Adsorption mechanism of trivalent metal ions on chelating resins containing iminodiacetic Acid groups with reference to selectivity

The adsorption equilibria of seven trivalent metal ions (M(3+): Sc(3+), Y(3+), La(3+), Fe(3+), Al(3+), Ga(3+), and In(3+)) on chelating resins containing iminodiacetic acid groups (-LH(2)) were studied. Adsorption curves, measured under the conditions of metal ions in excess against chelating groups, directly indicated the metal-to-ligand ratio of the complexes formed in the resin phase. Iron and group 13 metal ions were adsorbed as (-L)(2)HM, while group 3 metal ions were adsorbed as (-L)(3)H(3)M and (-L)(2)HM. The adsorption constants for (-L)(2)HM found for all the metal ions were well correlated with the formation constants of iminodiacetate complexes in aqueous solutions. The actual adsorption of group 3 metal ions was significantly enhanced beyond that expected from this correlation because of the formation of (-L)(3)H(3)M. This is why the selectivity in the adsorption of trivalent metal ions differs from that in the complexation of iminodiacetate in aqueous solutions. The effects of anions and the number of iminodiacetic acid groups per unit weight of resins were also discussed.