Sorption of lead(II) on two chelating resins:from the exchange coefficient to the intrinsic complexation constant

The sorption of inorganic lead(II) on two cationic resins containing different complexing groups, the iminodiacetic Chelex 100 and the carboxylic Amberlite CG-50, was investigated. The Gibbs-Donnan model was used to describe and predict the sorption through the determination of the intrinsic complexation constants. These quantities, even though non-thermodynamic, characterize the sorption as being independent of experimental conditions. The sorption mechanism for metals on complexing resins was also studied by adding a competitive soluble ligand that shifts the sorption curves to higher pH. The ligand competes with the resin for complexation with the metal ion. Lead(II) is strongly sorbed on Chelex 100 through the formation of two complexes in the resin phase: MHL with logβ(111i) =−0.3 and ML with log β(111i) =−3.7. The presence of the competitive ligand in solution allows for the determination of a third complex. Furthermore, on Amberlite CG-50 the sorption is rather strong and involves the formation of the complex ML, in more acidic solution, with logβ(110i) =−2.0. In the presence of the ligand PyDA, the ML(OH) complex was characterized by logβ(11−1i)=−5.6. In all the experiments the hydrolysis reactions in the aqueous phase are considered quantitatively.