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The effect of chelation on the selective transport of alkaline earth metal ions in asymmetric cellulose acetate hyperfiltration membranes. II
Author(s) -
Lee K. L.,
Hopfenberg H. B.,
Stannett V. T.
Publication year - 1978
Publication title -
journal of applied polymer science
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.575
H-Index - 166
eISSN - 1097-4628
pISSN - 0021-8995
DOI - 10.1002/app.1978.070220418
Subject(s) - chelation , chemistry , egta , ethylenediaminetetraacetic acid , magnesium , inorganic chemistry , metal ions in aqueous solution , membrane , aqueous solution , calcium , metal , nuclear chemistry , alkaline earth metal , organic chemistry , biochemistry
The effects of chelation on the transport of calcium and magnesium, both separately and in a variety of admixtures, in a controlled series of asymmetric cellulose acetate membranes were characterized. Ethylenediaminetetraacetic acid (EDTA) and ethylenebis(oxyethylenenitrilo)tetraacetic acid (EGTA) were used as chelating agents for the alkaline earth metal ions. Asymmetric cellulose acetate membranes annealed at 70°, 75°, and 85°C were studied. Chelation of each of these alkaline earth metals ions in aqueous solutions at pH 6, by either EDTA or EGTA, significantly increased the overall hyperfiltration rejections of these metals by all the membranes studied. The increase in rejection varied montonically with the fraction of metal ion complexed. The higher rejection of metal chelates, compared to the rejection of unbound metal ions, was considered to be the result of the significantly larger size of the chelated species. Calculations suggested that selective (or competitive) chelation took place at pH 6 in a mixture of calcium and magnesium ions in the presence of a stoichiometrically limiting amount of chelating agent. Calcium successfully competed for most of the available chelating agent in equimolar aqueous solutions of chelating agent, calcium, and magnesium. The calcium rejection was explained primarily in terms of the effects of chelation per se on the effective size of the formed complex even in feeds comprised of these ternary solute mixtures. The complexation reaction between magnesium and EGTA is, however, so unfavorable at pH 6 that the Mg 2+ ion remains uncomplexed even in the presence of an equivalent amount of EGTA. The observed increased rejection of magnesium ions, therefore, in ternary systems was explained by electroneutrality criteria and by solute–membrane interactions involving the various calcium species and the membranes.