Ion-expulsion ultrafiltration (IEUF) is proposed as a new membrane separation method for removing ions from aqueous streams; ordinary ultrafiltration membranes, which block the passage of charged colloidal species on the basis of size exclusion alone, are used in the process. IEUF exploits the fact that the ion product of the counterion of a colloidal polyion and a co-ion, present at much lower concentration, will under favorable conditions be nearly the same in the reteníate solution as in the permeate solution in ultrafiltration. This effect leads to a considerable enhancement in concentration of the co-ion in the permeate solution. Semiequilibrium dialysis experiments show the large equilibrium extent of separation that can be achieved by using either micellar or polyelectrolyte solutions in membrane separations. Initial ultrafiltration results show that separation efficiencies can be quite large, although not as great as in equilibrium experiments.Research from our laboratories has indicated the effectiveness of colloid-enhanced ultrafiltration methods in removing solutes from aqueous streams.1 23"11 In these methods, small ions or molecules attach to macromolecular species (either surfactant micelles or polymer macroions)
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