Preparation and characterization of PVDF flat-sheet membranes for direct contact membrane distillation

The polyvinylidene fluoride (PVDF) flat-sheet membranes for direct contact membrane distillation (DCMD) were prepared through coating and wet phase inversion process. The effects of solvent, polymer concentration, non-solvent additive and nonwoven fabric on membrane morphology, porosity, pore size, pore size distribution, hydrophobicity and permeability were investigated. It was found that the membrane using N,N-dimethylacetamide as solvent obtained good permeability, and the polymer dope containing 20 wt.% PVDF was the most suitable for hydrophobic flat-sheet membrane preparation. The PVDF flat-sheet membranes with different non-solvent additive such as acetone, phosphoric acid, glycerine, LiCl and PEG 400 were prepared and the membrane using 5 wt.% acetone and 3 wt.% H3PO4 as mixed additive had the highest permeate flux. The nonwoven fabric could prevent membrane shrinkage, while both pore size and permeate flux of the PVDF/nonwoven fabric composite membrane were enhanced. Although the nonwoven fabric caused the membrane surface roughness decline, the contact angle only decreased slightly. The PVDF/nonwoven fabric composite membrane exhibited satisfying performance stability during the 240 h continuous desalination experiments, indicating that the resultant flat-sheet membrane may be of great potential to be utilized in DCMD process for desalination.