Optimising the synthesis of polyethyleneimine-based nanofiltration membranes for enhanced organic removal and divalent-to-monovalent ion separation from seawater

Nanofiltration (NF) membranes are increasingly being explored as seawater pretreatment technologies; however, most commercial membranes exhibit limited divalent-to-monovalent ion selectivity and ineffective removal of dissolved organic carbon (DOC) at high ionic strength. In this study, a positively charged polyethyleneimine–trimesoyl chloride (PEI–TMC) thin-film composite (TFC) NF membrane was systematically optimised via interfacial polymerisation (IP). Key synthesis parameters (PEI concentration: 0.75–1.75 g L −1 ; TMC concentration: 0.05–0.15 wt%; IP time: 2–4 min; curing time: 0–7.5 min; and curing temperature: 60–80 °C) were evaluated in detail to identify optimised membrane conditions. The optimised PEI–TMC membrane achieved 60–62% Mg 2+ and Ca 2+ rejection with only 7–8% Na + and K + rejection from seawater at 4 bar, yielding a Mg 2+ /Na + separation factor of 2.4, while maintaining a permeate flux of 5.5 L m −2 h −1 . Simultaneously, 72% DOC removal (including 97% rejection of humic substances) was achieved via electrostatic repulsion, in contrast to the commercial TS80 membrane, which promoted the fractionation of the organic fraction into low-molecular-weight fractions. Membrane characterisation revealed that the enhanced performance of PEI–TMC membrane results from a hydrophilic, smooth hydration layer and a positively charged surface, promoting synergistic Donnan exclusion and size-sieving, while suppressing divalent ion–organic bridging and surface adsorption. Continuous seawater filtration experiments over 72 h demonstrated stable flux and sustained ion selectivity, reinforcing the fouling resistance of the PEI–TMC membrane. These findings provide insights into the rejection and separation behaviour of positively charged NF membranes in saline environments and support their potential application for pretreatment in seawater desalination.