Molecular layer-by-layer assembled forward osmosis membranes

Abstract

A recently devised, molecular layer-by-layer (mLbL) approach based on the alternative cross-linking of monomers was employed to fabricate high-performance thin film composite (TFC) forward osmosis (FO) membranes with excellent rejection toward monovalent NaCl salt. An ultrathin and highly dense polyamide (PA) selective layer with a precisely controlled structure was created on a tailored porous support via mLbL. The intrinsic separation properties of the mLbL-assembled TFC membranes were tuned by adjusting the mLbL cycle number to optimize the FO performance. The best FO performance was achieved at 10 mLbL cycles (mLbL-10), where the permeability and selectivity were properly balanced. Importantly, the mLbL-10 membrane exhibited superior FO performance compared to the commercial HTI FO membranes as well as hand-cast TFC membranes prepared by the conventional interfacial polymerization: the mLbL-10 membrane showed ~3.5 times higher water flux, ~60% lower reverse salt flux and ~85% lower specific salt flux compared to the cellulose triacetate HTI membrane, with 0.5M NaCl draw solution and DI water feed solution in FO mode. The stability and the associated membrane performance of the mLbL-assembled membrane depending on the ionic strength of the environment were explained by the swelling behavior of the polyelectrolyte-assembled interlayer adhered underneath the PA selective layer.