Properties of cellulose acetate nanofiltration membranes. Application to brackish water desalination

Abstract

In this work, we report about cellulose acetate nanofiltration (NF) membrane preparation according to the phase inversion process. Pore size was monitored by using dope solutions of two polymer concentrations (20 and 22 wt−%) and annealing temperatures from 60–80°C. Membrane characterization included a morphologic analysis using scanning electron microscopy (SEM) and hydraulic permeability ( L p 0) determined from pure water filtration. SEM cross-section micrographs exhibited the expected asymmetric structure with more opened sub-layer morphology for nanofilters prepared using the lower annealing temperatures. The L p 0 values indicative of the pore size increased by decreasing the polymer concentration in dope solutions and the temperature during the annealing operation. Desalting efficiency of the obtained nanofilters was first evaluated through the filtration of synthetic solutions containing 500–2000 ppm of NaCl using a dead end process. The variation of salt rejection vs. the permeation rate allowed us to determine the operating conditions corresponding to the optimal material performances as shown by the plateau related to limit rejection. Desalination experiments for two Tunisian brackish waters with a salt content between 3500 and 4000 ppm were carried out under a transmembrane pressure of 16 bar. Annealed membranes at 75 and 80°C showed the better performances.