Abstract
Humic substances are natural polyelectrolytes that play an important role in the speciation of metal ions in soil and aquatic environments. In this paper electrostatic models developed for humic substances are reviewed and their validity is examined by comparing the model predictions with the results of the fluorescence quenching method. For humic acid, a kind of humic substances, the models, where the electrical double layer around a humic acid molecule is explicitly considered, are in good agreement with the experiments. On the other hand, the Donnan models tend to overestimate the negative potential of humic acid.
