ION-EXCHANGE MECHANISMS OF SOME TRANSITION-METALS ON A MIXED-BED RESIN WITH A COMPLEXING ELUENT

Ion-exchange chromatography was used for the simultaneous elution of transition metals. The mobile phase consisted of an acetate buffer with pyridine-2,6-dicarboxylic acid (PDCA) as a chelating agent; the stationary phase was a mixed-bed resin that displays both anion- and cation-exchange properties. Under the selected conditions, the only species of the analyte that occur in solution are the anionic chelates M(PDCA)(2) while both the free cations and the chelates may be present in the resin. The mechanisms involved in this complex chromatographic pattern are described, considering either cation or anion exchange. It was found that the chromatographic behaviour of the transition metals can be explained by electrostatic interactions, the prevailing mechanism being pure anion exchange. Selectivity coefficients between the eluted species and either PDCA or acetate were derived from the experiments. The results were compared with those obtained after replacement of the mixed-bed resin by a low capacity anion-exchange resin. In this case, additional mechanisms such as adsorption had to be taken into account in the explanation of the retention behaviour of the transition metals.