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

Abstract

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 °C). This investigation was done by using Schubert's ion-exchange equilibrium method, and its modified version. The derived conditional stability constants (log β 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 β n ) increased by increasing pH and temperatures for all metal–humate complexes. It was found that the conditional stability constant log β 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 Δ G, Δ H, and Δ S, the results indicate that the stability of these complexes derives from very favorable entropy.