Kinetics of oxidation and dissolution of uranium dioxide in aqueous acid solutions (original) (raw)
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Electrochemical Studies on Uranium in the Presence of Organic Acids
Journal of Nuclear Science and Technology, 2007
We examined electrochemical redox reactions of UO 2 2þ in perchlorate and organic acid (oxalic, malonic, succinic, adipic, L-malic, and L-tartaric acids) solutions using cyclic voltammetry to reveal the effects of complex formation with organic acids on the redox behavior. In the perchlorate and organic acid solutions, a redox reaction of UO 2 2þ /UO 2 þ and an oxidation reaction of U(IV) produced by a disproportionation of UO 2 þ were observed. The peak potentials of the UO 2 2þ reduction showed a good linear relationship with the stability constants of 1:1 UO 2 2þ -organic complexes. In the presence of malonic acid, the redox potential for UO 2 2þ /UO 2 þ was constant at pH 1-2 and 5-6 while it decreased with an increase in pH from 2 to 5. Additionally, it was independent of malonate concentration at 0.1-0.5 M while it decreased with an increase in the concentration from 0.005 to 0.1 M. Based on the experimental and the speciation calculation results, we determined the redox reactions of UO 2 2þ -malonate complexes as a function of pH and malonate concentration. We also determined the redox reactions of UO 2 2þ -oxalate complexes in the same way.
Surface Electrochemistry of Uranium Dioxide in Acidic Hydrogen Peroxide Solutions
MRS Proceedings, 2012
ABSTRACTThe electrochemical reduction of H2O2 on SIMFUEL was investigated over the pH range 1 to 4. The mechanism at pH 4 is known to occur on UV species incorporated into a surface layer of UIV1-2xUV2xO2+x. However, below pH 3, reduction occurs on an adsorbed UVO2(OH) state which is unstable and oxidizes to insulating UVI before dissolving as UO22+. Both schemes are observed at intermediate pH’s. The presence of both low and high acidic regions at the electrode surface is determined by the combination of peroxide concentration, bulk pH and the surface diffusion conditions.
Electrochimica Acta, 2013
The direct dissolution of UO 2 CO 3 in neat and "wet" ionic liquid (IL) trimethyl-n-butylammonium bis(trifluoromethansulfonyl)imide [Me 3 N n Bu][TFSI] is examined. The ionic liquid serves as both the solvent for the direct dissolution of UO 2 CO 3 (s) and the electrolyte solution for the electrochemical analysis of the soluble uranyl species. The solubility data indicate that displacement of the CO 3 2− occurs slowly due to the low concentration of protons available from residual water in the pristine IL. Enhanced dissolution of UO 2 CO 3 through the formation of carbonic acid H 2 CO 3 is achieved after the addition of acid, bis(trifluoromethanesulfonyl)amide (HTFSI) and water. The soluble uranyl cation can then coordinate with the TFSI anion in place of the displaced CO 3 2− anion following the decomposition of carbonic acid and purging of CO 2 (g) and water from the IL. The solubility of UO 2 CO 3 was examined using liquid scintillation counting of 233 U for the pristine ionic liquid. The "wet" ionic liquid containing HTFSI and soluble UO 2 CO 3 was evaluated using UV/vis spectroscopy before and after dissolution. The electrochemical deposition of uranium species from ionic liquid was evaluated using cyclic voltammetry. The potential mediated deposition of uranium species was achieved and verified using scanning electron microscopy (SEM) and the solid uranium deposits were evaluated using energy dispersive X-ray emission spectroscopy (EDX).
Inorganica Chimica Acta, 2015
The uranyl UO2 2+ (VI) cation (hydrated) exhibited strong charge-transfer absorptions at 350-400 nm in aqueous solutions containing bromide and iodide. The charge-transfer absorptions originate from a single-electron transfer from a halide anion to the uranium(VI) valence shell. Their intensities (represented by absorbance at 375 nm) were found to be directly proportional to molar concentrations of the halide (bromide or iodide) and UO2 2+ in solution, respectively, showing the nature of a bimolecular interaction in the charge-transfer absorption transition. The absorptions were also greatly enhanced by sulfuric acid, and their intensity (absorbance at 375 nm) increased linearly as a function of the acid molarity. An electron paramagnetic resonance (EPR) study has indicated that the charge-transfer also took place slowly in the dark, resulting in appreciable thermal chemical reduction of diamagnetic UO2 2+ (VI) (hydrated) to paramagnetic UO2 + (V) (hydrated) (g=2.08) by bromide and iodide. In the presence of sulfuric acid, CH3SOCH3 (DMSO) was shown by EPR to undergo a charge-transfer oxidation by UO2 2+ (VI) to a stable CH3SOCH2. (DMSO .) radical (singlet, g=2.01), and UO2 2+ (VI) was reduced to UO2 + (V). A possible mechanism for this oxidation-reduction has been proposed. The charge-transfer absorption transition (350-400 nm) between UO2 2+ (VI) and phenol (PhOH) in acetone was observed and characterized. A chemical oxidation-reduction of UO2 2+ (VI) [in the form of U VI O2(acetone)5 2+ ] and PhOH in acetone was found by EPR to give UO2 + (V) [in the form of U V O2(acetone)5 + ] and a stable phenoxyl (PhO .) radical (singlet, g=2.00) via a simultaneous charge-transfer and deprotonation pathway.
Studying the UO2 Electrochemistry In Situ Using SEM
Microscopy and Microanalysis
Repositories for the disposal of radioactive waste depend on a multi-barrier system to segregate the waste from the biosphere. The multi-barrier system generally consists of the natural (geological) barrier by the repository host rock and its surroundings and an engineered barrier system (EBS). In situ and laboratory testing and modelling constitute an important aspect of research to ensure that an EBS will perform to its desired functions to contain spent uranium oxide (UO2) fuel [1]. Of specific interest in the dissolution and oxidation of UO2 is the determination of its corrosion potential. Although such processes are described in the Mixed Potential Model (MPM) and Fuel Matrix Dissolution Model (FMDM), microscopic understanding of the electrochemical and diffusion processes is needed to validate advanced models such as MPM.
Effects of on the kinetics of UO2 oxidation by H2O2
Journal of Nuclear Materials, 2006
The effect of HCO À 3 on the kinetics of UO 2 oxidation by H 2 O 2 in aqueous solution has been studied using powder suspensions where the concentration of H 2 O 2 was monitored as a function of time. By varying the UO 2 surface to solution volume ratio second order rate constants were obtained for HCO À 3 concentrations ranging from 0 to 100 mM. The second order rate constant increases linearly with HCO À 3 concentration from 0 to approximately 1 mM. Above 1 mM HCO À 3 the rate constant is 4.4 · 10 À6 m min À1 independent of [HCO À 3 ]. This indicates that the kinetics of the reaction depends on both oxidation and dissolution below 1 mM HCO À 3 while at higher concentrations it is solely governed by oxidation. Hence, the rate constant obtained at HCO À 3 concentrations above 1 mM is the true rate constant for oxidation of UO 2 by H 2 O 2 . The results also imply that the reaction between HCO À 3 and oxidized UO 2 on the UO 2 surface (i.e. HCO À 3 facilitated dissolution) is limited by diffusion (ca 10 À3 m min À1 in the present system). Furthermore, the experimental results were used to estimate the oxidation site density of the powder used (126 sites nm À1 ) and the rate constant for dissolution of UO 2þ 2 from the UO 2 surface (7 · 10 À8 mol m À2 s À1 ).
Behavior of Uranium Dioxide: Chemistry and Catalysis in the UO2-water System
MRS Proceedings, 2003
ABSTRACTInteractions during extended exposure of UO2 to 2:1 H2+O2 mixtures at room temperature and 0.13 bar pressure are investigated in an effort to describe chemical and kinetic behavior of spent fuel following contact with groundwater. Oxidation of UO2 to UO2+x by O2 occurs initially when oxide is directly exposed to the gas mixture or submerged in water, but immersion is accompanied by a 25-fold reduction in the rate. The initial rate is proportional to [O2]2 for gasphase oxidation and to [O2]1.5 for the submerged oxidation. Continued measurement during direct oxide-gas contact indicates sequential reactions in which the UO2+x product is further oxidized by H2O and ultimately reacts with H2 to form an oxide hydride.
Journal of Nuclear Materials, 2005
To evaluate the release of uranium from natural ore deposits, spent nuclear fuel repositories, and REDOX permeable reactive barriers (PRB), knowledge of the fundamental reaction kinetics associated with the dissolution of uranium dioxide is necessary. Dissolution of crystalline uranium (IV) dioxide under environmental conditions has been studied for four decades but a cardinal gap in the published literature is the effect of pH and solution saturation state on UO 2 (cr) dissolution. To resolve inconsistencies, UO 2 dissolution experiments have been conducted under oxic conditions using the single-pass flow-through system. Experiments were conducted as a function of total dissolved carbonate ð½CO À3 3 T Þ from 0.001 to 0.1 M; pH from 7.5 to 11.1; ratio of flow-through rate (q) to specific surface area (S), constant ionic strength (I) = 0.1 M, and temperatures (T) from 23 to 60°C utilizing both powder and monolithic specimens. The results show that UO 2 dissolution varies as a function of the ratio q/S and temperature. At values of log 10 q/S > À7.0, UO 2 dissolution becomes invariant with respect to q/S, which can be interpreted as evidence for dissolution at the forward rate of reaction. The data collected in these experiments show the rate of UO 2 dissolution increased by an order of magnitude with a 30°C increase in temperature. The results also show the overall dissolution rate increases with an increase in pH and decreases as the dissolved uranium concentration approaches saturation with respect to secondary reaction products. Thus, as the value of the reaction quotient, Q, approaches equilibrium, K, (with respect to a potential secondary phase) the dissolution rate decreases. This decrease in dissolution rate (r) was also observed when comparing measured UO 2 dissolution rates from static tests where r = 1.7 ± 0.14 · 10 À8 mol m À2 s À1 to the rate for flow-through reactors where r = 3.1 ± 1.2 · 10 À7 mol m À2 s À1 . Thus, using traditional static test methods can result in an underestimation of the true forward rate of UO 2 (cr) dissolution. These results illustrate the importance of pH, solution saturation state, and the concentration of dissolved carbonate on the release of uranium from UO 2 in the natural environment. Published by Elsevier B.V.
Journal of The Electrochemical Society, 2013
The influence of trivalent dopants (rare-earths, RE) on the structure, composition and electrochemical reactivity of UO 2 has been investigated using scanning electron microscopy (SEM/XRD), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy and cyclic voltammetry (CV). This was achieved by comparing the behavior of undoped UO 2.002 , a slightly doped 1.5 at% SIMFUEL, and two rare-earth doped UO 2 (6 wt% Gd-UO 2 and 12.9 wt% Dy-UO 2) specimens. The reactivity decreased in the order UO 2.002 > SIMFUEL > Dy-UO 2 > Gd-UO 2 , showing that this decrease is a consequence of RE III doping. Raman spectroscopy showed this could be attributed to the formation of RE III-oxygen vacancy clusters whose formation decreases the availability of the vacancies required to accommodate the injection of oxygen interstitials during anodic oxidation. The behavior of SIMFUEL is complicated by the simultaneous formation of RE III-oxygen vacancy clusters and Zr-O 8 clusters.