The sorption of metal ions on chelating resins is believed to take place through complexation by the active groups. Thus the selectivity of the resin for different metals is expected to be comparable to the complexing properties of a monomer having a structure similar to that of the active groups. This correlation can be done in a quantitative way on the basis of the Gibbs-Donnan model, which allows prediction of the extraction coefficients of metal ions on a chelating resin from the complexation constants, and other quantities, such as the concentration of the counterion in solution. In the case of chelating resins containing iminodiacetic groups, the selectivity toward some divalent metal ions is much worse than expected from the complexation by iminodiacetate in aqueous solution. Calcium for instance is sorbed at much lower pH than anticipated. Its sorption on acommercial chelating resin, Chelex 100, was xamined by the Gibbs-Donnan model, and a complex Ca( HL)2 has been found to be formed, with a complexation constant log b2exLi = -5.1. Similar findings were obtained for zinc and cadmium. In this case, the complexes ML and ML2, analogous to those formed in aqueous solution, are not negligible. The equilibrium constants of M(HL)2 was found to be log b2exLi = -3.6 and -3.5, respectively. In the case of copper and nickel, the sorption mechanism involves only the formation of the complex ML and the extraction coefficients are in good agreement with those predicted from the complexation constants of the hydrosoluble analogue by the Gibbs- Donnan model, respectively, log b1exLi = -0.75 and log b1exLi L = -2.90, while log K1 = -0.68 and log K2 = -3.05 in aqueous solution.
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