Positively charged loose nanofiltration membranes prepared by a green ionic cross-link method

Positively charged loose nanofiltration membranes were prepared via a green ionic cross-link method, without using of harmful solvents and extra cross-linkers. Sulfonated poly-ether-ether-ketone (SPEEK) was incorporated into PES membrane to introduce negatively charged sites. Branched PEI, which contains primary amino groups and secondary amino groups, was used as polycation to deposit on the SPEEK functionalized PES support. A dense separation layer of polyelectrolyte complex was formed through the ionic cross-linking between the amine groups in PEI and the sulfuric acid groups in SPEEK, evidenced by the ATR-IR, XPS, SEM and AFM results. The membrane with water permeance around 20 L/(m²·h·bar) and molecular weight cut-off around 600 Da (characterized by PEGs) was obtained at optimized conditions. The membrane can reject dyes with molecular weight down to 300 Da, with lower rejections of monovalent and divalent salts. In addition, the influences of post-treatment and the PEI deposition parameters on the membrane performance were investigated. The heat treatment improved the membrane performance in terms of rejection by stabilizing the polyelectrolyte complexes. The glycerol treatment protected the membrane from pore collapses during thermal annealing, resulting in membranes with acceptable permeances.