Preparation of re-entrant and anti-fouling PVDF composite membrane with omniphobicity for membrane distillation

Superhydrophobic and oleophobic membrane applied in membrane distillation was fabricated by constructing a re-entrant composite structure via immobilizing silica nanoparticles (NPs) on the polyvinylidene fluoride (PVDF) membrane. The silica coated membrane with the assistance of polydopamine (PDA) was then fluorosilanized by means of a silane coupling agent 1H,1H,2H,2H-perfluorooctyltrichlorosilane. A series of characterizations were implemented to investigate the effects of modification process on the structure and performance of the PVDF composite membranes. The fouling and wetting behaviors of original and modified membranes were explored by direct contact membrane distillation (DCMD) tests using a mixed solution of inorganic salts, organic matter and surfactant as the feed. The results indicated that silica NPs and fluorinated silane were presented on the PVDF membrane substrate. After surface modification, the liquid entry pressure (LEP) increased by 120%, in addition, the water contact angle (WCA) increased from 127.0° to 169.0°, and the oil contact angle (OCA) increased from 2.8° to 112.1°. The modification process showed no effect on the pore size and pore structure. Furthermore, the composite membrane exhibited excellent chemical stability and prominent mechanical stability. The re-entrant structure resulting from nanoparticles and microspheres and the omniphobic feature were conferred on the composite membrane. The DCMD tests showed that the composite membrane kept a stable permeate flux in the long-term concentration experiment, with the salt rejection rate approximately 100%. What's more, the composite membrane did not suffer from fouling and wetting, and still dry and clean after the test. In comparison, the pristine membrane was seriously polluted during the test, and the permeate flux decreased to 28.3% of the initial value. Overall, the novel modified PVDF composite membrane reported in this work presents outstanding anti-fouling performances.