Fabrication and modification of PVDF/PSF hollow-fiber membranes for ginseng extract and saline water separations via direct contact membrane distillation

Poly(vinylidene fluoride) (PVDF)/polysulfone (PSF) hollow-fiber (HF) membranes with high mechanical strength and satisfactory water permeance using a green fabrication technique were investigated by varying the compositions of coating fluid and bore fluid during membrane spinning using a triple spinneret. The results demonstrated that the HF membranes using hydrophobic coating fluid provided a skinless structure with higher water permeance than those produced from a hydrophilic coating fluid or polymer solution. These skinless PVDF/PSF membranes showed 99.96% rejection of a ginseng extract solution with different concentrations via direct contact membrane distillation (DCMD) but relatively poor stability and rejection behavior for saline water due to the poor hydrophobicity of the membrane. To improve the hydrophobic properties of PVDF/PSF membranes for saline water, two hydrophobic modification methods (i.e., chemical modification and physical modification) were employed and compared. Modified membranes showed enhanced hydrophobic properties as characterized by XPS, FTIR analysis, and stability tests. In the separation of saline water (35 g/L salts and 0.1 mM sodium dodecyl sulfate), modified membranes showed higher water flux and rejection than pristine membrane. The modified membrane showed greater than 99.99% rejection and outstanding stability even after operation for 120 h. This work provides skinless fiber-aligned PVDF/PSF hollow-fiber membranes with different surface properties for diverse systems via DCMD, such as concentration of food extract or separation of saline water.