Evaluation of a novel polyamide-polyethylenimine nanofiltration membrane for wastewater treatment: removal of Cu²⁺ ions

This paper describes a novel approach for enhancing membrane technology for the removal of heavy metal cations in contaminated waters. A simple method of forming a positively charged polyamide (PA) nanofiltration (NF) membrane has been developed by attaching a layer of hyperbranched polyethylenimine (PEI) to the PA surface, involving the linking of PEI amino groups to the PA surface carboxyl groups. The nature of the PEI modified PA membrane, in terms of surface morphology, zeta potential and hydrophobicity was found to depend on the PEI molecular weight (MW), and the PEI concentration and membrane exposure time during preparation. In turn, the nature of the modified membrane determined its performance in terms of hydraulic flux and metal ion rejection. In tests using a model solution of 5 mg/L Cu²⁺ and a 70,000 MW PEI membrane the Cu rejection was >90%, with only a modest reduction in flux compared to blank water. The Cu²⁺ rejection was found to be a combination of electrostatic repulsion and adsorption, with the relative proportions depending on the nature of the PEI modified PA membrane. In addition, the Cu²⁺ rejection and membrane flux were found to be sustainable over repeated filtration cycles, and the rejection was not adversely affected by the presence of humic acid in solution (5 mg/L).