In subsurface environments at DOE nuclear facilities, chelating agents from decontamination operations along with natural and microbially produced reactions have the potential to complex with radionuclides and facilitate transport of radionuclides and toxic metals. Biotransformation of radionuclide complexes may result in retardation of nuclide migration by precipitating into water-insoluble hydroxides, oxides or associate with immobile inorganic and organic phases. There is a paucity of information concerning the biotransformation of organic ligand complexed with radionuclides and metals as well as their fate under anaerobic conditions, which limits the development and optimization of in-situ stabilization of radionuclides at these nuclear sites.

This research addresses the structure-function relationship for the complexed uranium. Humic substances found in the subsurface, which are products of decay or polymerization of chemicals found in biological materials, may also complex with uranium due to their high content of functional groups, including carboxylate, phenolic and alcoholic groups. The family of humic substances that will be the focus of this research will be fulvic acids because they are more soluble and would be a major concern in relation to the transport of uranium in the subsurface environment. Analogs of this large and complicated acid such as lactic acid, malic acid, catechol, and salicylic acid will be used to obtain a better understanding of how functional groups play a role in uranium complexation to fulvic acid. Once the binding information between uranium and organic ligands at different pH is obtained, the effect of zero-valent iron and anaerobic bacteria (Desulfibrio desulfuricans) on the stability of the U-organic complexes will be assessed. The main questions that will be asked are:
1. How does the structure of the complex relate to the bioreduction and precipitation of complexed uranium?
2. What is the fate of the complex after bioreduction? What role does the biofilm play?
3. What is the fate of the complex after exposure to zero-valent iron? How does surface area of the iron play a role?
4. What is the stability of the uranium precipitate after biotransformation and/or reduction by ZVI? How can we improve the stability?

Preliminary FTIR data indicates that uranium are complexed to malic acid by unidentate bonding to the carboxylate (Figure 1). Asymmetric and symmetric carboxylate stretch was 1609 and 1410 cm-1, respectively. The shift to lower frequency from the malic acid standard indicates uranium complexation to the ligand. The O=U=O asymmetric stretching frequency was found to be 908 cm-1. As for uranium complexed to catechol, FTIR data (Figure 2) indicates the O=U=O asymmetric stretching to be at 930 cm-1. A decrease in the stretching vibration of the C-O bond indicates a change in bond character when the organic ligand is exposed to uranium. It is most likely that the uranium is bound to the catechol in a bidentate mononuclear fashion. It is hypothesized that the nature of the bonding between the uranium and the organic ligand will affect the stability of the complex, which in turn will influence its interaction with zero valent iron and bacteria.

Further research will involve the structural analysis for U-lactate and U-salicylate as well as experiments involving Desulfibrio desulfuricans and steel will be conducted. Surface analytical techniques such as SIRMS, Far-IR, UV-vis, XPS, SIMS, and SEM/EDAX will be used in conjunction with HPLC and leach protocols (to determine stability) to form a foundation of basic knowledge, which must be established in order to better develop, and ultimately optimize, promising stabilization and bioremediation methods used to treat radionuclide-contaminated sites.