Green fabrication of PVDF superhydrophobic membranes using a green solvent triethyl phosphate (TEP) for membrane distillation

Abstract

To produce clean water from seawater desalination, superhydrophobic membranes are essential for membrane distillation to minimize wetting. However, superhydrophobic membranes are often prepared using nanomaterials and highly toxic solvents, posing significant concerns to environments. This study has explored the use of triethyl phosphate (TEP) as a green solvent to fabricate PVDF membranes by the recently invented spray-assisted nonsolvent induced phase separation (SANIPS) method. Surprisingly, membranes fabricated from PVDF/TEP and PVDF/n-methyl-2-pyrrolidone (NMP) dope systems under SANIPS exhibited quite different morphology and hydrophobicity. Nevertheless, a superhydrophobic PVDF membrane with a water contact angle (WCA) of 154° and self-cleaning properties was successfully produced by spraying water on the nascent membrane for 45 s before completing phase inversion. The mechanism of SANIPS in the PVDF/TEP system was thoroughly investigated. The resultant superhydrophobic membrane was evaluated by direct contact membrane distillation (DCMD) to treat a high salinity solution of 10 wt% NaCl and showed a much higher flux than the nonsolvent induced phase separation (NIPS) membrane (i.e., 22 vs. 12 kg/m2 h). In addition, it exhibited stable long-term performance. These results demonstrate the potential of using TEP in SANIPS to produce superhydrophobic membranes for membrane distillation of seawater and wastewater with improved performance and environmental sustainability.