Engineering of robust PFAS-free hydrophobic hybrid ceramic membranes for desalination

Hydrophobic organo-functionalized ceramic membranes represent a major advancement in membrane technology, providing robust solutions for separation and purification processes where wettability resistance is critical. In seawater desalination by membrane distillation (MD), hydrophobicity enables selective water vapor transport while preventing pore wetting. Traditionally, ceramic membranes are functionalized with fluorinated alkylsilanes, a type of per - and polyfluoroalkyl substances (PFAS) used as hydrophobizing agents due to their low surface energy and covalent bonding capability. However, their Si-O-metal bonds are prone to hydrolysis, and PFAS leaching poses environmental and health risks. In this study, macroporous α-Al 2 O 3 membranes were functionalized with PFAS-free alkylphosphonic acids bearing 12 or 18 carbon chains. Regardless of chain length, grafted membranes exhibited water contact angles up to 134° and high liquid entry pressures (>19 bar for disks, >8 bar for tubular membranes). FTIR, solid-state NMR, and XPS confirmed the formation of stable phosphonate bonds in mono- and bidentate configurations. To assess applicability under operational conditions, vacuum membrane distillation ( V -MD) tests were conducted with tubular membranes using 3.5 wt% NaCl feed and resulted in stable water vapor fluxes (3.9–9.5 L.m −2 .h −1 across temperatures tested), 99.9% salt rejection, and ≥ 94% flux recovery after saline operation. Static stability tests in 3 wt% NaCl at 80 °C for 72 h showed minimal loss of hydrophobicity (~7%) and only a minor reduction in phosphorus by XPS, indicating a chemical robustness that compares with PFAS-modified ceramic membranes reported in the literature under similar saline exposure conditions. These findings demonstrate that alkylphosphonic acid grafting produces PFAS-free, chemically robust hydrophobic ceramic membranes with strong potential for long-term seawater desalination applications.