(396f) Adsorption and on-Demand Desorption of Perfluoroalkyl Substances in Water By Thermo-Responsive Poly(N-isopropylmethacrylamide)

Per- and polyfluoroalkyl substances (PFAS) have been utilized in a wide range of applications including surfactants, aqueous film forming foams (AFFF), and oleophobic coatings due to their thermal stability, low surface free energy, and chemical inertness. Recent reports have revealed that soil and groundwater at which firefighting activities have been ­undertaken such as military base sites are contaminated. Also, industrial effluents and disposal of PFAS-containing products have contributed to the contamination. Paradoxically, PFASs' excellent chemical and thermal durability makes them persistent in both environment and the human body. While it still remains an active debate field, it is generally agreed that PFAS can cause a variety of human health disorders and pose adverse effects to both aquatic and terrestrial ecosystems. Thus, the U.S. Environmental Protection Agency (EPA) included perfluorooctanoic acid (PFOA) and perfluorosulfonic acid (PFOS) in the Contaminant Candidate List in its 2016 annual report.

A typical PFAS concentration detected in water is in the order of parts per trillion (10^-12, ppt) which can cause a grand challenge not only for removing them but also for their detection. To measure the concentration of PFAS by using commercial instruments such as high-performance liquid chromatography (HPLC), the concentration should be in the range of parts per billion (10^-9, ppb). Various methodologies including activated carbon, ion exchange, and reverse osmosis membrane have been employed to remove PFAS from water. While they have proven effective in removing and isolating PFAS, their regeneration after releasing them has been rarely reported.

Herein, a poly(isopropylmethacrylamide) (P-NIPMAM)-based adsorbent that can adsorb and retain PFAS from water at a temperature higher than its lower critical solution temperature (LCST) while releasing them at a lower temperature is reported. P-NIPMAM was prepared by a free radical polymerization of N-isopropylmethacrylamide (NIPMAM) with potassium persulfate (KPS) and N,N'-methylenebisacrylamide (MBAA) as an initiator and a cross-linker, respectively. The resulting P-NIPMAM exhibits a LCST behavior. At a temperature below the LCST, hydrogen bonding between hydrophilic groups (i.e., -NH₂) of P-NIPMAM and water dominates. When a temperature becomes above the LCST, the solvation entropy for hydrophobic moieties (i.e., isopropyl and methyl groups) of P-NIPMAM becomes dominant resulting in a globule-like collapsed structure. These hydrophobic moieties exhibit a low affinity to water and can partition with the compounds bearing non-polar hydrophobes such as PFAS as adsorbate.

The adsorption and desorption of P-NIPMAM were systematically studied by using PFAS possessing varied fluoroalkyl chain lengths. The adsorption capacity was measured at T = 50 °C (> LCST). The results show that P-NIPMAM exhibits a higher adsorption capacity for PFAS with longer fluoroalkyl chain length. This is a direct consequence of a stronger hydrophobic interaction between the fluoroalkyl chain and the hydrophobic moieties of P-NIPMAM. We also demonstrated that P-NIPMAM can release the adsorbed PFAS to water at T (= 23 °C) < LCST. The results indicate that PFAS with a shorter fluoroalkyl chain is released faster than that with a longer chain. This can be attributed to different hydrophobic interaction which governs the adsorption and desorption. Finally, we fabricated a prototype sampler by utilizing our P-NIPMAM that can adsorb and retain PFAS at an elevated temperature (T > LCST) while releasing them into the water at a lower temperature (T < LCST).