@ Separation in solution of preselected ions from other ions of the same sign is effected with at least one membrane comprising (a) an ion transport selective film matrix formed from one or more hydrophobic, film-forming polymers com- prsing covalently-attached ionizable radicals; and (b) a polymer effective to reduce the electrical resistance per micron thickness of film (a) by at least ℓ of an order of magnitude, and derived from units comprising monomers containing amino groups. A selective electrodialysis unit for effecting such separation comprises at least one thin, dense non-porous charged membrane defined as above, which has a dielectric constant and composition preselected to induce selective ion dissociation of a first group of charged ions upon the passage of current therethrough, so as to effect a substantial ion migration of the selected ion group across the membrane at a fraction larger than its relative concentration. The invention may be adapted for e.g. the selective transportation of nitrate ions from the diluting to the concentrating compartments of the unit, from an aqueous stream containing nitrate, bicarbonate and sulfate ions, the scale formation on the unit being thereby substantially reduced.
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