Omniphobic surface modification of silica sand ceramic hollow fiber membrane for desalination via direct contact membrane distillation

Omniphobic silica sand ceramic hollow fiber membrane (SCHFM) was successfully fabricated for the reduction of the wetting and fouling in direct contact membrane distillation (DCMD). Surface functionalization of the laboratory spun SCHFM was performed following a series of steps 1) deposition of spherical silica nanoparticles (SiNPs), 2) fluorination by 1H,1H,2H,2H perfluorodecyltriethoxysilane (FAS17) and 3) poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) coating to glue SiNPs to the SCHFM surface. Initially, the pristine SCHFM was hydrophilic, but when the surface modification was fully applied the hollow fiber became omniphobic, exhibiting high contact angles for all tested liquids of different surface tensions, such as distilled water, red palm oil and ethanol. For example, one of the surface modified hollow fibers, called SiNPs-FAS-PVDF/FAS, was superomniphobic toward water (167°) and red palm oil (157°), and near superomniphobic toward ethanol (146.1°). As for the DCMD experiments with the feed solution containing 35 g/L NaCl and 10 mg/L humic acid, two of the surface modified hollow fibers, called SiNPs-PVDF/FAS and SiNPs-FAS-PVDF/FAS, showed high anti-fouling and anti-wetting resistance during the entire DCMD operation period of 400 min with little flux reduction and no observable increase in the conductivity of the permeate stream. The anti-fouling resistance of these hollow fibers was further evidenced by the cleanness of the surface in the micrographic picture taken after the DCMD run. The omniphobicity of these hollow fibers is ascribed to the combined effects of surface tension decrease by the FAS17 fluorination and the formation of the re-entrant structure by the densely packed SiNPs. With reasonably high fluxes of 35.1 kg/m2∙h and 43.6 kg/m2∙h, for SiNPs-PVDF/FAS and SiNPs-FAS-PVDF/FAS, respectively, and 100% of salt rejection, these hollow fibers have potential for seawater desalination by DCMD.