Positively charged capillary nanofiltration membrane with high rejection for Mg2 + and Ca2 + and good separation for Mg2 + and Li +

High water permeability and good separation property are greatly desired in water production due to energy concerns. To explore nanofiltration (NF) membrane with high permeability for cations separation, a positively charged NF membrane was fabricated via interfacial polymerization using polyethersulfone (PES) three-channel capillary ultrafiltration (UF) membrane as substrate, polyethyleneimine (PEI) as the aqueous precursor. The NF membrane preparation conditions were optimized. To enhance the permeability of the prepared NF membrane, modified hydroxyl contained multi-walled carbon nanotubes (MWCNTs-OH), grafting with piperazine (PIP), were utilized. The resultant NF membrane showed increased water flux from 20.8 L·m− 2·h− 1 to 56.1 L·m− 2·h− 1 at 4 bar after adding 0.01 wt% modified MWCNTs-OH in aqueous solution. Interestingly, MgCl2 rejection of the membrane also increased from 94.2% to 96.9%. The positively charged NF membrane exhibited above 97% rejection for divalent cations (Mg2 + and Ca2 +) and low rejection (< 70%) for monovalent cations (Na+ and Li+), and it also showed long durability and good separation for Mg2 + and Li+ when the membrane was used to separate mixed salts solution simulated the composition of salt lake brine. The fabricated membrane would have potential for effective water softening and for reclamation of lithium from brine or seawater with high Mg2 +/Li+ ratio.