Sorption Properties of Peat for U(VI) and 226Ra in U Mining Areas

AbstractUnderstanding the environmental behavior of U and 226Ra is important for remediation of former mining sites and assessment of their durability. The aim of this study is to determine the sorption properties of an acidic peat for U and 226Ra, located near a former U mine. A 90% retention of U over a pH range of 3 to 10 was observed in Na- conditioned peat samples issued from the mesotelm, with a Rd between 1000 and 12000 mL/g. It seems that U sorption in peat is not only governed by organic matter. The Kd for 226Ra adsorption increases with pH from zero to 4849 mL/g. However in this mining environment, the peat bog proved to be significantly impacted by anthropogenic activities, with the presence of detrital components in the peat material that can also contribute to U and Ra sorption

Développement de modèles prédictifs décrivant le couplage chimie-transport dans les sédiments naturels

La gestion de sols ou sédiments contaminés par des métaux nécessite de pouvoir prédire la migration de ces métaux, dont la mobilité dépend des propriétés de transport du milieu ainsi que de la réactivité chimique de celui-ci (principalement réactions sorption/désorption). Pour étudier l'adsorption des cations métalliques (majeurs et traces) en condition dynamique, des expériences de transport en colonne ont été réalisées avec un sol pauvre en minéraux carbonatés et en matière organique. Considérant que la réactivité de ce sol était due essentiellement à la phase argileuse smectitique, un modèle d'adsorption basé sur les propriétés de rétention de la montmorillonite du Wyoming, utilisant la théorie des échangeurs d'ions multi-sites a été intégré à un code de transport 1D. Les prédictions calculées par ce modèle ont ensuite été comparées aux courbes de percées mesurées dans cette étude et reportées de la littérature. L'étude sur le transport réactif des cations majeurs a mis en évidence le rôle significatif des protons (et ce même à des pH proche de la neutralité), et a permis de valider le modèle pour le transport compétitif des cations majeurs (Na et Ca). Cependant, l'étude de la rétention de Zn(II) a mis en évidence une incohérence entre les résultats issus des expériences batch et ceux obtenus avec les expériences colonne, que l'on pourrait attribuer à la contribution d'une autre phase adsorbante (illite). Néanmoins, le modèle d'adsorption proposé a permis de reproduire avec une bonne satisfaction des données expérimentales de rétention de Zn(II) proposées dans la littérature et obtenues avec des expériences colonne.Management of soils or sediments contaminated by metals requires to predict the migration of metallic cations, whose mobility depends both on the transport properties of the medium and chemical reactivity of the system (principally sorption/desorption reactions). To study the sorption of metallic cations (major and trace) in dynamic condition, transport experiments using columns have been carried out with a soil poor in carbonated minerals and organic matter. Considering that the reactivity of this soil was mainly due to swelling clay minerals, a sorption model based on the sorption properties of the Wyoming montmorillonite and built according to a multi-site ion exchanger theory has been integrated into a 1D transport code. The predictions given by this model were then compared with the breakthrough curves measured in this study and those reported in the literature. The study of the reactive transport of major cations highlighted the significant role of protons (even at near neutral pH), and validated the model for major cations (Na and Ca). However, the study concerning Zn (II) showed a discrepancy between the results obtained from batch experiments and those issued from column experiments, which could be attributed to the contribution of another sorbent phase (illite). Finally, the proposed sorption model allowed reproducing with a good confidence experimental data reported from literature for sorption of Zn (II) in dynamic conditions.POITIERS-SCD-Bib. électronique (861949901) / SudocSudocFranceF

Antiretroviral-naive and -treated HIV-1 patients can harbour more resistant viruses in CSF than in plasma

Objectives The neurological disorders in HIV-1-infected patients remain prevalent. The HIV-1 resistance in plasma and CSF was compared in patients with neurological disorders in a multicentre study. Methods Blood and CSF samples were collected at time of neurological disorders for 244 patients. The viral loads were >50 copies/mL in both compartments and bulk genotypic tests were realized. Results On 244 patients, 89 and 155 were antiretroviral (ARV) naive and ARV treated, respectively. In ARV-naive patients, detection of mutations in CSF and not in plasma were reported for the reverse transcriptase (RT) gene in 2/89 patients (2.2%) and for the protease gene in 1/89 patients (1.1%). In ARV-treated patients, 19/152 (12.5%) patients had HIV-1 mutations only in the CSF for the RT gene and 30/151 (19.8%) for the protease gene. Two mutations appeared statistically more prevalent in the CSF than in plasma: M41L (P = 0.0455) and T215Y (P = 0.0455). Conclusions In most cases, resistance mutations were present and similar in both studied compartments. However, in 3.4% of ARV-naive and 8.8% of ARV-treated patients, the virus was more resistant in CSF than in plasma. These results support the need for genotypic resistance testing when lumbar puncture is performe

Application of the multi-site ion exchanger model to the sorption of Sr and Cs on natural clayey sandstone

International audienceTo ensure the environmental monitoring of nuclear sites, sorption models enabling the prediction of contaminant migration (such as $^{90}$Sr and $^{137}$Cs) in soils, sediments and aquifers need to be developed. This paper aims 1) at developing a database containing the sorption properties of the pure mineral phases commonly encountered in natural environments and 2) at using this database in a multi-site ion exchange model to be applied on natural sediments. In addition to some raw adsorption data issued from literature, different sorption experiments of Sr and Cs, on pure illite and smectite, in competition with calcium were carried out, as a function of pH and as well as concentrations of Sr and Cs. These sorption data were interpreted using a multi-site ion-exchange model and contributed to the elaboration of a thermodynamic database gathering the retention properties of illite and smectite (site capacities and selectivity coefficients) towards Cs$^+$, Sr$^{2+}$, H$^+$ and Ca$^{2+}$. A multi-site ion exchange model based on the mixture of pure mineral phases (illite and smectite) was successfully applied to simulate the Sr and Cs behavior in a natural clayey sandstone, indicating that the additivity of retention properties of the minerals constituting the sediment can be verified

Etude des processus rédox de l'uranium aux interfaces des composés du fer

Une des hypothèses concernant le devenir du combustible usé issu des centrales nucléaires est son stockage en couche géologique profonde. Il est généralement admis qu après environ 10000 ans les radioéléments, et parmi eux l uranium, pourraient être relâchés dans la biosphère. Afin d assurer la sûreté d un tel stockage, il est important de pouvoir évaluer la migration de l uranium dans l environnement. L objectif de ce travail est d étudier si la sidérite, composé du fer présent en champ proche et en champ lointain, peut limiter cette migration ainsi que le rôle des processus rédox dans cette rétention. La sidérite a été synthétisée par voie électrochimique sous forme de couche mince. Les dépôts obtenus sont adhérents et homogènes. Ils ont une épaisseur d environ un micron et sont constitués de grains sphériques. Les caractérisations réalisées montrent que la sidérite est exempte de toute impureté, ainsi que de toute trace d oxydation. Les interactions entre la sidérite et l uranium (VI) ont été étudiées dans des milieux considérés comme représentatifs des eaux d environnement, en terme de pH et de concentration en carbonates. La rétention de l uranium sur la couche mince est importante puisqu elle correspond, après 24 heures d interaction, à des capacités de rétention de plusieurs centaines de micromoles d uranium par gramme de sidérite. Des analyses XPS montrent que, dans l ensemble des conditions expérimentales, une partie de l uranium retenu à la surface de la couche mince est réduit en un composé du type UO2+x. Les processus d interaction diffèrent en fonction du milieu considéré, et notamment en fonction de la stabilité de la sidérite.An option investigated for the management of long-term nuclear waste is a repository in deep geological formations. It is generally admitted that the release of radionuclides from the spent fuel in the geosphere could occur several thousand years after the beginning of the storage. Therefore, to assess the safety of the long-term disposal, it is important to consider the phenomena that can reduce the migration, and in particular the migration of uranium. The aim of this work is to study if siderite, an iron compound present both in the near - and far -field, can limit this migration as well as the role played by the redox process. Siderite thin layers have been obtained by electrochemistry. The layers are adherent and homogeneous. Their thickness is about 1 m and they are composed of spherical grains. Analytical characterisations performed show that siderite is free of any impurity and does not exhibit any trace of oxidation. The interactions between siderite and uranium (VI) have been carried out in solutions considered as representative of environmental waters, in terms of pH and carbonate concentration. The retention of uranium on the thin layer is important since, after 24 hours of interaction, it corresponds to retention capacities of several hundreds of uranium micromoles per gram of siderite. XPS analysis show that, in any studied condition, part of uranium present on the thin layer is reduced into an overstoechiometric uranium dioxide. The process of interaction differs depending on the considered environment, specially on the stability of siderite.EVRY-Bib. électronique (912289901) / SudocSudocFranceF

Zn sorption on Ca-illite and Ca-smectite: experiment and modelling

International audienceTo predict Zn behaviour in soil, the retention properties of clay minerals plays a relevant role. In a continental environment Ca is the main cation in solution. Soil reactivity may be reduced to sorption properties of illite and smectite towards Zn and Ca, major clay minerals in soil components. With this assumption, a multi-site ion exchanger model has successfully been applied to the Zn sorption on Ca-illite and Ca-smectite. New batch experiments performed in this study enabled to collect sorption data for Zn on Ca-illite by concentration and pH isotherms. Zn sorption reversibility was then verified. These sorption data were modelled successfully with a multi-site ion exchanger formalism by using four sorption site types. Zn sorption isotherm on smectite were retrieved from literature and interpreted following the MSIE formalism The obtained selectivity coefficients may be thereafter put into ion exchange models to describe the Zn sorption in natural environments

Assessment of a predictive model to describe the migration of major inorganic cations in a Bt soil horizon

International audienceThe aim of this study was to test the ability of a previous published model describing the sorption properties of complex solids (Bt soil horizon, sediment) under static conditions (batch mode) to describe sorption data obtained under dynamic conditions. This model assumes that the sorption properties of the multicomponent solid can be described by those of smectites present in the mineralogical assemblage. In our case, the reference smectite is a Wyoming montmorillonite. To test the model, experimental breakthrough curves of some major cations were obtained using a Bt soil horizon in different physico-chemical conditions. The fairly good agreement between the different experimental data sets and predicted breakthrough curves demonstrates that our proposed model can be used to accurately predict ion exchange reactions occurring under dynamic conditions between Na+, Ca2+, and H+ cations in a complex mineralogical assemblage. In addition, this model is also able to accurately predict previously published experimental data obtained with another B soil horizons and using Na+, Ca2+, and Mg2+ as cations. Other models reported from the literature, based either on sorption properties of pure smectites or of complex assemblages, are not able to accurately interpret experimental data proposed in this study motivating our purpose to propose another model. Therefore, our predicted model represents an alternative to models based on the generalized composite approach, which describes the reactivity of a complex material using generic sorption sites for which reactivity is not explicitly related to the properties of the individual phases of the complex materia

Saline groundwaters in the hercynian granites (Chardon Mine, France): geochemical evidence for the salinity origin

International audienceIn this study, the chemical evolution of high Cl− Chardon mine groundwaters is modelled as a mixing between an oxidising recharge and an old marine component on which the water–rock interaction is superimposed. Chemical and isotopic similarities with saline Carnmenellis mine groundwaters are emphasised and a general comparison with other brines is discussed.The cation content of deep granitic groundwaters is indicative of the water–rock interaction. In the case of Chardon and Carnmenellis groundwaters, the high Na/Cl ratio can still be related to the contribution of a brine of sedimentary origin to the water salinity. The differences in the hydrochemistry related to their geological context only appears at the trace metals level. On the contrary, brines in plutonic rocks which exhibit a low Na/Cl ratio represent groundwaters having a residence time in the host rock, long enough to equilibrate with secondary aluminosilicates. In that case, the brine origin is difficult to assess if only based on the water cation content

Reactive transport of Sr at two laboratory-scale columns: experiments and modeling

International audienceIn support to the environmental monitoring of nuclear sites, predictions of reactive transport models used to simulate the contaminant migration such as 90Sr in soils, sediments and aquifers have to be improved. The objectives of this study are to evaluate the coupling between the multi-site ion exchanger model (MSIE), based on a component additivity approach, and the advection-dispersion equation (ADE) to simulate Sr transport in two-component materials. Non-reactive transport experiments with conservative tracers are used to determine transport parameters, such as total porosity and dispersivity. The reactive transport model was used to simulate Sr transport in a Bt soil and a clayey sandstone in centimeter and decimeter scale columns. Simulation results obtained with the additivity approach were compared to those obtained with a generalized composite approach (Kd). Simulations results shows that the Kd approach often fails to reproduce experimental data whereas the additivity approach successfully predicts the Sr reactive transport in a clayey sandstone both at the centimeter and decimeter scales without any adjustment of parameters. These results justify taking into account more complex retention model in reactive transport models to understand and improve the robustness of Sr transport prediction. Sorption property of a low capacity and high selectivity site on illite was revealed to be a determining parameter in Sr transport behavior. However, this database is restricted to minerals having similar reactivity than the references used and has to be extended to soil clay minerals

Role of trace elements in the 226^{226}Radium incorporation in sulfate minerals (gypsum and celestite)

International audienceIncorporation of $^{226}$Ra within gypsum (CaSO$_4$.2H$_2$O$_{(s)}$) and celestite (SrSO$_{4(s)}$) was assessed through dedicated batch experiments monitored over hundreds of days. Results indicate that the published value for the distribution coefficient of $^{226}$Ra between gypsum and an aqueous phase, D$_{Ra,}$$_{gypsum}$= 0.03 is an upper limit. On the other hand, celestite shows high incorporation of the radionuclide, with partition coefficient around 200. This high $^{226}$Ra uptake by celestite results from the existence of a solid solution between celestite and radium sulfate, as expected considering the similarities between the two solids, as between barite and radium sulphate. On the contrary, a solid solution between gypsum and radium sulfate (Ca,Ra)SO$_4$.2H$_2$O$_{(s)}$ cannot be considered per se, due mainly to the fact they do not behave in the same crystal family. However, $^{226}$Ra incorporation in gypsum can be enhanced by the presence of Sr impurities (from 0.1 molar %). In such conditions, the radium distribution coefficient is around 0.15 $\pm$ 0.09. This behavior can be explained by an ion-exchange mechanism between $^{226}$Ra and Sr. These results highlight the key role of trace elements in the incorporation of $^{226}$Ra in sulphate bearing minerals and bring new insights in our understanding of the $^{226}$Ra in environment.(as illustrated with an example in mining context