Morphological controlling of CTA forward osmosis membrane using different solvent-nonsolvent compositions in first coagulation bath

Abstract

Cellulose triacetate (CTA) membranes were fabricated via a modified nonsolvent induced phase separation (NIPS) method. Different solvent-nonsolvent compositions in first coagulation bath (FCB) were introduced to optimize CTA membrane structures. The effects of FCB compositions, immersion time and mass ratio of solvent (N-methyl-2-pyrrolidone, NMP) and nonsolvent (water, ethanol, ethylene glycol and glycerol) on membrane morphology and performance were systematically investigated. Prospective membranes with a dense bottom layer and a scaffold-like top layer were obtained under room temperature, owing to the low relative energy difference (RED) between nonsolvent and polymer as well as the high viscosity of FCBs. A high water flux Jv (12.6 L m−2 h−1) and a low reserve salt flux Js (1.32 g m−2 h−1) were obtained in the optimized membrane, with a structural parameter S of 119 μm. Compared with membranes prepared via conventional NIPS method and commercial CTA forward osmosis (FO) membranes, a remarkable improvement of Js/Jv value and S value was achieved, indicating membranes with single dense-layer structure might suffer less from internal concentration polymerization (ICP) which is the main obstacle for the development of FO process. This study might help us pave the way to design superior CTA membrane structures for forward osmosis application.