Comparative study of binding strengths and thermodynamic aspects of Cu(II) and Ni(II) with humic acid by Schubert's ion-exchange method

The complexation of humic acid from Azraq Oasis with two heavy metal ions Cu(II) and Ni(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 degreesC). 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 (log beta(n)) increased by increasing pH and temperatures for all metal-humate complexes, except for Ni-humate at pH 4 (log beta(n)) decreased by increasing temperature. It was also found that the conditional stability constant log beta(1) for Cu-humate is bigger than Ni-humate at all the desired temperatures and at pH 4 and 5. The derived constants and their temperature dependencies have been used to calculate the corresponding thermodynamic parameters DeltaG, DeltaH, and DeltaS, the results indicate that the stability of these complexes derives from a very favorable entropy. (C) 2003 Elsevier B.V. All rights reserved.