Incorporation of U(IV) in monazite-cheralite ceramics under oxidizing and inert atmospheres

This work is the first attempt to prepare Nd1-xCaxUxPO4 monazite-cheralite with 0 < x ≤ 0.1 by a wet chemistry method. This method relies on the precipitation under hydrothermal conditions (T = 110 °C for four days) of the Nd1-xCaxUxPO4·nH2O rhabdophane precursor, followed by its thermal conversion for 6 h at 1100 °C in air or Ar atmosphere. The optimized synthesis protocol led to the incorporation of U and Ca in the rhabdophane structure. After heating at 1100 °C for 6 h in air, single-phase monazite-cheralite samples were obtained. However, α-UP2O7 was identified as a secondary minor phase in the samples heated under Ar atmosphere. The U speciation in the samples converted in an oxidising atmosphere was carefully characterized using synchrotron radiation by combining HERFD-XANES and XRD. These results showed the presence of a minor secondary phase containing hexavalent uranium and phosphate with a stoichiometry of U : P = 0.78. This highly labile uranyl phosphate phase incorporated 21 mol% of the uranium initially precipitated with the rhabdophane precursor. This phase was completely removed by a washing protocol. Thus, single-phase monazite-cheralite was obtained through the wet chemistry route described in this work with a maximum U loading of x = 0.08.

Time-resolved laser-induced fluorescence spectroscopy and chemometrics for fast identification of U(VI)-bearing minerals in a mining context

We evaluated the potential of time-resolved laser-induced fluorescence spectroscopy (TRLFS) combined with chemometric methods for fast identification of U(VI)-bearing minerals in a mining context. We analyzed a sample set which was representative of several environmental conditions. The set consisted of 80 uranium-bearing samples related to mining operations, including natural minerals, minerals with uranium sorbed on the surface, and synthetic phases prepared and characterized specifically for this study. The TRLF spectra were processed using the Ward algorithm and the K-nearest neighbors (KNN) method to reveal similarities between samples and to rapidly identify the uranium-bearing phase and the associated mineralogical family. The predictive models were validated on an independent dataset, and then applied to test samples mostly taken from U mill tailings. Identification results were found to be in accordance with the available characterization data from X-ray diffraction (XRD) and scanning electron microscopy-energy dispersive X-ray spectrometry (SEM-EDX). This work shows that TRLFS can be an effective decision-making tool for environmental investigations or geological prospection, considering the large diversity of uranium-bearing mineral phases and their low concentration in environmental samples.

Effect of Annealing on Structural and Thermodynamic Properties of ThSiO4-ErPO4 Xenotime Solid Solution

The effect of annealing on structural and thermochemical properties of a thorite-xenotime solid solution Th_1-xEr_x(SiO₄)_1-x(PO₄)_x was assessed. The samples synthesized at low temperatures and stored at room temperature for 2 years retained their tetragonal structures. This structure was also maintained after heating to 1100 °C. During annealing, the structure lost water and exsolved some thorianite phases. The thermodynamic parameters did not change much after annealing, suggesting that xenotime was not a low-temperature metastable phase but rather a stable structure able to withstand elevated temperatures regardless of the thorium content. The solid solution exhibited subregular behavior with the Margules function W(x) = (73.1 ± 20.1) - (125.7 ± 49.8)·x.

Thermodynamics and Stability of Rhabdophanes, Hydrated Rare Earth Phosphates REPO4 · n H2O

Rare earth phosphates comprise a large family of compounds proposed as possible nuclear waste disposal forms. We report structural and thermodynamic properties of a series of rare earth rhabdophanes and monazites. The water content of the rhabdophanes, including both adsorbed and structural water, decreases linearly with increase in ionic radius of the rare earth. The energetics of the transformation of rhabdophane to monazite plus water and the enthalpy of formation of rhabdophane from the constituent oxides was determined by high temperature drop solution calorimetry. The former varies linearly with the ionic radius of the lanthanide, except for cerium. By combining the enthalpy of formation determined by high temperature drop solution calorimetry and the free energy of formation determined previously by solubility experiments, a complete set of thermodynamic data was derived for the rhabdophanes. They are thermodynamically metastable with respect to the corresponding monazites plus water at all temperatures under ambient pressure conditions. This conclusion strengthens the case for monazites being an excellent nuclear waste form.

Monazite, rhabdophane, xenotime & churchite: Vibrational spectroscopy of gadolinium phosphate polymorphs

Rare-earth phosphates with the general formula REEPO₄·nH₂O belong to four distinct structural types: monazite, rhabdophane, churchite, and xenotime. We report herein the first direct comparison between vibrational spectra of these compounds for the same metal cation i.e. gadolinium. The four GdPO₄·nH₂O samples were prepared through wet chemistry methods and first characterized by X-ray diffraction. Three distinct spectral domains, associated to the deformation and stretching modes of phosphate tetrahedra (PO₄) and to water molecules vibrations were then analyzed from FTIR and Raman data, and discussed regarding the structural characteristics of each sample. The most obvious differences between the spectra were associated to δ(H₂O) and δ_s(PO₄) modes and led to propose a simple method to rapidly and unambiguously discriminate the four polymorphs.

Vibrational spectroscopy of synthetic analogues of ankoleite, chernikovite and intermediate solid solution

Ankoleite (K(UO2)PO4·nH2O), chernikovite (H3O(UO2)PO4·nH2O) and intermediate solid solutions are frequently encountered in the uranium ores that result from the alteration of uranium primary minerals. This paper reports a thorough FTIR and Raman study related to synthetic analogues for these minerals. First, the vibration bands associated to the UO2(2 +) uranyl ion were used to calculate the U = O bond length which appeared in good agreement with the data coming from PXRD. Then, the examination of the phosphate vibration modes in both sets of spectra confirmed the general formulation of the samples and ruled out the presence of hydrogenphosphate groups. Finally, the presence of H2O as well as protonated H3O(+) and/or H5O2(+) species was also pointed out, and could be used to clearly differentiate the various phases prepared. Vibrational spectroscopy then appeared as an efficient method for the investigation of such analogues of natural samples. It should be particularly relevant when identifying these phases in mineral ores or assemblies.

From thorite to coffinite: a spectroscopic study of Th(1-x)U(x)SiO4 solid solutions

Coffinite (USiO4), along with Th(1-x)U(x)SiO4 uranothorite solid solutions, are frequently present in reduced economically exploitable uranium ores. They could also control the concentration of uranium in the environment in the case of accidental release from underground radwaste repository. This paper reports for the first time a thorough FTIR and Raman study relative to the Th(1-x)U(x)SiO4 system, including synthetic analogues of thorite and coffinite end-members. Both sets of spectra confirmed the formulation of the samples and allowed to rule out the presence of structural water molecules and/or hydroxyl groups in the coffinite. Also, no characteristic signal of UO2(2+) uranyl ion was recorded, ensuring that uranium was fully incorporated under its tetravalent oxidation state. The variation of the positions corresponding to SiO4 internal vibration modes was then followed versus the chemical composition of the samples. If the FTIR spectra did not revealed any significant shift in the bands position, several Raman modes followed a linear trend as a function of the uranium incorporation rate. On this basis, Raman spectroscopy could be considered as a promising tool for the semi-quantitative determination of chemical composition of uranothorite samples, particularly for those coming from mineral ores. Finally, the data collected for the coffinite end-member, as the first to be obtained on pure synthetic samples, allowed a review of the results previously reported in the literature for this compound.

Comparison of three labeled silica nanoparticles used as tracers in transport experiments in porous media. Part I: syntheses and characterizations

The synthesis and the characterization of three kinds of labeled silica nanoparticles were performed. Three different labeling strategies were investigated: fluorescent organic molecule (FITC) embedded in silica matrix, heavy metal core (Ag(0)) and radioactive core ((110m)Ag) surrounded by a silica shell. The main properties and the suitability of each kind of labeled nanoparticle in terms of size, surface properties, stability, detection limits, and cost were determined and compared regarding its use for transport studies. Fluorescent labeling was found the most convenient and the cheapest, but the best detection limits were reached with chemical (Ag(0)) and radio-labeled ((110m)Ag) nanoparticles, which also allowed nondestructive quantifications. This work showed that the choice of labeled nanoparticles as surrogates of natural colloids or manufactured nanoparticles strongly depends on the experimental conditions, especially the concentration and amount required, the composition of the effluent, and the timescale of the experiment.

Comparison of three labeled silica nanoparticles used as tracers in transport experiments in porous media. Part II: transport experiments and modeling

Three types of labeled silica nanoparticles were used in transport experiments in saturated sand. The goal of this study was to evaluate both the efficiency of labeling techniques (fluorescence (FITC), metal (Ag(0) core) and radioactivity ((110m)Ag(0) core)) in realistic transport conditions and the reactive transport of silica nanocolloids of variable size and concentration in porous media. Experimental results obtained under contrasted experimental conditions revealed that deposition in sand is controlled by nanoparticles size and ionic strength of the solution. A mathematical model is proposed to quantitatively describe colloid transport. Fluorescent labeling is widely used to study fate of colloids in soils but was the less sensitive one. Ag(0) labeling with ICP-MS detection was found to be very sensitive to measure deposition profiles. Radiolabeled ((110m)Ag(0)) nanoparticles permitted in situ detection. Results obtained with radiolabeled nanoparticles are wholly original and might be used for improving the modeling of deposition and release dynamics.

Size- and concentration-dependent deposition of fluorescent silica colloids in saturated sand columns: transport experiments and modeling

This study investigates the size and concentration effects on the transport of silica colloids in columns of sandy aquifer material. Colloid transport experiments were performed with specifically developed fluorescent labeled silica colloids in columns of a repacked natural porous medium under hydro-geochemical conditions representative of sandy aquifers. Breakthrough curves and vertical deposition profiles of colloids were measured for various colloid concentrations and sizes. The results showed that for a given colloid concentration injected, deposition increased when increasing the size of the colloids. For a given colloid size, retention was also shown to be highly concentration-dependent with a non-monotonous pattern presenting low and high concentration specificities. Deposition increases when increasing both size and injected concentration, until a threshold concentration is reached, above which retention decreases, thus increasing colloid mobility. Results observed above the threshold concentration agree with a classical blocking mechanism typical of a high concentration regime. Results observed at lower colloid concentrations were not modeled with a classical blocking model and a depth- and time-dependent model with a second order kinetic law was necessary to correctly fit the experimental data in the entire range of colloid concentrations with a single set of parameters for each colloidal size. The colloid deposition mechanisms occuring at low concentrations were investigated through a pore structure analysis carried out with Mercury Intrusion Porosimetry and image analysis. The determined pore size distribution permitted estimation of the maximal retention capacity of the natural sand as well as some low flow zones. Altogether, these results stress the key role of the pore space geometry of the sand in controlling silica colloids deposition under hydro-geochemical conditions typical of sandy aquifers. Our results also showed originally that colloid mobility in porous media is not only favored at high colloid concentrations, but also at very low concentrations, which are more likely to be observed in groundwater.

Dissolution of cerium(IV)-lanthanide(III) oxides: comparative effect of chemical composition, temperature, and acidity

The dissolution of Ce(1-x)Ln(x)O(2-x/2) solid solutions was undertaken in various acid media in order to evaluate the effects of several physicochemical parameters such as chemical composition, temperature, and acidity on the reaction kinetics. The normalized dissolution rates (R(L,0)) were found to be strongly modified by the trivalent lanthanide incorporation rate, due to the presence of oxygen vacancies decreasing the samples cohesion. Conversely, the nature of the trivalent cation considered only weakly impacted the R(L,0) values. The dependence of the normalized dissolution rates on the temperature then appeared to be of the same order of magnitude than that of chemical composition. Moreover, it allowed determining the corresponding activation energy (E(A) ≈ 60-85 kJ·mol(-1)) which accounts for a dissolution driven by surface-controlled reactions. A similar conclusion was made regarding the acidity of the solution: the partial order related to (H(3)O(+)) reaching about 0.7. Finally, the prevailing effect of the incorporation of aliovalent cations in the fluorite-type CeO(2) matrix on the dissolution kinetics precluded the observation of slight effects such as those linked to the complexing agents or to the crystal structure of the samples.

Effects of water content on reactive transport of 85Sr in Chernobyl sand columns

It is known that under unsaturated conditions, the transport of solutes can deviate from ideal advective-dispersive behaviour even for macroscopically homogeneous porous materials. Causes may include physical non-equilibrium, sorption kinetics, non-linear sorption, and the irregular distribution of sorption sites. We have performed laboratory experiments designed to identify the processes responsible for the non-ideality of radioactive Sr transport observed under unsaturated flow conditions in an Aeolian sandy deposit from the Chernobyl exclusion zone. Miscible displacement experiments were carried out at various water contents and corresponding flow rates in a laboratory model system. Results of our experiments have shown that breakthrough curves of a conservative tracer exhibit a higher degree of asymmetry when the water content decreases than at saturated water content and same Darcy velocity. It is possible that velocity variations caused by heterogeneities at the macroscopic scale are responsible for this situation. Another explanation is that molecular diffusion drives the solute mass transfer between mobile and immobile water regions, but the surface of contact between these water regions is small. At very low concentrations, representative of a radioactive Sr contamination of the pore water, sorption and physical disequilibrium dominate the radioactive Sr transport under unsaturated flow conditions. A sorption reaction is described by a cation exchange mechanism calibrated under fully saturated conditions. The sorption capacity, as well as the exchange coefficients are not affected by desaturation. The number of accessible exchange sites was calculated on the basis that the solid remained in contact with water and that the fraction of solid phase in contact with mobile water is numerically equal to the proportion of mobile water to total water content. That means that for this type of sandy soil, the nature of mineral phases is the same in advective and non-advective domains. So sorption reaction parameters can be estimated from more easily conducted saturated experiments, but hydrodynamic behaviour must be characterized by conservative tracer experiments under unsaturated flow conditions.

Reactive transport of 85Sr in a chernobyl sand column: static and dynamic experiments and modeling

The effects of nonlinear sorption and competition with major cations present in the soil solution on radioactive strontium transport in an eolian sand were examined. Three laboratory techniques were used to identify and quantify the chemical and hydrodynamic processes involved in strontium transport: batch experiments, stirred flow-through reactor experiments and saturated laboratory columns. The major goal was to compare the results obtained under static and dynamic conditions and to describe in a deterministic manner the predominant processes involved in radioactive strontium transport in such systems. Experiments under dynamic conditions, namely flow-through reactor and column experiments, were in very good agreement even though the solid/liquid ratio was very different. The experimental data obtained from the flow-through reactor study pointed to a nonlinear, instantaneous and reversible sorption process. Miscible displacement experiments were conducted to demonstrate the competition between stable and radioactive strontium and to quantify its effect on the 85Sr retardation factor. The results were modeled using the PHREEQC computer code. A suitable cation-exchange model was used to describe the solute/soil reaction. The model successfully described the results of the entire set of miscible displacement experiments using the same set of parameter values for the reaction calculations. The column study revealed that the stable Sr aqueous concentration was the most sensitive variable of the model, and that the initial state of the sand/solution system had also to be controlled to explain and describe the measured retardation factor of radioactive strontium. From these observations, propositions can be made to explain the discrepancies observed between some data obtained from static (batches) and dynamic (reactor and column) experiments. Desorbed antecedent species (stable Sr) are removed from the column or reactor in the flow system but continue to compete for sorption sites in the batch system. Batch experiments are simple and fast, and provide a very useful means of multiplying data. However, interpretation becomes difficult when different species compete for sorption sites in the soil/solution system. A combination of batches, flow-through reactor and column experiments, coupled with hydrogeochemical modeling, would seem to offer a very powerful tool for identifying and quantifying the predominant processes on a cubic decimeter scale (dm3) and for providing a range of radioactive strontium retardation factor as a function of the geochemistry of the soil/solution system.