Enhanced desalination performance of poly (vinyl alcohol)/carbon nanotube composite pervaporation membranes via interfacial engineering

The dispersion and interfacial interactions between nanofillers and the polymer are two crucial factors affecting the performance of mixed matrix membranes (MMMs). In this study, poly (vinyl alcohol) (PVA) based mixed matrix membranes containing multiwalled carbon nanotubes (MWCNTs) or carboxylic multiwalled carbon nanotubes (C-MWCNTs) were produced via interfacial adhesion, hydrogen bonding or covalent bonding. The desalination performances of the synthesized membranes were compared by the pervaporation (PV) process. A PVA membrane crosslinked with maleic acid (MA) was also prepared for comparison. FESEM and TEM images indicated that the C-MWCNTs were disentangled and dispersed uniformly in the PVA matrix whereas MWCNTs without functional groups readily aggregated. The incorporation of CNTs endowed the PVA/CNT membranes with improved thermal stability, which was confirmed using TGA and DSC measurements. In particular, the altered properties of the PVA/CNT composites enhanced the separation performance compared with the PVA membrane without CNTs. The performance tests showed that the PVA/CNT composite membranes featured remarkably larger water fluxes than control PVA/MA membrane (38.8%–154.1% increase) while maintaining high salt rejection. Thereinto, the overall best performance of 99.91% of salt rejection and 6.96 kg/m2 h of water flux at room temperature (22 °C) was obtained by the PVA/C-MWCNT/MA membrane (19 ± 1 μm thickness) when the feed was synthetic NaCl solution of 35,000 ppm. Kinetic desorption method was applied to compare salt transport properties of the resultant PVA/CNT composite membranes. The salt transport through the PVA/C-MWCNT and PVA/C-MWCNT/MA membranes was significantly restricted with CNT mass fraction. High retention of water flux and salt rejection rates were also obtained by the PVA/C-MWCNT/MA membrane over a 30-h durability test that processed seawater collected from Brighton Beach (Melbourne, VIC, Australia), while the salt rejection for the PVA/C-MWCNT membrane declined with extended operating time indicating that cross-linking of the PVA was necessary for stable MMM performance.