Charged membrabe ultrafiltration of inorganic ions in single annd multi-salt systems

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Abstract

An experimental study of the ultrafiltratin of several ionic inorganic soluted, using a negatively-charged membrane, is detailed. Both single salt and multi-salt systems are investigated in a continuous flow, thin chappel unit, with and average transmembrane pressure difference of 2.76 × 105N/m2 and a channel Reynolds number of 2800, The solute rejection is predictable to some extent in terms of Donnan ion exclusion theory, and the water flux is dictated by the combined effects of osmotic pressure and membrane-cation interaction.

In the single salt experiments, the anion rejections can be related to the inlet (feet) concentration by power functions of the form: rejection = 1 − K Cia, in which 0 < a < 1.0. The power a is 0.4 chloride salts 0.25 fo monovalent oxyanion salts, and 0.1 fir divalent oxynion salts, but a is indepedent of the cationn present (Na+, Ca2+, or La3+). The constant K is generally a function of both the anion (except in the case of SO42−, HPO42−, and CrO42−) and the cation, being determined by such factors as sistance to water flux is found to depend on cation charge and cation radius. For systems containing two or more salts, the presence of a divalent anion decreses the rejection of the monovalent anion. For multi-salt systems, a rejection of Ca2+ plus Mg2+ of 0.82 and a water flux of 12.9 × 10−4 cm/s (27.2 gal/ft2-day) are achieved at a transmembrane pressure difference of 4.10 × 105 N/m2 (4.1atm), indicating that the process has considerable potential for industrial water softening.

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