Hydrophobic–Hydrophilic Switching in Thermo-Responsive Hydrogels: A Novel Pathway for PFAS Control and Remediation

Per- and polyfluoroalkyl substances (PFAS) are persistent contaminants widely detected in terrestrial systems, where their high mobility and resistance to degradation pose long-term risks to soil and groundwater resources. Although traditional adsorbents such as activated carbon are widely used for PFAS remediation, their limited regenerability often necessitates high-temperature treatment or direct incineration with high CO₂ emissions, or the use of expensive and toxic organic solvents for desorption, thereby constraining their sustainable application in environmental systems. Here, we present a novel thermo-responsive hydrogel adsorbent that enables temperature-controlled adsorption and release of PFAS, offering a new pathway for the sustainable management and remediation of PFAS.

The hydrogel was synthesized via copolymerization of N-isopropylacrylamide (NIPAM) and 2-(methacryloyloxy)ethyltrimethylammonium chloride (MTAC) and exhibits a lower critical solution temperature (LCST) of approximately 35 °C. Above the LCST, the hydrogel surface becomes hydrophobic, promoting the adsorption of long-chain PFAS including perfluorooctanoic acid (PFOA) and perfluorooctanesulfonic acid (PFOS) through hydrophobic interactions. Below the LCST, the surface transitions to a hydrophilic state, weakening these interactions and enabling PFAS desorption. As a result, the adsorption coefficient (Kd) of the hydrogel for PFOA at 45 °C is 35-fold higher than that at 25 °C. By exploiting this reversible hydrophobic–hydrophilic transition, adsorption and desorption of long-chain PFAS can be achieved without the use of organic solvents. Lab-scale batch experiments demonstrated that approximately 80% of PFOA and PFOS could be desorbed through temperature control alone, with complete desorption achieved upon the addition of chloride ions (Cl⁻). Notably, after ten adsorption–desorption cycles, PFOS desorption efficiency remained above 80%, indicating excellent reusability.

The environmental relevance of this approach was further evaluated using rapid small-scale column tests with tap water as a proxy for natural water matrices. Through temperature modulation and the addition of 1% NaCl, 84% of PFOS and 75% of PFOA were desorbed, with enrichment factors of 32 and 15, respectively. These results demonstrate that thermo-responsive hydrogels can provide a controllable and solvent-free strategy for PFAS retention and release, offering new opportunities for sustainable remediation and management of PFAS.