New model for equilibrium sorption of metal ions on solid humic acids

Experimental results of sorption kinetics and equilibrium are compared with traditional Langmuir theory. Although, equilibrium data for sorption of metal ions on solid humic acids can be fitted relatively precisely by Langmuir isotherm, its utilization is connected with some inaccuracies, because it does not take into account the production of hydrogen ions during reactions of metal ions with acidic functional groups on the surface of humic particles. The new type of adsorption isotherm, respecting the nature of surface chemical interactions, was proposed for adsorption of metal ions on solid humic acids and proved experimentally. The new model divides binding sites in humic acids into two parts: acidic functional groups, which split of hydrogen ions during surface reaction in adsorption and other binding sites as e.g. aromatic structures, which are not able to change pH value in system. Obtained results are in very good agreement with proposed isotherm. One of the most important advantages of the model is that computed adsorption coefficients are not dependent on pH value as well as adsorption and desorption rate constants obtained from kinetic equation. The isotherm proposed in this article is derived for adsorption of bivalent cations, but it can be modified for ions of other positive valency.