Determination and Comparison of Uranyl Complexation Constants with Natural and Model Humic Acids

We have investigated the complexation behavior of natural humic acids (HA) with model substances. Our synthetic HA model substance, prepared from glutamic acid and xylose, shows operational properties comparable to those of natural HA in terms of water solubility at different pH and in the type and number of its functional groups. We investigated its complexation behavior with the UO₂ ²⁺ ion by time-resolved laser-induced fluorescence spectroscopy. For comparison, we used purified natural HA from Fluka and Aldrich. The experimental data were evaluated applying the metal ion charge neutralization model developed by Kim and Czerwinski. For our synthetic product, we determined a loading capacity of 23 ± 4 % and a complexation constant of log beta = 6.16 ± 0.22 (pH 3.90 ± 0.05; I: 0.1 M NaClO₄). The obtained values are comparable with experimental results determined for Fluka and Aldrich HA. These results lead to the conclusion that our synthetic product appropriately models the functionality of natural HA.