Layer-by-layer (LBL) hollow fiber nanofiltration membranes for seawater treatment: Ion rejection

Layer by layer (LBL) hollow fiber nanofiltration (NF) membranes assembled by poly(styrene sulfonate) (PSS)/ poly(allylamine) (PAH) with 2.5 bilayers, both uncrosslinked (PSS/PAH)(2.5) and crosslinked (PSS/PAH)(2.5)-X, were researched as a pretreatment for seawater. Focusing on the stability and ion rejections, PSS/PAH NF membrane demonstrated long time stable ion rejection after exposure in artificial seawater (ASW, > 300 h) and a unique ion rejection behavior. Overcompensation of PAH contributed to the excess cationic amine groups in LBL layers. This explains the high rejection of bivalent cations, Mg2+, Ca2+ and Sr2+, for both membranes size exclusion and Donnan effect for single salt feed. The extrinsic compensation of SO42- to cationic amine groups for (PSS/PAH)(2.5) resulted in unexpected lower rejection of SO42- than Cl-. In ASW, increased rejection to SO(4)(2-& nbsp;)and reduced rejection to Cl-, bivalent and monovalent cations caused by the extrinsic compensation of Cl- to the amine groups highlighted the dominance of the affinity or sorption effect over the size and Donnan effect. A coherent ion rejection mechanism incorporating anion extrinsic compensation, combined with the size and Donnan exclusion, constitutes the third dimension for ion transport across LBL membranes.

Automated summary

Layer by layer hollow fiber nanofiltration membranes assembled by poly(styrene sulfonate)/poly(allylamine) demonstrated stable ion rejection and unique ion rejection behavior. Excessive compensation of cationic amine groups contributed to high rejection rates, highlighting a third dimension in ion transport.