Thin-Film Composite Membranes and Formation Mechanism of Thin-Film Layers on Hydrophilic Cellulose Acetate Propionate Substrates for Forward Osmosis Processes

For the first time, the potential of using hydrophilic cellulose acetate propionate (CAP) as microporous substrates for the fabrication of thin-film composite (TFC) forward osmosis (FO) membranes has been explored. Two types of TFC flat sheet membranes with well-designed substrate structures were prepared. The CAP-TFC membranes show very low Js/Jw ratios (i.e., the ratio of reverse draw solute flux to water flux) of about 0.05 g/L with reasonably high water fluxes under the pressure-retarded osmosis (PRO) mode using 2 M NaCl as the draw solution and deionized water as the feed. When using model seawater as the feed, the water flux is about 12.3 LMH which is comparable to the best in the literature. These results, combined with positron annihilation spectroscopy (PAS) data, confirm the hypothesis that a finger-like morphology in the substrate is not crucial to the performance of TFC FO membranes. Moreover, the surface and skin morphology of the substrate may play essential roles in the formation of the polyamide layer as well as its perfectness and FO performance. A dynamic scheme to elucidate the evolution of forming the globular and worm-like structure during the interfacial polymerization has been proposed. Substrates with larger pores and broader distribution may facilitate rapid migration of amine molecules, induce more complicated convection at the interface, and enlarge reaction contact area, resulting in a rougher but more compact polyamide layer in the TFC membranes, while smaller surface pores of substrate favor simple convection at the interface during the interfacial reaction, producing nascent cross-linked films with smaller domain sizes, smoother and less dense structure, and less defects.