U redox state and speciation of U in contact with magnetite nanoparticles : High resolution XANES, EXAFS, XPS and TEM study

Long-term storage of high-level radioactive waste is associated with potential radioecological hazards.
One chemical element of high interest is uranium (U), which can mainly exists as a mobile U(VI)
(oxidizing conditions) and sparingly soluble U(IV) (reducing conditions) species. It is expected that
the main inorganic reducing agent for U(VI) in the environment are ferrous species in magnetite,
formed on the steel canisters surface as an intermediate iron (Fe) corrosion product [1]. Results
obtained from laboratory experiments for the interaction of U(VI) with magnetite nanoparticles point
to partial reduction of U(VI) [2] or the formation of ~3 nm uranium dioxide (UO₂) particles on the
surface layer [3]. The evidence for U(VI) reduction to intermediate U(V) state was found with no
direct evidence of U(IV), which is in contradiction with thermodynamic calculations [4]. Continuous
interaction and related phase dissolution/recrystallization processes can also lead to U redox changes
and structural U incorporation into Fe oxides, resulting in U immobilization [5]. U redox state and
speciation analyses are still very challenging due to simultaneous formation of several different
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species in such mineral systems. New advanced spectroscopic methods for characterization of such
systems will provide more precise results from speciation studies. The main goal of our investigation
is to assess the U M4 edge high energy resolution X-ray absorption near edge structure (HR-XANES)
spectroscopy technique for detection of U(V) possibly co-existing with U(IV) and U(VI) under
reducing conditions on/in Fe containing minerals. The U M4 edge HR-XANES has an advantage
compared to the conventional U L3 edge XANES, as the measured spectra are less dominated by corehole lifetime broadening effects and therefore have narrower spectral features [6-8]. This technique
facilitates the detection of minor contribution of one oxidation state in mixtures.
We have investigated the U redox states and speciation in a set of samples where U coprecipitated
with magnetite nanoparticles (~ 20 nm) with U concentrations varying in the 1000-10000 ppm range
(1000, 3000, 6000 and 10000 ppm). In addition to U M4 edge HR-XANES, U L3 edge extended X-ray
absorption fine structure (EXAFS), X-ray photoelectron spectroscopy (XPS) and transmission electron
microscopy (TEM) techniques have been applied. The studied system models the interaction of U(VI)
with magnetite in aqueous solution, important for the understanding of the retarding effect of Fe
corrosion products on U in the context of deep geological spent nuclear fuel disposal. These
spectroscopic results can be compared with thermodynamic calculations and geochemical models
describing this interaction.
After 10 days U interaction with magnetite U M4 edge HR-XANES results indicate the formation of
U(IV), U(V) and U(VI) mixtures in varying ratios, depending on the initial U loading. Going from
10000 to 3000 ppm, the U(VI) content decreases continuously and is no longer found in the 1000 ppm
sample. At the same time the U(IV) and U(V) fractions increase. U(V) is stabilized as the main U
redox state in the 1000 ppm sample along with a smaller U(IV) contribution. After 20 days of contact
time XPS data show the predominance of U(IV) and U(V) species in the 6000 ppm sample. However,
mostly U(V) and some U(IV) is found for the 1000 ppm sample. For all samples aged for 240 days U
L3 XANES and EXAFS strongly suggest the formation of a UO₂ phase, UO₂ is the dominating species
in the 10000 ppm sample with U-O bond distance 2.33. Å as determined by EXAFS. UO₂ crystalline clusters with about 5 nm size formed on the surface of the magnetite nanoparticles are also found by TEM in the 10000 and 3000 ppm samples. The major and minor contributions of U(V) and U(IV), respectively, for the 1000 ppm sample after 240 days confirm the assumption that the U redox kinetics has completed within less than 10 days at this U concentration. EXAFS analyses reveal U(V)-Fe interaction in the second U coordination sphere, which substantially increases from the 10000 to 1000 ppm sample and is the dominating species in the 1000 ppm sample.