Abstract:International audienceThe evolution of plutonium dioxide surface due to water adsorption seems to influence H-2 generation through the radiolysis of adsorbed water. Surface evolution of ceria, a non-radioactive surrogate for plutonium dioxide, was investigated using Inverse Gas Chromatography (IGC), Raman spectroscopy, Environmental Scanning Electronic Microscopy (ESEM) and Atomic Force Microscopy (AFM). IGC highlights the complexity of ceria surface revealing three different adsorption sites on surface and in… Show more
“…24 It was found that the Ce(IV) complexes have comparable behaviors in term of structures, charge distribution and stabilities to those of Pu(IV), and that Ce(IV) is a priori a better surrogate for Pu(IV) for experimental studies than Th(IV) or U(IV). This conclusion is in line with the observed applicability of cerium dioxide CeO 2 as a surrogate for plutonium dioxide PuO 2steaming from their structural similarities and comparable redox behavior [64][65][66][67].…”
The prediction of cerium (Ce) aqueous speciation is relevant in many research fields. Indeed, Ce compounds are used for many industrial applications, which may require the control of Ce aqueous chemistry for their synthesis. The aquatic geochemistry of Ce is also of interest. Due to its growing industrial use and its release into the environment, Ce is now considered as an emerging contaminant. Cerium is also used as a proxy of (paleo)redox conditions due to the Ce(iv)/Ce(iii) redox transition. Finally, Ce(iv) is often presented as a relevant analogue of tetravalent actinides (An(iv)). In the present study, quantum chemical calculations were conducted to highlight the similarities between the structures of Ce(iv) and tetravalent actinide (An(iv); An = Th, Pa, U, Np, Pu) aqua-ions, especially Pu(iv). The current knowledge of An(iv) hydrolysis, solubility and colloid formation in water was briefly reviewed but important discrepancies were observed in the available data for Ce(iv). Therefore, new estimations of the hydrolysis constants of Ce(iv) and the solubility of Ce(iv)-(hydr)oxides are proposed, by analogy with Pu(iv). By plotting pH-Eh (Pourbaix) diagrams, we showed that the pH values corresponding to the onset of Ce(iv) species formation (i.e. Ce(iv)-(hydr)oxide or dissolved Ce(iv)) agreed with various experimental results. Although further experimental studies are required to obtain a more accurate thermodynamic database, the present work might yet help to predict more accurately the Ce chemical behavior in aqueous solution.
“…24 It was found that the Ce(IV) complexes have comparable behaviors in term of structures, charge distribution and stabilities to those of Pu(IV), and that Ce(IV) is a priori a better surrogate for Pu(IV) for experimental studies than Th(IV) or U(IV). This conclusion is in line with the observed applicability of cerium dioxide CeO 2 as a surrogate for plutonium dioxide PuO 2steaming from their structural similarities and comparable redox behavior [64][65][66][67].…”
The prediction of cerium (Ce) aqueous speciation is relevant in many research fields. Indeed, Ce compounds are used for many industrial applications, which may require the control of Ce aqueous chemistry for their synthesis. The aquatic geochemistry of Ce is also of interest. Due to its growing industrial use and its release into the environment, Ce is now considered as an emerging contaminant. Cerium is also used as a proxy of (paleo)redox conditions due to the Ce(iv)/Ce(iii) redox transition. Finally, Ce(iv) is often presented as a relevant analogue of tetravalent actinides (An(iv)). In the present study, quantum chemical calculations were conducted to highlight the similarities between the structures of Ce(iv) and tetravalent actinide (An(iv); An = Th, Pa, U, Np, Pu) aqua-ions, especially Pu(iv). The current knowledge of An(iv) hydrolysis, solubility and colloid formation in water was briefly reviewed but important discrepancies were observed in the available data for Ce(iv). Therefore, new estimations of the hydrolysis constants of Ce(iv) and the solubility of Ce(iv)-(hydr)oxides are proposed, by analogy with Pu(iv). By plotting pH-Eh (Pourbaix) diagrams, we showed that the pH values corresponding to the onset of Ce(iv) species formation (i.e. Ce(iv)-(hydr)oxide or dissolved Ce(iv)) agreed with various experimental results. Although further experimental studies are required to obtain a more accurate thermodynamic database, the present work might yet help to predict more accurately the Ce chemical behavior in aqueous solution.
“…Particularly, cerium dioxide has been largely investigated in the replacement of plutonium dioxide due to structural similarities and comparable redox behavior. [40][41][42][43][44] CeO 2 crystallizes in the fluorite-type structure (Fm3 ˉm space group) and exhibits close lattice parameters with a similar octacoordinated ionic radius (Ce 4+ = 0.97 Å and Pu 4+ = 0.96 Å). 45 Also, cerium can exist under +III and +IV oxidation states, in solid or in liquid media, which is very interesting to simulate PuO 2 behavior during reducing or complexing dissolutions.…”
PuO2 is considered an important material for current and future nuclear fuel; however it is a very refractive compound towards dissolution. Among other techniques, its reprocessing can be performed via complexing dissolution in concentrated and boiling nitric acid containing hydrofluoric acid, or via oxidant dissolution in the presence of reagents with redox couples having high potentials such as Ce(iv)/Ce(iii), or Ag(ii)/Ag(i). Reductive dissolution can be performed under softer conditions and is considered an alternative to these methods which may suffer from several drawbacks (corrosion, effluent management, compatibility with nuclear waste disposal, etc.). In this study, a sonochemical and reductive approach is investigated for PuO2 dissolution under relatively mild conditions. At the first stage, the experiments are performed with CeO2 as an inactive surrogate for PuO2. The quantitative dissolution of both oxides can be achieved under ultrasound (20 kHz, 0.35-0.70 W mL(-1)) in 0.5 M HNO3/0.1 M [N2H5NO3]/2 M HCOOH sparged with Ar at 33-35 °C in the presence of Ti particles as a generating source of reductive species. Ultrasound enables the depassivation of the Ti surface (usually strongly passivated in nitric solutions) through acoustic cavitation which then allows further generation of the intermediate Ti(iii) reductive species. Dissolution rates and yields can be further increased with the injection of dilute fluoride aliquots (NH4F or HF) in the sonicated solution to favor Ti chemical depassivation. The rapid and complete dissolution of PuO2 under selected conditions is accompanied by Pu(iii) accumulation in solution.
“…A comparison of Raman spectra (Fig. 6) of the Si substrate obtained by its successive dipping at first into the 3% H 2 O 2 solution for 30 sec and then into the Ce(NO 3 ) 3 and Mohr's salt solution has shown that besides the Fe-O stretching vibrations (221 and 287 cm −1 ) in iron (III) oxide-hydroxide [31, 32] and Ce-O stretching vibrations (402 and 490 cm −1 ) in cerium (II) oxide-hydroxide [17, 33], the mode at 830 cm −1 attributed to the stretching vibrations of Ce-O 2 - peroxide group [34] was present in the spectrum in the first case and was absent in the second one.Fig. 6Raman spectra of the initial film of hydroxide NPs synthesized via SILD at III step of the 29-th cycle and I step of the 30-th cycle.…”
An ultrafine α-CeO
2
–α-Fe
2
O
3
nanocomposite was prepared from the ultradispersed nanoparticles of cerium (IV) and iron (III) amorphous hydroxides heat-treated at 600 °С and 900 °С in the air. The initial composites were obtained by the successive ionic layer deposition (SILD) method. According to scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX) and powder X-ray diffraction (PXRD), the cerium/iron ratio in the synthesized nanocomposite is close to 1:2, and the α-CeO
2
and α-Fe
2
O
3
nanocrystals are isometrically shaped and have an average size of 4 ± 1 and 7 ± 1 nm (600 °С) and 24 ± 2 and 35 ± 3 nm (900 °С), respectively. Transmission electron microscopy (TEM) and selected area electron diffraction (SAED) have shown that nanocrystals are evenly distributed in the composite volume and are spatially conjugated. The formation mechanisms of both initial amorphous composites of cerium (IV) and iron (III) hydroxides and of α-CeO
2
and α-Fe
2
O
3
nanocrystals were established. It was shown that synthesis of the initial hydroxide composite using the SILD method proceeds via the formation of amorphous cerium hydroxo-peroxide (CeO(OOH)
2
). As a result of the study, a schematic mechanism for the formation of a composite based on ultrafine nanocrystals of cerium (IV) and iron (III) oxides has been proposed.
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