Bicontinuous porous substrates by polysulfone/2-pyrrolidone binary system for high-performance forward osmosis membranes

Forward osmosis (FO) has shown great promise in concentrating enriched lithium brine during lithium extraction from salt-lake brine due to low energy consumption and ecologically friendliness. The effective utilization of enormous osmotic pressure of freely available MgCl 2 onsite requires highly selective FO membranes with high flux, which calls for thin-film composite forward osmosis (TFC-FO) membranes of highly interconnected substrate and defect-free separation layer. Herein, for the first time, we employed 2-Pyrrolidone (2-PD) as the solvent to prepare FO substrates via the non-solvent induced phase separation (NIPS) for preparing TFC FO membranes. Owing to its weak solubility, the PSf/2-PD polymer solution shows high viscoelasticity, resulting in spinodal demixing and a bicontinuous porous structure, unlike the closed finger-like pore structure observed in the PSf/DMAc binary system. By further reducing the casting humidity and coagulation bath temperature, the substrate surface pore size was optimized to prepare high-performance TFC-FO membranes: the TFC-30%-3 exhibits a water flux of 20.0 LMH and low specific reverse salt flux of 0.23 g/L at AL-FS mode, showing competitive performance compared to the state-of-the-art membranes. These results imply that 2-PD, as an environmentally friendly alternative to traditional solvents, shows significant promise for FO substrate preparation using a simple polymer/solvent binary system. • Green solvent (2-pyrrolidinone, 2-PD) was for the first time adopted to prepare forward osmosis membrane substrate. • Bicontinuous pores in PSf/2-PD membranes attribute low structure parameters and high FO flux. • Substrates surface morphology was optimized by lowering ambient humidity and coagulation bath temperature.