Eco-Friendly Nonfluorinated Coating of Omniphobic Membranes With ZnO Nanoparticles Re-Entrant Structures: Design, Characterization, and Desalination Performance via Membrane Distillation

Abstract

Membrane distillation commonly faces challenges such as wetting, fouling, and scaling, which limit its long-term desalination performance. In this study, an omniphobic membrane was developed via an ecofriendly coating approach, a nonfluorinated omniphobic PVDF hollow fiber membrane via dip coating of zinc oxide nanoparticles with various concentrations of 1–3 wt% as re-entrant structures, followed by stearic acid grafting. The proposed strategy enhanced surface roughness, reduced surface free energy, and promoted stable omniphobicity without fluorination. A comprehensive characterization was conducted using SEM, AFM, FTIR, XRD, contact angle goniometry, LEP testing, and ICP-OES analysis to confirm surface morphology, roughness, chemical bonding, and coating stability. The results show that increasing ZnO NPs concentration up to 3 wt% produced a surface roughness of 100.43 nm and contact angle above 120° for all tested liquids. The best membrane exhibited excellent desalination performance with water flux above 14 L/m²h and salt rejection of ~99.97% for five consecutive days under 30,000 ppm NaCl feed and 100 ppm humic acid. Accelerated chemical exposure under hot synthetic seawater showed less than 5% salt deposition while water contact angle remained above 90° after 48 h, demonstrating strong resistance to surface coating degradation.