Polyamide nanofiltration membrane with high mono/divalent salt selectivity via pre-diffusion interfacial polymerization

Fabricating membranes with homogenous pore size at sub-nanometer scale is highly demanded for precise separation but remains a challenge. Herein, we propose a new strategy for preparing polyamide (PA) nanofiltration membranes with high solute-solute selectivity via a pre-diffusion interfacial polymerization (PDIP) process. The enrichment of amine monomers on the organic phase side of the oil-water interface facilitates a stoichiometric interfacial polymerization reaction with acyl chloride and endows the PA active layer more uniform pores with sub-Angstrom separation precision. The nanofiltration membrane prepared by PDIP strategy has lower surface roughness and higher degree of polymerization reaction inside the PA active layer compared with the membrane prepared by the traditional method. The pre-diffusion process enables PA nanofiltration membranes to have a 99.7% rejection rate of Na2SO4 and 98.7% of MgCl2 with no loss of flux while the rejection of that is 96.7% and 62.4% for traditional IP. Most importantly, the membrane exhibits fairly high mono/divalent salt selectivity of both cations and anions (e.g. 246 for NaCl/Na2SO4 and 51 for NaCl/MgCl2), which is one of the highest mono/divalent salt selectivity among all the reported polymer membranes. The PDIP process with improved ion selectivity is expected to change the traditional manufacturing mode of PA thin film composite (TFC) membranes, including nanofiltration and reverse osmosis membranes those are the most important members in the family of filtration membranes for water purification.