The purpose of this work is to investigate the preparation of composite anisotropic membranes in a single stage, through the technique of simultaneous casting of two polymer solutions (cellulose acetate (CA)/acetone/formamide (26.3/50/23.7 % w/w) and CA/polyvinylpyrrolidone/dimethyl formamide (15/10/75 % w/w)). The precipitation of the solutions was followed using light transmission experiments. The effect of the exposition period precipitation bath on the membrane properties is also reported. All the membranes were characterized by scanning electron microscopy and by permeation experiments. The RO and NF experiments are carried out using 5,000 mug/g (ppm) of NaCl aqueous solution and 5 g/L of polyethyleneglycol (PEG), 6 KDa, aqueous solution, respectively. Moreover, to reduce defects in the upper layer, the membranes were immersed in a water bath at 90°C for 60 minutes. The membrane morphology analysis indicates that is possible to obtain complete adhesion of the two layers. This result is related to the mass transfer conditions, which should delay the precipitation in the regions near the polymer solutions interface. Regarding the performance of the membranes in RO and NF experiments, the permeate fluxes are comparable to those observed with commercial membranes (7--465 L/h.m²). The solute retention varied in the range of 24 to 63% for NaCl, and in the range of 53 to 82% for PEG. These results point out the potential of technical scale application of the double layer membranes.
Cellulose acetate; reverse osmosis; nanofiltration; phases inversion; simultaneous casting
