None of the many elements of the periodic table affected the course of history as plutonium did: the Manhattan project during the Second World War, the Trinity project, the Cold War, the Chernobyl disaster. All these historical events have accumulated a desire for greatness, which too often involves great sacrifices. Unfortunately, some big discoveries lead to big consequences. Environmental pollution by radioactive waste is far from the first of many environmental problems at this stage of human development, but you should not forget about it. Today we will meet with you an amazing discovery that happened absolutely by accident, as a result of which a new solid and stable phase of plutonium - Pu (V) was revealed. How did this random discovery happen, what is so unusual in the new phase of plutonium, and what is the significance of this study for ecology? We learn about this from the report of the research group. Go.
Random randomness
The protagonist of this discovery is the physicist from the Helmholtz Center Dresden-Rossendorf (HZDR) Christina Kvashnina, who, together with her team, conducted a series of experiments within the walls of the ESRF (European Synchrotron in Grenoble, France).
Dr. Kvashnina at work.
The main objective of the study by Kvashnina was the creation of a plutonium dioxide nanoparticle using various precursors * and their further study.
Precursor * - a substance involved in the reaction, which leads to the formation of the target substance.During the next experiment, the Pu (VI) precursor was used, but the reaction halfway went wrong. At first, most of the scientists in her group decided that the experiment had failed, and it was worth starting over. But a more detailed examination of the results of this “failure” led to the identification of Pu (V), pentavalent plutonium, which was not previously observed.
Nobody was ready to believe in the discovery of the stable phase of Pu (V) without solid evidence, because additional experiments, measurements and analysis of the data were carried out. The main method for confirming the discovery was HERFD (high-precision fluorescence analysis).
Study basis
One of the unpleasant features of plutonium is that it can easily spread in the form of colloids * for many kilometers from the point of entry into the ground through groundwater.
Colloid * is a cross between a true solution and a suspension.In addition, plutonium is readily absorbed by clays, iron oxides, or natural organic substances. Thus, in places where plutonium has been around for quite some time (storage of nuclear fuel or nuclear waste, etc.), PuO 2 is often formed.
Plutonium
Scientists are very interested in studying this kind of colloidal nanoparticles in order to find a way to reduce the degree of radioactive contamination. The most discussed issues in this area are the structure of nanoparticles (crystalline or still amorphous), as well as the presence of Pu (V) in small nanoparticles (<3 nm). More and more research is being carried out, but there are no less questions.
One of the main properties of the chemical behavior of Pu is the diversity of its oxidation states, which are determined by the number of electrons removed from the valence orbitals of the neutral atom. Pentavalent plutonium in the oxidized state has 3 electrons in the 5f shell, leaving the 6d orbitals empty. Therefore, the oxidation state of Pu determines its chemical behavior and speciation. Four oxidation states (III to VI) can coexist under environmental conditions. There is an assumption that the degree of (VII) and even (VIII) may be stable in highly alkaline conditions.
The oxidation states of the aqueous, solid, and interfacial Pu species were previously determined by XANES * spectroscopy using the L3 edge of Pu.
XANES * is a fine X-ray absorption structure.The application of this method allows one to determine Pu (V), since its spectrum always shows a characteristic shift of energy toward lower energies, in contrast to the spectra of Pu (IV) and Pu (VI).
The edge of the absorption band * is the value of the energy of electromagnetic radiation, in the case of exceeding which there is a sharp increase in the absorption of this radiation by the investigated substance.
The decoding of XANES energy data can be improved by applying HERFD fluorescence analysis. However, there are some difficulties: at the L3 edge of plutonium, electrons are excited from the 2p level of the nucleus to the 6d level, which is always not occupied regardless of the degree of Pu oxidation. Therefore, applying the L3 edge will not give accurate results. Therefore, it was decided to apply the edge of M4. X-ray absorption at the M4 edge of actinides explores the 5f states through transitions from the 3d core level.
Image No. 1
Figure 1a shows the first experimental HERFD data for Pu IV O 2 and KPuVO 2 CO 3 (s) systems with oxidation states of Pu (IV) and Pu (V), respectively.
* (s) - (solid - solid phase)Data was collected using an X-ray emission spectrometer with a set radiation index of 3534 eV. The HERFD spectrum of PuO 2 clearly shows two intense peaks, at ~ 3970.2 eV and ~ 3971.8 eV. According to the results of calculations ( 1b ) carried out within the framework of the Anderson solitary impurity model, the intensity and energy of these two peaks are the result of many factors: the strength of intraatomic and crystalline interactions, and the degree of hybridization Pu 5f - 2p ligand in the ground and final states of the spectroscopy process. Compared to PuO 2 , the HERFD spectrum of KPuO 2 CO 3 (s) shifts toward higher energy and shows a narrow profile with an asymmetric shape and a shoulder on the higher energy side.
Figure 1b shows the results of calculations using the Anderson model, which are in excellent agreement with the experimental HERFD spectrum of KPuO 2 CO 3 (s) , confirming the presence of a pentavalent oxidation state of Pu in KPuO 2 CO 3 (s) .
Due to the rules for choosing dipoles (J = 0; ± 1), it is expected that the HERFD shape of the Pu M4 and M5 junctions will be different. At the M5 edge of Pu, unoccupied 5f electron levels with J = 5/2 and 7/2 can be reached by an electron excited from the Pu 3d 5/2 state, while only the state J = 5/2 can be reached at the M4 edge of plutonium .
It was found that the energy shift between Pu (III), Pu (IV) and Pu (V) in solid compounds is about 2 eV between Pu (III) and Pu (IV) and 0.4 eV between Pu (IV) and Pu (V ) It follows that in order to more accurately determine the degree of plutonium oxidation, it is necessary to improve the energy resolution of the absorption spectra.
So, as we already know, image 1a shows the experimental HERFD data recorded at different stages during the synthesis of PuO 2 nanoparticles from the aqueous Pu (VI) precursor.
First, ammonia was added to the Pu (VI) solution, resulting in a pH value of 11. The kinetics of the conversion of Pu (VI) to PuO 2 demonstrates a two-stage process. During the first minutes, the formation of an intermediate Pu phase consisting of yellow sludge was observed (image below).
Image No. 2: kinetics of the conversion of Pu (VI) to PuO 2 .
Later, during the formation of PuO 2 nanoparticles, this intermediate phase was dissolved and another equilibrium phase formed, called the “final phase”. The blue line in 1a shows the HERFD spectrum recorded in the intermediate reaction step. From this spectrum, the oxidation state of Pu (V) is visible.
In addition, the HERFD spectrum of the final reaction product formed after 3 weeks of precipitation shows a profile identical to that found for the PuO 2 single crystal. This confirms that the reaction ends with the formation of PuO 2 nanoparticles with a cubic structure and with an oxidation state of Pu (IV).
Then, scientists carried out an ITFA (iterative analysis of the conversion factor) analysis, which allows to evaluate the contribution of various chemical states to the obtained HERFD data. The analysis results showed that the spectrum of the intermediate Pu phase contains 87% Pu (V) and 13% Pu (IV). A significant contribution of Pu (V) in the final phase was not observed, nor was the quantitative determination of Pu (VI). In other words, after the formation of PuO 2, Pu (V) nanoparticles were absent in the final phase altogether, in contrast to Pu (IV). Scientists also found that the restoration of Pu (VI) to Pu (IV) does not occur in one step. Pu (VI) is first reduced to Pu (V), and then to Pu (IV).
It was decided to conduct a series of additional HERFD and EXAFS (extended fine X-ray absorption fine structure) experiments at the L3 edge of Pu, which would allow us to identify the intermediate phase that manifests itself during the formation of PuO 2 nanoparticles.
Image No. 3
The graph above shows a comparison of the HERFD data for the L3 edge of Pu recorded for PuO 2 and the intermediate phase of Pu during the formation of PuO 2 nanoparticles.
As discussed earlier, the L3 spectrum of Pu (V) compounds always shows a characteristic shift toward lower energies and a decrease in the white line intensity * L3 compared to the systems Pu (IV) and Pu (VI).
White line * - in some XANES spectra, the growing edge of the absorption band can lead to a sharp intense peak, which is called the "white line".The chemical shift of the intermediate phase of Pu is clearly visible in the HERFD data and indicates the degree of oxidation of Pu (V).
Then, calculations based on the basic laws of the natural sciences were performed to identify the intermediate phase Pu during the synthesis of PuO 2 nanoparticles. The HERFD spectra of several compounds containing Pu were modeled. The best agreement was found for NH 4 PuO 2 CO 3 in which Pu is present in the pentavalent state. EXAFS analysis also confirmed that the intermediate Pu phase formed during the synthesis of PuO 2 nanoparticles is compatible with NH 4 PuO 2 CO 3 .
The intermediate phase NH 4 PuO 2 CO 3 was completely dissolved within ~ 10 hours, and then, as a result of longer redox reactions, PuO 2 nanoparticles were formed.
Finally, part of the intermediate phase Pu (V) was centrifuged from the suspension and dried at room temperature to check its stability. Surprisingly, the dried NH 4 PuO 2 CO 3 phase turned out to be quite stable even after a few months.
In all the experiments described above, the pH was 11. To check how much this affects the results, scientists conducted similar experiments, but with a pH of 8. As a result, it was found that the kinetics of Pu deposition is quite similar to the previous data, but the amount of intermediate Pu (V ) phase in this case is less.
A comparison of the experimental conditions with the available thermodynamic data shows that the Eh / pH values correspond to the stability region of the Pu (IV) phase. That is, the formation of the intermediate Pu (V) phase is quite possible, but the high thermodynamic stability of PuO 2 and its extremely low solubility lead to the further conversion of the Pu (V) phase to PuO 2 .
The result of this study is that although solid Pu (V) is always regarded as exotic compounds, a thermodynamically metastable solid phase of Pu (V) is formed by the reductive deposition of PuO 2 nanoparticles from the Pu (VI) precursor at pH 11. Scientists have also characterized for the first time an intermediate Pu (V) phase by HERFD.
For a more detailed acquaintance with the nuances of the study, I recommend that you look into the report of scientists and additional materials to it.
Epilogue
At first glance, it seems that such a discovery, even if by accident, does not carry any benefit in itself. However, the more we know about plutonium, the better will be our understanding of the processes that occur with its participation. Therefore, we will be able to more effectively eliminate the effects of radiation pollution.
In this particular study, it became known that the redox processes behind the conversion of Pu (VI) to PuO 2 nanoparticles and the formation of Pu (V) have a strong effect on the increase in solubility.
Studying the properties of plutonium allows us to understand how nuclear waste will behave after a long time, even after a million years. And the discovery of the stable solid phase Pu (V) unambiguously introduces certain corrections to such predictions.
Thank you for your attention, remain curious and have a good working week, guys. :)
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