Sorption of trivalent actinides onto gibbsite, γ-alumina, and kaolinite : A spectroscopic study of An(III) interactions at the mineral-water interfaces

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

The effect of UV-C irradiation and EDTA on the uptake of Co2+ by antimony oxide in the presence and absence of competing cations Ca2+ and Ni2+

In nuclear power plants and other nuclear facilities the removal of cobalt from radioactive liquid waste is needed to reduce the radioactivity concentration in effluents. In liquid wastes containing strong organic complexing agents such as EDTA cobalt removal can be problematic due to the high stability of the CoEDTA complex. In this study, the removal of cobalt from NaNO3 solutions using antimony oxide (Sb2O3) synthesized from potassium hexahydroxoantimonate was investigated in the absence and presence of EDTA. The uptake studies on the ion exchange material were conducted both in the dark (absence of UVlight) and under UV-C irradiation. Ca2+ or Ni2+ were included in the experiments as competing cations to test the selectivity of the ion exchanger. Results show that UV-C irradiation noticeably enhances the cobalt sorption efficiency on the antimony oxide. It was shown that nickel decreased the sorption of cobalt to a higher extent than calcium. Finally, the sorption data collected for Co2+ on antimony oxide was modeled using six different isotherm models. The Sips model was found to be the most suitable model to describe the sorption process. The Dubinin-Radushkevich model was further used to calculate the adsorption energy, which was found to be 6.2 kJ mol-1. (c) 2021 Korean Nuclear Society, Published by Elsevier Korea LLC. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).Peer reviewe

Neptunium(V) transport in granitic rock : A laboratory scale study on the influence of bentonite colloids

In the present study neptunium(V) uptake by crystalline granitic rock (Kuru Grey granite) and the role of stable and mobile bentonite colloids (MX-80) on the migration of neptunium(V) was investigated. Two different experimental setups were utilized, batch-type experiments under stagnant conditions and column experiments under flowing water conditions. The uptake of 10(-6) M neptunium(V) by 40 g/L crushed granite in 10 mM NaClO4 was found to be pH-dependent, whereas neptunium(V) uptake by MX-80 bentonite colloids (0.08-0.8 g/L) was pH-independent up to a pH-value of approximately 11. Column experiments were conducted in the presence and absence of colloids at two pH values (pH = 8 and 10) and two flow rates (0.3 and 0.8 mL/h) in 10 mM NaClO4. The injected neptunium(V) concentration was 2x10(-4) M and the colloid concentration ranged from 0.08 to 0.32 g/L. The properties of the flow field in the columns were investigated with a conservative chloride tracer, at the same two flow rates of 0.8 and 0.3 mL/h. The resulting breakthrough curves were modeled using the analytical solution of advection-matrix diffusion equation. A tailing of neptunium(V) breakthrough curves in comparison to the conservative tracer was observed, which could be explained by a slightly higher retardation of neptunium(V) in the column caused by sorption on the granite. The sorption was in general lower at pH 8 than at pH 10. In addition, the tailing was almost identical in the absence and presence of MX-80 bentonite colloids, implying that the influence of colloids on the neptunium(V) mobility is almost negligible.Peer reviewe

A Combined Extended X-ray Absorption Fine Structure Spectroscopy and Density Functional Theory Study of Americium vs. Yttrium Adsorption on Corundum (α–Al2O3)

Adsorption reactions on mineral surfaces are influenced by the overall concentration of the adsorbing metal cation. Different site types (strong vs. weak ones) are often included to describe the complexation reactions in the various concentration regimes. More specifically, strong sites are presumed to retain metal ions at low sorbate concentrations, while weak sites contribute to metal ion retention when the sorbate concentration increases. The involvement of different sites in the sorption reaction may, thereby, also be influenced by competing cations, which increase the overall metal ion concentration in the system. To date, very little is known about the complex structures and metal ion speciation in these hypothetical strong- and weak-site regimes, especially in competing scenarios. In the present study, we have investigated the uptake of the actinide americium on corundum (α–Al2O3) in the absence and presence of yttrium as competing metal by combining extended X-ray absorption fine structure spectroscopy (EXAFS) with density functional theory (DFT) calculations. Isotherm studies using the radioactive 152Eu tracer were used to identify the sorption regimes where strong sites and weak sites contribute to the sorption reaction. The overall americium concentration, as well as the presence of yttrium could be seen to influence both the amount of americium uptake by corundum, but also the speciation at the surface. More specifically, increasing the Am3+ or Y3+ concentrations from the strong site to the weak site concentration regimes in the mineral suspensions resulted in a decrease in the overall Am–O coordination number from nine to eight, with a subsequent shortening of the average Am–O bond length. DFT calculations suggest a reduction of the surface coordination with increasing metal–ion loading, postulating the formation of tetradentate and tridentate Am3+ complexes at low and high surface coverages, respectively

Investigations towards incorporation of Eu3+ and Cm3+ during ZrO2 crystallization in aqueous solution

Nuclear energy provides a widely applied carbon-reduced energy source. Following operation, the spent nuclear fuel (SNF), containing a mixture of radiotoxic elements such as transuranics, needs to be safely disposed of. Safe storage of SNF in a deep geological repository (DGR) relies on multiple engineered and natural retention barriers to prevent environmental contamination. In this context, zirconia (ZrO2) formed on the SNF rod cladding, could be employed as an engineered barrier for immobilization of radionuclides via structural incorporation. This study investigates the incorporation of Eu3+ and Cm3+, representatives for trivalent transuranics, into zirconia by co-precipitation and crystallization in aqueous solution at 80 °C. Complementary structural and microstructural characterization has been carried out by powder X-ray diffraction (PXRD), spectrum imaging analysis based on energy-dispersive X-ray spectroscopy in scanning transmission electron microscopy mode (STEM-EDXS), and luminescence spectroscopy. The results reveal the association of the dopants with the zirconia particles and elucidate the presence of distinct bulk and superficially incorporated species. Hydrothermal aging for up to 460 days in alkaline media points to great stability of these incorporated species after initial crystallization, with no indication of phase segregation or release of Eu3+ and Cm3+ over time. These results suggest that zirconia would be a suitable technical retention barrier for mobilized trivalent actinides in a DGR

Rare-Earth Orthophosphates From Atomistic Simulations

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

Data for "Complexation of trivalent lanthanides (Eu) and actinides (Cm) with aqueous phosphates at elevated temperatures"

At the Institute of Resource Ecology, the complexation of Eu(III) and Cm(III) with aqueous phosphate was studied using laser-induced luminescence spectroscopy at low pH ([H+] = 0.1 M), different ionic strengths (0.6 to 3.1 M) and different temperatures (25 to 80 °C). The data set contains the species distribution, slope analysis and the extrapolation to zero ionic strength using R.The authors would like to thank the German Federal Ministry of Education and Research (BMBF) for funding the SEM2 (033R127D) and ThermAc (02NUK039B) projects