One-step preparation of omniphobic membrane with concurrent anti-scaling and anti-wetting properties for membrane distillation

Membrane scaling represents one of the significant challenges for membrane distillation (MD), especially when treating hypersaline brines. This work presents a one-step dip-coating method to develop an omniphobic membrane. The commercial polyvinylidene fluoride (PVDF) hydrophobic porous membranes were immersed into the epoxy acrylic (EA) resin and fluorosilane (1H,1H,2H,2H-Perfluorodecyltrimethoxysilane, PFTS) mixture followed by a heating cross-linking process. EA/PFTS ratio of 1:1 (v/v) gave the best performance regarding surface morphology, surface chemical composition, surface energy, and mechanical property. The resultant PVDF-(EA/PFTS)-1:1 membrane demonstrated excellent wetting resistance and omniphobicity during the penetration of deionized water, 30% and 60% ethanol in water, and kerosene, giving 141°, 124°, 106°, and 116° contact angles, respectively. It also exhibited excellent scaling resistance in concentrating 14.7 mM gypsum solution and actual reverse osmosis (RO) brine. The permeate flux and conductivity were maintained at about 14 kg m−2 h−1 and below 3 μS cm−1, respectively. More than 99% salt rejection (actual RO brine) was achieved during a long-term continuous MD operation for 80 h. The PVDF-(EA/PFTS)-1:1 membrane exhibited excellent anti-scaling and anti-wetting properties. A series of capillary experiments confirmed that the membrane's surface pores achieved the gas-wetting state. The increase in the membrane surface hydrophobicity, the reduction in the surface pore size, and the toughening of the surface pores contributed to the formation of a stable gas-liquid interface on the modified membrane surface that can enhance the anti-scaling and anti-wetting performance.