Selective removal of heavy metals from saline water by nanofiltration

Polyethylenimine (PEI) cross-linked P84 nanofiltration membranes were successfully developed through a facile stirring cross-linking method. The separation performance of the membrane was tailored by changing the concentration of the P84 casting solution. A systematic characterization, including chemical structure, surface morphology, surface charge, and separation performance, was carried out. Nanofiltration membranes with a high positive charge and hydrophilic surface were successfully fabricated. All the PEI cross-linked membranes showed high isoelectric points at pH from 8 to 8.9 and a mean pore radius from 0.389 to 0.474 nm. Under the influence of the Donnan effect and size exclusion, the 20%P84/PEI membrane showed an excellent water permeance of 27.7 ± 1.5 L m⁻² h⁻¹ bar⁻¹ and a high rejection (>90%) for single-component heavy metal salts (ZnCl₂, PbCl₂, Ni(NO₃)₂ and Cu(NO₃)₂). A variety of mixed salt separation performance tests were performed. These results clearly indicate that the 20%P84/PEI and 22%P84/PEI membranes have a huge potential to remove and concentrate copper ions from aqueous solutions. In particular, the 24%P84/PEI membrane is promising in the selective removal of various heavy metals from highly saline water.