Santiago Leguey

Hydrothermal alteration of “La Serrata” bentonite (Almeria, Spain) by alkaline solutions

Abstract Bentonite and concrete are considered as backfilling and sealing materials in a deep geologic repository of high-level radioactive waste (HLRW). The pH of the pore waters leached during concrete degradation is >11 for a long period of time. Under these alkaline conditions, bentonite is susceptible to dissolution–precipitation reactions. The aim of this work is to study mineralogical changes in bentonite in contact with synthetic pore waters of cements. The temperature effect has been taken into account in order to consider the thermal impact of radioactive decay. The bentonite used comes from “La Serrata de Nijar” (Almeria, Spain) and is mainly composed of montmorillonite. Bentonite was submitted to alkaline solutions at 35, 60 and 90 °C up to 365 days. Alkaline solutions were similar to the pore waters leached in the first stage of cement degradation. The zeolite crystallization and the increase of magnesium in a non-exchangeable form in the smectitic clay fraction are the main mineralogical changes detected in the bentonite. Both processes are enhanced with the duration time and temperature of reaction.

Zeolite formation during the alkaline reaction of bentonite

The cement-bentonite interface is a highly dynamic region in engineered barrier systems (EBS) designed to isolate radioactive wastes. Two very different chemical environments are held together: a near-neutral clay mineral-calcite equilibrium-dominated system (hydrated bentonite system) and a thermodynamically unstable hyperalkaline one (cement). The interface is, therefore, in permanent disequilibrium due to the conditions of changing pH that are related to the different degradation steps of the cement. The formation of phillipsite-(Na,K), (Si/Al ratio of 1.8-2.4), has been found in a series of closed-system hydrothermal tests at 35-90 °C when the resulting pH was within the 11.7-12.6 range. The chemistry of the equilibrium solutions, rather than the crystallization substrata, controls the Si/Al atomic ratio and the type of zeolite that has been formed. This is based on the decrease of Si/Al at higher pHs and the predominance of phillipsite-Na. The formation of phillipsite-Na in these experiments is in agreement with the available thermodynamic data on zeolites.

The performance of natural clay as a barrier to the diffusion of municipal solid waste landfill leachates

In this paper, the diffusion of solutes in natural clay from a concentrated solution consisting primarily of ammonium, sodium and chloride ions at a pH level of 8 was studied and was based on an existing 20-year-old landfill. Contaminant transport through clay liners was predicted using transport and reaction geochemical codes to help explain the experimental data. The model predicted the chloride anion diffusion and cation exchange processes for three different experiments: (1) small-scale interactions in compacted clay, (2) 1:1 European Union (EU) Directive demonstration experiments (0.5-m-thick clay barrier), and (3) analysis of a bore hole with core recovery drilled in an old landfill located above a similar type of clay as that studied in (1) and (2). Orders of magnitude between 10(-10) and 10(-9) m(2) s(-1) were used for the apparent diffusion coefficient to fit the chloride profiles at the different scales; however, at larger space and time scales, diffusion was retarded due to the presence of more consolidated, non-mechanically disturbed clay materials at large depths in a natural clay-rock emplacement.

Diffusion of soluble salts under a temperature gradient after the hydration of compacted bentonite

Abstract The transport of the soluble salts contained in a compacted bentonite has been studied in a hydration and heating experiment. The objective is to follow the processes of transport produced during and after the hydration of compacted bentonite in order to detect the generation of localized saline environments under the presence of a temperature gradient. Permanent saline environments may affect the surfacial and swelling properties of a bentonite when used as a clay liner for the isolation of high-level nuclear waste (HLNW). Several hermetic cells have been designed to constrain an unsaturated compacted bentonite cylinder (dry density of 1.65 g/cm3). The bentonite is moisturized by injecting an aqueous solution in an opposite direction to a heat source. The tests are conducted during pre-saturation to post-saturation stages. The hydration is carried out with two different solutions, synthetic granitic water of low salinity and saline water. Two gradients with different average temperatures are generated: 100–60°C (16°C/cm) and 60–35°C (10°C/cm). Convection and diffusion processes govern the transport of salts in the compacted bentonite. The temperature is the variable that mostly influences the processes of transport, while the concentration of the injected solution had smaller importance. The convective transport of salts, driven by the hydration, occurs in the presence of the thermal gradient. By means of this mechanism, the dissolved ions concentrate on the heated area of the cell. The process permits the measurement of apparent diffusion coefficients (Da) for ions and salts during the homogenization of concentration profiles in the post-saturation stages. Da for Cl−, SO42−, Br−, Ca2+, Mg2+, Na+, K+, Sr2+ and Zn2+ have been calculated. The values vary in the range of 1.5×10−10–4×10−12 m2/s; the ions Cl−, Na+ and Ca2+ being of higher mobility. Codiffusion of Ca2+ and Na+ with Cl− is the main mechanism found for salt transport in the compacted bentonite.

CHEMISTRY OF Mg SMECTITES IN LACUSTRINE SEDIMENTS FROM THE VICALVARO SEPIOLITE DEPOSIT, MADRID NEOGENE BASIN (SPAIN)

The chemical and structural properties of Mg smectites in the Vicálvaro sepiolite deposit have been studied in detail. The characterization was performed on different size-fractions of selected smectitic samples (5−2 µm; 2−1 µm; 1−0.5 µm; <0.5 µm and <0.1 µm). The chemical compositions of individual particles (5−1 µm) and of bulk undifferentiated fine fractions (1−<0.1 µm) were determined by energy dispersive spectroscopy-scanning electron microscopy and interpreted with the aid of X-ray diffraction (XRD) and infrared spectroscopy (IR) methods. The XRD and IR data demonstrate that all of the Mg smectite materials studied are mainly composed of a complex mixture of stevensite, saponite and mica-type minerals. Although the presence or absence of saponite cannot be confirmed absolutely, stevensite is a significant component of these Mg smectites. This is proven by the calculated layer charge reduction after the Hofmann-Klemen effect. The results are in close agreement with the suggested mechanism of topotactic overgrowth of stevensite on pre-existing phyllosilicate templates. This characterizes clay diagenesis in saline-lake systems.

The role of biomineralization in the origin of sepiolite and dolomite

Sepiolite is an important industrial mineral whose mechanism of formation and detailed origin are still not clear. Sepiolite is frequently found with dolomite, Mg-smectites or palygorskite in the Intermediate Unit of Miocene sediments of the Madrid basin. Herein, we describe mineralogy, texture and mineral chemistry of sepiolite-rich layers (up to 90% concentration) in marl-clay sediments, either with gypsum, chert (micro-crystalline quartz), or dolomite, that are up to 2 to 3 m thick. The materials studied were formed in shallow lake environments or mudflats. These sediments contain dolomite aggregates resembling mineralized microorganisms (biomorphs) with ovoid morphologies and spherical to tubular internal voids. The regular size and relatively uniform shape of the aggregates are likely controlled by the shape and size of the microorganisms. The paper describes the presence of organic debris, silicification processes and potassium silicate depletion related to the basal sediments that preceded sepiolite rich layers. Potassium depletion, as a nutrient, in detrital components and the occurrence of organic and reduced sediment (U and sulfide concentration) at the interface with sepiolite also can be linked to biosignatures related to the origin of sepiolite. In addition, the occurrence of dolomite with carbon that has δ13C PDB values of < −7.5 permil confirms the possibility of an organic carbon source. High concentrations of sepiolite are observed where the amount of dolomite is small. The remaining dolomite in virtually pure sepiolite layers is present as partially dissolved biomorphs or fully recrystallized isolated crystals. Thus, a general process for dolomite dissolution and recrystallization is associated with sepiolite differentiation. In addition, the formation of sepiolite related to the biomineralization of dolomite is likely. The potential role of biomineralization in the formation of these high-grade sepiolite deposits depends on two factors: (1) specific organic-inorganic interactions and (2) the highly porous nature of the sepiolite precursor dolomite during the diagenetic evolution of these sediments. The confirmation of these processes awaits the future development of sepiolite synthesis strategies that incorporate the appropriate biogeochemical reactions.

The alkaline reaction of FEBEX bentonite: a contribution to the study of the performance of bentonite/concrete engineered barrier system

Desde 1997, el desarrollo de dos proyectos de la Union Europea, Effects of Cement On CLAY barrier performance (ECOCLAY, 1997-2000) y ECOCLAY phase II (2000-2003), ha permitido abordar la investigacion del efecto de la pluma alcalina inducida por la lixiviacion del hormigon sobre la bentonita. Ambos materiales forman parte de un concepto de barrera compuesta de hormigon y bentonita, en el contexto del almacenamiento de residuos a una profundidad dada. El grupo de geoquimica aplicada a las arcillas de la Universidad Autonoma de Madrid (UAM) ha participado en ambos proyectos, y ha conseguido aportar experiencia y cono¬cimiento al estudio del comportamiento de la bentonita de referencia espanola (FEBEX) bajo condiciones de elevada alcalinidad. Este articulo ofrece una sintesis del trabajo desarrollado, mencionando las lecciones aprendidas sobre la reactividad alcalina de la bentonita, y su enfoque practico hacia el analisis del comportamiento de uno de los conceptos de referencia, el almacenamiento de residuos en formaciones arcillosas. Se han seleccionado las principales contribuciones cientificas a este respecto, a juicio de los autores: (1) la naturaleza particular de los productos generados en la reaccion alcalina de la bentonita FEBEX (zeolitas, arcillas magnesicas y geles de silicato aluminato calcico hidratado (CASH)); (2) la aproximacion mediante mineralogia cuantitativa al ritmo de reaccion de la montmorillonita FEBEX a pHs elevados, y (3) la validacion experimental de la reactividad alcalina de la bentonita a partir de experimentos en columna.

Characterization of Mg-clays from the Neogene of the Madrid Basin and their potential as backfilling and sealing material in high level radioactive waste disposal

Abstract Three types of sedimentary materials are studied, referred to as green, brown and pink clays. They are exploited in a number of 0.25-3 m thick layers and contain magnesium smectites. The smectite content in the green and brown clays averages over 80%, with illite and sepiolite and small quantities of quartz and feldspar. The pink clay composition corresponds to disordered interstratified kerolite-stevensite, with a varying stevensite content (20dash45%). The structural formula of the smectite in the Adsorption, external specific surface and swelling pressure are very similar in the saponitic clays, with, respectively, average values of: 73–75 meq/100 g (CEC), : 160–170 m2/g (SBET) and 4.6-5.28 MPa (Ps) (precompacted at dry density 1.4 g.cm−3). When there is more than 15–20% sepiolite in the green clays, however, the CEC drops, the external specific surface increases and with unvarying density the swelling pressure doubles. This increase in swelling pressure is related to an additional increase in osmotic pressure produced in the external surfaces. This reaches 15–16 MPa in the pink clays with a specific surface of 250–260 m2/g. Maximum values of 19.4 MPa are reached in discrete levels of green clays containing 70% sepiolite and with a specific surface of 287 m2/g. The genesis of these materials is related to weathering processes in the fluviolacustrine environment in the different zones (lake centre and edge, swamp soils and vadose zone), associated with the evolution of reducing conditions and an alkalinity increase in the water table. The kinetics of these processes would appear to be extremely slow, as long periods without sedimentation are required in order to form significant quantities of magnesium clays and especially saponite. Saponitic clays display qualities that were suitable for their possible use as backfilling and sealing material, although the green clays appear to provide better conditions regarding the rehydration properties associated with the exchange complex and their lower carbonate content.

Thermo-hydraulic gradients on bentonite: distribution of soluble salts, microstructure and modification of the hydraulic and mechanical behaviour

Abstract In the context of an investigation of the near field for a repository of high-level radioactive waste (HLW), the FEBEX Project, a set of laboratory tests has been designed to give a better understanding of the thermo-hydro-mechanical and geochemical behaviour of the compacted bentonite. Small compacted samples of bentonite were heated during variable periods of time of up to 80 days under different thermal gradients. The hydration water was either granitic, simulating the conditions of the outer part of the barrier, or saline, which simulates the chemistry of the pore waters inside the bentonite barrier. At the end of the thermo-hydraulic (TH) treatment, a geochemical characterisation was performed in different sections, both of the soluble components and of the solid phase. The microstructure was analysed by means of optical microscopy and determination of the BET surface and pore size distribution. The swelling capacity of the whole sample and its permeability were also checked. The results of the permeability and swelling tests were compared to those obtained in experiments performed under the same conditions with nontreated samples. As a result of hydration, there exists a rapid movement of chloride towards the heater. This ion is progressively excluded of the bentonite after saturation that makes the bulk salinity of the clay decrease. Anion exclusion should prevent the anion transport once the bentonite is saturated, but the existence of preferential passages is postulated to explain the continuous salinity decrease. This phenomenon makes the salt content in the compacted bentonite very limited, with localised anomalies prior to saturation. All the physico-chemical parameters that have been determined are virtually unchanged during the TH treatment. An increase of the hydraulic conductivity after the TH treatment with saline water has been observed, while the swelling capacity of the samples treated with granitic water slightly increases after treatment.

Saponitic clays from the Madrid basin: Accessory-minerals influence in hydrothermal reactivity

Abstract Some hydrothermal alteration tests have been carried out on two selected clays which contain saponite as major component with illite or sepiolite representing the main accessory minerals in the bulk composition of saponitic clays. Both samples show an important increment of silica in solution and in removable amorphous silica when temperature rises from 60°C to 175°C. The same trend is observed related to K + adsorption. Occurrence of sepiolite causes different behaviour in irreversible K + fixation processes. A drastic degradation of sepiolite at 175°C, was detected by means of XRD together with slight increases in CEC while no irreversibly fixed K + was detected. On the opposite, significative amounts of fixed K + increasing with reaction time and lower levels of removable amorphous silica were the main features observed in the illite-saponite sample, where CEC remains almost constant with temperature and reaction time.