Complexation of low-valent actinides (uranium(IV), plutonium(III)) by organic model ligands and humic acid in aqueous solution

Risk assessments predicting the transport of actinides under environmentally relevant conditions require basic knowledge of their solubility, their interaction with complexing ligands, their sorption and redox behavior as well as their ability to form colloids. Under reducing conditions as prevalent in deep underground nuclear waste repositories and in the depth of flooded uranium mines, actinide species occur in lower oxidation states. Both the speciation and the mobility of actinides in aquatic systems strongly depend on their oxidation state. For instance, in contrast to the soluble and mobile uranium(VI), uranium(IV) is much less mobile due to the low solubility of uranium(IV) hydrous oxide (UO2•xH2O(am)). However, in the presence of inorganic and organic ligands also uranium(IV) may become mobile due to formation of soluble complexes. Also the mobility of plutonium depends strongly on the existing oxidation states and the chemical species formed. Thus, the speciation of low-valent actinides in aqueous solution has to be studied to predict their migration behavior in natural environments.
Data for the complexation of uranium(IV) and plutonium(III) with inorganic and organic ligands are scarce or vary strongly. In the present study, the complexation behavior of the model ligands citric acid, succinic acid, mandelic acid and glycolic acid is studied and compared to that of humic acid. These model ligands stand for a variety of organic ligands in aqueous systems. The actinide complexation by model ligands and humic acid is studied applying UV-vis absorption spectroscopy and ultrafiltration, respectively.
The stability constants for 1:1 and 1:2 uranium(IV) citrate complexes of the type MpHqLr were determined with log ß101 = 13.5 ± 0.2 and log ß102 = 25.1 ± 0.2 [1]. This shows a strong interaction between uranium(IV) and the ligand. The uranium(IV) complexation with mandelic and glycolic acid is weaker. Stability constants for 1:1 and 1:2 uranium(IV) ligand complexes of the type MpHqLr were determined with log ß101 = 4.53 ± 0.09 and log ß102 = 8.02 ± 0.13 for mandelate and with log ß101 = 4.71 ± 0.08 and log ß102 = 8.25 ± 0.15 for glycolate (I = 1.0 M). Speciation calculations show that due to complexation with organic ligands the solubility and thus, the mobility of uranium(IV) in aquatic systems is increased.
Also plutonium(III) is strongly complexed by organic ligands and humic acid under environmentally relevant conditions which shows the need for speciation studies of plutonium in aqueous solutions containing various complexing agents.

[1] Schmeide, K., Bernhard, G.: Spectroscopic Study of the Uranium(IV) Complexation by Organic Model Ligands in Aqueous Solution. In: Uranium, Mining and Hydrogeology (Merkel, B.J.; Hasche-Berger, A. eds.), Springer Verlag, Berlin, 591-598 (2008).