Competitive ion-exchange reactions of Pb(II) (Pb2+/PbCl+) and Ra(II) (Ra2+) on smectites: Experiments, modeling, and implication for 226Ra(II)/210Pb(II) disequilibrium in the environment.

In this study, new experimental data for the adsorption of lead onto a swelling clay mineral with a tetrahedral charge (beidellite) at the ultratrace level (<10-10 M) are presented. The data were interpreted using an ion-exchange multisite model that considers the sorption of major cations (including H+), which always compete with trace elements for sorption onto mineral surfaces in natural environments. The ability of the proposed model to predict experimental Kd values under various conditions of ionic strength (fixed by NaCl solutions) and aqueous cation compositions (including Pb2+ and PbCl+) was tested. The proposed model was applied to experimental data previously published for other types of swelling clay minerals, and the results were compared with the results obtained using previously published models. The preferential adsorption of chloride ion pairs, as well as the effect of the swelling clay crystal chemistry on lead adsorption, were assessed. Finally, the selective adsorption behavior of 226Ra compared to 210Pb was demonstrated, which has implications for the study of many environmental processes using isotope partitioning.

Assessment of 226Ra and U colloidal transport in a mining environment.

The colloidal transport of trace (Fe, Al, Ba, Pb, Sr, U) and ultra-trace (226Ra) elements was studied in a mining environment. An original approach combining 0.45 µm filtered water sampling, the Diffusive Gradient in Thin films (DGT) technique, mineralogical characterization, and geochemical modelling was developed and tested at 17 sampling points. DGT was used for the truly dissolved fraction of the elements of interest, while the 0.45 µm filtration includes both colloidal and truly dissolved fractions (together referred to as total dissolved fraction). Results indicated a colloidal fraction for Al (up to 50%), Ba (up to 86%), and Fe (up to 99%) explained by the presence of submicrometric grains of kaolinite, barite, and ferrihydrite, respectively. Furthermore, the total dissolved 226Ra concentration in the water samples reached up to 10-25 Bq/L (1.2-3.0 10-12 mol/L) at 3 sampling points, while the truly dissolved aqueous 226Ra concentrations were in the mBq/L range. Such high total dissolved concentrations are explained by retention on colloidal barite, accounting for 95% of the total dissolved 226Ra concentration. The distribution of 226Ra between the truly dissolved and colloidal fractions was accurately reproduced using a (Rax,Ba1-x)SO4 solid solution, with values of the Guggenheim parameter a0 close to ideality. 226Ra sorption on ferrihydrite and kaolinite, other minerals well known for their retention properties, could not explain the measured colloidal fractions despite their predominance. This illustrates the key role of barite in such environments. The measured concentrations of total dissolved U were very low at all the sampling points (<4.5 10-10 mol/L) and the colloidal fraction of U accounted for less than 65%. U sorption on ferrihydrite could account for the colloidal fraction. This original approach can be applied to other trace and ultra-trace elements to complement when necessary classical environmental surveys usually performed by filtration on 0.45 µm.

Ion exchange model for reversible sorption of divalent metals on calcite: implications for natural environments.

Most of the thermodynamic models available in the literature describing the speciation of the calcite surface do not predict a significant concentration of sorbed Ca(II), whereas previous electrokinetics studies clearly show that Ca(2+) is the main cation determining the potential of the calcite surface. This study proposes a new thermodynamic model based on ion-exchange theory that is able to describe the reversible sorption of Ca(2+) on calcite. To constrain the model, concentrations of Ca(II) sorbed reversibly on the mineral surface were obtained as a function of pH. Such experimental data were obtained using solutions in equilibrium with both calcite and fixed p(CO2(g)) values (from 10(-5) to 10(-2) atm). The concentration of (de)sorbed Ca(II) is almost constant in the [7-9.5] pH range, having a value of approximately 1.2 × 10(-6) ± 0.4 × 10(-7) eq·g(-1). Such a value agrees with total sorption site densities that were previously calculated by crystallography and is used to obtain a selectivity coefficient between H(+) and Ca(2+) species by fitting the experimental data. Then, selectivity coefficients between H(+) and different metallic cations (Zn(2+), Cd(2+), Pb(2+)) that are able to accurately describe previously published data are proposed. Finally, the model is used to predict the contribution of calcite in the overall sorption of Cd(II) on a natural and complex solid (calcareous aquifer sand).

Reactive transport of strontium in two laboratory-scale columns: Experiments and modelling.

To support the environmental monitoring of nuclear sites, reactive transport models used to predict the migration of contaminants such as strontium-90 (90Sr) in soils, sediments and aquifers are developed, continuously tested and improved. This study aims at assessing the adequacy of the multi-site ion exchanger model (MSIE) based on a component "additivity approach" and coupled to the advection-dispersion equation (ADE) to simulate Sr transport in a clayey sandstone and a Bt soil horizon. We have also compared the performance of the modelling approach with simulation results obtained by considering a Kd approach (constant Kd). Transport experiments were performed in centimetre- and decimetre-scale columns in order to test the model sensitivities to the mineral abundance and the specificities of their reactive parameters. Non-reactive transport experiments with conservative tracers allowed us to determine the transport parameters, such as porosity and dispersivity. In this paper, we have compared the Sr transport simulation results with Sr experimental breakthrough curves acquired at various flow velocities. The simulations results show that the Kd approach can reproduce experimental data in the case of the clayey sandstone when a certain amount of uncertainty is accepted, whereas the additivity approach better fits the Sr reactive transport in both columns (especially the maximum value) without it being necessary to adjust the parameters. These results advocate for more complex retention models than Kd to better understand and improve the robustness of Sr transport predictions. The clay content, the relative abundance of illite and smectite, and the clay mineral specificity, are all sensitive parameters when it comes to defining the reactive system involved in Sr transport simulation. The results highlight the influence of illite in the spreading of the Sr breakthrough curve, especially through its low-capacity and high-selectivity site. This implies having access to a robust and extensive set of retention parameters acquired on reference minerals. In this study, the results obtained for the clayey sandstone confirm the robustness of our selected parameters when clay minerals have similar reactivity levels as the reference minerals. This set of parameters appears more limited in the case of the Bt soil containing weathered or evolved minerals. The choice of modelling approach is therefore crucial for accurately modelling and predicting Sr transport behaviour in porous media, as is the representativeness of the minerals in the database.

Stability constants determination of successive metal complexes by hyphenated CE-ICPMS.

The study of radionuclides speciation requires accurate evaluation of stability constants, which can be achieved by CE-ICPMS. We have previously described a method for 1:1 metal complexes stability constants determination. In this paper, we present its extension to the case of successive complexations and its application to uranyl-oxalate and lanthanum-oxalate systems. Several significant steps are discussed: analytical conditions choice, mathematical treatment by non-linear regression, ligand concentration and ionic strength corrections. The following values were obtained: at infinite dilution, log(beta(1) degrees (UO(2)Oxa))=6.93+/-0.05, log(beta(2) degrees (UO(2)(Oxa)(2) (2-)))=11.92+/-0.43 and log(beta(3) degrees (UO(2)(Oxa)(3) (4-)))=15.11+/-0.12; log(beta(1) degrees (LaOxa(+)))=5.90+/-0.07, log(beta(2) degrees (La(Oxa)(2) (-)))=9.18+/-0.19 and log(beta(3) degrees (La(Oxa)(3) (3-)))=9.81+/-0.33. These values are in good agreement with the literature data, even though we suggest the existence of a new lanthanum-oxalate complex: La(Oxa)(3) (3-). This study confirms the suitability of CE-ICPMS for complexation studies.

Geochemical characterization of uranium mill tailings (Bois Noirs Limouzat, France) highlighting the U and 226Ra retention.

As with other metals, the management of tailings from former uranium (U) mines requires a good knowledge of the geochemical mechanisms governing the retention of radioelements of interest: U and 226Ra. This article presents the results of the study of the bearing phases featuring these two radioelements within the Bois Noirs Limouzat tailings storage facility (Loire), the only site in France where the tailings (a sandy silt facies and a clayey silt facies) are currently stored only under water. The aim is to gain a better understanding of their respective mobility under current storage conditions. For this purpose, a multi-scale approach was adopted combining historical research and airborne image analysis to select the core location, chemical and radiological analyses, mineralogical characterizations supplemented by sequential extractions (two specifically developed protocols). The results show that U and 226Ra are mainly found in the clayey silt facies with an average U concentration of 243.3 ppm (132.3 ppm in the sandy silt facies) and an average 226Ra mass activity of 64.7Bq/g (18.0Bq/g in the sandy silt facies). These results are in accordance with the initial U grade of the ore (2‰), the extraction efficiency of the ore processing plant (95%) and the age of mineralization (305 Ma). The approach adopted made it possible to highlight several mineralogical traps available for each radioelement, regardless of the facies type. Thus, a significant part of the U is still trapped within the primary phases, resistant to treatment and therefore relatively immobile under current storage conditions (49.6%-77.8% for the sandy silt facies and 27.2%-36% for the clayey silt facies). Most of the leached U is mainly associated with weakly crystalised iron oxyhydroxides (8.7%-42.4% for the sandy silt facies and 50.9%-71.8% for the clayey silt facies) and to a lesser extent with clay minerals (5%-12.3% for the sandy silt facies and 0.8%-11.5% for the clayey silt facies). For the 226Ra, irrespective of the facies type, a significant part remains trapped within phosphate phases, resistant to the leaching process and therefore also relatively immobile under storage conditions (24.4%-38.9% for the silty sandy silt facies and 39.9%-98.9% for the clayey silt facies). Sequential extractions revealed a different geochemistry of 226Ra depending on the facies. For the silty sandy silt facies, most of the 226Ra is mainly associated with the clay minerals (6.4%-69.2%) and to a lesser extent with iron oxyhydroxides, barite or aluminum phosphate sulphate minerals (APS) (6.4%-33.9%). For the clayey silt facies, most of the 226Ra is mainly associated with iron oxyhydroxides, barite or APS (6.4%-53.3%) and to lesser extent clay minerals (0.4%-6.8%). The leaching process did not allow the differentiation between the contributions of each of these phases to the retention of 226Ra. At last, all the identified bearing phases demonstrate that the U is relatively immobile under the current storage conditions, irrespective of the facies. For the 226Ra, the bearing phases differ according to the facies. Within the sandy silt facies, the 226Ra is mainly borne by clay minerals and can be mobilised more easily. However, the sandy silt facies represents only one third of the tailings currently.

Quantitative imaging of 226Ra ultratrace distribution using digital autoradiography: Case of doped celestines.

The ability of the autoradiographic device BeaQuant™ is evaluated herein to quantitatively map the ultratrace element 226Ra distributed spatially in celestine (SrSO4) grains/crystals. 226Ra doped celestines have been obtained from coprecipitation and recrystallization experiments, and have been characterized with high purity germanium gamma detector (HPGe), giving specific activities ranging from 3251 to 32523 Bq.g-1. Alpha autoradiographs of polished sections from doped celestines have been obtained using BeaQuant™. These alpha maps have been compared to the celestine grains/crystals arrangement observed with a scanning electron microscope (SEM). At the sample scale, celestine grains are responsible of an alpha signal, indicating that 226Ra is detectable in celestine from its alpha emissions. 226Ra distribution has also been investigated at the celestine grains/crystals scale: the crystal/grain properties do not allow to decide if the distribution process is homogeneous or not, i.e. if there is a chemical zoning into the crystal/grain. The counting of alpha particles by autoradiography has been compared with the total activity of the 226Ra doped celestines by gamma counting (HPGe technique). This comparison was performed by standardizing the measured activities to the same celestine volume, which has been determined by performing a threshold on SEM grey level images to assess to the celestine surface and using Geant4 Monte Carlo simulation toolkit to assess to the emission depth of the particles in celestine. A very good linear correlation between gamma activity and alpha counting from autoradiographs is obtained for all the samples, demonstrating the ability of BeaQuant™ to quantify 226Ra in any points of the millimetric section samples, at a resolution of 20 µm.

Metal complexes stability constant determination by hyphenation of capillary electrophoresis with inductively coupled plasma mass spectrometry: the case of 1:1 metal-to-ligand stoichiometry.

Nuclear energy development has raised new issues like radionuclides biogeochemistry. The modelling of their biochemical properties involves the accurate determination of thermodynamical data, like stability constants. This can be achieved by using hyphenated capillary electrophoresis (CE)-ICPMS and the method was applied successfully on 1:1 lanthanum-oxalate and uranyl-oxalate complexes. Several significant steps are discussed: choice of analytical conditions, electrophoretic mobility calculation, mathematical treatment of experimental data by using linear regressions, ligand concentration and ionic strength corrections. The following values were obtained with a good precision for lanthanum-oxalate and uranyl-oxalate complexes: log(K degrees (LaOxa(+)))=6.10+/-0.10 and log(K degrees (UO(2)Oxa))=6.40+/-0.30, respectively, at infinite dilution. These values are consistent with the literature data, showing CE-ICPMS potential for metal complexes stability constants determination.

Environmental impact of uranium mining and ore processing in the Lagoa Real District, Bahia, Brazil.

Uranium mining and processing at Lagoa Real (Bahia, Brazil) started in 2000. Hydrogeochemical monitoring carried out from 1999 to 2001 revealed generally good quality of the water resources outside and inside the mineralized area. No chemical contamination in waters for domestic uses was observed. Hydrochemical characteristics did not vary significantly after 1 year of U exploitation, as compared to premining conditions. Due to the short time of mining, the results cannot exclude future variations in water quality. Leaching experiments helped to describe processes of ore and waste degradation. Sulfate was identified as an indicator for different types of contamination. Potential hazards related to local climate (hot rainy season) were identified. They indicate that tailings derived from the ore processing, destabilized by sulfuric acid attack, may induce acidification and salinization in the surrounding environment. Another potential source of environmental impact could be linked to local radium-rich mineralization, originating radon emission.