adsorptive capabilities of polyaniline (PANI) and studies of its injectability in a porous media. PANI is a cationic hydrophobic polymer that can be dispersed in water to form a stable suspension of colloidal particles. In situ it adheres to soil particles and provides adsorption sites for anionic PFAS. Results include its PFAS adsorptive capacity, adsorption mechanisms, and migration in sand columns.

Approach/Activities:

Initial testing focused on the synthesis of cationic polymers including PANI and poly-o-toluidine (POT), the ability to adsorb PFAS, adsorption mechanisms using infrared spectroscopy, regeneration, and the effect of NOM and cations on PFAS adsorption. The next phase of studies focused on developing the colloidal PANI (<1 µm) for direct injection, testing for adhesion to aquifer materials as well as adsorption of PFAS in a flow through system. Dynamic light scattering and scanning electron microscopy were used to characterize the suspended polymer and to visualize adhesion to soil particles, respectively. Adsorption and migration were tested in soil columns using both washed silica sand and the same sand coated with aluminum hydroxide to simulate diverse aquifer materials. Soil columns tested the migration of PANI under a variety of conditions of pH, as well as with coated and uncoated particles, and determined an equilibrium concentration of PANI adsorbed to the sands. Soil columns were then used to test the migration of PFAS in soil treated with PANI to quantify the potential impact of PANI on PFAS migration.

Results/Lessons Learned:

Based on the experimental results, these polymers are well characterized, and their behavior is understood. While both PANI and POT were found to adsorb PFAS, PANI was selected for additional testing based on superior adsorption and dispersion qualities. Transport and adherence of PANI was impacted by the aquifer material surface chemistry and pH, and pH appears to be a useful tool for tuning migration of the PANI and improving injectability. An optimal loading for PANI was determined by dissecting the soil columns and measuring the adsorbed PANI and this optimal loading was used to design advanced column tests. The average loading ranged from 2 to 25 mg/g_Sand during the tests and, under a constant infusion of PFAS, the PANI-sand matrix was able to retain and remove around 90% of the PFAS concentration for over 6000 bed volumes.

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Reyes Sierra-Alvarez