Single-step synthesis of a polyelectrolyte complex hollow-fiber membrane for forward osmosis

We present the simultaneous synthesis of a hollow fiber membrane with a selective layer created by means of polyelectrolyte complexation (PEC), to be used as a membrane in forward osmosis. The aim of this single-step approach was to create a defect-free robust selective layer and circumvent the challenges associated with coating via interfacial polymerization. The nascent hollow fiber membrane with a PEC layer was characterized by SEM imaging as well by determining the streaming potential and pure water permeance. We also evaluated several electrolytes as potential draw solutes in combination with the developed membrane and selected trisodium citrate (TSC) as it showed a very high rejection of 97 ± 2%. Using 1 M TSC as draw solution showed promising osmotic performance in selective layer facing feed solution (FO) mode, having a water flux of 7.8 ± 0.2 (L·m⁻²⋅h⁻¹) and a reverse salt flux of 2.1 ± 0.7 (g·m⁻²⋅h⁻¹). A significantly higher reverse salt flux of was gained in PRO mode which was attributed to the high ionic strength of the charged draw solute near the PEC layer. It is highlighted that the choice of draw solute as well as process orientation (FO or PRO mode) are crucial for charged selective layers such as our PEC selective layer. We conclude that our approach shows substantial promise for use in FO processes using TSC as the draw solution. In addition, the taken approach successfully eliminates the time-consuming and challenging extra step of coating hollow fibers through interfacial polymerization, opening up opportunities for the cost-effective synthesis of FO hollow-fiber membranes.