Kinetic Study of the Oxidative Dissolution of UO₂ in Aqueous Carbonate Media

Abstract

The oxidative dissolution of uranium(IV) dioxide powder at room temperature in aqueous carbonate media has been investigated. Kinetic studies evaluating the efficacy of various oxidants, including K2S2O8, NaOCl, and H2O2, for dissolving UO2 in alkaline solution have been performed, with H2O2 exhibiting the most rapid initial dissolution at 0.1 M oxidant concentrations. This result is due in part to the ability of peroxide to act as both an oxidant and a ligand under alkaline conditions. A spectrophotometric titration was used to confirm peroxide coordination to the U(VI) metal center. The disappearance of characteristic absorbance maxima associated with UO2(CO3)3 4- (e.g., 448.5 nm) and a subsequent change in solution coloration upon titration with hydrogen peroxide indicated a change in speciation. Optimization of the hydrogen peroxide concentration indicated that the initial rate of uranium oxidation increased with increasing peroxide concentration, with a maximum reaction rate estimated at about 0.9 M peroxide. In addition, the effects of both the carbonate countercation and the carbonate concentration were also studied. It was determined that for 40 mg UO2 0.5 M Na2CO3 was the most propitious choice, exhibiting both a high initial dissolution rate and the highest UO2 dissolution capacity among the systems studied.

Reaction mechanism of uranyl in the presence of zero-valent iron nanoparticles

“…Previous studies have demonstrated that carbonate leaching can remove sorbed uranium and solid-phase U(VI) from contaminated sediments (Gu et al, 1998), but such complexation is less likely for precipitated UO 2 and Fe/U oxides (Casas et al, 1998;Duff et al, 2002;Peper et al, 2004). Therefore, in terms of retarding U transport in the subsurface, presence of these phases is preferable.…”

Oxidation of UO2(s) in aqueous solution

“…Oxidative UO 2 dissolution rates under various conditions have been reported in numerous publications [4,5,7,9,10,13,14,[21][22][23][30][31][32][33][34][35]44,48,50,53,54]. The studies considered here were all performed at room [7,10,34,44].…”

“…The oxidant concentration and the pH or the carbonate concentrations employed in each experiment are given in table 2. The ionic strength and/or the pH were in some cases adjusted by various additives; NaClO 4 /HClO 4 [4,5,9,13,21,23,35], NaCl [21,22,30,31], NaOH [4,35], NH 4 OH [53]. The UO 2 materials used were powders, with particle size ranging from ~1 μm to ~1 mm, UO 2 discs or pellets.…”

The electrochemical reduction of hydrogen peroxide on uranium dioxide under intermediate pH to acidic conditions

“…That HCO 3 − /CO 3 2− and H 2 O 2 react synergistically on UO 2 is also suggested by the UV/vis spectrophotometric identification of soluble uranyl-carbonate-peroxide complexes formed as the product of the dissolution process [35],…”