SORPTION MECHANISM OF TRACE AMOUNTS OF DIVALENT METAL-IONS ON A CHELATING RESIN CONTAINING IMINODIACETATE GROUPS

The sorption of metal ions on chelating resins is believed to take pla ce through complexation by the active groups. Thus the selectivity of the resin for different metals is expected to be comparable to the com plexing 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 c omplexation constants, and other quantities, such as the concentration of the counterion in solution. In the case of chelating resins contai ning iminodiacetic groups, the selectivity toward some divalent metal ions is much worse than expected from the complexation by iminodiaceta te in aqueous solution. Calcium for instance is sorbed at much lower p H than anticipated. Its sorption on a commercial chelating resin, Chel ex 100, was examined by the Gibbs-Donnan model, and a complex Ca(HL)2 has been found to be formed, with a complexation constant log beta2exL i = -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 beta2exLi = -3.6 and -3.5, respectively. In the case o f copper and nickel, the sorption mechanism involves only the formatio n of the complex ML and the extraction coefficients are in good agreem ent with those predicted from the complexation constants of the hydros oluble analogue by the Gibbs-Donnan model, respectively, log beta1exL = -0.75 and log beta1exL = -2.90, while log K1 = -0.68 and log K2 = -3 .05 in aqueous solution.