Enhancing DCMD vapor flux of PVDF-HFP membrane with hydrophilic silica fibers

Abstract

Hydrophobicity is an important property for membranes used in direct contact membrane distillation (DCMD). The higher the hydrophobicity the better the membrane capability in preventing liquid crossover. On the other hand, membrane pore hydrophilicity enhances water vapor transport and increases membrane permeability. This tradeoff is resolved by incorporating electrospun silica fibers (SiF) as fillers in electrospun poly(vinylidenefluoride-co-hexafluropropylene) (PVDF-HFP). Composite PVDF-HFP/SiF membranes were fabricated and applied in DCMD to desalinate salt water. Various SiF loadings from 0.5 wt% to 7.0 wt% were studied. Although no hydrophobic modification was carried out on the SiF before use, they still allowed the composite membranes to maintain water contact angles above 90° due to their fibrous structure (Cassie-Baxter state). The addition of silica in fibrous form to the membrane prevented agglomeration in the composite membrane and decreased overall porosity, but resulted in higher water flux for the composite membranes compared to the neat PVDF-HFP membrane when tested for MD over a time period of 11 h with 35 g/L saline feed. Despite the hydrophilic nature of SiF, all the composite membranes maintained perfect salt rejection throughout the experiment.