Complexation of Metal Ions with Solid Humic Acids, Humic Colloidal Solutions, and Humic Hydrogel

Complexation kinetics of Co(II), Cu(II), and Ni(II) ions with humic acids was studied. Humic acids were used in three different forms: solid powder, colloidal solution (sol), and hydrogel. Pseudo-second-order rate equation was used as the basic equation for the mathematical description of the complexation kinetics in the humic systems. Three different methods were applied to monitor the concentration changes in studied systems (conductometry, potentiometry, and UV/VIS spectroscopy). A double-linear fit was obtained in all cases. This is an indication that there are two principal types of metal binding sites in humic acids in all used forms. The strongest active centers are probably occupied in the first step, and slower interactions with less preferable binding sites and the rearrangement of formed complex follow in the second step. Values of rate constants were much higher for the first step in all cases. The highest values were obtained for humic sol and Cu(II) ions and the lowest ones for humic solid powder and Ni(II) ions. In general, the complexation rate increased in the following order: humic powder, sol, and hydrogel for all used metal ions and analytical methods.