Metal Ion Binding by Humic Acid: Application of the NICA-Donnan Model
Résumé
A Donnan-type model for nonspecific binding of
electrolyte ions has been combined w ith the non-
ideal competitive adsorption (NICA) model for specific
binding to produce a model for ion binding to humic
substances. The model considers site
heterogeneity, non-ideality, multicomponent competition,
and electrostatic interactions. The NICA-Donnan
model w as fitted to data for H, Ca, Cd, Cu, and Pb
binding by a purified peat humic acid. The model
fits w ere good and covered a w ide range of pH and
free metal concentrations. The parameters from
these single metal data sets w ere then used to predict
the competitive effect of Ca on Cd and Cu binding
at various pHs. These predictions agreed w ell w ith the
experimental data although there w ere some small
but systematic differences. The new NICA-Donnan
model also predicted reasonably w ell the increase
in Cd and Cu binding on changing from a 0.1 M KNO3
background electrolyte to 0.01 M KNO3. A shortcom-
ing of the model is that in some cases it significantly
underestimated the H+/M 2+ exchange ratio, especially
at high pH and for Cu binding.