Fabrication of novel omniphobic polypropylene membrane for efficient direct contact membrane distillation of produced water

Among the main challenges in the membrane distillation (MD) process are the membrane wetting and fouling, which significantly hinder the long-term treatment of produced water containing high salinity, hydrocarbons and surfactants. To address these limitations, a robust omniphobic polypropylene (PP) membrane through a two-step modification process was fabricated. First, a PP membrane, prepared via the thermally induced phase separation (TIPS) method, was spray-coated with a solution containing amine-functionalized silica (NH₂-SiO₂) nanoparticles (NP) and an epoxy polymer resin, followed by a curing step. NH₂-SiO₂ NP were immobilized onto the PP substrate via crosslinking between the amine groups and the epoxy groups of the polymer resin and adhesion property of the epoxy resin. In the second step, surface fluorination was performed using 1H,1H,2H,2H-perfluorodecyltrimethoxysilane (PFDTES) to produce a hierarchically rough coating with low surface energy. The influence of NH₂-SiO₂ NP concentration in the spray-coating dispersion was thoroughly evaluated with respect to its effect on the morphology, pore size distribution, surface chemical structure and water contact angle of the membranes, as well as separation performance. It was demonstrated that at NP concentrations of 0.5 and 0.75 wt%, the resulting PP-based membranes exhibited high contact angles (>120 $°$) against liquids with diverse surface tensions, indicating excellent omniphobicity. The anti-wetting and anti-fouling performance of all membranes were assessed via direct contact membrane distillation (DCMD) over 50 h of continuous operation, using both synthetic and real produced water as feed. The PP membranes modified with 0.5 and 0.75 wt% NP demonstrated outstanding resistance to wetting and fouling, maintaining salt rejection above 99.99 %, whereas the pristine PP membranes exhibited wetting within 13 h under the same conditions. The feasibility of pure water recovery (71 % over a 50-h DCMD run) from a real highly saline produced water originating from an oil field was also demonstrated. In summary, the developed PP membrane showed a great potential for membrane distillation in the practical treatment of challenging oily wastewater.