Zwitterionic carbon nanotube assisted thin-film nanocomposite membranes with excellent efficiency for separation of mono/divalent ions from brackish water

Zwitterionization of multiwalled carbon nanotubes (ZCNTs) is initially conducted by grafting poly(4-styrenesulfonic acid) and poly(4-vinylpyridine) copolymers via an atom transfer radical polymerization (ATRP) reaction, and the products obtained are further used as an aqueous additive to fabricate thin-film nanocomposite (TFN) nanofiltration (NF) membranes. The most optimal membrane with a ZCNT dosage of 0.01%, Z-TFN 0.01% shows nearly 31% enhancement in water permeability (14.9 ± 0.5 L m−2 h−1 bar−1), probably due to the shortened ZCNT, which could create a larger number of inorganic–organic interfaces in the barrier layers. Besides, it also exhibits MgSO4/NaCl selectivity (5.6 ± 0.8), 2.8 times higher than that of the pristine one (2.0 ± 0.2), which is attributed to the formation of ion-response channels and the stretching effect of the side chains. By using model brackish water as the feed solution and subjecting it to a two-stage NF process, the outstanding separation ability of the Z-TFN 0.01% membrane for the ions with the final Na+/Mg2+ concentration ratio in the permeate reaching as high as 589.2, as against only 111.7 for the control, is displayed. Furthermore, due to the strong bacteriostasis ability of quaternary ammonium compounds, Z-TFN 0.01% also shows excellent antibacterial properties, as evidenced by the mortality of Escherichia coli to 97% after remaining in contact with the membrane surface.