Novel dissolution-phase inversion strategy for the green and high-flux PVDF membranes for membrane distillation

Membrane surface microstructures played an important role in constructing the high-permeance polymer membranes for membrane distillation (MD). However, conventional phase inversion method often results in a low-porosity membrane layer due to the rapid solvent/non-solvent exchange. Herein, a novel and universal “dissolution-phase inversion” strategy was proposed to construct porous surface and mico-nano structures on the “dense” PVDF membrane layer that was particularly suitable for MD. In addition, green solvent was employed in the whole membrane preparation process, including the base membrane and porous layer. First of all, the properties of the base membrane were investigated in detail. It was demonstrated that the base membrane surface showed a little porous during the restricted shrinkage of the non-woven fabrics (NWFs) if the NWFs were used as support, and thus the base membrane showed a high water flux than those without using NWFs as supports. Then, the detailed parameters, such as the PVDF concentrations in the dilute solution and the immersion time during the dissolution-phase inversion process were investigated. The results showed that when the PVDF concentration was 1 wt% and the immersion time was 5s, the resulting membrane showed a water flux of 32 kgm⁻²h⁻¹ while the base membrane just obtained a water flux of 1.2 kgm⁻²h⁻¹. During the MD process, these membranes achieved a vapor flux of 32.6 kgm⁻²h⁻¹ with a NaCl rejection exceeding 99.9 %. Additionally, the membranes showed effective performance in dye separation with a water flux surpassing 19.8 kg m⁻²h⁻¹. This work introduced an interesting method for the fabrication of green and high-flux PVDF membranes, which presented a novel strategy to enhance the surface porosity and hydrophobicity.