Construction of wetting resistance surface of highly hydrophobic PVDF-HFP/rGO coated supported PVDF hollow fiber composite membrane for membrane distillation

In this research, the construction of a highly wetting-resistance membrane surface was achieved through an integrated dip-coating phase inversion process to accelerate water mass transfer of membrane distillation (MD). The synergistic effect of incorporating reduced graphene oxide (rGO) embedded Poly (vinylidene fluoride-cohexafluoropropylene) (PVDF-HFP) on the surface of a PVDF hollow fiber support membrane using a dilute coating solution was studied. Various loadings of rGO nanosheets were incorporated into the dilute PVDF-HFP coating to systematically investigate its physicochemical characteristics, including surface morphology, hydrophobicity and wetting resistance which could be analyzed using scanning electron microscopy (SEM), atomic force microscopes (AFM), water contact angle (WCA), porosity measurement, pore size, x-ray diffraction (XRD), and liquid entry pressure (LEP) tests. The membrane separation performance for desalination was evaluated using direct contact membrane distillation (DCMD) process. Compared to the PVDF hollow fiber support membrane, the rGO-embedded PVDF-HFP coating layer demonstrates outstanding enhancements. The membrane surface exhibits enhanced hydrophobicity, as evidenced by an increased WCA from 105.37 degrees (for the PVDF support membrane) to 148.37 degrees (for the rGO/PVDF-HFP composite membrane). Additionally, the water permeability significantly improved, achieving a water flux of 25.9 kg m-2 h-1 (compared to 11.55 kg m-2 h-1 for the PVDF support membrane). These enhancements occur without compromising NaCl rejection, which remains consistently above 99.99 % (3.5 wt% NaCl aqueous solution at a feed temperature of 70 degrees C and a permeate temperature of 18 degrees C). These findings underscore the potential of rGO/PVDF-HFP in enhancing MD performance. The composite HF membrane demonstrates remarkable stability during continuous MD operation over a 48 h period. Pore wetting is effectively avoided, indicating that the composite membrane has superior resistance to wetting and excellent stability.