Functionalized open-ended vertically aligned carbon nanotube composite membranes with high salt rejection and enhanced slip flow for desalination

[Display omitted] •Water flux in COOH functionalized VACNTs membrane reveals 103 enhancement factor.•The functional groups with different charges and lengths enhance the rejection.•An increase in the feed concentration improves the salt rejection.•Glycine at VACNTs/PS membrane exhibits a minimum rejection at the isoelectric point. Vertically aligned carbon nanotubes (VACNTs) have shown unexpectedly high liquid flux. In this research, we investigated the effect of functionalization on the performance of VACNTs/polystyrene membranes, in which densely packed VACNTs with 2–11 nm inner diameters and areal density around 3.6 × 1010 cm−2 act as the only mass transport paths. The measured water permeance in the open-ended VACNTs membrane revealed an enhancement factor of three orders of magnitude higher than the continuum-based no-slip model. Different functional groups including carboxylic, glycine, octadecylamine, and 1,6-diaminohexane were attached to the VACNTs tips through carbodiimide coupling method, which were characterized by ATR-FTIR. Salt exclusion as a function of ion valence ratios, ionic strength, and pH value of the feed solution was studied. The results were strongly supported by Donnan exclusion mechanism. Interestingly, an increase in the concentration of salts from 500 to 5000 ppm improved the rejection, which is in contrast with other reports. The glycine functionalization was repeated two to three times to find the effect of the functional groups length as well as charge density on the rejection. pH-dependent measurements of glycine functionalized membrane demonstrated that both carboxyl and amide groups are responsible for the ion rejection characteristics of the membrane.