Enhanced performance of cellulose triacetate membranes using binary mixed additives for forward osmosis desalination

In this study, on a basis of the single additive optimization experiments of the cellulose triacetate (CTA) forward osmosis (FO) membranes, the CTA FO membranes with binary mixed pore-forming additives were designed and fabricated to retrofit the membrane structure and separation performance for FO desalination. The binary mixed additives such as acetic acid-lactic acid (AA-LA), acetic acid-maleic acid (AA-MA) as well as zinc chloride-lactic acid (ZnCl2-LA) were selected via combining a single additive with the capacity to improve the salt rejection with another single additive with the capacity to improve the water flux. The effectiveness of the binary mixed additives on the FO performance was extensively evaluated and confirmed. For example, with the ZnCl2-LA as binary additive, the resulting CTA membrane exhibited a much improved water flux of ~ 11.5 L/m2 h and an excellent salt rejection of ~ 98.3% under the fixed FO operating condition (0.1 M NaCl solution as feed, 2 M glucose solution as draw solution, FO mode, 2 h), which were increased by approximately 51.3% and 8.1% compared to the commercial CTA membrane from HTI, respectively. Compared to the single additives, the binary mixed additives could significantly enhance the water flux for the CTA membranes with an insignificant reduction in salt rejection, which might depend on the interaction between two additives as well as the interaction between additives and solvent during the phase inversion. It indicated that the trade-off effect between the selectivity and permeability of a polymer membrane was partially broken for our membranes with binary mixed additives. This study helps us to pave the way to design highly efficient additives for high-performance FO membranes.