The influence of bicarbonate concentration and ionic strength on peroxide speciation and overall reactivity towards UO2

H2O2 produced from water radiolysis is expected to play a significant role in radiation induced oxidative dissolution of spent nuclear fuel under the anoxic conditions of a deep geological repository if the safety-barriers fail and ground water reaches the fuel. It was recently found that the coordination chemistry between U(vi), HCO32- and H2O2 can significantly suppress H2O2 induced dissolution of UO2 in 10 mM bicarbonate. This was attributed to the much lower reactivity of the U(vi)O22+-coordinated O22- as compared to free H2O2. We have extended the study to lower bicarbonate concentrations and explored the impact of ionic strength to elucidate the rationale for the low reactivity of complexed H2O2. The experimental results clearly show that dissolution of U(vi) becomes suppressed at [HCO3-] < 10 mM. Furthermore, we found that the reactivity of the peroxide in solutions containing U(vi) becomes increasingly more suppressed at lower carbonate concentration. The suppression is not influenced by the ionic strength, which implies that the low reactivity of O22- in ternary uranyl-peroxo-carbonato complexes is not caused by electrostatic repulsion between the negatively charged complex and the negatively charged UO2-surface as we previously hypothesized. Instead, the suppressed reactivity is suggested to be attributed to inherently higher stability of the peroxide functionality as a ligand to UO22+ compared to as free H2O2.

Formation and stability of studtite in bicarbonate-containing waters

Studtite and meta-studtite are the only two uranyl peroxides found in nature. Sparsely soluble studtite has been found in natural uranium deposits, on the surface of spent nuclear fuel in contact with water and on core material from major nuclear accidents such as Chernobyl. The formation of studtite on the surface of nuclear fuel can have an impact on the release of radionuclides to the biosphere. In this work, we have experimentally studied the formation of studtite as function of HCO3- concentration and pH. The results show that studtite can form at pH ≤ 10 in solutions without added HCO3-. At pH ≤ 7, the precipitate was found to be mainly studtite, while at 8 ≤ pH ≤ 9.8, a mixture of studtite and meta-schoepite was found. Studtite formation from UO22+ and H2O2 was observed at [HCO3-] ≤ 2 mM and studtite was only found to dissolve at [HCO3-] > 2 mM.

Exploring the Change in Redox Reactivity of UO2 Induced by Exposure to Oxidants in HCO3- Solution

Understanding the possible change in UO₂ surface reactivity after exposure to oxidants is of key importance when assessing the impact of spent nuclear fuel dissolution on the safety of a repository for spent nuclear fuel. In this work, we have experimentally studied the change in UO₂ reactivity after consecutive exposures to O₂ or γ-radiation in aqueous solutions containing 10 mM HCO₃^-. The experiments show that the reactivity of UO₂ toward O₂ decreases significantly with time in a single exposure. In consecutive exposures, the reactivity also decreases from exposure to exposure. In γ-radiation exposures, the system reaches a steady state and the rate of uranium dissolution becomes governed by the radiolytic production of oxidants. Changes in surface reactivity can therefore not be observed in the irradiated system. The potential surface modification responsible for the change in UO₂ reactivity was studied by XPS and UPS after consecutive exposures to either O₂, H₂O₂, or γ-radiation in 10 mM HCO₃^- solution. The results show that the surfaces were significantly oxidized to a stoichiometric ratio of O/U of UO_2.3 under all the three exposure conditions. XPS results also show that the surfaces were dominated by U(V) with no observed U(VI). The experiments also show that U(V) is slowly removed from the surface when exposed to anoxic aqueous solutions containing 10 mM HCO₃^-. The UPS results show that the outer ultrathin layer of the surfaces most probably contains a significant amount of U(VI). U(VI) may form upon exposure to air during the rinsing process with water prior to XPS and UPS measurements.

Stability of Studtite in Saline Solution: Identification of Uranyl-Peroxo-Halo Complex

Hydrogen peroxide is produced upon radiolysis of water and has been shown to be the main oxidant driving oxidative dissolution of UO₂-based nuclear fuel under geological repository conditions. While the overall mechanism and speciation are well known for granitic groundwaters, considerably less is known for saline waters of relevance in rock salt or during emergency cooling of reactors using seawater. In this work, the ternary uranyl–peroxo–chloro and uranyl–peroxo–bromo complexes were identified using IR, Raman, and nuclear magnetic resonance (NMR) spectroscopy. Based on Raman spectra, the estimated stability constants for the identified uranyl–peroxo–chloro ((UO₂)(O₂)(Cl)(H₂O)₂^–) and uranyl–peroxo–bromo ((UO₂)(O₂)(Br)(H₂O)₂^–) complexes are 0.17 and 0.04, respectively, at ionic strength ≈5 mol/L. It was found that the uranyl–peroxo–chloro complex is more stable than the uranyl–peroxo–bromo complex, which transforms into studtite at high uranyl and H₂O₂ concentrations. Studtite is also found to be dissolved at a high ionic strength, implying that this may not be a stable solid phase under very saline conditions. The uranyl–peroxo–bromo complex was shown to facilitate H₂O₂ decomposition via a mechanism involving reactive intermediates.

Aqueous solutions containing UO₂²⁺ and H₂O₂ are stabilized by the presence of chloride. This is attributed to the formation of uranyl−chloro and uranyl−peroxo−chloro complexes preventing the precipitation of studtite. The existence of these complexes was confirmed using IR, Raman, and NMR spectroscopies.

Kinetic Effects of H2O2 Speciation on the Overall Peroxide Consumption at UO2-Water Interfaces

The interfacial radiation chemistry of UO₂ is of key importance in the development of models to predict the corrosion rate of spent nuclear fuel in contact with groundwater. Here, the oxidative dissolution of UO₂ induced by radiolytically produced H₂O₂ is of particular importance. The difficulty of fitting experimental data to simple first-order kinetics suggests that additional factors need to be considered when describing the surface reaction between H₂O₂ and UO₂. It has been known for some time that UO₂ ²⁺ forms stable uranyl peroxo-carbonato complexes in water containing H₂O₂ and HCO₃ ^-/CO₃ ^2-, yet this concept has largely been overlooked in studies where the oxidative dissolution of UO₂ is considered. In this work, we show that uranyl peroxo-carbonato complexes display little to no reactivity toward the solid UO₂ surface in 10 mM bicarbonate solution (pH 8-10). The rate of peroxide consumption and UO₂ ²⁺ dissolution will thus depend on the UO₂ ²⁺ concentration and becomes limited by the free H₂O₂ fraction. The rate of peroxide consumption and the subsequent UO₂ ²⁺ dissolution can be accurately predicted based on the first-order kinetics with respect to free H₂O₂, taking the initial H₂O₂ surface coverage into account.

On the Stability of Uranium Carbide in Aqueous Solution-Effects of HCO3 - and H2O2

Uranium carbide (UC) is a candidate fuel material for future Generation IV nuclear reactors. As part of a general safety assessment, it is important to understand how fuel materials behave in aqueous systems in the event of accidents or upon complete barrier failure in a geological repository for spent nuclear fuel. As irradiated nuclear fuel is radioactive, it is important to consider radiolysis of water as a process where strongly oxidizing species can be produced. These species may display high reactivity toward the fuel itself and thereby influence its integrity. The most important radiolytic oxidant under repository conditions has been shown to be H₂O₂. In this work, we have studied the dissolution of uranium upon exposure of UC powder to aqueous solutions containing HCO₃ ^- and H₂O₂, separately and in combination. The experiments show that UC dissolves quite readily in aqueous solution containing 10 mM HCO₃ ^- and that the presence of H₂O₂ increases the dissolution further. UC also dissolves in pure water after the addition of H₂O₂, but more slowly than in solutions containing both HCO₃ ^- and H₂O₂. The experimental results are discussed in view of possible mechanisms.

Meta-studtite stability in aqueous solutions. Impact of HCO3-, H2O2 and ionizing radiation on dissolution and speciation

Two uranyl peroxides meta-studtite and studtite exist in nature and can form as alteration phases on the surface of spent nuclear fuel upon water intrusion in a geological repository. Meta-studtite and studtite have very low solubility and could therefore reduce the reactivity of spent nuclear fuel toward radiolytic oxidants. This would inhibit the dissolution of the fuel matrix and thereby also the spreading of radionuclides. It is therefore important to investigate the stability of meta-studtite and studtite under conditions that may influence their stability. In the present work, we have studied the dissolution kinetics of meta-studtite in aqueous solution containing 10 mM HCO3-. In addition, the influence of the added H2O2 and the impact of γ-irradiation on the dissolution kinetics of meta-studtite were studied. The results are compared to previously published data for studtite studied under the same conditions. 13C NMR experiments were performed to identify the species present in aqueous solution (i.e., carbonate containing complexes). The speciation studies are compared to calculations based on published equilibrium constants. In addition to the dissolution experiments, experiments focussing on the stability of H2O2 in aqueous solutions containing UO22+ and HCO3- were conducted. The rationale for this is that H2O2 was consumed relatively fast in some of the dissolution experiments.

On the change in UO2 redox reactivity as a function of H2O2 exposure

To assess the long-term leaching behaviour of UO₂, the main constituent of spent nuclear fuel, the oxidative dissolution of UO₂ pellets was studied at high H₂O₂ exposures ranging from 0.33 mol m^-2 to 1.36 mol m^-2. The experiments were performed in aqueous media containing 10 mM HCO₃^- where the pellets were exposed to H₂O₂ three consecutive times. The results indicate that the dissolution yield (amount of dissolved uranium per consumed H₂O₂) at high H₂O₂ exposures is significantly lower compared to previous studies of both pellets and powders and decreases for each H₂O₂ addition for a given pellet. This implies a change in redox reactivity, which is attributed to irreversible alteration of the pellet surface. Surface characterization after the exposure to H₂O₂, by SEM, XRD and Raman spectroscopy shows, that the surface of all pellets is significantly oxidized.

Alkali-metal ion coordination in uranyl(VI) poly-peroxo complexes in solution, inorganic analogues to crown-ethers. Part 2. Complex formation in the tetramethyl ammonium-, Li(+)-, Na(+)- and K(+)-uranyl(VI)-peroxide-carbonate systems

The constitution and equilibrium constants of ternary uranyl(vi) peroxide carbonate complexes [(UO2)p(O2)q(CO3)r](2(p-q-r)) have been determined at 0 °C in 0.50 M MNO3, M = Li, K, and TMA (tetramethyl ammonium), ionic media using potentiometric and spectrophotometric data; (17)O NMR data were used to determine the number of complexes present. The formation of cyclic oligomers, "[(UO2)(O2)(CO3)]n", n = 4, 5, 6, with different stoichiometries depending on the ionic medium used, suggests that Li(+), Na(+), K(+) and TMA ions act as templates for the formation of uranyl peroxide rings where the uranyl-units are linked by μ-η(2)-η(2) bridged peroxide-ions. The templating effect is due to the coordination of the M(+)-ions to the uranyl oxygen atoms, where the coordination of Li(+) results in the formation of Li[(UO2)(O2)(CO3)]4(7-), Na(+) and K(+) in the formation of Na/K[(UO2)(O2)(CO3)]5(9-) complexes, while the large tetramethyl ammonium ion promotes the formation of two oligomers, TMA[(UO2)(O2)(CO3)]5(9-) and TMA[(UO2)(O2)(CO3)]6(11-). The NMR spectra demonstrate that the coordination of Na(+) in the five- and six-membered oligomers is significantly stronger than that of TMA(+); these observations suggest that the templating effect is similar to the one observed in the synthesis of crown-ethers. The NMR experiments also demonstrate that the exchange between TMA[(UO2)(O2)(CO3)]5(9-) and TMA[(UO2)(O2)(CO3)]6(11-) is slow on the (17)O chemical shift time-scale, while the exchange between TMA[(UO2)(O2)(CO3)]6(11-) and Na[(UO2)(O2)(CO3)]6(11-) is fast. There was no indication of the presence of large clusters of the type identified by Burns and Nyman (M. Nyman and P. C. Burns, Chem. Soc. Rev., 2012, 41, 7314-7367) and possible reasons for this and the implications for the synthesis of large clusters are briefly discussed.

Alkali-metal ion coordination in uranyl(vi) poly-peroxide complexes in solution. Part 1: the Li+, Na+ and K+ – peroxide–hydroxide systems

The complex Na[(UO₂)(O₂)(OH)]₄³⁻ (aq.) can be modeled by a solid state structure element (a) and by quantum chemistry using a solvent model (b).

Carbonate–H2O2 leaching for sequestering uranium from seawater

Uranium adsorbed on amidoxime (AO)-based polyethylene fiber in simulated seawater can be quantitatively eluted at room temperature using 1 M Na₂CO₃ containing 0.1 M H₂O₂ without significant damage to fiber.

Biogeochemical behaviour and bioremediation of uranium in waters of abandoned mines

The discharges of uranium and associated radionuclides as well as heavy metals and metalloids from waste and tailing dumps in abandoned uranium mining and processing sites pose contamination risks to surface and groundwater. Although many more are being planned for nuclear energy purposes, most of the abandoned uranium mines are a legacy of uranium production that fuelled arms race during the cold war of the last century. Since the end of cold war, there have been efforts to rehabilitate the mining sites, initially, using classical remediation techniques based on high chemical and civil engineering. Recently, bioremediation technology has been sought as alternatives to the classical approach due to reasons, which include: (a) high demand of sites requiring remediation; (b) the economic implication of running and maintaining the facilities due to high energy and work force demand; and (c) the pattern and characteristics of contaminant discharges in most of the former uranium mining and processing sites prevents the use of classical methods. This review discusses risks of uranium contamination from abandoned uranium mines from the biogeochemical point of view and the potential and limitation of uranium bioremediation technique as alternative to classical approach in abandoned uranium mining and processing sites.