Evaluation of the sorption of metal ions on a complexing resin in contact with different solutions based on the Gibbs-Donnan model

The intrinsic protonation and complexation constants of an ion-exchange complexing resin, previously determined in
condition of trace metal ions on the basis of the Gibbs–Donnan model of ion-exchange resins, were here used to evaluate the
concentration of metal ions sorbed on the resin when the metal ion is in excess. The resin considered was a commercial one,
21 containing carboxylic groups at a concentration of 10 mmol g of dry resin. Metal ions complexable by the active groups
present in the resin phase, i.e. alkaline earth metals and cadmium(II), were investigated, together with a not complexable
metal ion, sodium, which enters in the resin phase only by diffusion and ion-exchange. The calculation method is an iterative
one, taking into account the diffusion and ion-exchange in the case of the not complexable metals, and the complexation in
the case of the complexable metal ions. The concentration of metals in the resin phase calculated by the proposed procedure
was compared with that obtained experimentally in some representative solutions at different composition and acidity. The
agreement was acceptable, showing that the intrinsic complexation constants are of help not only in conditions of trace
metal, but also for evaluations at completely different conditions. In the case of complexable metal ions it was shown that the
complexation in the resin phase is the most important sorption mechanism, even if ion-exchange takes place at high acidity,
when the complexation is negligible.