Utilization of the Donnan effect for improving electrolyte separation with nanofiltration membranes

The possibility of utilizing the Donnan effect to enhance NaCl removal from a solution of a water-soluble organic dye (Procion Red H-E7B, ICI) using a nanofiltration membrane was examined. The increased salt removal is obtained by the addition of an ionized polyelectrolyte (Na salt of polyacrylic acid, PNa) having a molecular weight of 60 000 dalton. Salt rejection data and permeate flux were measured in three solution system: NaClH2O, NaClH2O-dye and NaClH2O-dye-PNa. The solutions were fed through a parallel plate osmotic cell having an area of 200 cm2, fitted with an MPF-44 nanofiltration membrane. Polyelectrolyte addition was found to provide a substantial enhancement of salt removal. For example, chloride rejection in a binary system with a salt weight concentration of 1% was around 50% (i.e. permeate salt concentration is about half that of the feed). Addition of 4.4% PNa decreased the salt rejection to a negative value of −68%, signifying that permeate salt concentration was almost 1.7 times higher than the feed concentration. However, polyelectrolyte addition reduced permeate flux and induced a flux limiting phenomenon, similar to that commonly encountered in ultrafiltration. A simple two parameter model is presented which successfully correlates the salt rejection data measured in this work, as well as results reported in the literature. The model enables prediction of salt rejection in multi-ionic systems from data measured easily in a single salt system and can be usefully applied for quantifying NF processes utilizing the Donnan effect.

[1]  A. Fane,et al.  Negative Rejections of Cations in the Ultrafiltration of Gelatin and Salt Solutions , 1980 .

[2]  T. Stocker,et al.  Comparison of two thin-film composite membranes: low pressure FT-30 to very low pressure NF40HF , 1987 .

[3]  Milton Blander,et al.  Chemical physics of ionic solutions: edited by B. E. Conway and R. G. Barradas. 622 pages, diagrams, illustr. 6 × 9 in. New York, Wiley and Sons, 1966. $25.00 , 1967 .

[4]  Shoji Kimura,et al.  ANALYSIS OF SOLUTES REJECTION IN ULTRAFILTRATION , 1981 .

[5]  Shoji Kimura,et al.  Calculation of ion rejection by extended nernst-planck equation with charged reverse osmosis membranes for single and mixed electrolyte solutions , 1991 .

[6]  P. Schirg,et al.  CHARACTERISATION OF NANOFILTRATION MEMBRANES FOR THE SEPARATION OF AQUEOUS DYE-SALT SOLUTIONS , 1992 .

[7]  Shoji Kimura,et al.  Reverse Osmosis of Single and Mixed Electrolytes with Charged Membranes: Experiment and Analysis , 1991 .

[8]  Shoji Kimura,et al.  REJECTION OF INORGANIC SALTS BY CHARGED ULTRAFILTRATION MEMBRANES MADE OF SULFONATED POLYSULFONE , 1983 .

[9]  A. R. Cooper.  Ultrafiltration membranes and applications , 1981 .

[10]  P. Eriksson.  Water and salt transport through two types of polyamide composite membranes , 1988 .

[11]  C. Linder,et al.  Intermediate reverse osmosis ultrafiltration (RO UF) membranes for concentration and desalting of low molecular weight organic solutes , 1989 .

[12]  Robert Rautenbach,et al.  Separation potential of nanofiltration membranes , 1990 .

[13]  O. Kedem,et al.  Hyperfiltration in charged membranes: the fixed charge model , 1967 .