Natural sintering of carbonate powder for use as a carbon-14 immobilization matrix

International audienc

Coupling between SAXS and Raman spectroscopy applied to the gelation of colloidal zirconium oxy-hydroxide systems

International audienceThe colloidal sol–gel transition based on zirconyl nitrate solution systems is investigated in this work. The different steps occurring in the transition have been identified by coupling small angle X-ray scattering with Raman spectroscopy and rheology measurements. The effect of the experimental conditions, such as the zirconium precursor concentration and pH, on the transition is studied. The precise mechanisms involved during the transition are based on a detailed understanding of the nanostructure of these systems. In particular, the dissolution of the zirconium salt leads to the formation of cyclic tetramers that self-organize into a cylindrical shape. We clearly demonstrate that increasing the pH induces a strong attractive interaction between the cylinders, giving rise to a mass fractal dimension. For each system, two characteristic pH values have been determined via rheological measurements analysis, where gelation is notably slow below the first pH value and precipitation occurs above the second one. The complete description of the quaternary system (zirconyl nitrate + acetylacetone + ammonia + water) is an efficient formulation guide for the further combination with a templating route leading to structured Zr-based materials

Matériaux hybrides inorganiques-organiques pour l'extraction d'uranium en milieu acide phosphorique

Les minerais de phosphates, principalement exploités pour la production d'acide phosphorique et d'engrais, contiennent une quantité non-négligeable d'uranium (50 à 300 ppm) qui suscite l'intérêt de l'industrie nucléaire. Notre étude s'inscrit dans ce contexte de valorisation de l'uranium en tant que sous-produit de l'industrie des fertilisants.L'objectif de ce travail de thèse a été de mettre au point un matériau hybride, constitué d'un support inorganique sur lequel est greffée une molécule complexante, capable d'extraire sélectivement l'uranium du milieu acide phosphorique. La première étape de notre démarche a consisté à identifier un support inorganique capable de résister aux conditions particulières du milieu acide phosphorique (acidité élevée et milieu très complexant). Pour ce faire, la stabilité chimique et mécanique de différents matériaux, silice, verre et carbone mésoporeux, a été étudiée. Dans un deuxième temps, nous nous sommes intéressés à l'identification et l'optimisation de molécules complexantes spécifiques de l'uranium en milieu acide phosphorique. Ces dernières ont ensuite été greffées sur les supports les plus stables. Enfin, l'efficacité de ces systèmes hybrides a été évaluée lors de tests d'extraction, de sélectivité et de désextraction.Phosphate rocks are industrially processed in large quantities to produce phosphoric acid and fertilisers. These rocks contain significant concentration of uranium (50 to 300 ppm) which could be interesting for nuclear industry. This work deals with the valorisation of uranium as a by-product from fertiliser industry.The aim of this study is to develop a hybrid material, constituted of an inorganic solid support grafted with an extractant (complexing molecule), which can extract selectively uranium from phosphoric acid medium. The first step of our approach was to identify an inorganic support which is stable under these particular conditions (strong acidity and complexing medium). The chemical and mechanical stability of different mesoporous materials, such as silica, glass and carbon was studied. In a second phase, we focused on the identification and the optimisation of complexing molecules, specific of uranium in phosphoric acid. These ligands were then grafted on the most stable solids. Finally, the efficiency of these hybrid systems was evaluated through different tests of extraction, selectivity and desextraction.MONTPELLIER-BU Sciences (341722106) / SudocSudocFranceF

Remediation of 137^{137}Cs-contaminated concrete gravels by supercritical CO2_2 extraction

International audienceThe removal of cesium contamination is a critical issue for the recycling of concrete gravel in most decommissioning sites . The high solvent strength and diffusivity of supercritical CO$_2$ make it an attractive choice as vector for extractant system in this context. Experimental extraction runs have been carried out in a radioactive environment on gravels contaminated with $^{137}$Cs. The best extraction system was found to be CalixOctyl (25,27-Bis(1-octyloxy)calix(4)arene-crown-6, 1,3-alternate) with pentadecafluorooctanoic acid as a modifier. The effects of various operating parameters were investigated, namely the coarseness of the gravel, the temperature of SC-CO$_2$, the residual water and initial cesium concentrations, and the amounts of extractant and modifier used. The yields from direct extraction were low (< 30percent), because of the virtually irreversible sorption of Cs in concrete. The best extraction yield of $\sim$55percent was achieved by leaching the concrete gravel with nitric acid prior to supercritical CO$_2$ extraction

The effect of pore diameter in the arrangement of chelating species grafted onto silica surfaces with application to uranium extraction

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Effects of Impregnated Amidophosphonate Ligand Concentration on the Uranium Extraction Behavior of Mesoporous Silica

A series of solid-phase uranium extractants were prepared by post-synthesis impregnation of a mesoporous silica support previously functionalized with octyl chains by direct silanization. Five materials were synthesized with 0, 0.2, 0.3, 0.4 and 0.5 mmol of the amidophosphonate ligand DEHCEBP per gram of functionalized solid, and the effect of the ligand concentration on the uranium extraction efficiency and selectivity of the materials was investigated. Nitrogen adsorption&ndash;desorption data show that with increasing ligand loadings, the specific surface area and average pore volume decrease as the amidophosphonate ligand fills first the micropores and then the mesopores of the support. Acidic uranium solutions with a high sulfate content were used to replicate the conditions in ore treatment leaching solutions. Considering the extraction kinetics, the equilibration time was found to increase with the ligand concentration, which can be explained by the clogging of micropores and the multilayer arrangement of the DEHCEBP molecules in the materials with their highest ligand contents. The fact that the equilibrium ligand/uranium ratio is about 2 mol/mol regardless of the ligand concentration in the material suggests that all the ligand molecules remain accessible for extraction. The maximum uranium extraction capacities ranged from 30 mg&#8729;g&minus;1 at 0.2 mmol&#8729;g&minus;1 DEHCEBP to 54 mg&#8729;g&minus;1 in the material with 0.5 mmol&#8729;g&minus;1 DEHCEBP. These materials could therefore potentially be used as solid-phase uranium extractants in acidic solutions with high sulfate concentrations

Solubility and solution enthalpy of a cesium-selective calixarene in supercritical carbon dioxide

International audienceThis paper investigates the solubility of the cesium-selective extractant 25,27-Bis(1-octyloxy)calix[4]arene-crown-6, 1,3-alternate (CalixOctyl) in supercritical CO$_2$ (SC-CO$_2$). A dynamic setup with a solvent loop was used, avoiding any plugging of the solute and allowing precise quantification by UV/visible spectrophotometry. The solubility measurements were carried out between 10.3 and 27.4 MPa at 314, 324 and 334 K. The solubility of CalixOctyl in SC-CO$_2$ ranged from (0.20 $\pm$ 0.02)·10$^{−6}$ mol·mol$^{−1}$ at 334 K and 10.4 MPa to (72.78 $\pm$ 0.31)·10$^{−6}$ mol·mol$^{−1}$ at 324 K and 27.4 MPa, approximately 100 times higher than that of simple $p-t$-butylcalix[4,6,8]arenes and calix[4]arene. The solution enthalpy of CalixOctyl in SC-CO$_2$ was also studied. The heat flow during solubilization at 24.0 MPa and 314, 324 and 334 K was measured by differential scanning calorimetry. The temperature dependence of the solution enthalpy and its extrapolation to any pressure are discussed

Purification of Uranium from acidic liquid wastes and ore solutions by selective hybrid materials

International audienc

Thermodynamics methodology for ternary chelatant/cationic exchanger/SCCO2 system applied for the remediation of 137Cs contaminated soil.

International audienceIn March 2011, Fukushima Daiichi nuclear power plant accident led to the release of a large quantity of volatile fission products such as 137Cs in the environment. Despite the numerous existing soil decontamination technologies, there is a need for environmentally friendly process that will limit the production of unsuitable secondary waste. Extraction in supercritical CO2 (SCCO2) solvent is one of these promising, smooth technologies to be applied to soil remediation [1].SCCO2 exhibits gas-like mass transfer rates and yet have liquid-like solvating abilities. Its high diffusivity and low viscosity enable it to penetrate and transport solutes from porous solid matrices to the SCCO2 phase. Furthermore, its flexibility in terms of ability to adjust solvating power by tuning operating pressure and temperature is quite interesting. Direct extraction of metal ions by SCCO2 is inefficient due to charge neutralization requirement and weak ionic solute-solvent interactions. Hence, organic ligands are needed in order to chelate these ions and make them soluble in SCCO2. Crown ethers and calixarenes have been intensively studied for their selectivity in the extraction for alkali metal cations such as cesium in liquid-liquid processes. Crown ethers have also been successfully used to extract cesium in SCCO2 with perfluorocarboxylic acid as a cationic exchanger. A general methodology to study ternary system extractant/cationic exchanger/ SCCO2 is needed to improve the efficiency of the extraction process. The aim of this study is to optimize amounts of extractant and exchanger, considering decontamination yield, energetical and economic process optimization in the aim of scale-up. This work is divided into the following steps determination of pure component properties, acquisition of data for each binary system, model implementation by binary interaction parameter fitting and validation with typical ternary data. Solubilities data of considered extractant and cationic exchanger in SCCO2 have never been published. Feasibility of SCCO2 extraction processing for decontamination of contaminated soils has been studied using simulated sand soil and non-radioactive 133Cs as contaminant. Extractant system considered was a mixture of DB18C6 crown ether and perfluorooctanoic acid. Extraction yields up to 95% have been obtained. Process optimization in terms of extraction yield was carried out by setting suitable operating conditions such as pressure, temperature, extractant system concentration and soil moisture.From experimental results and ternary system thermodynamics data, extraction process scale-up is in progress in the aim of transportation on contaminated site. CO2 and extractant fluxes management, expansion/compression cycles have to be considered carefully in order to minimize financial and energy costs. Feasibility of back extraction step is also studied.As a conclusion, SCCO2 soil remediation cleaning remains a promising technique that could be combined with other pre or post treatment processes

Effect of composition on ionic transport in SiO2-B2O3-Na2O glasses

International audienceIonic diffusion was investigated in the SiO2B2O3Na2O glass system over a wide composition range by impedance spectroscopy measurements. The Na+ cation transport mechanism was described by an interstitial pair migration model based on Frenkel defects in ionic crystals. The activation energy of the static electrical conductivity is shown to be correlated with the boron coordination number in these glasses. Published 11B NMR results were used to calculate the activation energies of sodium cations acting as charge compensators for the (BO4-2)- tetrahedron and of sodium cations bonded to nonbridging oxygen atoms . These values are in agreement with the activation energies of the Na2OB2O3 and Na2OSiO2 binary systems, respectively