A study of complexation thermodynamic of humic acid with cadmium (II) and zinc (II) by Schubert's ion-exchange method

The complexation of humic acid from Azraq Oasis with two heavy metal ions Cd(II) and Zn(II) was investigated at pH 4 and 5 under constant ionic strength of 0.1 and at different temperatures (25, 35, 45, 55 and 65 degrees C). This investigation was done by using Schubert's ion-exchange equilibrium method, and its modified version. The derived conditional stability constants (log beta(n)) for these two metal-humate complexes were determined; they formed 1: 1 and 1:2 complexes. It was found that the conditional stability constants (logo,,) increased by increasing pH and temperatures for all metal-humate complexes. It was found that the conditional stability constant log beta(1) for Cd-humate is bigger than Zn-humate at all the desired temperatures and at pH 4 and 5. The derived constants and their temperature dependences have been used to calculate the corresponding thermodynamic parameters Delta G, Delta H, and Delta S, the results indicate that the stability of these complexes derives from very favorable entropy. (c) 2005 Elsevier B.V. All rights reserved.