Investigation of PFAS rejection by closed-circuit reverse osmosis and nanofiltration and sorption to treatment materials during groundwater treatment: a pilot demonstration

Abstract

The rejection of a broad range of per- and polyfluoroalkyl substances (PFAS) by reverse osmosis (RO) and nanofiltration (NF) was evaluated using a pilot closed-circuit membrane system operating at three recoveries (80, 85, 90 %) treating aqueous film-forming foam (AFFF) impaired groundwater (total PFAS ∼ 14.3 µg/L). Evaluation of the membranes focused on 15 PFAS measured in the groundwater above 75 ng/L including carboxylates, sulfonates, fluorotelomer sulfonates, and sulfonamides, dominated by perfluorooctane sulfonate (PFOS). RO required higher pressures and energy to reach recovery setpoints than NF, in exchange for PFAS specific rejections greater than 99 %. Rejection by NF ranged from 97.9 to 99.8 % and was impacted by functional group (carboxylates > sulfonates, fluorotelomer sulfonates > sulfonamides) and increased by increasing chain lengths. Overall PFAS rejection by RO decreased between 85 and 90 % recovery, with discrete sampling demonstrating a decrease in rejection after 87 % recovery, indicating that the tradeoff between reduced retentate volume and decreased permeate quality is an important operational consideration. To evaluate PFAS sorption to treatment materials, methanol extractions were performed on pretreatment materials and one BW30 element. Adsorbed total PFAS mass was dominated by PFOS, with long-chain PFAS exhibiting preferential adsorption. Per gram of material extracted, a 0.2 µm cartridge filter accumulated the most PFAS. The membrane and IX softening resin had similar accumulation of PFAS, while accumulation was the lowest on greensand. PFAS chain length had the greatest impact on adsorption to the filter, membrane, and IX softening resin, sorption to greensand was more impacted by functional group.

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