Perfluorooctanesulfonic Acid Removal by Delaminated Titanium Carbide MXene: Impact of MXene Surface Chemistry on Adsorption Mechanisms and Removal Efficiency

Titanium carbide (MXene) is characterized by unique surface terminations such as ═O, −OH, and −F, which make it a tunable material capable of interacting with diverse compounds, including per- and polyfluoroalkyl substances (PFAS). Selective removal of PFAS from water remains challenging due to their low concentrations (∼50 ppb) and limited reactivity with most adsorbents. Modifying the surface chemistry of MXene holds excellent potential for improving the adsorption of PFAS. However, the precise relationship between MXene surface chemistry and PFAS adsorption remains largely unexplored. In particular, the fluorine-rich surface of MXene provides an opportunity to probe fluorophilic interactions in addition to conventional hydrophobic effects. In this study, two MXene variants were synthesized to evaluate their performance in removing perfluorooctanesulfonic acid (PFOS). The first, F-MXene, was prepared using hydrofluoric acid and possessed a surface rich in fluorine terminations. The second, O-MXene, was obtained through a mild delamination method, resulting in a higher oxygen-to-fluorine ratio. Adsorption tests were conducted under different pH values (4.5 and 10), PFOS concentrations (50 ppb and 2 ppm), and contact times (1, 3, 5, and 24 h). At 50 ppb PFOS, both variants performed comparably, with F-MXene and O-MXene showing adsorption efficiencies of 92.6 ± 7.9% and 88.5 ± 5.6%, respectively. At 2 ppm PFOS, however, F-MXene exhibited a markedly higher efficiency (69.8 ± 15%) compared to O-MXene (27.6 ± 15%). This highlights the contribution of fluorine–fluorine interactions at elevated PFOS concentrations, where adsorption sites become more competitive. Kinetic studies further revealed that F-MXene reached equilibrium within 5 h, while O-MXene required 24 h. Additionally, F-MXene demonstrated good reusability, retaining high adsorption performance after five regeneration cycles with only a 13% decline when treated with 80% methanol.