Multi-walled carbon nanotube/PVDF blended membranes with sponge- and finger-like pores for direct contact membrane distillation

Abstract

Polyvinylidene fluoride (PVDF) flat sheet membranes were blended with multi-walled carbon nanotubes (MWCNTs) by the phase inversion method for direct contact membrane distillation (DCMD) of salty water (35gL−1 NaCl). The membrane properties and performances depended markedly on the synthesis parameters such as MWCNT loading, polyvinylpyrrolidone (PVP) addition and MWCNT surface chemistry. MWCNT/PVDF membranes prepared with functionalized MWCNTs have a smaller pore size (determined by bubble point measurements) and lower contact angles, thus, functionalization of MWCNTs is not recommended for this application. For membranes presenting the same pore size, the pore morphology and the membrane thickness are crucial for efficient salt rejection. Sponge-like pores and the smallest possible thickness (i.e., membranes prepared without PVP) allow complete salt rejection (i.e., 100%), in contrast with larger thickness and elongated finger-like pores (resulting from PVP addition) whose salt exclusion ranged from 88.8 to 98.6%. Overall, the MWCNT/PVDF blended membrane prepared with 0.2wt.% optimal content of pristine MWCNTs (without adding PVP) exhibited the best performance in DCMD, presenting total salt rejection and a higher permeate flux (9.5×10−3kgm−2s−1) than that obtained with a commercial PVDF membrane (7.8×10−3kgm−2s−1).