Method for the preparation of cellulose acetate flat sheet composite membranes for forward osmosis—Desalination using MgSO4 draw solution

A lab scale method for the preparation of defect free flat sheet composite membranes for forward osmosis (FO) has been developed. Membranes containing a thin layer of cellulose acetate (CA) cast on a nylon fabric of 50 μm thick were prepared by phase inversion in water. Cellulose acetate (CA) membranes with an overall thickness of 70–80 μm have been prepared with lactic acid, maleic acid and zinc chloride as pore forming agents, at different annealing temperatures, for forward osmosis. These membranes have been tested in the desalination of saline feeds (35 g·L− 1 of NaCl) using magnesium sulphate solution (150 g·L− 1) as the draw solution. The water flux, and rejection of NaCl, were compared with those of commercially available membranes tested under the same FO conditions. The commercially available FO membrane from Hydration Technologies Inc, OR (M1) has a permeability of 0.13 L·h− 1·m− 2·bar− 1 with a NaCl rejection of 97% when tested with 150 g·L− 1 of MgSO4 in the draw solution. Another commercially available membrane for FO from Hydration Technologies Inc, OR, M2 has a water permeability of 0.014 L·h− 1·m− 2·bar− 1 with NaCl rejection of 100%. The flux and rejection of the CA membranes prepared in this work are found to be dependent on the nature of the pore forming agent, and annealing temperature. Impregnation of an inorganic filler, sodium montmorrillonite in CA membranes and coating of CA membranes with hydrophilic PVA did not enhance the flux of base CA membranes. Cellulose acetate membranes cast from dope solutions containing acetone/isopropanol and lactic acid, maleic acid and zinc chloride as pore forming agents have water permeabilities of 0.13, 0.09 and 0.68 L·h− 1·m− 2·bar− 1 respectively, with NaCl rejections of 97.7, 99.3 and 88% when annealed at 50 °C. CA membranes prepared with zinc chloride as a pore forming agent have good permeability of 0.27 L·h− 1·m− 2·bar− 1 with a NaCl rejection of 95% when annealed at 70 °C.

Research Highlights
► Flat sheet composite membranes for forward osmosis (FO) were prepared.
► Membrane thick ness is around 70–80 µm.
► Membranes were tested with magnesium sulphate as the draw solute.
► Lactic acid, maleic acid and zinc chloride were used as poreforming agents.
► High permeability was obtained when zinc chloride was used as poreforming agent.