(454a) Functionalized Fluorinated-Aminated Mxene Membranes for Enhanced and Selective PFAS Removal

Per- and perfluoroalkyl substances (PFAS) are a group of synthetic compounds that are major emerging contaminants in water and have attracted widespread attention nowadays. The U.S Environmental Protection Agencies have set the permissible level of Perfluoro octane sulfonate (PFOS), a major concern among PFAS, at 4 ppt, which poses a challenge for effective removal. While various techniques, such as sorption and membrane separation, have been employed for PFOS removal, they remain inefficient in selectively and completely eliminating these contaminants. MXene (Ti₃C₂Tₓ) is a two-dimensional metal carbide/nitride material with surface termination groups such as oxygen, hydroxyl, and fluorine, which facilitate its engineered functionalization for the selective removal of analytes such as PFAS. In this work, 2D MXene nanosheets were functionalized with silane-based reagents to introduce amine (NH₂-MXene) and fluoride (F-MXene) groups, aiming to enhance their selectivity toward long- and short-chain PFAS through improved fluorophilic and electrostatic interactions.

The zeta potential for pristine MXene is negative (- 30 mV), while NH₂- MXene and F-MXene became positively charged and showed zeta potential of 42 mV and 4.97 mV, respectively, at pH 5. Surface functionalization was confirmed through FTIR and XPS, while XRD, SEM, and AFM were used to analyze the d-spacing, morphology, and surface roughness of the MXene sheets. Batch adsorption experiments showed that the equilibrium adsorption capacity of PFOS for pristine MXene was 26 (± 0.8) mg/g at a 1.8 ppm equilibrium concentration. However, the composite of F-MXene and NH₂-MXene exhibited significantly higher adsorption due to a combination of unique C-F^...F-C interactions (fluorophilic) between the F-MXene and molecule and electrostatic interactions between the positively charged of F-NH₂-MXene surface and the negatively charged PFOS molecules. This demonstrates the synergistic effect of the functionalized MXene composites. In addition, to facilitate continuous PFOS removal, the functionalized MXene nanosheets were fabricated into 2D-structured membranes, and their performance, specifically water permeability, PFAS rejection, and selectivity in the presence of competing compounds such as natural organic matter, was investigated to provide effective membrane for water treatment tailored for generation of PFAS-free permeate. Overall, the selective adsorption and removal of PFOS through electrostatic attraction and size exclusion, combined with interlayer spacing that facilitates water transport, make the functionalized F-NH₂-MXene membrane an effective solution for water treatment aimed at producing PFAS-free permeate.