Titanium carbide (MXene), a member of the large family of 2D transition metal carbides, carbonitrides, and nitrides is characterized by its unique surface chemistry, which includes a variety of functional groups such as =O, -OH, and -F. Modifying the surface chemistry of MXene holds great potential for improving the adsorption of per- and polyfluoroalkyl substances (PFAS), yet the precise relationship between MXene surface chemistry and PFAS adsorption remains largely unexplored. In this study, two distinct MXene variants were synthesized to assess their effectiveness in removing perfluorooctane sulfonic acid (PFOS) from water. The first MXene, termed F-MXene, was synthesized using hydrofluoric acid (HF) and predominantly featured a surface terminated with high fluorine content. The second variant, O-MXene, was synthesized through a mild delamination method and had a higher oxygen-to-fluorine ratio. The adsorption performance of these two MXenes was evaluated under varying pH levels (4.5 and 10; acidic and basic conditions), PFAS concentrations (0.05 ppm and 2 ppm), and contact times. The results showed that PFOS adsorption was significantly influenced by pH, with maximum adsorption observed at acidic conditions (pH 4.5). At an initial PFOS concentration of 0.05 ppm, F-MXene and O-MXene achieved similar adsorption efficiencies of 92.6 ± 7.9% and 88.5 ± 5.6%, respectively. However, at a higher PFOS concentration of 2 ppm, adsorption sites became saturated. In this case, F-MXene outperformed O-MXene, achieving an adsorption efficiency of 69.8 ± 15%, while O-MXene showed a much lower efficiency of 27.6 ± 15%. The interaction mechanisms for PFOS adsorption on the MXenes likely involved hydrophobic interaction and hydrogen bonding playing key roles. Kinetic studies further demonstrated the superior performance of F-MXene, which reached adsorption equilibrium within 5 h, much faster than O-MXene. This study provides valuable insights into the design of advanced MXene-based materials for water purification applications.
