Advanced FO membranes from newly synthesized CAP polymer for wastewater reclamation through an integrated FO-MD hybrid system

A new cellulose acetate propionate (CAP) polymer has been synthesized and used to prepare high-performance forward osmosis (FO) membranes. With an almost equal degree of substitution of acetyl and propionyl groups, the CAP-based dense membranes show more balanced physicochemical properties than conventional cellulose acetate (CA)-based membranes for FO applications. The former have a lower equilibrium water content (6.6 wt. %), a lower salt diffusivity (1.6×10¹⁴ m² s^-1) and a much lower salt partition coefficient (0.013) compared with the latter. The as-prepared and annealed CAP-based hollow fibers have a rough surface with an average pore radius of 0.31 nm and a molecular weight cut off of 226 Da. At a transmembrane pressure of 1 bar, the dual-layer CAP-CA hollow fibers show a pure water permeability of 0.80 L m^-2 h^-1 bar^-1 (LMH/bar) and a rejection of 75.5% to NaCl. The CAP-CA hollow fibers were first tested for their FO performance using 2.0 M NaCl draw solution and deionized water feed. An impressive water flux of 17.5 L m^-2 h^-1 (LMH) and a reverse salt flux of 2.5 g m^-2 h^-1 (gMH) were achieved with the draw solution running against the active CAP layer in the FO tests. The very low reverse salt flux is mainly resulting from the low salt diffusivity and salt partition coefficient of the CAP material. In a hybrid system combining FO and membrane distillation for wastewater reclamation, the newly developed hollow fibers show very encouraging results, that is, water production rate being 13-13.7 LMH, with a MgCl₂ draw solution of only 0.5 M and an operating temperature of 343 K due to the incorporation of bulky propionyl groups with balanced physiochemical properties.