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International audience

International audience

International audience; The incorporation of actinides in lanthanide phosphate matrices crystallizing in the monazite structure has been intensely investigated in the past decades due to the relevance of these monazites as potential ceramic host phases for the immobilization of specific high level radioactive waste (HLW) streams [1-3]. In recent years, understanding the incorporation behaviour of trivalent dopants in the LnPO4•xH2O rhabdophane structure has been given more attention [4,5]. Rhabdophane is the hydrated phosphate precursor in the synthesis of monazites through precipitation routes and a potential secondary mineral controlling actinide solubility in dissolution and re-precipitation reactions of monazite host-phases. Despite the large interest in lanthanide phosphates and the interaction of actinides with these solids, very little data [6-8] is available on the complexation of lanthanides and actinides with aqueous phosphates, even though these complexation reactions precede any aqueous synthesis of monazite ceramics and are expected to occur in natural waters as well as in the proximity of monazite-containing HLW repositories. In many cases, an independent spectroscopic validation of the stoichiometry of the proposed complexes, is also missing. Both from the perspective of aqueous rhabdophane synthesis, which is often carried out at elevated temperatures, and heat-generating HLW immobilization in monazites, the lanthanide and actinide complexation reactions with aqueous phosphates under ambient conditions should be complemented with data obtained at higher temperatures.In the present work, laser-induced luminescence spectroscopy was used to study the complexation of Cm(III) (1.15×10-8 to 1.15×10-7 M) as a function of total phosphate concentration (0 to 0.08 M) in the temperature regime 25-90 °C, using NaClO4 as a background electrolyte (I = 0.5 to 3.0 M). These studies have been conducted in the acidic pH-range (−log10 [H+] = 1.00, 2.52, 3.44, and 3.65) to avoid precipitation of solid Cm rhabdophane. For the first time, in addition to the presence of CmH2PO42+ already evidenced before [6,7], the formation of Cm(H2PO4)2+ was unambiguously established from the luminescence spectroscopic data collected at the various H+ concentrations previously mentioned [8].The conditional complexation constants of both aqueous complexes were found to increase upon rising ionic strength and temperature. Extrapolation of the obtained complexation constants to infinite dilution at 25 °C was performed by applying the Specific Ion Interaction Theory (SIT) [9]. The obtained log10 β° values for CmH2PO42+ and Cm(H2PO4)2+ were 0.45 ± 0.04 and 0.08 ± 0.07 [8], respectively, for reactions 1 and 2 below:Cm3+ + H3PO4 ⇌ CmH2PO42+ + H+ (1)Cm3+ + 2 H3PO4 ⇌ Cm(H2PO4)2+ + 2 H+ (2)The ion interaction coefficients ε(CmH2PO42+;ClO4-) = 0.17 ± 0.04 and ε(Cm(H2PO4)2+;ClO4-) = −0.10 ± 0.06 were derived at 25 °C [8]. Temperature-dependent conditional complexation constants for the identified species were obtained from the recorded luminescence emission spectra. They were subsequently extrapolated to I =0 M, assuming that the ion interaction parameters obtained at 25 °C are not significantly impacted by the temperature increase from 25 °C to 90 °C [6]. Using the integrated van´t Hoff equation, both the molar enthalpy of reaction ΔrHm° and entropy of reaction ΔrSm° values were found to be positive for the two complexes, namely CmH2PO42+ and Cm(H2PO4)2+ [8].Relativistic quantum chemical investigations revealed a monodentate binding of the H2PO4- ligand to the central Cm3+ ion to be the most stable configuration for both complexes. By combining ab initio calculations with a thorough analysis of the obtained luminescence spectroscopic data, both CmH2PO42+ and Cm(H2PO4)2+ complexes with an overall CN of 9 were shown to be stable in solution at 25 °C. However, a different temperature-dependent evolution of the coordination of the Cm3+ ion to hydration water molecules could be derived from the electronic structure of the Cm(III)-phosphate complexes. More specifically, an overall coordination number of 9 was retained for the CmH2PO42+ complex in the investigated temperature range (25 to 90 °C), while a coordination change from 9 to 8 was established for the Cm(H2PO4)2+ species with increasing temperature [8]. This change of coordination upon increasing temperature, which has not been investigated in detail in the past, might also be relevant in the complexation of other f-elements with inorganic and/or organic ligands and deserves further exploration.[1] R. C. Ewing, Proc. Natl. Acad. Sci. USA 96, 3432 (1999).[2] D. Bregiroux et al., J. Nucl. Mater. 366, 52 (2007).[3] N. Huittinen et al., J. Nucl. Mater. 486, 148 (2017). [4] E. Du Fou de Kerdaniel, J. Nucl. Mater. 362, 451 (2007).[5] N. Huittinen et al., Inorg. Chem. 57, 6252−6265 (2018).[6] H. Moll et al., Radiochim. Acta, 99, 775−782 (2011).[7] N. Jordan et al., Inorg. Chem. 57, 7015-7024 (2018).[8] N. Huittinen et al., Inorg. Chem. 60, 10656−10673 (2021).[9] I. Grenthe et al., Second update on the chemical thermodynamics of uranium, neptunium, plutonium, americium and technetium, OECD Nuclear Energy Agency Data Bank, Eds., OECD Publications, Paris, France, (2020).

International audience; Thermodynamic databases are essential for the safety assessments of radioactive waste repositories. They have to be reliable, comprehensive, and describe the key mechanisms controlling the mobility of contaminants in the environment. However, in many cases these prerequisites are not fulfilled. An important example is the complexation of actinides with aqueous phosphates, for which this work provides complexation constants for spectroscopically identified species at 25 °C and at elevated temperature.The complexation of Cm(III) was studied at sub micromolar concentrations by laser induced luminescence spectroscopy as a function of total phosphate concentration (0-0.06 M ΣPO4) in the temperature range 25-90°C, using NaClO4 as a background electrolyte at –log[H+] ranging from from 2.5 to 3.6. The formation of both CmH2PO42+ and Cm(H2PO4)2+ complexes was revealed, the latter being spectroscopically evidenced for the first time. Complexation constants were found to increase when raising the ionic strength from 0.5 to 3.0 M.Temperature-dependent (25 to 90 °C) complexation constants for the identified species were derived, and were recalculated to standard conditions with the van´t Hoff equation and the Specific Ion Interaction Theory. Endothermic and entropy driven reactions were established for both complexes. In addition, relativistic quantum chemical investigations were performed to study the complexation strength of Cm(III) with aqueous phosphates and to provide insight in potential changes of the coordination number with increasing temperature and to probe the character of the Cm water and Cm phosphate bonds.

International audience; Thermodynamic databases are essential for the safety assessments of radioactive waste repositories. They have to be reliable, comprehensive, and describe the key mechanisms controlling the mobility of contaminants in the environment. However, in many cases these prerequisites are not fulfilled. An important example is the complexation of actinides with aqueous phosphates, for which this work provides complexation constants for spectroscopically identified species at 25 °C and at elevated temperature.The complexation of Cm(III) was studied at sub micromolar concentrations by laser induced luminescence spectroscopy as a function of total phosphate concentration (0-0.06 M ΣPO4) in the temperature range 25-90°C, using NaClO4 as a background electrolyte at –log[H+] ranging from from 2.5 to 3.6. The formation of both CmH2PO42+ and Cm(H2PO4)2+ complexes was revealed, the latter being spectroscopically evidenced for the first time. Complexation constants were found to increase when raising the ionic strength from 0.5 to 3.0 M.Temperature-dependent (25 to 90 °C) complexation constants for the identified species were derived, and were recalculated to standard conditions with the van´t Hoff equation and the Specific Ion Interaction Theory. Endothermic and entropy driven reactions were established for both complexes. Eventually, relativistic quantum chemical investigations were performed to study the complexation strength of Cm(III) with aqueous phosphates and to provide insight in potential changes of the coordination number with increasing temperature and to probe the character of the Cm water and Cm phosphate bonds.

Lanthanide phosphates (LnPO4) are considered as a potential nuclear waste form for immobilization of Pu and minor actinides (Np, Am, and Cm). In that respect, in the recent years we have applied advanced atomistic simulation methods to investigate various properties of these materials on the atomic scale. In particular, we computed several structural, thermochemical, thermodynamic and radiation damage related parameters. From a theoretical point of view, these materials turn out to be excellent systems for testing quantum mechanics-based computational methods for strongly correlated electronic systems. On the other hand, by conducting joint atomistic modeling and experimental research, we have been able to obtain enhanced understanding of the properties of lanthanide phosphates. Here we discuss joint initiatives directed at understanding the thermodynamically driven long-term performance of these materials, including long-term stability of solid solutions with actinides and studies of structural incorporation of f elements into these materials. In particular, we discuss the maximum load of Pu into the lanthanide-phosphate monazites. We also address the importance of our results for applications of lanthanide-phosphates beyond nuclear waste applications, in particular the monazite-xenotime systems in geothermometry. For this we have derived a state-of-the-art model of monazite-xenotime solubilities. Last but not least, we discuss the advantage of usage of atomistic simulations and the modern computational facilities for understanding of behavior of nuclear waste-related materials.

The incorporation of actinides in solid lanthanide phosphates crystallizing in the monazite structure has been intensely investigated in the past decades due to the relevance of these monazites as potential ceramic phases for the immobilization of specific high level radioactive waste (HLW) streams [1-3]. In recent years, understanding the incorporation behaviour of trivalent dopants in the LnPO4×nH2O rhabdophane structure, which is the hydrated phosphate precursor in the synthesis of monazites through precipitation routes and a potential secondary mineral controlling actinide solubility in dissolution and re-precipitation reactions of monazite host-phases, has been given more attention [4,5]. Despite the large interest in lanthanide phosphates and the interaction of actinides with these solids, very little data is available on the complexation of lanthanides and actinides with aqueous phosphates, even though these complexation reactions precede any aqueous synthesis of monazite ceramics and are expected to occur in natural waters as well as in the proximity of monazite-containing HLW repositories. It also suffers from an almost systematic absence of independent spectroscopic validation of the stoichiometry of the proposed complexes. Both from the perspective of aqueous rhabdophane synthesis, which is often carried out at elevated temperatures, and heat-generating HLW immobilization in monazites, the lanthanide and actinide complexation reactions with aqueous phosphates under ambient conditions should be complemented with data obtained at higher temperatures. In the present work, laser-induced luminescence spectroscopy (LIL) was used to study the complexation of Eu(III) (5×10 6 M) and Cm(III) (5×10 7 or 1×10 8 M) as a function of total phosphate concentration (0-0.3 M ΣPO4) in the temperature regime 25-90°C, using NaClO4 as a background electrolyte (I = 0.5 to 3.1 M). These studies have, in a first step, been conducted in the acidic pH-range (pH = 1) to avoid precipitation of solid Eu or Cm rhabdophane. Both trivalent metal cations form a complex with the anionic H2PO4 species, i.e. EuH2PO42+ and CmH2PO42+. The conditional complexation constants were found to increase upon rising ionic strength and temperature. Extrapolation of the obtained complexation constants to infinite dilution at 25 °C was performed by applying the Specific Ion Interaction Theory (SIT) [6]. The obtained log β° values for EuH2PO42+ and CmH2PO42 were 0.89 ± 0.13 and 0.45 ± 0.19, respectively, for reaction 1 below: Me3+ + H3PO4 ⇌ MeH2PO42+ + H+ (Me = Eu or Cm) (1) The ion-ion interaction coefficients ε(EuH2PO42+;ClO4 ) = 0.20 ± 0.08 and ε(CmH2PO42+;ClO4 ) = 0.16 ± 0.12 were derived at 25 °C. Temperature-dependent conditional complexation constants for the identified species were obtained from the recorded luminescence emission spectra. They were subsequently extrapolated to I =0 M, assuming that the ion-ion interaction parameters obtained at 25 °C are not significantly impacted by the temperature increase from 25 °C to 90 °C [6]. Using the extended van´t Hoff equation, the molal enthalpy ΔRHm° and entropy of reaction ΔRSm° values were both found to be positive. Exactly the same combination of batch, spectroscopic, and thermodynamic studies was used at lower H+ concentrations ( log[H+] = 2.52, 3.44, and 3.65). Our results clearly showed the presence of Eu(H2PO4)2+ and Cm(H2PO4)2+ species, so far never reported in the literature. In addition Eu(HPO4)+ and Cm(HPO4)+ species were identified. Conditional complexation constants for these species will be derived and extrapolated to infinite dilution using the SIT approach. Finally, relativistic quantum chemical investigations will be performed to shed light on the observed differences in the complexation strength of Eu(III) and Cm(III) with aqueous phosphates. They will also provide insight on the role of spin-orbit coupling and serve to probe the character of the metal water and metal phosphate bonds.

This Ph.D. thesis concerns investigations on the interactions of trivalent actinides with the mineral phases γ-alumina, gibbsite, and kaolinite. Spectroscopic methods, namely time-resolved laser fluorescence spectroscopy (TRLFS) and nuclear magnetic resonance (NMR) have been employed together with macroscopic sorption investigations to obtain molecular level process understanding of the solid-water interface reactions occurring in the mineral suspensions. Results obtained in the macroscopic sorption investigations with gadolinium(III) and the spectroscopic (TRLFS) investigations with curium(III) onto the aluminum hydroxide phase gibbsite, α-Al(OH)3 showed that two interaction mechanisms, inner sphere surface complexation and incorporation, are required to explain the metal ion speciation at the gibbsite surface over the investigated pH range 5-11.5. Sorption of Gd3+/Cm3+ commences at pH values close to 5 through formation of an inner sphere surface complex. When the pH is raised towards the alkaline pH range another inner sphere complex can be detected from the acquired TRLFS data. The continuous increase of suspension pH, however, also influences the solubility of aluminum with a direct impact on the trivalent metal ion speciation. Upon moving from higher aluminum solubility regions in the acidic pH range towards the solubility minimum at pH 6.3, aluminum hydroxide precipitates from oversaturated solutions and consequently incorporates surface bound curium on the gibbsite surface. When further increasing the pH towards the alkaline pH range, aluminum solubility increases again which results in dissolution of the incorporating precipitate uncovering the curium surface complex. This sorption behavior has not been encountered previously in investigations with other aluminum oxide/hydroxide minerals as sorbent phases for trivalent metal ion attachment. Identification of an incorporated curium species as a consequence of pH variations is a clear proof that mineral surfaces cannot be considered as inert with regard to chemical variations as done in many studies. In macroscopic (Eu3+) and TRLFS (Cm3+) sorption investigations with kaolinite as sorbent phase, both outer sphere and inner sphere surface complexes could be identified. Outer sphere complexation was pronounced in background electrolyte concentrations of 1 mM NaClO4, while outer sphere attachment was found only for low (less than 10-7 M) europium concentrations on a natural kaolinite batch and not at all in synthetic kaolinite suspensions when 100 mM NaClO4 was used in the experiments. Three inner sphere curium complexes differing only in the degree of hydrolysis were found to dominate the speciation over a large pH range (5.5-11.5). These species were assigned to >Cm(H2O)52+, >Cm(OH)(H2O)4+ and >Cm(OH)2(H2O)3 based on fluorescence lifetime data and they correlate very well in terms of emission peak positions and fluorescence lifetimes values to curium species found on other clay mineral surfaces. Above pH 11.5 a further curium complex that could not be assigned to a pure hydrolyzed curium surface complex became prevailing. The TRLFS experiments showed that the dissolution of kaolinite in the alkaline pH-range (>10) resulted in the formation of a ternary complex between surface-sorbed curium and dissolved silicates in solution. Thus, the dissolution of the sorbent phase could again be shown to influence the metal ion speciation verifying the need for spectroscopic investigations for a correct assignment of the formed complexes at the solid/water interface. The sorption of diamagnetic yttrium(III) and paramagnetic europium(III) on the various surface hydroxyl groups of γ-alumina (γ-Al2O3) and kaolinite was investigated in NMR studies. Upon Eu3+/Y3+ attachment onto the mineral surfaces the 1H NMR signal could be seen to decrease. In difference spectra produced by subtracting the acquired proton spectra of samples with high metal ion concentrations from spectra of clean mineral samples or samples with lower metal ion loadings, a manifold of surface hydroxyl groups influenced by the metal ion attachment to the surface could be identified. These surface groups could be assigned to doubly coordinated Al2-OH and singly coordinated Al-OH groups on the respective mineral surfaces. In the γ-alumina study the wide distribution of chemically very similar OH protons was attributed to variations in O-H bond lengths and to variations in the Al coordination (AlIV/AlVI). In the kaolinite study removed protons in the metal ion sorption reaction were assigned to singly coordinated Al-OH groups at the kaolinite edge surfaces. In addition, indications of trivalent metal ion attachment to bridging Al-OH-Al groups at the gibbsite-like basal plane of the mineral were obtained. Det använda kärnbränslet från de finska kärnreaktorerna kommer att slutförvaras i berggrunden i Olkiluoto. Syftet med slutförvaret är att isolera det radioaktiva avfallet från den levande naturen i flera hundratusen år. I slutförvaret kommer det använda kärnbränslet att skyddas av tre barriärer: kopparkapseln, bufferten och själva urberget. I denna avhandling har växelverkningar mellan trevärda aktinider som förekommer i det använda kärnavfallet samt mineralfaserna γ-alumina [γ-Al2O3], gibbsit [α-Al(OH)3] och kaolinit [Al2Si2O5(OH)4] undersökts. Syftet med arbetet har varit att erhålla information om hur dessa aktinider adsorberas på mineralytorna, vilket i berggrunden kommer att fördröja eller hindra dessa radioaktiva ämnens transport med grundvattnet upp mot markytan. För att erhålla information om de processer som sker vid mineral/vatten gränsytan har de spektroskopiska metoderna tidsupplöst laserinducerad fluorescensspektroskopi (TRLFS) samt kärnmagnetisk resonansspektroskopi (NMR) tillämpats. De spektroskopiska undersökningarna har kompletterats med makroskopiska, så kallade batchundersökningar, där trevärda lantanider har används som analoger till de radioaktiva aktiniderna. Sorptionsundersökningarna med lantaniden gadolinium(III) eller europium(III) och de spektroskopiska (TRLFS) undersökningarna med aktiniden curium(III) har visat att de trevärda metallerna adsorberas på mineralytorna gibbsit och kaolinit som såkallade innersfärkomplex, där en kemisk bindning mellan metallen och mineralytan bildas. Dessutom sker adsorptionen av Eu3+/Cm3+ även som yttersfärkomplexbildning på kaolinitytan i det sura pH-området. Yttersfärkomplexbildning är en elektrostatisk växelverkan mellan den negativa mineralytan och den positiva metalljonen. I TRLFS undersökningarna kunde den kontinuerliga pH-ökningen från pH 5 till 11.5 under experimenten ses påverka gibbsitlösligheten i mineralsuspensionerna med en direkt inverkan på den trevärda metalljonens speciation. Då experimenten utfördes från ett pH-område med högre aluminiumlöslighet till ett med lägre löslighet, skedde en utfällning av aluminiumhydroxid från den övermättade lösningen. Denna fällning inkorporerade det innersfäradsorberade curiumkomplexet vid gibbsitytan. I kaolinitstudierna kunde ett curium-innersfärkomplex mellan adsorberat curium på kaolinitytan samt lösta silikater i mineralsuspensionen, som uppkommit som följd av en märkbar upplösning av kaolinitmineralen då pH överskrider 10, påvisas. Identifieringen av den inkorporerade curium-specien på gibbsitytan och komplexet mellan curium och lösta silikater på kaolinitytan är ett tydligt bevis på att mineralytor inte kan anses vara inerta med avseende på kemiska variationer, utan förändringar i de kemiska förhållandena under undersökningarna måste beaktas. Dessutom framhävs behovet av spektroskopiska undersökningar för en korrekt beskrivning av de bildade komplexen vid mineral/vatten gränsytan. Adsorptionen av diamagnetiskt yttrium(III) och paramagnetiskt europium(III) på de olika yt-hydroxylgrupperna hos γ-alumina (γ-Al2O3) och kaolinit undersöktes i NMR-studier. Vid adsorption av Eu3+/Y3+ på mineralytorna, kunde protonsignalen i samtliga 1H NMR spektra ses avta i jämförelse med den erhållna protonsignalen från de rena mineralerna. 1H NMR signalens avtagande sker som följd av metallkomplexbildningen på ytan som samtidigt lösgör en eller flera hydroxylprotoner från mineralytan. Hydroxylgrupper på de respektiva mineralytorna som påverkades av sorptionreaktionen identifierades som dubbelt koordinerade Al2-OH grupper, där en hydroxylgrupp är förknippad med två aluminiumatomer i mineralstrukturen, samt enkelt koordinerade Al-OH grupper, där varje hydroxyl är förknippad med enbart en aluminiumatom. Erhållna NMR spektra visade dessutom att det är fråga om ett flertal, mycket liknande hydroxylgrupper som deltar i sorptionsreaktionen.