Construction of rough and porous surface of hydrophobic PTFE powder-embedded PVDF hollow fiber composite membrane for accelerated water mass transfer of membrane distillation

The construction of rough and porous hydrophobic membrane surface is expected to overcome the obstacles including low permeate water flux and membrane pore wetting which greatly restrict the development of membrane distillation (MD) technology. In this study, a rough and porous polytetrafluoroethylene (PTFE) powder-embedded polyvinylidene fluoride (PVDF) hydrophobic coating layer was compounded on the outer surface of PVDF hollow fiber support membrane by the dilute solution coating-phase inversion method. PVDF hollow fiber support membrane was fabricated by the dry-jet wet-spinning technique. PTFE powder was directly incorporated in PVDF dilute coating solution and embedded in the porous PVDF coating layer after the nonsolvent induced phase separation (NIPS). The variations of membrane morphology, surface chemical compositions, hydrophobicity and wetting resistance were investigated by scanning electron microscope (SEM), Attenuated total reflection-Fourier transform infrared spectroscopy (ATR-FTIR), dynamic water contact angle (WCA) and liquid entry pressure (LEP) analysis. Membrane separation performance including desalination and ionic dyes removal properties was evaluated by VMD experiment. Compared with PVDF hollow fiber support membrane, both the surface hydrophobicity and the water permeability of the PTFE powder-embedded PVDF hollow fiber composite membrane (HFC) had obvious improvement. The surface WCA and permeate water flux increased from 92.6° and 11.3 kg/m2.h for PVDF support membrane to 133.6° and 26.8 kg/m2.h for the PTFE powder-embedded HFC membrane meanwhile NaCl rejection can be maintained above 99.9% (3.5 wt% NaCl aqueous solution at 50 °C and permeate pressure at 31.3 kPa). The separation performance of HFC membrane can remain stable without obvious pore wetting during the continuous MD operation for 36 h. Different from the desalination, porous hollow fiber membrane would adsorb two different charged dyes, Congo red (CR) and methylene blue (MB) to a certain extent, resulting in the decrease of membrane flux and the change of permeate water quality. Finally, the MD separation mechanism of inorganic salt, anionic dye and cationic dye by PTFE powder-embedded HFC membrane was proposed.