The role of sulphonated polymer and macrovoid-free structure in the support layer for thin-film composite (TFC) forward osmosis (FO) membranes

Abstract

A new approach to fabricate thin film composite (TFC) membranes via interfacial polymerization for forward osmosis (FO) applications has revealed that it is possible to design TFC-FO membranes with fully sponge-like structure and likely anti-fouling characteristics while maintaining a high water flux. Not only does the sulphonated material in the substrate of TFC-FO membranes play the key role to create macrovoid-free structure but also induces hydrophilic properties with enhanced water fluxes. It is found that the TFC-FO membranes containing a 50 wt% sulphonated material in the membrane substrate exhibit a fully sponge-like structure, while those with lower or without sulphonated content show finger-like structures. In terms of FO performance, the TFC-FO membranes with 50 wt% sulphonated material content can achieve the highest water flux of 33.0 LMH against DI water and 15 LMH against the 3.5 wt% NaCl model solution using 2 M NaCl as the draw solution tested under the pressure retarded osmosis (PRO) mode. The value of 15 LMH for seawater desalination is the highest reported so far. Despite the debates on whether TFC-FO membranes should possess a finger-like or sponge-like structure, it is proven that the degree of hydrophilicity of membrane substrates is a much stronger factor enhancing the water flux in FO tests. Meanwhile, a fully sponge-like structure with expected anti-fouling property is preferred for long-term membrane stability. Furthermore, the structural parameter indicating the internal concentration polarization (ICP) can be remarkably decreased with an increase in sulphonated material content in membrane substrates.