Modeling of metal-humate complexation based on the mean molecular weight and charge of humic substances:: Application to Eu(III) humate complexes using ion exchange

A general model, the so called Mean Molecular Weight Model (MMWM), of complexation of metal cations (Mez+) with macromolecular polyanions of humic acid (HAS) is proposed. The model is based on the results of previous studies of the electrophoretic mobility of humate complexes and assumes that the complexation proceeds by consecutive neutralization of the dissociated carboxyl groups of the central polyanion HA(p-) with Mez+ cations. It reflects the macromolecular character of humic acid, applies molar concentrations of reacting components with equations for stability constants and incorporates also the mean charge of humic macromolecules. The model has been verified with experimental data obtained in the study of complexation of Eu(III) with Aldrich humic acid using ion exchange (Amberlite IR-120), over a broad range of [Eu] to [HA] ratio, at pH 4 and 7.