STABILITY-CONSTANTS OF METAL-HUMATE COMPLEXES - TITRATION DATA ANALYZED BY BIMODAL GAUSSIAN DISTRIBUTION

A continuous multiligand model was applied to the study of the systems Cu(II)-,Pb(II)-, Mn(II)-, and Cd(II)-humate. The model is essentially based on a bimodal Gaussian distribution of the binding sites. It all ows the calculation of pH-independent metal-binding thermodynamic cons tants which are more representative than simple empirical curve-fittin g parameters. This model was applied to the study of the potentiometri c titration data of an humic acid extracted from an Andosol of Macomer (Sardinia, Italy) in order to calculate the formation constants of th e complexes of humic acids with four metal ions; Its application to th e potentiometric titration experiments showed that different classes o f sites are involved in the binding of the metal ions considered. The affinity to the humic matrix followed the order: Cu(II) > Pb(II) > Mn( II) >> Cd(II); phenolic groups play a relevant role in Cu binding whil e Cd appears to bind exclusively to carboxylic groups.