Triethanolamine modification produces ultra-permeable nanofiltration membrane with enhanced removal efficiency of heavy metal ions

Nanofiltration membranes are attractive for the removal of the heavy metal ions but limited by the trade-off between permeance and solute rejection. Membrane permeance is closely correlated to hydrophilicity and solute rejection is closely linked to size exclusion and Donnan electrostatic repulsion. Here, triethanolamine was employed as a crosslinking agent to prepare modified polyethyleneimine/trimesoyl chloride nanofiltration (PEI/TMC NF) membranes with excellent separation performance for the removal of heavy metal ions from contaminated water. The permeance was more than 2 times increased from 6.4 Lm⁻²h⁻¹bar⁻¹ to 13.6 Lm⁻²h⁻¹bar⁻¹ due to the increase in membrane hydrophilicity resulting from the incorporation of hydrophilic hydroxyl groups on the membrane surface; the rejection of metals was high (around 97% rejection for Zn²⁺, Cd²⁺, Ni²⁺, Cu²⁺; around 92% rejection for Pb²⁺) due to the enhancement of positive charges on the membrane surface resulting from lone pair electrons of N elements on triethanolamine and the decrease in pore size of the membrane (MWCO from 280 Da to 220 Da). The short-term filtration test (72 h) of the simulated wastewater with various metal ions shows that the prepared membranes have excellent stability for industrial application. Unlike the traditional superior permeance achieved by ultrathin membranes with weakness and defects, the triethanolamine modification endows the pristine PEI-TMC NF membranes with superior performance without the complicated methods for thickness reduction.