Enhanced removal of per- and polyfluoroalkyl substances by crosslinked polyaniline polymers

Polyaniline (PANI) and other amine-containing materials have recently been reported as high-affinity, selective adsorbents for per- and polyfluoroalkyl substances (PFAS) remediation, with surface area normalized adsorbed mass significantly exceeding that of granular activated carbon. However, most of these materials have low specific surface area (SSA), suggesting that increasing SSA could result in further enhancement of their ability to sequester PFAS. Herein, we employed a crosslinking method with paraformaldehyde (PFA) to improve the SSA and porous structure of PANI. Several characterization techniques, including elemental composition, N₂ adsorption–desorption, zeta potential titration, X-ray photoelectron spectroscopy, and Fourier transform infrared spectroscopy, confirmed the success of the crosslinking process. The characterization results demonstrated that crosslinking was a very effective method to synthesize hyper-crosslinked, permanently porous polymers with high SSA (490.3 m² g⁻¹ for crosslinked PANI_PFA_2.4 compared to 26.3 m² g⁻¹ for original PANI). Most importantly, crosslinked PANI presented rapid kinetics and high removal efficiency toward perfluorooctanoic acid (PFOA) at environmentally relevant concentrations, the maximum adsorption capacity of crosslinked PANI_PFA_2.4 (98.4 mg g⁻¹) for PFOA remarkably exceeded that of PANI (41.1 mg g⁻¹). Both PANI and crosslinked PANI showed higher sequestration for perfluorosulfonates than their corresponding perfluorocarboxylates, and adsorption increased with increasing fluorocarbon chain length. However, PANI_PFA_2.4 showed lower removal efficiency of multicomponent PFAS mixtures, relative to PANI, which is likely due to competitive PFAS adsorption and to the formation of narrow pores during crosslinking that were easily blocked by preferentially adsorbed long-chain PFAS molecules. The results in this study indicate that crosslinked PANI polymers are promising adsorbents for in situ remediation of PFAS-contaminated groundwater.