Influence of Phenolic OH Groups on Humic Acid Complexation with Uranium(VI)

The study of the interaction processes between humic acids (HAs) and actinide ions is important for a reliable long-term risk assessment with regard to the behavior of actinides in the environment. However, due to the heterogeneous nature of HAs the description of their complex formation with metal ions is difficult and not completely understood up to now.
To further improve the understanding of the HA-metal ion-interaction we studied the influence of phenolic OH groups on the complexation behavior of HAs with uranium(VI) using chemically modified HAs with blocked phenolic OH groups (synthetic HA type M1 and natural HAs Aldrich and Kranichsee). These modified HAs were synthesized by permethylation of the original HAs with diazomethane, and subsequent alkaline saponification of the permethylated HAs. The modified and unmodified HAs were characterized regarding their functional group content. Extensive comparative 13C-CP/MAS-NMR measurements were performed to verify the modification process and to detect whether or not significant structural changes of the HAs had occurred during their modification. The complexation behavior of the modified HAs with UO22+ at pH 4 was studied by time-resolved laser-induced fluorescence spectroscopy (TRLFS) and compared with the results of the UO22+ complexation of the original, unmodified HAs.
The NMR spectra of chemically modified 13C-labeled synthetic HAs type M1 as well as of the modified HAs with carbon-13 at natural abundance level have shown that the permethylation of the HAs with diazomethane and the subsequent alkaline saponification causes the intended structural changes on carboxyl and acidic OH groups. No significant structural changes of the HAs occur during modification.
For all HAs we determined comparable complexation constants by TRLFS. However, significant lower loading capacities with UO22+ were determined for the modified HAs type M1 and Aldrich (10.5 ± 0.9 % and 9.7 ± 1.6 %, respectively) than for the corresponding original, unmodified HAs (18.0 ± 2.0 % and 17.5 ± 1.6 %, respectively). That indicates a lower amount of maximal available complexing sites of the modified HAs compared to the unmodified HAs. Hence, it can be concluded that the modification of phenolic OH groups changes the complexation behavior of HAs with UO22+. Already at pH 4, phenolic OH groups contribute to the interaction between HA and UO22+ ions.