HomeChemistryUranium oxides structural transformation in human physique liquids

Uranium oxides structural transformation in human physique liquids

The dissolution experiments have been carried out for following oxides: UO2.05, U4O9, U3O8, UO3 as a result of their noticeable distinction of their Raman, XANES and EXAFS spectra. The construction of the synthesized oxides UO2+x sequence was additionally studied.

Raman spectroscopy and scanning electron microscopy (SEM)

Raman spectra of stoichiometric UO2, hyperstoichiometric UO2+x oxides and U4O9 together with SEM photos are proven in Fig. 1. Raman spectra of UO2, U4O9, U3O8 and UO3 earlier than and after dissolution experiment together with SEM photos earlier than and after dissolution are proven in Fig. 2.

Determine 2
figure 2

Raman spectra of uranium oxides earlier than and after publicity in simulated lungs (L) and gastro-intestinal (G) liquids: (a) UO2.05; (b) U4O9; (c) U3O8; (d) UO3. (e) SEM photos of UO2.05 earlier than and after dissolution.

Raman spectroscopy for characterization of synthesized uranium oxides

The Raman spectrum (see Fig. 1a) of stoichiometric uranium dioxide exhibits peaks at 445 and 1150 cm−1 similar to the triple degenerate Raman energetic vibration U–O T2g and the 2LO peak, which is dedicated to Γ5–Γ3 crystal area digital transition44. That proves its stoichiometry22,23,26,45,46,47. For hyperstoichiometric oxides, the looks of the height at 560 cm−1 was noticed, which corresponds to a first-order longitudinal optical phonon LO ranging from UO2.05. With additional saturation of the oxygen sublattice, a peak at 630 cm−1 is noticed, similar to the stretching vibration U–O A1g. Its look is as a result of distortion of the anionic sublattice within the fluorite construction because of incorporation of oxygen27,48. On this case, for UO2.10 oxide, the looks of the second peak seems like a broadening of the LO line. For UO2.15 the LO and A1g traces have the identical depth, whereas for UO2.20 the depth of the second peak turns into larger. That is in a great settlement with the peaks deconvolution outcomes (Desk 2, Appendix A4.1). Solely the traces similar to the T2g and A1g vibrations stay within the spectrum of U4O9. Raman shift presence in uranium oxides spectra is proven in Desk 2.

Desk 2 Uranium oxides Raman shift peaks traits.

The Raman spectrum (for Raman spectra of UO2.05, U4O9, U3O8 and UO3 see Appendix A5) of U3O8 accommodates traces similar to the vibrational modes B1g, B2g, B3g, and A1g within the area from 100 to 500 cm−149. The height within the area of 800 cm−1 corresponds to vertical vibrations of the uranyl cation or impurity uranium trioxide26,45,46. The Raman spectrum of uranium trioxide accommodates vibrational modes B1g, B2g, B3g and peaks in areas 590 cm−1 and 852 cm−1, which is in good settlement with the outcomes of earlier research of α-UO317.

Raman spectroscopy allows to detect adjustments within the crystal lattice, which is proven by a easy transition from the 560 cm−1 to the 630 cm−1 peaks within the UO2 to U4O9 vary and the looks of the brand new scattering traces within the spectra of oxides in larger oxidation states (see Appendix A8).

Comparability of uranium oxides Raman spectra earlier than and after dissolution experiment

The common particle dimension for the preliminary pattern UO2.05 was about 3 μm, as decided by dynamic mild scattering methodology, which signifies that the scale of particle aggregates was decided, since in accordance with the SEM the particle dimension is way smaller—round 500 nm. The values of the particular free floor established by processing the whole isotherms of nitrogen sorption at 77 Ok within the framework of the BET mannequin was slightly low—about 3 m2 g−1.

The particle dimension and the morphology adjustments in another way for the studied oxides (see Fig. 2e for SEM photos). Morphology of UO2.05 adjustments insignificantly, whereas the typical dimension modified from 500 to 300 nm. U4O9 particles morphology adjustments significantly—e.g. after gastrointestinal liquids particles develop into sharper, and after lung fluid it develop into flake-like. The common particle dimension decreases from 7 to 2 μm. For U3O8 common particle dimension adjustments insignificantly as do the morphology. UO3 morphology adjustments after the expose to lung fluid—it develop into needle-like. The common particle dimension adjustments insignificantly.

The spectra (see Fig. 2a and Appendix A7) of UO2.05 present the distinction in 445 cm−1 and 560 cm−1 peaks depth after publicity to each liquids. Whereas the 1050 cm−1 peak has roughly the identical depth, the 560 cm−1 peak will increase relative to 445 cm−1 after publicity indicative the elevated diploma of non-stoichiometry within the fluorite lattice as a result of a lower within the contribution of T2g vibration. The peaks maxima have the identical positions indicative the soundness of the crystal lattice and consequently the resistance of the UO2.05 in the direction of dissolution in physique fluids. The U4O9 spectra (see Fig. 2a and Appendix A7) earlier than and after the publicity to liquids of the gastrointestinal tract exhibits the height improve at 620 cm−1 relative to 445 cm−1, which signifies a rising diploma of non-stoichiometry within the lattice because it was proven for UO2.05 oxide.

The spectra (see Fig. 2a for Raman spectra, Appendix A6 for deconvolution outcomes, A7 for residual of spectra) of U3O8 develop into noisy after the exposition in liquids, particularly in lungs liquids. The 454 cm−1 mode, which isn’t detected within the preliminary and lung spectra, develop into a separate peak after the gastrointestinal tract liquid, in accordance with deconvolution outcomes. The Raman spectrum after the publicity to lung fluid can also be noisier than of preliminary U3O8. After the publicity to the gastrointestinal tract liquids, a noticeable line seems within the spectrum of U3O8 within the 500 cm−1 area, which can correspond to the first-order longitudinal optical phonon LO, which additionally seems in hyperstoichiometric UO2+x oxides along with defects within the fluorite construction or a threefold degenerate T2g vibration, which additionally indicative the looks of the UO2+x part. The positions of the traces (see Fig. 2a and Appendix A7) for UO3 after dissolution additionally adjustments considerably. After publicity to gastric fluid, the height within the 400 cm−1 area turns into narrower, which corresponds to Eg stretching within the P42/nmc lattice as a result of formation of cuboctahedral Willis clusters25. After publicity to the lung fluid, peaks at 818 cm−1 seem, as an alternative of the height at 852 cm−1 within the preliminary oxide and after gastrointestinal fluids, and 990 cm−1, which can correspond to the peaks of natural impurities50. Due to the noise showing within the spectra after pulmonary fluids as a result of presence of natural compounds, the proper interpretation of the spectra is troublesome, since low-intensity modes will likely be leveled by the sign from natural impurities.

Raman spectroscopy exhibits the best susceptibility to adjustments within the oxygen sublattice in uranium oxides, which is in line with the outcomes of earlier research17,22,25. The soundness of UO2.05 in all media contradicts with earlier research, which confirmed that because of stoichiometric uranium dioxide soaking, a UO2.25 part is fashioned on the floor51,52. This contradiction may very well be defined by the truth that in our work, the flow-through reactor was used within the experiment and pH of all of the liquids was decrease than it was launched in Torrero et al.52. Then the kinetics has extra affect in uranium dioxide floor oxidation. Ulrich et al. established the looks of a passivating UO2+x layer on the floor of the granules beneath oxidizing situations. Leinders et al.10 confirmed the formation of disordered U4O9 initially of the oxidation means of UO2.03 granules, after which ordering and additional oxidation occurred to type inclusions of amorphous uranium trioxide on the floor. As Raman spectroscopy is delicate in the direction of oxidative processes from the very starting within the case of hyperstoichiometric uranium dioxide, the absence of adjustments within the Raman spectra signifies the absence of oxidation processes on the particles floor. No floor adjustments in U4O9, in accordance with Raman spectroscopy are noticed, although the lattice dysfunction diploma will increase. For U3O8 we seen the presence of impurities in a extra diminished type of the oxide on the granules floor (peak within the area round 520 cm−1).

In keeping with the outcomes of Raman spectroscopy, publicity to lung fluid had the best impact on all oxides. This may very well be defined by the truth that the length of the experiment with lung fluid was for much longer: 34 days in comparison with 2 + 4 h in gastrointestinal fluids, which is in line with the earlier research of uranium ore dissolution in simulated lung fluids, the place the dissolution equilibrium was reached after 72–120 h53. Contemplating the pH of simulated liquids, we should always have anticipated the better adjustments after gastrointestinal liquids then after lung fluids. Above pH 7 the solubility of uranium dioxide decreases greater than 3 orders of magnitude52. Right here we will conclude that the length of the dissolution experiment has extra vital affect than pH on the adjustments of oxide floor properties.


To find out the oxidation state of uranium and its native molecular surroundings in oxides, X-ray absorption spectroscopy (XAS) measurements have been carried out earlier than and after the solubility experiment. XANES and EXAFS spectra together with becoming curves are proven in Figs. 3 and 4.

Determine 3
figure 3

(a) XANES spectra on the U L3-edge for uranium oxides in numerous oxidation states. (b, c) EXAFS spectra on the U L3-edge for uranium oxides in numerous oxidation states and their Fourier transforms. Stable traces are for experimental knowledge, dotted traces are for suits.

Determine 4
figure 4

Comparability of U L-3 edge XAS knowledge for preliminary and uncovered to lung (L), gastric and intestinal (G) simulated fluids uranium oxides: (a)—XANES; (b, d, f, h)—EXAFS spectra; (c, e, g, i)—corresponding Fourier transforms of the EXAFS spectra.

XANES and EXAFS for characterization of synthetized uranium oxides

The place of the white line of the spectra of uranium oxides from UO2 to U4O9 noticeably shifts (by 1–2 eV) in the direction of larger power when the sublattice is saturated with oxygen inside 5 eV, which is in line with the outcomes of earlier research14,15,54,55,56,57,58,59. The post-edge buildings of those spectra have an identical form with an area most at 17,209 eV. Upon oxidation to U3O8, the power place of the white line adjustments extra dramatically—i.e. by 10 eV in comparison with UO2. On this case, the height form is similar, and the primary post-edge characteristic is within the area of 17,230 eV, which is within the larger power area in comparison with oxides with a decrease oxidation state of uranium. The place of the white line within the spectrum of uranium trioxide stays the identical as within the spectrum of U3O8, nevertheless, the form of the primary absorption peak is much less symmetrical: the correct edge is flatter, so the height has the form of a triangle. Submit-edge buildings usually are not noticed within the spectrum of uranium trioxide (Fig. 3a).

As further oxygen atoms are integrated into the interstitial positions of fluorite kind crystal lattice of the UO2.0 ensuing within the formation of non-stoichiometric sequence UO2+x, the coordination quantity (CN) of O atoms of the primary coordination sphere will increase. To keep in mind the rise of CN as a result of incorporation of further O within the construction, a linear dependence of the CN as a perform of the diploma of oxide of oxidation, x, is launched within the becoming fashions of UO2+x. The reference factors for the linear extrapolation are the stoichiometric oxides UO2.0 (CN = 8) and U4O9 (CN = 11), as their construction and CN are well-known from diffraction knowledge.

In keeping with EXAFS (Appendix A9), UO2.05 stands out of your complete sequence of oxides when it comes to the primary coordination sphere parameters. Within the Fourier remodel magnitude of the spectrum, a peak with double most is clearly noticed at a distance of 1.4–2.2 Å, which corresponds to separate O coordination sphere with distances of two.24 and a couple of.39 Å. The inclusion of a small quantity of further oxygen causes the displacement of O atoms from common positions of fluorite-type lattice (2.34 Å) each up and down. On this case, the shift to the smaller distances is 2 occasions larger (~ 0.1 Å) then to the bigger aspect (0.05 Å). Quite the opposite, in oxides with x = 0.10–0.20, the native surrounding stays roughly unchanged and is extra in line with the idea of together with further atoms within the fluorite construction. For these oxides, the fluorite oxygen sublattice is retained at a distance attribute for UO2 with CN of about 8. An extra subshell seems at a ~ 2.8 Å similar to O atoms in defect positions. It needs to be famous that the Debye–Waller parameter will increase with the rise of oxides defectiveness, which is a pure consequence of construction disordering because the variety of defects will increase. The space to U coordination sphere is similar for all oxides of the sequence and coincides with the worth identified for the stoichiometric UO2 (3.87 ± 0.01 Å) from earlier EXAFS calculations60. A shift of 0.01 Å seems solely in UO2.20. Nevertheless, its significance is uncertain as a result of uncertainty of interatomic distances calculation from EXAFS being ± 0.01–0.02 Å. Based mostly on the values of the Debye–Waller elements, dysfunction within the uranium sublattice will increase considerably when x = 0.15 is reached, which is expressed by dramatic improve of σ2.

Becoming the EXAFS spectra of the sequence of non-stoichiometric oxides UO2+x with x starting from 0.05 to 0.20 and evaluating the parameters of the native surroundings with stoichiometric UO2 confirmed the next regularities (see Fig. 3b, c for EXAFS experimental and match knowledge and Appendix A9 for becoming parameters):

  • Small diploma of oxidation (x = 0.05) causes distortion of the preliminary fluorite construction and splitting of the oxygen sublattice into two with roughly equal CNs at distances 2.24 and a couple of.39 Å, whereas the uranium sublattice stays unchanged.

  • In oxides with excessive oxidation diploma (x = 0.10–0.20), the fluorite oxygen sublattice is steady and extra oxygen atoms seem in interstitial positions at a distance of ~ 2.8 Å from U. The Debye–Waller parameter of O subshells will increase with the defectiveness, which correlates with the rising distortion.

  • The uranium sublattice stays unchanged at a distance of three.87 ± 0.01 Å, constant to stoichiometric UO2. Vital disordering of this sublattice happens at x = 0.15, which is expressed by a big improve within the Debye–Waller parameter for UO2.15 and UO2.20.

XANES makes it doable to exhibit small adjustments within the oxidation state of uranium by the distinction within the place of the white line at a decision higher than 1 eV. EXAFS detected adjustments within the oxygen sublattice because of uranium oxidation for your complete sequence of oxides, which was additionally beforehand broadly mentioned20,54,58,59,61,62.

Uranium oxides XANES and EXAFS spectra earlier than and after dissolution experiment

In keeping with XANES spectra, the white line positions and the post-edge buildings of the spectra are preserved for all oxides earlier than and after publicity to gastrointestinal and lung fluids. This means the soundness of uranium oxidation state throughout soaking in liquids of organic significance for a time frame near the actual time the meals is within the gastrointestinal tract and for a couple of month within the lung fluid (Fig. 4a).

EXAFS spectra of UO2.05 earlier than and after publicity in gastrointestinal and lung liquids don’t present any vital adjustments within the native surroundings of uranium indicative the small affect of gastrointestinal or lungs liquid compositions on oxide conduct (see Fig. 4b, c for experimental and match EXAFS knowledge and Appendix A9 for becoming parameters).

An appropriate match of U4O9 was obtained on the construction solved from neutron diffraction (Appendix A2). At a distance of two.2–2.8 Å, contributions from three coordination spheres of O atoms are current, with a complete CN of 8. Further coordination spheres with a hard and fast whole CN of three seem at a bigger distance from U: 3.16 and three.39 Å. The uranium sublattice in U4O9 is preserved as in case of excellent fluorite construction and accommodates 12 U atoms at a distance of three.89 Å. The U-U distance will increase by 0.02 Å compared to UO2. That may be conditionally vital due to the uncertainty of interatomic distances dedication by the EXAFS on the stage of ± 0.01 Å.

The Fourier remodel of U4O9 after gastrointestinal liquids (U4O9_G) spectrum is visually nicely akin to the spectrum of preliminary U4O9. The positions of the maxima of the 2 fundamental peaks differ inside 0.01 Å, whereas within the case of U4O9_L (after publicity to lung fluid), the primary peak is shifted to decrease R-values by ~ 0.2 Å. The half-width of the peaks in U4O9_G is bigger indicative a rise of the Debye–Waller parameter. Consideration of the actual half individually highlights the coincidence of the peaks place similar to the contribution of the U-U scattering in all U4O9 samples.

Visible comparability of the EXAFS spectra of preliminary U4O9 and U4O9_G reveals no vital adjustments within the construction. Due to this fact, the becoming parameters obtained for untreated U4O9 are utilized to the spectrum of U4O9_G with the next adjustments, contemplating the chemistry of the processes and conclusions from the visible comparability of the spectra. ∆E0 was mounted to scale back the variety of variable parameters, which is motivated by the absence of serious structural adjustments within the oxide throughout publicity to gastrointestinal liquids. The approximation for the Debye–Waller parameter of O coordination shells in U4O9_G was taken from untreated U4O9. The coordination numbers of O coordination spheres are allowed to differ, retaining the general sum at CN = 11, as it’s decided from neutron diffraction of untreated U4O9.

The ensuing becoming mannequin is according to the anticipated barely affected construction of U4O9. The Debye–Waller parameter for U atoms will increase by 0.002 Å2, the space decreases by 0.01 Å. Such a change within the distance, once more, isn’t vital, contemplating the uncertainty in figuring out distances by EXAFS of ± 0.01 Å. The parameters of the oxygen spheres change extra noticeably: as an alternative of two distant subshells in untreated U4O9 at R > 3 Å, just one remained at 3.38 Å. The distances to the closest ones initially discovered at 2.82, 2.46 and a couple of.26 Å decreased by 0.26, 0.09, and 0.06 Å, respectively. With the size of the U–O bond lower, the distortions as a result of therapy with gastrointestinal fluid descends, which will be related to a stronger power of quick U–O bonds relative to elongated ones.

Thus, therapy of U4O9 with gastrointestinal fluids doesn’t result in vital adjustments in its construction. This course of has a big impact on the oxygen sublattice: the distances to the closest subshells diminished, and this impact is extra pronounced for coordination spheres with lengthy U–O bond lengths. This result’s defined by the better energy of the shortened U–O bonds, which signifies that they’re much less vulnerable to the publicity in studied liquids (see Fig. 4d, e for experimental and match EXAFS knowledge and Appendix A9 for becoming parameters).

The spectrum of U4O9 interacted with lung fluid, quite the opposite, exhibits a big impact of therapy on the native molecular surroundings. The actual half and the magnitude of the Fourier remodel exhibits vital variations each within the positions and intensities of the primary peaks. Because of the visually distinguishable variations between the 2 spectra, when becoming the EXAFS of the uncovered oxide, the construction of the preliminary U4O9 isn’t used as the primary approximation. Within the U4O9 oxide uncovered to lung fluid a coordination sphere of two.5 O atoms seems at a a lot shorter distance of two.12 Å, which drops out of the vary of attribute values within the preliminary construction of U4O9. Such quick distances usually are not described throughout the framework of the mannequin of formation of cuboctahedral clusters when further O atoms are included within the fluorite construction. The looks of a coordination sphere, uncharacteristic for U4O9, suggests {that a} rearrangement of the construction upon extended interplay with lung fluid happens. Related quick distances to O atoms, nevertheless, are current within the buildings of U4O9-y defect phases constructed throughout the mannequin of ordered quad-interstitials clusters62. Based mostly on DFT calculations, non-stochiometric U4O9-y phases are predicted to be extra steady, than U4O9. Specifically, it’s proven that U4O8.889 compound is essentially the most steady on this sequence63. Its construction was taken because the preliminary approximation when becoming the U4O9 after lung fluid spectrum. On this mannequin the entire CN of all subshells of O atoms, aside from the primary one, was mounted at 10, in order that the sum throughout the error is near 11. Splitting of U coordination shell considerably improved the match. Whole CN of two U subshells was constrained to 12. In consequence, O coordination shells are positioned at distances of two.12, 2.31, 2.54, and a couple of.76 Å. Within the uranium sublattice, on the “fluorite” distance of three.88 Å, similar to the construction of the preliminary oxide, solely part of U atoms stays with a CN of about 5. The remaining 7 U atoms contribute to the spectrum at a distance of 4.09 Å. The end result obtained is in better settlement with the construction of the nonstoichiometric U4O9-y part than with the construction of the stoichiometric U4O9. Contemplating that63, it may be assumed that because of a long-term therapy of U4O9 with lung fluid, the construction is reformed right into a extra steady one, which isn’t noticed within the case of short-term therapy with gastrointestinal fluid.

EXAFS spectra of the preliminary U3O8 is fitted in accordance with the construction by Siegel42. The very best match is obtained contemplating solely a single coordination shell of equatorial O atoms at 2.23 Å and a couple of axial O at 1.98 Å. Though calculated interatomic distances are ~ 0.1 Å shorter than structural ones, the becoming curve agrees nicely with the info. Elongation of U–O distances and splitting of Oeq. coordination sphere has resulted in worse match. Debye–Waller parameters are fairly massive, which suggests excessive disordering of O atoms. The very best match of the U coordination sphere is obtained with 3 U subshells in accordance with the layered construction of U3O8: 2 and 4 atoms at 3.70 and three.91 Å, respectively, signify neighboring U atoms inside a layer, 2 U atoms at 4.20 Å are U of the adjoining sheets. The obtained becoming mannequin agrees nicely each with knowledge and identified structural info.

The comparability of an actual elements of Fourier remodeled EXAFS spectra of U3O8 samples earlier than and after therapy clearly signifies the ordering of O subshells after gastrointestinal liquids. Inconsistencies at 3.5–4 Å counsel that some adjustments happen within the nearest U subshell that accommodates the contribution of in-layer U atoms. The general form of the EXAFS spectra signifies that no vital adjustments within the construction occurs after the publicity of U3O8 to gastrointestinal and lung liquids. Due to this fact, EXAFS mannequin obtained for untreated U3O8 is utilized to samples of handled U3O8. In U3O8_L, O coordination spheres appeared at 0.03–0.05 Å shorter distances and with decrease Debye–Waller elements, verifying the suggestion drawn from the form of FT actual half spectra. Distance to the primary U subshell will increase by 0.03–0.05 Å, whereas Debye–Waller elements stays the identical (see Fig. 4f, g for experimental and match EXAFS knowledge and Appendix A9 for becoming parameters).

In keeping with the EXAFS evaluation, soaking course of results in ordering of U–O subshells, lower of U–O distances and barely impacts U sublattice in U3O8. That’s in line with the suggestion about larger resistance of U3O8 beneath oxidizing situations64.

Fourier transforms of α-UO3 samples additionally look equally, besides of the pattern handled with lung fluid, which demonstrates larger background at low R. The untreated pattern of α-UO3 is fitted contemplating layered construction with 2 axial O atoms at shorter distance and 6 O atoms in equatorial airplane at 2.2–2.8 Å. CN for the axial O subshell is saved mounted equal to 2. For equatorial O CN is allowed to differ in order that the sum stays equal to six. Break up of the equatorial subshell is allowed provided that it led to the development of the match high quality. Following this method, 3 coordination subshells are launched within the remaining match: at 2.28, 2.56 and a couple of.76 Å. CNs for U-U are saved fixed in accordance with the construction and obtained R and σ2 are affordable. The similarity of spectra in R-space signifies the absence of serious structural adjustments after soaking in gastrointestinal and lung liquids. Due to this fact, becoming mannequin obtained for α-UO3 is used as the primary approximation within the becoming process of the spectra of handled samples. Calculated parameters exhibit the absence of basic structural adjustments after the publicity of UO3 to gastrointestinal and lung liquids, with solely minor adjustments in CN ratios in Oeq. subshells (see Fig. 4h, i for experimental and match EXAFS knowledge and Appendix A9 for becoming parameters).

Options at low R round 1 Å usually are not described by EXAFS equation and are attributed to background or truncation results. They haven’t any bodily sense and due to this fact weren’t thought of within the becoming process.

The obtained knowledge confirms the soundness of α-UO3 with respect to the therapy with human physique liquids, which isn’t the case of the opposite oxides, i.e. U4O9, U3O8 and UO2.05. In keeping with the EXAFS becoming outcomes, in U3O8 and UO2.05 the next ordering of O sublattice occurred. Essentially the most outstanding adjustments are noticed in U4O9 uncovered to lung liquid for 34 days: the preliminary construction endure transformations and type totally different, however extra steady construction presumably of non-stochiometric U4O9-y part. Vital lattice ordering as a result of fluid publicity occurred just for U4O9 within the simulated lung fluid, whereas the lattice parameters modified for different oxides just for 0.01–0.02 angstroms. Throughout oxidation in a sequence UO2+x a lower within the interatomic distance happens with a rise within the saturation of the lattice with oxygen58.



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