(146a) Development of Composite Hollow Fiber Membranes for Aqueous and Non-Aqueous Nanofiltration

Nanofiltration (NF) has been growing rapidly in pharmaceutical separation, wastewater treatment, seawater desalination and many other applications. This presentation focuses on the new developments of composite NF hollow fiber membranes from both co-extrusion and interfacial polymerization (IP). Firstly, a highly water permeable dual-layer NF hollow fiber membrane fabricated by the simultaneous co-extrusion of Torlon^® polyamide-imide (PAI) and cellulose acetate (CA) polymer solutions through a triple-orifice spinneret in a dry-jet wet phase inversion process. As compared to conventional membranes, the dual-layer hollow fiber membranes developed here have a unique structure which consists of (1) controllable thin selective outer layer; (2) innovatively designed interface, which is finely connected between outer and inner layers, does not contribute significant transport resistance while still maintains good mechanical strength; (3) fully porous substructure with reduced transport resistance. The cross-section struction and the nanopores of dual-layer hollow fiber membranes were molecularly designed by controlling the phase inversion process with the aid of various non-solvent additives into the polymer solutions. The newly developed NF dual-layer hollow fiber membrane with methanol as additive has a relatively high pure water permeability of 11.93 l m^-2bar^-1h^-1 with a mean effective pore radius of 0.63 nm and a more than 96.5% rejection of Na₂SO₄.

Secondly, a novel thin-film composite NF membrane was fabricated by IP of hyperbranched polyethyleneimine (HPEI) and isophthaloyl chloride (IPC) on a dual-layer hollow fiber membrane. The substrate possesses a unique cross-section structure comprising a layer full of macrovoids located in the middle and sandwiched by two thin spongy-like layers. Such a sandwich-like sublayer structure provides minimal transport resistance and sufficient mechanical strength for water permeation under high pressures. After IP, the NF membrane possesses a negatively charged substrate and a positively charged selective layer with a mean effective pore radius of 0.36 nm, molecular weight cut off of 500 Da, and pure water permeability of 4.9 lm^-2bar^-1h^-1. Due to this double-repulsion effect, together with the steric-hindrance and the solute electro-neutrality effects, by adjusting the pH of cephalexin aqueous solution to modify the ionization states of this zwitterionic molecule, the NF membrane shows high rejections over a wide pH range. Furthermore, the latest preliminary data shows the membrane is very stable in harsh organic solvents such as dimethyl sulfoxide. Therefore, the NF membrane may potentially be useful to reduce waste, recycle valuable pharmaceuticals and reuse solvents for the pharmaceutical industry.