Investigation of Humic Acid Complexation Behavior with Uranyl Ions Using Modified Synthetic and Natural Humic Acids

We investigated the influence of phenolic OH groups on the complexation behavior of humic acids (HA's) with UO22+ ions at pH 4. Starting from the synthetic HA type M1, the natural HA Aldrich, and the natural HA Kranichsee, we synthesized modified HA's with blocked phenolic OH groups by derivatization with diazomethane. The partial blocking of phenolic OH groups was confirmed by a radiometric method which quantitatively determined the functional groups and by FTIR spectroscopy. The complexation behavior of the modified and unmodified HA's with UO22+ ions was investigated by time-resolved laser-induced fluorescence spectroscopy. The experimental data were evaluated with the metal ion charge neutralization model. We determined comparable complexation constants for all HA's. Two modified HA's (type M1 and Aldrich) had significantly lower loading capacities with UO22+ ions (10.5 ± 0.9 % and 9.7 ± 1.6 %, respectively) than the corresponding unmodified HA's (18.0 ± 2.0 % and 17.5 ± 1.6 %, respectively). This indicates that the blocking of the phenolic OH groups changes the complexation behavior of the HA's.