Preparation, evaluation and modification of PVDF-CTFE hydrophobic membrane for MD desalination application

Abstract

Desalination by membrane distillation (MD) have found rapid growth and diversified applications in recent decade, associated with it was the demand for hydrophobic membranes. Poly (vinylidene fluoride-co-chlorotrifluoroethylene) (PVDF-CTFE) was used for hydrophobic membrane preparation with LiCl additive by non-solvent induced phase inversion (NIPS), the effect of polymer concentration was investigated in terms of membrane morphology, pore structure, hydrophobicity, thermal property, and membrane distillation (MD) performance to evaluation its potential in MD desalination. It was found that the solid-liquid demixing process showed more influence with increasing polymer content due to the delayed phase inversion process, which was responsible for the suppressed macrovoids, reduced pore size, higher hydrophobicity and pore interconnectivity. The melting temperature and crystallinity also increased due to the crystallization during membrane formation. M3 with 12% polymer content presents the best MD performance (permeate flux of 20.65kg/m2·h with a salt rejection higher than 99.95%). Ethanol and sodium hydroxide (NaOH) solution were applied as the second immersion bath to post-treat the nascent membranes. Pore enlargement effect was found for both post-treated membranes. However, the dehalogenation process was the most important mechanism for NaOH, while the obstruction of partial shrinkage was the mechanism of ethanol solution. In a word, PVDF-CTFE hydrophobic membrane show potential in MD desalination, especially when a second solution bath is used for membrane modification as the flux enhancement without the sacrifice of salt rejection.