Cellulose acetate nanofiltration hollow fiber membranes for forward osmosis processes

Cellulose acetate (CA) nanofiltration (NF) hollow fiber membranes have been fabricated and tested in the forward osmosis (FO) process. A two-step heat-treatment, i.e., 60 min at 60 °C and 20 min at 95 °C, effectively shrinks the membrane mean pore radius from 0.63 to 0.30 nm. The molecular weight cut off (MWCO) of the resultant CA NF membrane is 186 Da. In the NF experiments under 1 bar transmembrane pressure, the newly developed CA NF membrane shows a pure water permeability (PWP) of 0.47 L m^-2 bar^-1 h^-1 and rejection levels of 90.17% to NaCl and 96.67% to MgCl₂. The water flux and the salt leakage of the CA NF hollow fiber are measured in the FO process using NaCl or MgCl₂ draw solutions. With increasing the draw solution concentration, the water flux increases. However, the increase is almost linear when the draw solution flows in the shell side and is nonlinear when the draw solution flows in the lumen side. With 2.0 M MgCl₂ draw solution (osmotic pressure ≈ 258.3 bar) flowing in the shell side, the water flux and the salt leakage are 7.3 L m^-2 h^-1 (LMH) and 0.53 g m^-2 h^-1 (gMH), respectively. When using saline water as the feed solution, the efficiency of the osmotic pressure decreases with increasing feed concentration due to the more severe internal concentrative concentration polarization. These preliminary results indicate that CA NF membrane is a promising candidate for FO processes.