Thermodynamics of Clay Minerals-Humic Acids Interaction

The sorption equilibrium of humic acids (HA) onto kaolinite and montmorillonite clays and, for comparison, onto a zeolite-rich tuff has been analyzed by the batch technique. The sorbents were both in the native and calcium enriched forms. The data fit well with the Henry model for sorption onto clay, and with the Freundlich model for sorption onto tuff. The Gibbs energy of sorption (Delta(s)G degrees) in the standard state, i.e., low-concentration HA solution and completely free sorbent, was markedly lower for sorption onto either type of clay than for the zeolitic tuff. Delta(s)G degrees was higher for the Ca2+-enriched form than for the native form.