In this work, we have studied the reaction between H 2 O 2 and UO 2 with particular focus on the nature of the hydroxyl radical formed as an intermediate. Experiments were performed to study the kinetics of H 2 O 2 consumption and uranium dissolution at different initial H 2 O 2 concentrations. The results show that the consumption rates at a given H 2 O 2 concentration are different depending on the initial H 2 O 2 concentration. This is attributed to an alteration of the reactive interface, likely caused by blocking of surface sites by oxidized U/surface-bound hydroxyl radicals. The dissolution yield given by the amount of dissolved uranium divided by the amount of consumed hydrogen peroxide was used to compare the different cases. For all initial H 2 O 2 concentrations, the dissolution yield increases with reaction time. The final dissolution yield decreases with increasing initial H 2 O 2 concentration. This is expected from the mechanism of catalytic decomposition of H 2 O 2 on oxide surfaces. As the experiments were performed in solutions containing 10 mM HCO − 3 and a strong concentration dependence was observed in the 0.2-2.0 mM H 2 O 2 concentration range, we conclude that the intermediate hydroxyl radical is surface bound rather than free.
Radiation-induced oxidative dissolution
of uranium dioxide (UO2) is one of the most important chemical
processes of U driven
by redox reactions. We have examined the effect of UO2 stoichiometry
on the oxidative dissolution of UO2 in aqueous sodium bicarbonate
solution induced by hydrogen peroxide (H2O2)
and γ-ray irradiation. By comparing the reaction kinetics of
H2O2 between stoichiometric UO2.0 and hyper-stoichiometric UO2.3, we observed a significant
difference in reaction speed and U dissolution kinetics. The stoichiometric
UO2.0 reacted with H2O2 much faster
than the hyper-stoichiometric UO2.3. The U dissolution
from UO2.0 was initially much lower than that from UO2.3 but gradually increased as the oxidation by H2O2 proceeded. Increase in the initial H2O2 concentration caused decrease in the U dissolution yield
with respect to the H2O2 consumption both for
UO2.0 and UO2.3. This decrease in the U dissolution
yield is attributed to the catalytic decomposition of H2O2 on the surface of UO2. The γ-ray irradiation
induced the U dissolution that is analogous to the kinetics by the
exposure to a low concentration (2 × 10–4 mol
dm–3) of H2O2. The exposure
to higher H2O2 concentrations caused lower U
dissolution and resulted in deviation from the U dissolution behavior
by γ-ray irradiation.
In
radiolysis of water, three molecular products are formed (H2O2, O2, and H2). It has previously
been shown that aqueous hydrogen peroxide is catalytically decomposed
on many oxide surfaces and that the decomposition proceeds via the
formation of surface-bound hydroxyl radicals. In this work, we have
investigated the behavior of aqueous H2 and O2 in contact with ZrO2. Experiments were carried out in
an autoclave with high H2 pressure and low O2 pressure (40 and 0.2 bar, respectively). In the experiments the
concentration of H-abstracting radicals was monitored as a function
of time using tris(hydroxymethyl)aminomethane (Tris) as scavenger
and the subsequent formation of formaldehyde to probe radical formation.
The plausible formation of H2O2 was also monitored
in the experiments. In addition, density functional theory (employing
the hybrid PBE0 functional) was used to search for reaction pathways.
The results from the experiments show that hydrogen-abstracting radicals
are formed in the aqueous H2-/O2-system in contact
with solid ZrO2. Formation of H2O2 is also detected, and the time-dependent production of hydrogen-abstracting
radicals follows the time-dependent H2O2 concentration,
strongly indicating that the radicals are produced upon catalytic
decomposition of H2O2. The DFT study implies
that H2O2 formation proceeds via a pathway where
HO2
• is a key intermediate. It is interesting
to note that all the stable molecular products from aqueous radiolysis
are precursors of quite intriguing radical reactions at water/oxide
interfaces.
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