Humic acid-divalent cation interactions

The adsorption behavior of divalent cations M2+ (Cu, Ni, Co and Zn) with commercial humic acid (HAAl) and also with an extracted fraction of peat soil (HAPs) was followed in aqueous solution. The series of adsorption isotherms were fitted to a modified Langmuir equation. The maximum number of moles adsorbed gave: 0.55±0.02, 0.66±0.02, 0.54±0.02, 0.40±0.02 mmol per gram for HAAl and 0.63±0.03, 0.61±0.06, 0.55±0.02, 0.54±0.03 mmol/g for solid HAPs, for copper, nickel, zinc and cobalt, respectively. The same interaction followed calorimetrically gave endothermic values: 2.4±1.0, 8.4±0.9, 18.3±0.9, 10.6±0.9 kJ mol−1 and 18.4±1.2, 15.9±1.4, 15.4±1.2, 15.0±1.2 kJ mol−1 for HAAl and HAPs, respectively, for the same sequence. Because all Gibbs free energies were negative. Complexation must be accompanied by an increase in entropy.