Uranium remediation using iron chemistry [electronic resource]

The refining of nuclear materials for energy and military applications often leads to subsurface contamination. In-situ remediation via biological or chemical reduction has been proposed and demonstrated uranium sequestration, preventing the movement of uranium with groundwater. The sequestration method most commonly investigated entails reduction of uranium from its soluble hexavalent state, U(VI), to its sparingly soluble tetravalent state, U(IV). Extensive research at the US DOE Oak Ridge Field Research Center (FRC) ar Oak Ridge, TN, and other sites have demonstrated the potential for long-term sequestration and suggested a reaction mechanism involving iron. This dissertation explores the feasibility of uranium reduction by ferrous iron in well-defined uranium-iron-carbonate reaction systems. The results show that Fe(II) reduction of uranium is thermodynamically controlled and may be useful for U sequestration. Simple reaction systems containing calcium are also analyzed to investigate the effect of calcium on the thermodynamics and kinetics of U(VI) reduction. The thermodynamic properties of a simple reaction system of UO2Cl2 (0.2 mM) and FeCl2 (1 mM) were evaluated by coupling all possible reactive species of U(VI) and Fe(II) under specific pH conditions. These calculations indicated that Fe(II) reduction of U(VI) is thermodynamically feasible when pH > 5 for a simple system in which all Fe was initially present as Fe(II) and all U was initially present as U(VI). Reduction of U(VI) was predicted at higher pH levels. These results were confirmed experimentally by creating a low pH solution then triggering reduction of U(VI) and oxidation of Fe(II) by rapidly raising the pH. Acidity production was expected as the reaction advanced based on the stoichiometry of the overall reaction. This was confirmed experimentally: equilibrium was observed at a final pH of 5.5; partial reduction occurred when the initial pH was increased to intermediate levels, such as 6.2; complete reduction occurred when the pH was increased to levels above 8. Synchrotron analyses of the uranium oxidation state in the solid phase revealed complete reduction of uranium at pH levels above 8. Long term sequestration of uranium was achieved by Fe(II) remediation of uranium at high pH (> 9.5). For this condition, the U(IV) remained sequestered in the solid phase when exposed to pure oxygen for 20 minutes, and U(VI) concentrations in the aqueous phase remained below the detection limit for 500 days. Synchrotron measurements demonstrated that the U(VI) percentage in the solid phase shifted from 87% following initial exposure to oxygen to 100% after 500 days of sustained oxygen exposure. Selected area diffraction pattern image indicates that the iron uranium matrix has a crystal structure like magnetite, Fe3O4. EDX analyses indicated that the solid phase was composed of iron and uranium, and TEM revealed a layered iron-uranium structure. A stoichiometric excess of Fe(II) was required for complete reduction of U(VI) when reduction was thermodynamically feasible. This result is consistent with the solid phase iron matrix stoichiometry of Fe3O4. Uranium reduction by Fe(II) was less favorable when calcium was present. When 1 mM calcium was introduced into the iron-uranium system, the aqueous uranium concentration decreased rapidly when pH > 8. When pH > 6, reduction was feasible in the iron-uranium-carbonate-calcium system but the kinetics of reduction were slow, suggesting kinetic in the presence of significant calcium.

Advisor: Criddle Craig primary advisor. Thesis advisor
Bargar John advisor. Thesis advisor
McCarty Perry L. advisor. Thesis advisor