Investigation on sorption equilibria of Mn(II), Cu(II) and Cd(II) on a carboxylic resin by the Gibbs–Donnan model

Sorption mechanism of bivalent metal ions on a weak cationic resin containing the carboxylic group is studied. The Gibbs–Donnan model is used to describe and then to predict the sorption through the determination of the intrinsic complexation constants. These quantities characterize the sorption being independent of experimental conditions. They are determined according to a well established procedure and using a recently proposed iterative method for calculation of counter ion concentration in the resin phase. Sorption mechanisms are also studied adding appropriate soluble ligands whose complexing properties are exactly known to the solution containing the resin and the metal ion. Competing with the resin for the complexation of the metal, they shift the sorption curve to higher pH and often this allows detecting other complexes between the metal and the resin. In this way for Mn(II), besides the 1:1 complex formed in the more acidic solution, with log β110=−4.55, the complex ML2, characterized by log β120=−9.80, is found; for Cd(II), besides the ML complex, with log β110=−3.01, at pH higher than 7, the specie MLOH with log β11–1=−8.28. For Cu(II) the complex ML2 is detected, confirming previous findings, with log β120=−7.24. In the presence of two different ligands, sulphosalicylic and malonic acid, a different complex, ML2OH, is identified, with the same intrinsic complexation constant for the two ligands, log β12–1=−13.35. As expected from the model, the intrinsic complexation constants, especially for the 1:1 complex, are in a good agreement with the complexation constants of acetic acid.