A Novel Metastable Pentavalent Plutonium Solid Phase on the Pathway from Aqueous Plutonium(VI) to PuO2 Nanoparticles

Abstract Here we provide evidence that the formation of PuO2 nanoparticles from oxidized PuVI under alkaline conditions proceeds through the formation of an intermediate PuV solid phase, similar to NH4PuO2CO3, which is stable over a period of several months. For the first time, state‐of‐the‐art experiments at Pu M4 and at L3 absorption edges combined with theoretical calculations unambiguously allow to determine the oxidation state and the local structure of this intermediate phase.

Plutonium plays ap rominent role in nuclear energy production but nuclear accidents and nuclear weapons tests have led to the release of Pu and other hazardous isotopes into the environment in the past, and Pu contamination has been detected in waters and soils. [1] Based on such cases, several countries decided to shut down the operation of the oldest nuclear facilities and put effort into improving the safety of nuclear waste storage in order to prevent further release of radioactive nuclides into the environment. To progress in this direction, it is fundamental to deepen our basic knowledge of the chemistry of actinides in environmentally relevant conditions by making compounds,c haracterizing them, and understand them experimentally and theoretically.T hanks to the increased experimental sensitivity,r ecent cross-activities between theory and experiment, and different synthetic approaches,s uch ag oal becomes reachable.
In spite of the low solubility of the most prevalent environmental species,P uh as been shown to be transported by groundwater from contaminated sites for several kilometers in the form of colloids,w ith Pu being absorbed on clays, [2] iron oxides, [3] or natural organic matter. [4] In the nearfield conditions of geological repositories of spent nuclear fuel and other radioactive wastes,t he formation of intrinsic PuO 2 colloids is ak ey scenario. [5] Therefore,t he characterization of such intrinsic colloidal nanoparticles (NPs) in aqueous solution has recently received much attention. [6][7][8][9][10] Them ost debated question is the structural nature of these NPs (crystalline vs.amorphous) as well as the presence of Pu V and other oxidation states in small NPs (< 3nm). [9,[11][12][13][14][15] Va rious studies used different synthetic approaches and different solution conditions to examine aprecipitated product, either amorphous or crystalline.T his has led to ac ontroversy which has not been resolved. Fore xample,W alther et al. [14] observed evidence for multiple Pu oxidation states (III, IV,V )i nt he early stages of hydrolysis and polymerization of PuO 2 colloids at pH 0.5-1.0, while Rothe et al. [9] reported Pu IV oxyhydroxide-colloid formation. Conradson et al. [11] examined solid precipitates prepared by av ariety of synthetic approaches and argued for the presence of Pu V in nonstoichiometric PuO 2+x solids.
One of the most fundamental properties of the chemical behavior of Pu is the variety of its oxidation states.T he oxidation state is defined by the number of electrons that are removed from the valence orbitals of an eutral atom. In the pentavalent oxidation state,P uh as three electrons in the 5f shell, leaving the 6d orbitals empty.The oxidation state of Pu determines its chemical behavior and reactivity.F our oxidation states (from III to VI) may co-exist under environmental conditions,while oxidation states VII and VIII are proposed to be stable under highly alkaline oxidative conditions. [16] Oxidation states of aqueous,s olid-state,a nd interfacial Pu species have been previously determined using Pu L 3 edge [6,7,17] X-ray absorption near edge structure (XANES) spectroscopy.The Pu edge of the L 3 XANES spectrum of Pu V always shows ac haracteristic energy shift towards low energies compared to Pu IV and Pu VI XANES spectra. The experimental energy resolution of the recorded XANES data can be improved if the spectra are recorded in the high energy resolution fluorescence detection (HERFD) mode. [8] Nevertheless,a tt he Pu L 3 edge,t he electrons are excited from the 2p core level to the 6d level, which is always unoccupied independent of the Pu oxidation state.F or uranium systems, we have previously shown that HERFD experiments at the U M 4 edge [18][19][20] are much more informative on the oxidation state and electronic structure than measurements at the L edges.X-ray absorption at the M 4 edge of actinides probes 5f states via transitions from the 3d core level. To our knowledge,H ERFD data at the Pu M 4 edge have never been reported in the literature and have never been exploited. Figure 1a shows the first experimental HERFD data at the Pu M 4 edge for the Pu IV O 2 and KPu V O 2 CO 3(s) (solid) systems with Pu IV and Pu V oxidation states,respectively.Data were collected with an X-ray emission spectrometer [21] set to the maximum of the Mb emission line at 3534 eV.S ynthesis procedures and the characterization of both materials are reported in the Supporting Information. TheH ERFD spectrum of PuO 2 clearly shows two intense peaks,at% 3970.2 eV and % 3971.8 eV.A ccording to the results of calculations carried out in the framework of the Anderson impurity model (AIM;F igure 1b), [22][23][24] the intensity and energy of these two peaks are ar esult of multiple factors,s uch as the strength of the intra-atomic and crystal-field interactions,and the degree of the Pu 5f/ligand 2p hybridization in the ground and final states of the spectroscopic process.Incomparison with PuO 2 , the HERFD spectrum of KPuO 2 CO 3(s) shifts towards higher incident energies and shows an arrow profile with an asymmetric shape and as houlder at the higher incident energy side.T he results of the AIM calculations reported in Figure 1b show ag ood agreement with the experimental KPuO 2 CO 3(s) HERFD spectrum, confirming the presence of the pentavalent Pu oxidation state in KPuO 2 CO 3(s) .
Due to dipole selection rules (J = 0;AE 1), the shape of the Pu M 4 and M 5 HERFD transitions is expected to be different. At the Pu M 5 edge,t he unoccupied 5f electronic levels with J = 5/2and 7/2can be reached by an electron excited from the Pu 3d 5/2 state,whereas only the J = 5/2state can be reached at the Pu M 4 edge. [25] Ac omparison between Pu M 4 and Pu M 5 spectra for several Pu systems is shown in Figure S1 (Supporting Information). Thee nergy shifts between Pu III ,P u IV , and Pu V in solid compounds are found to be in the order of 2eV( between Pu III and Pu IV )a nd 0.4 eV between Pu IV and Pu V (Table S1). Ac orrect determination of the Pu oxidation state therefore requires the improved energy resolution of the absorption spectra provided by HERFD. Figure 1a shows experimental HERFD data recorded at different stages during the synthesis of PuO 2 NPs from the aqueous Pu VI precursor.F or this purpose,as olution of Pu VI was added to an excess of ammonia. Themeasured pH value of the solution was 11. We kinetically traced the route of the Pu VI -to-PuO 2 transformation as atwo-step process:during the first minutes,w eo bserved the formation of an intermediate Pu phase consisting of yellow sludge (see Figure 2). Later, during the formation of PuO 2 NPs,t he intermediate phase dissolved and ad ifferent equilibrium phase (named "final phase" in the following) was formed. [26] TheP uM 4 HERFD spectrum recorded at the intermediate stage of the reaction is represented by the blue curve in Figure 1a.T he spectrum clearly indicates the presence of the Pu V oxidation state.T his is supported by the good correspondence between energy and the relative intensities of the main features of the Pu M 4 edge spectrum for KPuO 2 CO 3(s) (green curve) and the Pu intermediate phase (blue curve).
Furthermore,t he HERFD spectrum of the final product of the reaction, formed after 3weeks of the precipitation Figure 1. a) ExperimentalH ERFD data at the Pu M 4 edge from two plutonium phases obtained during the synthesis of PuO 2 nanoparticles (NPs) from aPu VI precursor at pH 11. Blue curve:s pectrum of the intermediateP u V solid phase appearing during the synthesis of the PuO 2 NPs;r ed curve:spectrum of the final phase of PuO 2 NPs. The spectra of aPuO 2 bulk sample (grey curve) and of KPuO 2 CO 3(s) (green curve) are also shown as references for Pu IV and Pu V oxidation states, respectively.Data were collected with an X-ray emissions pectrometer set to the maximum of the Mb emission line at 3534 eV.b )Experimental HERFD spectra of PuO 2 and KPuO 2 CO 3(s) compared with the results of Anderson impurity model calculations.
reaction, shows an identical profile to the one detected for PuO 2 single crystal, confirming that the reaction terminates with the formation of PuO 2 NPs with cubic structure and with the Pu IV oxidation state,asreported by Soderholm et al. [15] for Pu 38 clusters (Li 14 8 ]) isolated from the initially alkaline peroxide solution. [15] Thee xperimental data collected for the intermediate phase during the PuO 2 NPs synthesis show evidence of the Pu V oxidation state.T he exact contribution of the different chemical states in the Pu M 4 HERFD data reported in Figure 1a was estimated by the ITFAprogram. [27] Theresults indicate that the spectrum of the intermediate Pu phase contains 87 %o fP u V and 13 %o fP u IV (with an estimated root-mean-square error of less than 2%,s ee Figure S2). We did neither observe as ignificant contribution of Pu V in the final phase (after the PuO 2 NPs were formed) nor aquantifiable amount of Pu VI (Table S2). Thea bsence of Pu V in the final phase and the 100 %presence of the Pu IV oxidation state after the PuO 2 NPs formation is an important result. At the same time,our data demonstrate that Pu VI -to-Pu IV reduction does not occur in asingle step. [26] ThePu VI is first reduced to Pu V and then to Pu IV .
Moreover,additional HERFD and EXAFS (extended Xray absorption fine structure) experiments at the Pu L 3 edge gave us the opportunity to identify the intermediate phase forming in the course of the PuO 2 NPs growth. Figure 3shows the comparison of the Pu HERFD L 3 edge data recorded for PuO 2 and the intermediate Pu phase during the PuO 2 NPs formation. As discussed previously,t he L 3 spectrum of Pu V compounds always shows av ery characteristic energy shift towards low energies and ad ecrease of the L 3 white line intensity compared to Pu IV and Pu VI systems [6][7][8]17] (Figure S3). Thec hemical shift of the intermediate Pu phase is clearly resolved in the HERFD data reported in Figure 3a nd indicates the presence of the Pu V oxidation state,i na greement with the Pu M 4 HERFD results.H owever, for actinide systems,H ERFD at the L 3 edge is not as sensitive as the M 4 edge HERFD to the presence of minor contributions (< 10 %) from different oxidation states. [28,29] HERFD at the L 3 edge is,however, extremely sensitive to the local structure around the absorber,w hich results in specific post-edge features. [19,30,31] Ab-initio calculations on different structures were used to identify the intermediate Pu phase during the synthesis of the PuO 2 NPs.W es imulated the HERFD spectra of several compounds containing Pu (Figures S4 and S5) in order to determine the Pu speciation of the intermediate Pu phase structure.T he best agreement is found for NH 4 PuO 2 CO 3 in which Pu is present in the pentavalent state.T he HERFD spectral shape reflects the d-density of states (DOS) of Pu apart from the small shoulder at the absorption edge,which is barely visible in the data but well resolved in the simulation and represents the Pu 5f DOS ( Figure S6). ThePud -DOS is involved in the bonds with O, C, and N. TheP uL 3 EXAFS data confirmed that the intermediate Pu phase formed during the PuO 2 NPs synthesis is compatible with NH 4 PuO 2 CO 3 . Furthermore,the EXAFS spectrum ( Figure S7 and Table S3) could be fitted with amodel based on the crystal structure of NH 4 PuO 2 CO 3 that was previously published. [32] Thef itted PuÀOd istance of the triple-bond group is 1.82 ,i ng ood agreement with previously determined distances of 1.80-1.81 for Pu V compounds [7,17,33] (see Table S4), while the crystallographic distance of 1.93 is most likely biased by the very weak scattering of oxygen in comparison to Pu. [33] We also found that the PuÀPu coordination number in the experimental EXAFS spectra of the intermediate phase is lower than for the structural data, which can be explained  either by a( partially) amorphous nature or by nano-sized particles.
Thei ntermediate NH 4 PuO 2 CO 3 phase was completely dissolved within % 10 h, after which the PuO 2 NPs were formed as ar esult of longer redox reactions (see Figure 2). Finally,apart of the intermediate Pu V phase was centrifuged out of suspension and dried at room temperature in order to check its stability over months.S urprisingly,t he dried NH 4 PuO 2 CO 3 phase was found to be stable over months. We recorded additional Pu L 3 HERFD spectra after 3months and the spectral shape remained the same ( Figure S8). Therefore,t he method reported here can be used to synthesize this Pu V phase.
To understand the pH influence,w ep erformed as imilar experiment at pH 8. Thek inetics of the Pu precipitation is very similar to the experiment at pH 11, whereas the quantity of the intermediate Pu V phase is lower ( Figure S9). Comparison of the experimental conditions with the available thermodynamic data shows that Eh/pH values during our synthesis correspond to the area of stability of the Pu IV phase close to the phase boundary ( Figure S10). It makes the formation of the intermediate Pu V phase possible,b ut at the same time,t he high thermodynamic stability of PuO 2 and its extremely low solubility lead to af urther transformation of the Pu V phase into PuO 2 .
We show here for the first time that while Pu V solid-state complexes are always viewed as exotic compounds,athermodynamically metastable Pu V solid phase is formed during the reductive precipitation of PuO 2 NPs from aPu VI precursor at pH 11. Thei ntermediate Pu V phase is characterized for the first time using HERFD at the Pu M 4 edge and model calculations in the framework of AIM. TheP uM 4 HERFD method allows for the unambiguous identification of the Pu oxidation state,i td emonstrates the Pu V existence,a nd provides quantitative estimates for varying Pu oxidation states.T he local structure of the intermediate Pu V phase, similar to NH 4 PuO 2 CO 3 ,isidentified by acombination of the Pu L 3 HERFD experiment and ab-initio calculations,a nd is found to be stable over aperiod of several months.The redox reactions behind aqueous Pu VI ÀPuO 2 NPs and the formation of Pu V cause the substantial increase of the solubility.T his finding provides as ignificant step towards ab etter understanding of Pu chemistry and emphasizes the value of the HERFD technique for studies of PuO 2 NPs formation under different conditions.