(340e) Investigation of Cellulose Acetate-Based Forward Osmosis Hollow Fiber Membranes with Dense Inner Layer

Ideal membranes for forward osmosis (FO) processes should be semi-permeable and hydrophilic, allowing water to permeate through while rejecting other components. Normally, FO membranes have an asymmetric structure, i.e., a dense skin layer acting as the selective barrier and a less dense or porous sublayer serving as the mechanical support. The advantage of this morphology is that the water flux is improved with decreasing the resistance at the sublayer without significantly sacrificing the solute rejection. It has been found in our study that no matter the draw solution flows against the selective layer, i.e., the pressure-retarded osmosis (PRO) mode, or flows against the porous sublayer, i.e., the forward osmosis mode, concentration polarization occurs at both sides of the membrane. The occurrence of concentration polarization greatly reduces the effective osmotic pressure difference (driving force) across the membrane, resulting in a much lower water flux than that expected.

Nanofiltration-based FO single-layer hollow fiber membrane has been successfully developed from cellulose acetate in our study. The resulted CA membrane has a very thin outer skin and a highly porous sublayer. The mean radius of the membrane surface pores is 0.30 nm and the pore size distribution is very narrow. Using 2M MgCl2 draw solution, the water flux obtained in the FO test is about 31 L m-2h-1 with a reverse salt passage of 0.37 g m-2h-1. Based on these promising results, novel asymmetric FO hollow fiber membrane with dense inner layer and porous outer layer is to be developed. Several important spinning conditions such as bore fluid, coagulation temperature and air-gap distances are to be investigated for the formation of dense inner layer. The morphology of the novel FO hollow fiber membrane will be thoroughly characterized, while the performance will be examined in the FO process in order to evaluate how the variation of the membrane morphology affects concentration polarization. The research work related to this project is being carried out and the results will be reported in due time.