Influence of salinity gradients on the diffusion of water and ionic species in dual porosity clay samples.

Most of the available data on diffusion in natural clayey rocks consider tracer diffusion in the absence of a salinity gradient despite the fact that such gradients are frequently found in natural and engineered subsurface environments. To assess the role of such gradients on the diffusion properties of clayey materials, through-diffusion experiments were carried out in the presence and absence of a salinity gradient using salt-diffusion and radioisotope tracer techniques. The experiments were carried out with vermiculite samples that contained equal proportions of interparticle and interlayer porosities so as to assess also the role played by the two types of porosities on the diffusion of water and ions. Data were interpreted using both a classical Fickian diffusion model and with a reactive transport code, CrunchClay that can handle multi-porosity diffusion processes in the presence of charged surfaces. By combining experimental and simulated data, we demonstrated that (i) the flux of water diffusing through vermiculite interlayer porosity was minor compared to that diffusing through the interparticle porosity, and (ii) a model considering at least three types of porous volumes (interlayer, interparticle diffuse layer, and bulk interparticle) was necessary to reproduce consistently the variations of neutral and charged species diffusion as a function of salinity gradient conditions.

Role of zeolite content on the sorption properties of analcime-rich rocks from the Abinky Formation (Niger).

The Abinky formation, composed of analcimolites (i.e., rocks with <70 wt% analcime), underlies Tchirezrine II, which hosts the Imouraren (Niger) uranium deposit. A potential mining project is under consideration to recover U by in situ acid leaching. Analcimolites are uncommon rocks, and assessing their ion-exchange properties is the first step to understand and predict the mobility of aqueous species in these formations. The objective of this study is then to understand the link between the Cation Exchange Capacities (CEC) of analcimolites as a function of their analcime content and associated crystal chemistry. Mineral quantification was performed by Rietveld refinement constrained by local chemical analysis with scanning electron microscopy coupled with Energy Dispersive Spectrometry. CEC were obtained at neutral pH by performing NH4+-for-Na+ exchange (CECNa/NH4), and Na+/H+ ion exchange experiments were performed with 4 analcimolites. Results showed that the analcime crystal chemistry deduced from Rietveld refinement was in good agreement with that obtained from SEM analysis (1.99 < Si/Al < 2.53). The results showed that all samples had a positive correlation between CECNa/NH4 and analcime content until ~30 meq/100 g for a sample containing ~85wt%Riet. of analcime, and that ~6 % of the total amounts of Na+ present in the analcime could be exchanged by NH4+ and H+. Based on Si and Al aqueous measurements, results showed that exchange with Na+ is the main process consuming H+ during Na+/H+ exchange when pH > 3.5. These experimental data were then interpreted by considering a single site equal to the CECNa/NH4 value, specific for each analcimolite, and a selectivity coefficient equal to log KNa/H = 1.3 (Gaines Thomas convention) being equal for all samples investigated. Finally, these data were used to assess the role played by Na+/H+ exchange in the pH evolution of the pore water of an analcime-rich rock subjected to dynamic acidification.

Quantifying 226Ra activity in a complex assemblage of 226Ra-bearing minerals using alpha autoradiography and SEM/EDS.

226Ra is an ultra-trace element with important environmental implications for many industries (including water treatment and oil and mineral extraction). Its extremely low concentrations in natural environments do not allow for direct observation and measurement of the 226Ra-bearing minerals governing 226Ra mobility. To better understand the retention processes for 226Ra in rocks and soil, a synthesized assemblage of 226Ra-doped minerals was made, combining montmorillonite, ferrihydrite and barite. A new methodology was developed using alpha activity maps acquired using alpha autoradiography, and elemental maps by using SEM/EDS. These maps were processed using a global approach, considering the entirety of the signal. The comparison of the alpha activity map and the elemental map enabled a correlation to be established between the 226Ra activity and the chemical composition and identification of the main 226Ra-bearing mineral of the assemblage, from which we were able to estimate the contribution of each mineral to the total activity of the assemblage, and to quantify the 226Ra-activity for each mineral. This methodology makes it possible to link mineralogy and occurrence of 226Ra at the scale of the mineral (tens of µm). It can be applied to natural samples, including fine-grained samples with a complex mineralogy.

Effect of solutes structure and pH on the n-octanol/water partition coefficient of ionizable organic compounds.

Liquid-liquid partition coefficient is a useful tool to predict biological and environmental fate of organic compounds, for example bioaccumulation or toxicity of lipophilic contaminants. Conversely, the partitioning of ionizable compounds is poorly studied in contrast to that of neutral compounds. Yet, such topic deserves attention, since numerous organic contaminants are ionizable as well as their degradation products. Hence, the contribution of charged species has to be considered in order to model accurately the mass balance or partition of ionizable compounds. In this context, we investigated the liquid-liquid partition of 13 ionizable compounds (oxalic acid, histidine, benzimidazole, etc.), covering various classes of compounds (carboxylic acids, amino-acids, etc.). The n-octanol/water partition coefficient was measured from pH 1 up to 13, in order to fully gather the distribution of both neutral and charged species. Empirical models describing these results are reviewed and partition parameters adjusted for charged species. The study of benzoic acid derivatives (benzoic, salicylic, ortho- and iso-phthalic acids) provides insights on the influence of chemical groups on the partitioning. In the case of tryptophan, the use of acid/base microconstants allowed to estimate the partition of both the zwitterion and its neutral tautomer. Despite a major zwitterionic form (log PZ(tryptophan) = -1.58 ± 0.30), the minor but neutral tautomer (log PN(tryptophan) = +0.03 ± 0.30) drives the partition equilibrium. Overall, the provided data may be useful to assess the retention of contaminants, its dependency on pH and salinity variations, and thus understanding their environmental fate. Such data may also be useful as well for molecular simulation involving solvation of organic ions in aqueous and non-aqueous solvents.

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.

Diffusion of Water through the Dual-Porosity Swelling Clay Mineral Vermiculite.

Prediction of water and solute migration in natural clay-based materials requires a detailed understanding of the roles played by different porosity types (around or inside clay particles) on the overall transfer process. For smectite, a reference material for the design of migration models, this discrimination is complex because of osmotic swelling of the structure under water-saturated conditions. Diffusion experiments with a water tracer (HDO) were conducted on 0.1-0.2, 1-2, and 10-20 µm size fractions of Na-vermiculite, a swelling clay mineral with no osmotic swelling. Results obtained for the two finest fractions suggest that osmotic swelling and the associated impact on pore structure are responsible for the low De values reported in the literature for smectite compared to those of vermiculite. When considering only interparticle porosity, De values for vermiculite are similar to those reported for nonporous grains (Na-kaolinite and Na-illite). This indicates that interparticle porosity has a primary effect on the overall water diffusion process, whereas interlayer porosity is shown to imply a small proportion of HDO adsorption. This study provides evidence that vermiculite is a promising reference mineral for the understanding of the roles played by pore structure and mineral-water interaction in the transport properties of water in claystones and for associated refinement of dual-porosity diffusion models.

How mobile is iodide in the Callovo-Oxfordian claystones under experimental conditions close to the in situ ones?

The iodide behaviour towards the Callovo-Oxfordian claystone was studied using batch and diffusion experiments under conditions which limited the artefacts cited in the literature to be responsible for the iodide uptake (i.e. the experiments were carried out under anoxic conditions with N(2)/CO(2) atmosphere with a monitoring of the iodine redox-state). The results show that all the radioactive iodine was (125)I(-), with no measurable activity for (125)IO(3)(-), which is known to have a higher affinity for the rock than iodide. Moreover, the batch experiments revealed no sorption, independently of the initial iodide concentration (from 10(-6) to 10(-3) mol L(-1)) and the contact time (up to 106 days). Conversely, the diffusion experiments indicated a weak but measurable retention. The through-diffusion experiments led to distribution ratio values only significant (R(D)~0.05 mL g(-1)) for initial iodide concentration ≤ 10(-4) mol L(-1). Higher R(D) values were estimated from out-diffusion experiments, ranging from about 0.05 mL g(-1) for an initial concentration of 10(-3) mol L(-1) to 0.14 mL g(-1) for the lowest one. A retention phenomenon that could be reversible and kinetically-controlled was proposed to explain the differences in the extent of the iodide retardation of the two types of diffusion experiments.

New experimental approach for studying diffusion through an intact and unsaturated medium: a case study with Callovo-Oxfordian argillite.

The diffusion of tritiated water and anionic species was studied in an unsaturated core of Callovo-Oxfordian claystone, which is a potential host-rock for disposal of high-level radioactive wastes. The diffusion parameters in such conditions were determined using modified through-diffusion cells in which the suction is generated by the osmosis process. This specific device leads to values of saturation degree ranging from 81% to 100%. The results show that the diffusion through unsaturated samples is clearly slower than that in fully saturated samples, with steady-state fluxes decreasing by a factor up to 7 for tritium and up to 50 for anionic species. While tritium porosity values follow volumetric water contents (from 21 to 16%), the porosity accessible to anionic species significantly decreases (from 7.5 to 0.7%). Such diffusive behaviors have been modeled by means of a modified Archie's law, taking into account a critical water saturation below which no tracer can percolate. These results indicate that the largest pores, which are initially affected by dehydration, would play an important role on the connectivity of the porous medium. This would especially affect anionic species diffusion behavior because they are constrained to diffuse into the largest pores first.

Contribution of the diffusive exchange method to the characterization of pore-water in consolidated argillaceous rocks.

Consolidated argillaceous rocks, potential host rocks for nuclear waste disposal, display particular properties (low water content, very small pore size) that require specific methods, especially for the characterization of their pore-water. The diffusive exchange method was applied to the claystone of Tournemire by taking into account the effect of swelling or micro-cracks induced by drilling. The good agreement of diffusion parameters with those obtained from the through-diffusion method showed that the extent of this disturbance was low. This method led to stable isotope content values in pore-water in good agreement with those measured in the fracture fluids located in the vicinity, validating the method. On the contrary, the vacuum distillation technique gave a systematic depletion in heavy isotopes of water, probably induced by an incomplete extraction of pore-water. For the chloride tracer, the determination of the accessible porosity on each sample allowed the estimate of chloride concentration of pore-water to be improved, by taking into account all the petrophysical heterogeneities.