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enresa publication tecnica 09/2004
FEBEX II Project THG Laboratory Experiments
Edited by: T. Missana CIEMAT
enresa ENRESA Direction de Ciencia y Tecnologia Emilio Vargas n° 7 28043 Madrid - Espana Tfno.: 91 5 668 1 00 Fax: 91 5 668 169 www.enresa.es Disen o y production: TransEdit Imprime: GRAFISTAFF, S.L. ISSN: 7 734-380X D.L.M-54 7 73-2004 Diciembre de 2004 This report has been drawn up within the context of the FEBEX project. Its contents represents only the opinion of the authors, which do not necessarily coincide with those of the other participants in the said project. Authors:
CIEMAT: A M3. Fernandez, M. Garcia-Gutierrez, Ni. Mingarro, l Missana, P. Rivas.
CSIC: E. Caballero, F. J. Huertas, S. Garcla-Palma, C. Jimenez de Cisneros, J. Linares, M. L. Rozalen.
PSI: B. Baeyens, M. H. Bradbury.
VET: A Muurinen.
EMPRESARIOS AGRUPAOOS: J. L. Cormenzana fable of contents Table of contents Table of contents
RESUMEN...... 1
ABSTRACT...... 5
1. INTRODUCTION...... 9
2. FEBEX BENTONITE...... 13
3. POREWATER IN THE CLAY BARRIER...... 19 3.1 Experimental methods for the physico-chemical charaterisation of the FEBEX bentonite and porewater analysis ...... 21 3.1.1 Introduction ...... 21 3.1.2 Experimental Techniques ...... 22 3.1.3 Solid phase characterisation ...... 24 3.2 FEBEX bentonite porewater chemistry modelling ...... 35 3.2.1 Introduction ...... 35 3.2.2 Porewater chemistry calculations in FEBEX bentonite ...... 38 3.3 Physicochemical properties of bentonite: effect of the exchangeable cations ...... 40 3.3.1 Introduction ...... 40 3.3.2 Effect of the exchange complex on the bentonite hydration properties ...... 42 3.3.3 Isotopic study of adsorption water in bentonite. A preliminary study ...... 47 3.4 Experiences obtained in previous studies with MX-80 clay...... 51 3.4.1 Introduction ...... 51 3.4.2 Equipment used for squeezing of bentonite porewaters ...... 51 3.4.3 Analysis methods for porewaters ...... 52 3.4.4 Experiences of pore water studies ...... 54 3.5 Pore water obtained by squeezing as a function of the applied pressure ...... 58 3.5.1 Introduction ...... 58 FEBEXII project. THG Laboratory Experiments
3.5.2 Hydrodynamic results ...... 58 3.5.3 Analytical results ...... 62
4. EFFECT OF INTERLAYER CATIONS ON THE RHEOLOGICAL PROPERTIES OF BENTONITE...... 65 4.1 Viscosity of clay suspensions ...... 61 4.2 Materials and methods ...... 61 4.3 Results ...... 61 4.3.1 Fitting of the flow curves ...... II 4.3.2 Apparent viscosity...... II 4.4 Discussion and conclusions ...... 14
5. GEOCHEMICAL PROCESSES AT THE SMECTITE-SOLUTION INTERFACE: DISSOLUTION, TRANSPORT AND PRECIPITATION...... 75 5.1 Introduction ...... 77 5.2 Materials...... 77 5.3 Methods ...... 19 5.4 Results and discussion ...... 81
6. RADIONUCLIDE SORPTION AND MIGRATION IN BENTONITE...... 93 6.1 Diffusion experiments in compacted clay...... 95 6. 1.1 Introduction ...... 95 6.1.2 Materials and methods ...... 95 6.1.3 Theoretical description ...... 96 6.1.4 Experimental methods ...... 91 6.2 Batch sorption experiments in purified clay...... 101 6.2.1 Experiments with FEBEX clay...... 101 6.2.2 Experiments with MX-80 clay...... ii9
1. CONCLUSIONS...... 125 1.1 Porewater in the clay barrier...... 126 1.2 Effect of the interlayer cations on the rheological properties of bentonite ...... 121 1.3 Geochemical processes at the solution/bentonite interface ...... 121 1.4 Sorption and migration in bentonite ...... 128
8. REFERENCES...... 131
IV Resumen Resumen Resumen
Uno de los objetivos principales de la bentonita en □ Estudio de los efectos del complejo de cambio un almacenamiento de residues radiactivos es la de de la arcilla en sus propiedades reologicas; actuar como barrera geoqufmica a la migration de □ Identification y modelacion de los procesos que los radionucleidos. La capacidad de esta barrera tienen lugar en la interfase bentonita/agua; geoqufmica depende de sus propiedades superficia- determination de las constantes de solubilidad les y del entorno ffsico-qufmico generado par las in- de la esmectita y de formation de complejos teracciones agua/roca. superficiales; En el marco del proyecto FEBEX, se diseno un pro- □ Estudio de los mecanismos de adsorcion de grama de ensayos de laboratorio para entender los distintos radionucleidos en la bentonita. procesos fundamentales que tienen lugar en la ba rrera de bentonita. Desde las primeras Loses del □ Estudio de la difusion de especies neutras y proyecto, estos ensayos de laboratorio ban permiti- anionicas para profundizar en los mecanismos do aislar distintos procesos y facilitar su interpreta de exclusion anionica y obtener valores de po- tion, edemas de proporcionar parametros utiles rosidad accesible en funcion de la densidad de para su aplicacion en los modelos THM y THG. Los la bentonita; dates experimentales tambien ban side utiles para □ Completar la base de datos de coeficientes de validar los modelos THM y THG ya existentes. difusion para los trazadores utilizados en los En la segunda Lose del proyecto, los ensayos de la experimentos "in-situ" y "mock-up". boratories se centraron fundamentalmente en los as- El objetivo de este informe tecnico es el de resumir pectos relevantes que no babfan tenido un suficiente los principales resultados obtenidos durante la se desarrollo en la Lose anterior. En particular, se desa- gunda Lose del proyecto FEBEX, por todos los grupos rrollaron las siguientes Ifneas de investigation: de investigation involucrados en el programa de en □ Obtencion de una description Liable del agua sayos THG. El informe se estructurara en cuatro blo- de pore de la bentonita, en distintos condicio- ques principales, en los que se reunen las contribu- nes geoqufmicas; ciones de los distintos grupos. CIEMAT (Espana), PS I (Suiza), CSIC (Espana) y VTT (Finlandia) ban contri- □ Identification de los distintos tipos de agua buido al estudio del agua de poro en la barrera de presentes en la bentonita y determination de arcilla; CSIC (Espana) ha contribuido al estudio de la cantidad de agua que esta realmente dispo- los procesos geoqulmlcos en la Interfase arcilla/ nible para el transpose de solutos; agua y al estudio de los efectos de los catlones de □ Evaluation de los potenciales efectos que la cambio en las propiedades reologicas de la arcilla. presion de extraction tiene en la composition Finalmente, CIEMAT (Espana) y PS I (Suiza) ban con qufmica del agua obtenida por consolidation tribuido a los estudios sobre la sorclon y migracion a alto presion; de radionucleidos en la bentonita.
3
Abstract Abstract Abstract
The main roles of the bentonite in a radioactive □ Study of the effects of the exchange complex in waste repository is to act as a geochemical barrier the rheological properties of the clay. against the radionuclides migration. The effective □ Identification and modelling of the surface pro ness of this geochemical barrier depends on the sur cesses occurring in smectite, determination of face properties of the solid phases and on the the solubility constants of smectite and the for physico-chemical environment generated by the in mation constants of the surface complexes. teraction of the solid phases with the groundwater. □ Understanding of the mechanisms involved in Within the FEBEX (Full-scale Engineered Barriers the sorption of different radionuclides in the Experiment) project, a program of laboratory tests bentonite. was designed to study and to understand the pro cesses taking place in the clay barrier. Since the first □ Investigation of the diffusion mechanisms of stages of the project, these laboratory tests enabled conservative neutral and anionic species to to isolate different processes, making easier their in have a deeper insight on the anionic exclusion terpretation, and provided fundamental parameters process and determine the accessible porosity to be used in the Thermo Hydro Mechanical (THM) to diffusion at different clay densities. and Thermo Hydro Geochemical (THG) models. □ To complete the diffusion coefficients database Additionally, experimental data enabled to check for the tracers used in the "mock-up" and "in the predictive capability of these models. situ" tests.
In the second phase of the project, laboratory tests The aim of this report is to summarise the main re focused on all those relevant aspects not sufficiently sults obtained by all the research groups involved in covered during FEBEX I. Particularly, the following the THG Laboratory Experiments programme, during main objectives were proposed for the THG investi the second phase of the FREBEX project. The report gations during FEBEX II: will be organised in four main blocks in which the □ Attainment of a reliable description of the po- contributions of different institutions will be collected. rewater chemistry at different geochemical con CIEMAT (Spain), PS I (Switzerland), CSIC (Spain) and ditions. VTT (Finland) contributed to the study of the pore water in the clay barrier; CSIC (Spain) contributed to □ Identification of the different types of water pres the study of the geochemical processes at the solu ent in the bentonite and to determine the amount tion/bentonite interface and to the study of the ef of available water for the solute transport. fects of the interlayer cations on the rheological pro □ Evaluation of the potential effects of the extrac perties of bentonite. Finally CIEMAT (Spain) and PS I tion pressure in the chemical composition of (Switzerland) contributed to investigate radionucli the water obtained by squeezing methods. des sorption and migration in bentonite.
7
1. Introduction 1. Introduction 1. Introduction
The aim of the FEBEX (Full-scale Engineered Barriers ved also to check the predictive capacities of the ex Experiment) project is to study the behaviour of istent THM and THG numerical models. components in the near-field for a high-level radio FEBEX I showed that the THM and THG models are active waste repository (HLWR) in crystalline rock. very sensible to small variations in some of the pa The experimental work consists of three main parts: rameters that represent the properties of the mate □ an "in situ" test, under natural conditions and rials. It became also clear that such parameters are at full scale, located at the Grimsel Test Site not constant, but they may vary as a result of the (Grimsel, Switzerland); changing conditions in the clay buffer.
□ a "mock-up" test, at almost full scale located For these reasons, the FEBEX II project included also at CIEMAT (Madrid, Spain); a wide laboratory tests programme, which focuses on those aspects not sufficiently covered during FEBEX I, □ a series of laboratory tests to complement the and on the new processes and scenarios that are be information from the two large-scale tests. ing considered in this second phase of the project. The project is based on the Spanish reference con This information will help in the interpretation of the cept for radioactive waste disposals in crystalline results obtained in the "mock-up" and "in situ" tests. rocks, in which the waste canisters are placed hori One of the main roles of the bentonite in the reposi zontally in drifts and surrounded by a clay barrier tory is to function as a geochemical barrier against constructed from highly-compacted bentonite blocks the migration of radionuclides. The effectiveness of (ENRESA, 1995). this geochemical barrier depends on the surface properties of the solid phases and on the physico The engineered barriers (waste, canister, and clay chemical environment generated by the interaction barrier) are key elements in the final disposal con of the solid phases and water. The importance of cept for high level radioactive waste. The clay ba this issue makes necessary to extend the research rrier has the multiple purpose of providing mechani work to the investigation of basic processes whose cal stability for the canister, by absorbing stress and importance was demonstrated by FEBEX I but that deformations, of sealing discontinuities in the adja were not sufficiently studied. cent rock and retarding the arrival of groundwater at the canister. In addition, these barriers are expec The laboratory tests programme of FEBEX II conti ted to retain radionuclides or retard their migration, nues in part with the work carried out during FEBEX once failure of the canister and lixiviation of the I, in those areas where more data about material spent fuel have occurred. properties are required, but the programme also in cludes tests on new areas. The proposed program The behaviour of a HLWR is determined, to a large me is divided into three main fields: Tests related to extent, by the characteristics of the design and con THM processes, tests related to THG processes, and struction of the engineered barriers and especially tests in connection with the generation of gases by the changes that may occur in the mechanical, from the bentonite. This document refers to the THG hydraulic, and geochemical properties as a result of tests, that will be focused on the study of the physi the combined effects of heat generated by the ra co-chemical properties of the bentonite and pore dioactive decay and by the water and solutes from water in the clay barrier, the geochemical processes the surrounding rock. Therefore, in FEBEX I, it was at the bentonite/solution interface, and the radionu considered of fundamental importance to under clide sorption and transport processes in the bento stand and quantify the processes taking place in the nite. The work planned for the 2nd phase of FEBEX is near-field, for the evaluation of the HLWR long-term therefore divided in 3 main sub-programmes. behaviour. As a consequence, the program of labo □ CIEMAT (Spain), PS I (Switzerland), CSIC (Spain) ratory tests was designed to study and comprehend and VTT (Finland) contributed to the program the processes that take place in the clay barrier un me devoted to the study of the pore water in der simple and controlled conditions and to develop the clay barrier. the governing equations. These laboratory tests ena bled to isolate the different processes, making easier □ CSIC (Spain) contributed to the study of the their interpretation, and they provided fundamental geochemical processes at the solution/bentonite data concerning the parameters to be used in the interface and to the study of the effects of the thermo-hydro-mechanical (THM) and thermo-hidro- interlayer cations on the rheological properties geochemical (THG) models. The laboratory tests ser of bentonite.
11 FEBEXII project. THG Laboratory Experiments
□ CIEMAT (Spain) and PS I (Switzerland) contributed □ To understand the mechanisms involved in the to investigate radionuclides sorption and migration sorption of different radionuclides, in order to in bentonite. identify and quantify to what extent these mecha nism occur in different experimental conditions.
The following main objectives were proposed for the □ To investigate the diffusion mechanisms of THG investigations during FEBEX II: conservative neutral and anionic species to have a deeper insight on the anionic exclusion □ To attain a reliable description of the porewater process and determine the accessible porosity chemistry at different geochemical conditions. to diffusion.
□ To identify the different types of water present □ To complete the diffusion coefficients database in the bentonite, and to determine the amount for the tracers used in the "mock-up" and "in of available water for the solute transport. situ" tests. The aim of this report is to summarise the main re □ To evaluate the potential effects of the extrac sults obtained by the research groups involved in the tion pressure in the chemical composition of THG Laboratory Experiment Work-Package. It will be the water obtained by squeezing methods. organised following the structure of the above-men □ To identify and model the surface processes tioned sub-programmes, in which the contributions of occurring in smectite, determining the solubility each institution will be presented separately. A constants of smectite and the formation cons chapter that shortly describes the main properties of tants of the surface complexes. the FEBEX bentonite will be also included.
12 2. FEBEX bentonite 2. FEBEX bentonite 2. FEBEX bentonite
The FEBEX bentonite comes from the Cortijo de Ar- titanium and rare earth. They appear in a quantity chidona deposit (Almeria, Spain). It has been selec around 0.8 percent. ted by ENRESA as suitable material for the back-fill The mineralogical composition has been also ob ing and sealing of HLRW repositories. This mate served and quantified by optical microscopy study of rial was used both in laboratory tests and in the "in thin sections (Cuevas et al. 2000). The textural hete situ" and "mock-up" tests (ENRESA 1998 and 2000). rogeneity itself is the main feature that can be des The processing at the factory has consisted in dis cribed in the sample. The FEBEX bentonite is mainly aggregation and gently grinding, drying at 60 °C composed of clay aggregates whose aspect ranges and sieving by 5 mm. Thus, the "as received" mate between dark isotropic low crystalline size ones to rial is a bentonite granulate. those presenting preferred orientation and relatively The physico-chemical properties of the FEBEX ben large (sub-micrometric) crystals. The remaining ele tonite, as well as its most relevant termo-hydro-me- ments of the texture are glassy materials, volcanic chanical and geochemical characteristics obtained rock fragments and individual accessory minerals during FEBEX I are summarised in the final report of (quartz and feldspars). Calcite is usually present as the project (ENRESA 2000) and are shown with sparitic crystals replacing feldspars, but it has been more detail in ENRESA (1998). Several laboratories observed also as isolated micritic cements. participated in these characterisation tasks. A sum Table 2 shows the average content values of the ma mary of the results obtained is given below. jor elements and Table 3 shows the average values of the minor and trace elements of the FEBEX bento The mineralogical composition of the FEBEX ben nite. Table 4 shows the average content values of the tonite was analysed by X-ray diffraction (XRD). The exchangeable cations, along with the cation exchan content of the mineral montmorillonite is higher ge capacity (CEC). The cation exchange capacity (CEC) than 90 percent. However, the smectite phase is ac of the FEBEX clay, was determined in the FEBEX I as tually made up of a smectite-illite mixed layer, with the sum of exchangeable cations displaced by 1 M 10-15 percent of illite layers. Besides, the bentonite ammonium acetate at pH 8. It is of 1 02 ±4 meq/ contains variable quantities of quartz, plagioclase, 1 OOg, and the major exchangeable ca- tions are: K-feldspar, calcite and opal-CT (cristobalite). By weight Co (42 %), Mg (33 %), No (23 %) and K (2 %). New from dense concentrates and SEM observation, the techniques were applied in the frame of the FEBEX II following minerals have been identified: mica (bio- project to optimise the evaluation of the exchange tite, sericite, muscovite), chlorite, non-differentiated able cations, as described in Chapter 3. silicates (Al, K, Fe, Mg, Mn), augite-diopside, hy- persthene, hornblende, oxides (ilmenite, rutile, mag The liquid limit of the bentonite is 1 02 ± 4 percent, netite, Fe-oxides), phosphates (apatite, xenotime, the specific gravity 2.70+0.04, and 67±3 percent monacite) and other non differentiated minerals of of particles are smaller than 2 / Table 1 Content of the main and accessory minerals of the FEBEX bentonite, in wt% (Fernandez e/a/., 2001; 2004). Main minerals Accessory minerals Poorly ordered minerals Smectite 92 ± 3 Organic Matter (as C02) 0.35 ± 0.05 Si02 0.038 ± 0.005 Quartz 2 ± 1 Carbonates (calcite, dolomite) 0.60 ±0.13 Plagioclase 2 ± 1 Soluble sulphates (gypsum) 0.14 ±0.01 ai2o3 0.035 ± 0.005 Cristobalite 2 ± 1 Low soluble sulphates (barite, celestite) 0.12 ±0.05 K-Feldspars Traces Sulfures (pyrite) 0.02 ± 0.01 Tridymite Traces Fe 203 0.105 ±0.009 Chlorides (halite) 0.13 ±0.02 Calcite Traces 15 FEBEXII project. THG Laboratory Experiments Table 2 Chemical composition of FEBEX bentonite, in %. CSIC CIEMAT Si02 58.92 ±1.74 58.89 ± 1.55 ai2o3 19.48 ±1.05 17.95 ±0.71 fe 203 total 3.48 ± 0.63 2.84 ± 0.12 MgO 4.83 ± 0.27 4.21 ± 0.21 MnO 0.06 ± 0.02 0.04 ± 0.00 CaO 2.51 ± 0.09 1.83 ±0.10 Na20 2.28 ±0.11 1.31 ±0.09 K20 1.21 ±0.08 1.04 ±0.05 Ti02 0.27 ± 0.06 0.23 ± 0.01 P205 0.06 ± 0.02 0.03 ± 0.01 H20"(,) 5.07 ± 0.76 8.66 ±2.88 h 2o+|2) — 4.31 ± 0.41 C02 org 0.19 ±0.04 0.35 ± 0.05 C02 inorg 0.52 ± 0.07 0.26 ± 0.06 S02 total — 0.21 ±0.10 F" 0.21 ± 0.03 0.18 ±0.01 !1> Determined at220°C, Table 3 Minor and trace elements of the FEBEX bentonite, in ppm. water content in equilibrium with the laboratory at the intra-aggregate pores (smaller than 0.006 /cm) mosphere (relative humidity 50± 1 0 %, temperature represent the 73-78 percent of total pore volume 21 ±3 °C, total suction about 100 MPa) is 13.7± 1.3 %. when the bentonite is compacted at a dry density of The value obtained in FEBEX I for the external spe 1.7 g/cm3 (Villar 2000). cific surface using BET technique is 32±3 m2/g and More details on the physico-chemical characterisa the total specific surface obtained using the Keeling tion of FEBEX bentonite with new results obtained in hygroscopicity method is about 725 m2/g. The the second phase of the project will be given in analysis of the mercury intrusion data reveals that Chapter 3. 16 2. EEBEX bentonite Table 4 Average values of cation exchange complex (CEC), in meq/lOOg. CSIC CIEMAT Ca2+ 43 ± 5 42 ± 3 Mg2+ 32 ± 3 32 ±2 Na+ 24 ±4 25 ±2 K+ 2.1 ± 0.2 2.5 ± 0.3 CEC 101 ±4 102 ± 4 17 3. Pore water in the clay barrier 3. Pore water in the clay barrier 3. Porewater in the clay bam 3.1 Experimental methods composition of the pore water. There are numerous papers concerning the modelling of the pore water in for the physico-chemical compacted bentonite and argillaceous rocks (Brad charaterisation of the FEBEX bury and Baeyens, 1998 and 2002a,b; Fernandez ef a/., 2001; Wanner ef a/., 1999; Muurinen and bentonite and porewater analysis Lehikoinen, 1999; Pusch ef a/., 1999; Wieland ef a/., 1994; Curti, 1993). The methodology for deter mining the pore water composition of the FEBEX 3.1.1 Introduction bentonite under the conditions expected in a HLW repository is presented in this Chapter. The work described in Section 3.1 has been carried out by A.M. Fernandez and P. Rivas (C IE MAT). The pore water chemistry in bentonites is the result of different interactions occurring in the clay-water Compacted bentonites are being considered in many system: interactions between water, solutes and clay. countries as a backfill material in high-level radio For this reason, it is necessary to know the mineral- active waste (HLW) disposal concepts because of its ogical and chemical components of the clay system; low permeability, high swelling capacity, high plasti their physico-chemical characteristics, the hydration city and high sorption. A knowledge of the pore wa mechanisms, as well as the types of waters, porosity, ter chemistry in the clay barrier is essential for perfor microstructure, and the ion diffusion pathways of mance assessment since the pore water composition compacted bentonites. is an important parameter influencing the release and transport of the radionuclides, canister corro The main objectives of this work are: sion, dissolution of the waste matrix, sorption on i) to investigate the water-FEBEX bentonite inter mineral surfaces, solubility of radionuclides, etc. action processes controlling the physico-che However, obtaining reliable data on the pore water mical parameters and the chemistry of the sys chemistry of compacted bentonite (dry density of 1.65 tem; g/cm3) under initial and saturated conditions, where ii) to create a database containing the main phys the water contents (w.c.) are so low (14 wt% and ical, mineralogical and geochemical parame 23.8 wt% of the dry mass, respectively), is very diffi ters of the bentonite, and cult. Many of the different laboratory techniques em ployed so far to obtain pore water compositions tend iii) to obtain the best possible estimation of the to perturb the system and introduce sampling arte bentonite porewater composition under initial facts into the measured data. For example, squee repository conditions (w.c. of 14 wt. %) and at zing at high pressures may produce oxidation and saturation (w.c. of 23.8 wt%) in the com dissolution of the accessory minerals present in the pacted state (dry density of 1.65 g/cm3). bentonite, the outgassing of CO? and chemical In order to achieve these aims, it was necessary to fractionation (Pearson ef a/., 2003). Furthermore, use a combination of methodologies based on dif squeezing techniques can not extract pore water from ferent porewater extraction methods, physico-chemi bentonites with water contents below 20 wt% (Fer cal and mineralogical characterisation and geoche nandez ef a/., 2001 and 1999; Cuevas ef a/., 1997). mical modelling. These include: Also, pore water chemistries for compacted ben a) determination of the physical and physico tonite cannot be obtained directly from aqueous chemical properties of the bentonite: porosity, extraction experiments. They are generally carried grain density, total and BET specific surface out at unrealistically low solid to liquid (S:L) ratios areas; and the unconstrained dissolution of highly soluble b) determination of the accessory and trace min salts and sparingly soluble minerals, together with erals in equilibrium with the bentonite pore cation exchange reactions on the montmorillonite, water; leads to water compositions and cation occupancies which are very dependent on the experimental con c) aqueous extracts at different S:L ratios to de ditions (Bradbury and Baeyens, 1998; Fernandez ef termine the concentration of dissolved and ex of., 2001). changeable ions, Rather, indirect methods based on geochemical d) determination of the cation exchange occu modelling must be applied to deduce the chemical pancies using different index cations and 21 EEBEXII project. THG Laboratory Experiments e) determination of selectivity coefficients for the Cu (CuKa), using an acceleration voltage of 35 Kv cation exchange reactions. and an current intensity in the filament of 40 mA. The apertures of divergence and reception were 1 Also, an analysis of the pore water at high solid to and 0.2°, respectively, using a Ni filter. The explora liquid ratios extracted from squeezing tests at diffe tion velocity was of 2°/min for the disoriented powder rent pressures was performed. sample (total analysis of the sample) and 1 °/min for An important issue in calculating the pore water the randomly oriented preparations with the < 2/ Scanning electron microscopy (SEM). A ZEISS, DSM 3.1.2 Experimental Techniques 960 SEM microscope coupled to an energy disper sive X-ray spectrometer (EDX) (LINK, EXE model) was The dry density of the samples was determined by used to identify the accessory and trace minerals. the mercury displacement method. Classical N2 adsorption/desorption isotherms will be The percentage water content is defined as the ratio obtained at 77 K (nitrogen liquid temperature) on a between the weight of the water lost after heating discontinuous volumetry sorptometer, Micromeritics the sample to 110 or 150°C for 24 hours and the ASAP 201 0. Before measurement, the samples were weight of the dried clay. outgassed by heating to 90 °C under a residual The specific weight or grain density of the solid was vacuum of 0.01 Pa. Depending on the porosity, measured on a powder of oven-dried specimen with about 0.05 to 0.25 grams of a sample was used. a pycnometer. Surface areas will be determined using the standard N2-BET method. The presence of micropores in the The total specific surface area was determined by samples will be assessed using the t-plot method (de Keeling's higroscopicity method by an adsorption Boer ef at, 1 966). isotherm of water in a constant relative humidity at mosphere (90%) of saturated NaCI. In this method, The soluble salts are analysed in aqueous extract the weight variation of a dried clay material was fo solutions. Crushed rock samples were placed in llowed during a period of one week. contact with de-ionised and de-gassed water, sha ken end-over-end at a solid to liquid ratios of 1:4 The mineralogical analysis was performed by X-ray and allowed to react for 2 days under atmospheric diffraction techniques. Diffractometer patterns from conditions. After phase separation by centrifuging samples dried at 60°C and powdered to a particle (30 minutes at 12500 r.p.m., the supernatant solu size below 60 / 22 3. Porewater in the clay bam nium acetate IN at pH = 7.0 (Rhoades, 1982; The squeezing rig at CIEMAT is similar to that devel Thomas, 1 982). oped by Peters efal. (1992) and Entwisle and Reeder (1993). The squeezer has been designed to allow a In order to determine the exchangeable cations, a one-dimensional compression of the sample (Cuevas C sNC>3 solution was used (Sawhney, 1970). Cs act ef ah, 1 997; Fernandez ef ah, 2001) by means of a as a high selective cation to displace all exchangea automatic hydraulic ram operating down-wards with ble cations from the montmorillonite if its concentra squeezed water expelled into polypropylene syringes at tion is sufficiently high. Bentonite samples was equi both top and bottom of the cell (Figure 1). The com librated with 0.5 M C sNC>3 at a S:L ratio of 0.25 paction chamber is made of type AISI 329 stainless kg • L“1 in a glove box. Samples were shaken end- steel (due to high tensile strength and resistance to over-end for 2 day. After phase separation by cen corrosion) with an internal diameter of 70 mm. Two trifuging, the supernatant solutions were analysed. compaction chambers were designed. One of them is 250 mm high with 20 mm wall thickness and al Squeezing technique. Squeezing is analogous to the lows pressures up to 100 MPa. The other one is natural process of consolidation, caused by the de 500 mm high with 45 mm wall and allows pressures position of material during geological time, but at a up to 200 MPa. greatly accelerated rate. The squeezing process in volves the expulsion of interstitial fluid from the satu The filtration system allows the extraction of intersti rated argillaceous material being compressed (Ent- tial water by drainage at the top and at the bottom wisle and Reeder, 1993). In squeezing experiments, of the sample. This system comprises of a 0.5 / Figure I. Squeezed water extraction apparatus used at CIEMAT. 23 EEBEXII project. THG Laboratory Experiments 25°C) and not under anoxic atmosphere, although colourimetric method. It was estimated that the max at the first time of the test the system is flushed with imum analytical errors were ±10% for the major nitrogen gas. ions. Before squeezing, sub-samples of each sample were taken for different physical determinations (grain den 3.1.3 Solid phase characterisation sity, dry density (oj, gravimetric water content^)). The samples for squeezing were trimmed in sepa Primary clay particles consist of a coherent stack of rate pieces using a large knife, fragmented ideally silicate layers. These stacks are called quasicrystals to lumps fitted to the squeezing cell or small pieces which often occurs as aggregates displaying various of about 25-200 grams. The trimming was carried textures determined by the shape and arrangement of the constituent primary particles. The cluster of out in plastic bags flushed with N2. Then the sample was weighed and placed into the body of the cell. A the clay aggregates may generate a micro-structure small stress was applied initially to remove most of (fabric) often with large pores between the constitu the air from the cell and allow the sample to bed in. ent clay aggregates. Thus, a clay may develop three Then the syringe was assembled to collect the kinds of pores: interlamellar pores within the pri squeezed water. The applied stress was increased mary particles, intraaggregate pores (0.01-0.2 / CEiemical Analysis. The total alkalinity of the sam 2) deprotonation of the two types of edge-sur ples (expressed as mg/L of HCO3) was determined face hydroxyl groups (silanol and aluminol). by potentiometric titration using a Metrohm 682 Clay hydration involves adsorption of water molecu titrator. The major cations were analysed by Induc les on the clay surfaces that are exposed in different tively Coupled Plasma-Atomic Emission Spectrometry pore spaces of the clay. As a consequence, the spe (ICP-AES) in a Perkin-Elmer Elan 5000 spectrometer. cific surface area will influence the amount of ad Sodium and potassium were determined by flame sorbed water. Three modes of clay hydration can be atomic emission spectrometry, AAS-Flame, in a Perkin distinguished and may take place simultaneously Elmer 2280 spectrometer, and trace elements were with the increasing water activity or relative humidity determined by ICP-MS (Finningan Mat SOLA). An (Guven, 1 993): ions were analysed by ion chromatography (Dionex DX-4500i). The silica was determined using a UV-Vis a) interlamellar hydration which involves the ad spectrophotometer by means of the silico-molybdic sorption of limited amounts of water mole 24 3. Porewater in the clay bam cules on the internal surfaces of primary clay on the external surfaces of quasicrystals (Guven, particles, named interlamellar water; 1993). b) continuous (osmotic) hydration which is re As mentioned before, the major mineral phase (90 - lated to an unlimited adsorption of the water 92 wt%) of the FEBEX bentonite is a montmorillonite on the internal and external surfaces of pri (dioctaedric aluminic smectite with a 2:1 structure). mary particles, named intraparticle water; and This bentonite originates from alteration of pyro c) capillary condensation of free water in micro clastic volcanic rocks, such as poligenitic tuffs and pores within the clay fabric (i.e., in the inter porphyritic agglomerates of riolitic character (Ca aggregate and intra-aggregate pores), named ballero ef a/., 1985; Fernandez Soler, 1992; Del interparticle water. gado, 1993). The specific weight is 2.70 ± 0.04 g/cm3. The equilibrium gravimetric water content of The swelling pressure is due to interlamellar hydra the clay under laboratory conditions (relative humid tion of expandable minerals and to osmosis related ity 50 ±10%) is about 13.7 ± 1.3 % (Villar, 2002). to interparticle forces. The first mentioned depends on the type of adsorbed interlamellar cation, which In spite of its high smectite content, the bentonite determines the maximum number of hydrates and contains numerous accessory minerals (Table 1). hence the maximum c-dimension of the stacks of la Some are neoformed minerals, others are the re mellae (Push, 1999). In the first stage of hydration mains of the original volcanic rock which appear of the dry clay particles, water is adsorbed in suc nearly unaltered. Other accessory minerals are mica cessive monolayers on the surfaces and pushes the (biotite, sericite, muscovite), chlorite, non-differenti- particles or the layers of a montmorillonite clay ated silicates of Al, K, Fe, Mg and Mn, augite-diop- apart. In this stage, the principal driving power is the side, hypersthene, hornblende, oxides (ilmenite, adsorption energy of the water layers on the clay rutile, magnetite, Fe-oxides), phosphates (apatite, surface (mainly solvating the exchangeable cations). xe- notime, monacite) and non differentiated of tita The second stage of swelling is due to double-layer nium and rare earths, which have been determined repulsion. In this stage one usually speaks about os by weight from dense concentrates and SEM identifi motic swelling, that was found to be inversely pro cation. Their total content is 0.8 %. The poorly or portional to the square-root of the salt concentration dered minerals have been determined by selective in the solution (Norrish, 1954). At plate distances chemical methods. Some minerals, such as carbon beyond about 10 A (equivalent with four monomo- ates, chlorides and sulphates, have been deter lecular layers of water), the surface hydration energy mined by a normative calculation and SEM identifi is no longer important, and the electrical dou cation (Table 1). Attention is brought to the content ble-layer repulsion becomes the major repulsive for of these latter minerals present in the bentonite as ce between plates. trace minerals, because of their influence on the Large volumes changes accompany this stage of chemistry of the porewater. swelling, but there often a limit to the swell caused by the formation of cross links by edge-face (EF) Based on XRD and the positions (CufQ,) of the 001/002 and face-face (FF) associated particles leading to and 002/003 reflections (Moore and Reynolds, the formation of gels. The hydration complexes bet 1989), the smectitic phases of the FEBEX bentonite ween these distant layers are not organised as in the are actually made up of a smectite-illite mixed layer interlamellar swelling, but consist of a continuous (Cuadros and Linares, 1996), with ~11 % of illite la diffuse double layer. Such a set of smectite layers yers (A20 = 5.502). The thickness of FEBEX smectite that are separated by their overlapping double la quasicrystals is around 1 02 A, calculated by means yers is called tactoid. In a quasicrystal the silicate la of the Scherrer equation, and quasicrystals of satu yers are held together by attractive electrostatic for rated FEBEX smectite consist on 6 lamellae or layers ces related to the coupling by the hydration stacked along the crystallographic c-axis. The Bisca- complexes of interlayer cations. The tactoid, on the ye index of smectite crystallinity is 0.97. other hand, maintains its integrity by long-range re pulsive forces due to overlapping double layers of Structural formula smectite lamellae. The individual smectite layers are separated by their diffuse double layers in a tactoid Based on chemical analyses, the structural formula but by their interlamellar hydration complexes in a or unit-cell formula of the Co conditioned FEBEX quasicrystal. Diffuse double layer may develop only smectite is: 25 FEBEXII project. THG Laboratory Experiments (Si7 78 AI022)lv (AI2.78 Feg +33 Fe 20+02 Ti002 Mg08])vl trated with ferrous ammonium sulphate (Mohr O20 (OH)4 (Ca050Na0 08K0a)] J9 salt). The amount of oxidable species is propor TetraEiedraI charge: -0.22, tional to the potassium dichromate consumed: Octahedral charge: -0.97, interlayer charge: +1.19 + 14H+ * 20-3+ + 7HP 18 % of layer the charge arises from tetrahedral subs The total reduction capacity evaluated by this titution, and 82 % of the charge from octahedral method is 0.23 ± 0.19 meq/g. The values, substitution. According to the chemical analysis, the expressed as % of organic carbon and as % of theoretical exchange capacity is 1.05 eq/Kg and organic matter are 0.07±0.06 and 0.12 ± therefore, the total charge per unit-cell is 0.79. The 0.1 0 respectively. 0.1 1 K ions per formula unit are not exchangeable and belong to the 1 1 wt % illitic layers of the smec- b) The other method arises from the chemical tite-illite mixed layers of FEBEX bentonite. The sur analysis of the bentonite samples. According to face charge density (a), i.e., the excess charge per the total chemical analyses, an approximated unit surface area of this smectite is 0.1 36 C/m2. value to the total reducing capacity (TRC) of the FEBEX clay samples can be calculated. The Surface area TRC of a solid is related to the amount of reduc ible species (sulphide, ammonium, oxidable or The total specific surface area was determined by ganic carbon (COOx), Fe(ll) and Mn(ll) (Heron two methods. The first is based on the unit cell para ef a/., 1994): meters (a = 5.156 and b = 9.0), obtained from the XRD analyses, and the unit weight (752.46 g per 44 TRC = 2[Mn 2+ ] + [Fe 2+ ] + 4[COOx] + 8 [S2"] + atoms of oxygen). The calculated value was 746 + 7[S"] + 8 [NH4+] m2/g. The second is based on the Keeling's hygros copic method from an adsorption isotherm of water The total reducing species obtained from the in a constant relative humidity atmosphere (75 %) of chemical analyses of the FEBEX bentonite is saturated NaCI. In this method, the weight variation 0.59 ±0.12 meq/g which is very low. In these of a dried clay material is analysed over a period of calculations, the amount of sulphates (SO2-) in several weeks. The total specific surface area of the total S has been subtracted and the amount of FEBEX bentonite obtained by this method was NH)( has not been taken into account. Stotal — 725 ±47 m2/g. XRD, ATD/TG and N2 adsorption This total specific surface area is obtained when the for characterising the dry state of FEBEX bentonite water molecules not only cover the external surface The determination of the amount of water actually but are adsorbed into interlamellar space, i.e., are available for chemical reactions/solute transport is introduced between the sheets, expanding all the one of the main issues in bentonite pore water mod sheets until the clay adsorbs the maximum quantity elling. Different techniques have been used to ana of water that it can, with the given composition of lyse the different types of waters, e g. XRD analysis, interlayer cations. thermogravimetric (TG) analysis and specific surface area determinations. Redox capacity of FEBEX bentonite The dry state can be further characterised by nitro Due to the importance of redox measurements, the gen adsorption-desorption isotherms. In the adsorp redox buffering capacity and the knowledge of re tion of non-polar molecules such as N?, the nitro dox processes in clayey environments, different met gen molecules do not penetrate between layers. The hods are required to obtain reliable data: external surface area measured corresponds to the external faces and the edges of the montmorillonite a) One method to determine the total reducing particles. The BET specific surface areas (micro (< 2 capacity of the FEBEX bentonite consists on oxi nm), meso (2-50 nm) and total) were determined dising the sample with potassium dichromate in according to t-plots and Harkins-Jura methods (De presence of concentrated sulphuric acid. The Boer, 1966; Cases ef a/., 1 992). oxidable components are destroyed by boiling sulphuric acid. The sample is refluxed with Figure 2 shows the N? adsorption-desorption iso known amounts of potassium dichromate and therms obtained on FEBEX bentonite total fraction, sulphuric acid and the excess dichromate is ti expressed in terms of the volume of gas adsorbed 26 3. Porewater in the clay bam per unit mass, Va. The adsorption isotherm has the dooi in height (thickness being 9.6 A)) are perfectly characteristic features of the type IV isotherm stacked as in a deck of cards. It is possible to deter (Fernandez and Rivas, 2003) with a very important mine the number of layer in a quasicrystal using the hysteresis loop due to capillary condensation in relationship between the total (746 m2/g) and the mesopores formed between different quasicrystals. BET surface area (Cases et al., 1992): The curves exhibit a H3 hysteresis loop in desorption S6ET(m2 / g)= 56.38 = 746 / n + 5.13 => characteristic of the presence of slit-shaped pores. Table 5 shows the relevant parameters deduced =>n=l 4 layers (E.3.1.1) form the BET and t-plot treatments. The specific sur face area calculated from the BET method is 56.38 Thus, when the bentonite is dry, quasicrystals of m2/g while that obtained from the B point method about 14 elementary condensed montmorillonite lay (P/P0 = 0.057) is 57.0 m2/g. The adsorption iso ers are formed on average. When FEBEX clay pow therm was replotted following the procedure of De der is brought into equilibrium with saturated water Boer ef al. (1 966) in the form Va versus t (not shown), vapour, the mean size of the quasicrystals remains where t is the mean thickness of the adsorbed layer constant during the filling of the interlayer spaces, calculated according to the Harkins-Jura method. and the size corresponds to particles about six layers This method has been used for analysing the mi thick with two or three monolayers of water in the cro-porosity. The original slope of the t-plot indi interlamellar space. It seems that during the first cates the total specific surface area to be 61.53 stage of water adsorption, the original 14-layer-thick m2/g. The FEBEX bentonite is micro-porous with the quasicrystals are split into smaller ones approxi micro-porous volumes accounting for —43 % of the mately 6 layers thick (Cases ef al., 1 992). BET monolayer capacity. Under room conditions (RH = 50 - 60 %) the FEBEX The average number of clay layers per quasicrystal smectite interlayers contain two monolayers of wa was calculated from the nitrogen adsorption data ter, as deduced from the measured basal spacing of assuming that the plates (square parallelepipeds = around 15.2 A (FEBEX bentonite is a calcium-mag 3000 A in the lateral direction and a multiple of the nesium-sodium smectite). The swelling due to inter- ; •• o < V > ' <> V <■■■ : > . ■ • ' > A " :■> vv .,0,Wy.>' F 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 P/Po Figure 2. Adsorption-desorption isotherms of nitrogen at 77 K onto FEBEX bentonite. 27 FEBEXII project. THG Laboratory Experiments laminar hydration at different water contents, and 100-110 °C and the structure collapsed to a the shrinkage as a function of the temperature, were c-spacing of 9.6 A at 150-200 °C, i.e., essentially also analysed. In wetting experiments, the FEBEX no water in the interlayer space. montmorillonite expands in the c-dimension to an extent equivalent to the interlayer space containing It is worthwhile noting that the FEBEX bentonite is a maximum of three monolayers of water (see next not fully dehydrated at the standard temperatures at section). In drying experiments (Figure 3), the basal which the water content is determined (105-110 spacing corresponded to one monolayer of water at °C), since one monolayer of water remains at this Table 5 Parameters deduced from BET and t-plot treatment on the adsorption of N% at 77K. r Vm Sbet St„, v,„, ^ext micro ^ext meso v BET (cmVg) (m2/g) (m2/g) (cm 3/g) (m2/g) (cm 3/g) (m2/g) (m2/g) (cm 3/g) 388 1.99 10^(1) 56.38 61.53 9.6-1 0"2 20.73 8.6-1 0"3 (1) 35.65 29.35 1.2-1 O’2 (1) Vm : monolayer capacity derived from the BET treatment; expressed os liquid (I); S: BET Surface area St„ : Total surface area derived from the slope of the straight line passing through the origin of the t-plot lmja : Liquid microporous volume derived from the ordinate at the origin in the second straight line of the t-plot E> ex mm •' Surface area out of micropores derived from the slope of the second straight line of the t-plot Zm : Total pore volume, derived from the amount of nitrogen adsorbed at p/p of 0.98 Sea „„ ; Surface area out of mesopores derived from the slope of the third straight line of the t-plot !mesa : Liquid mesoporous volume derived from the ordinate at the origin in the third straight line of the t-plot -*mm-- ^BEt'^msso~ -*bei' L^exmm -*exmesJ■ Surface area ofthe micropores 16 2 monolayers 15 14 13 monolayer 12 10 closed state 9 Figure 3. Evolution of the water content and the d (001) basal spacing as a function of temperature. 28 3. Porewater in the clay bam temperature. Only at temperatures above 1 50 °C is populations with quasicrystals formed with two and the total interlayer water lost. The structural water, three monolayers of water. or de-hydroxylation water, is about 4.3 ±0.6 wt%, The amount of water adsorbed and the c-spacings calculated from mass losses between 300 and 950 of the FEBEX bentonite for each point on the water °C as determined from TG analysis (discounting CO? vapour isotherm were obtained (Figure 5). The and SO? contents). Frenkel, Halsey and Hill (FHH) formalism was used (Frost ef a/., 1 998) for identifying and describing the XRD, ATD/TG and water vapour adsorption for different states and locations of water and the characterising the saturated state of FEBEX bentonite mechanism involved in the water retention phenom The saturated state of the FEBEX bentonite was enon as a function of the activity of water, aw (or characterised by water-vapour adsorption isotherms. P/P0 or relative humidity). The FHH curves were ob Also, the basal spacing of powdered FEBEX benton tained by plotting log w as a function of log ite samples was analysed as a function of the water (log(P Q/P)). In Figure 5, six domains were distin content, from an initial water content of 14 % up to guished from an analysis of the slope changes, the liquid limit of this bentonite, i.e., 102 %. In these which are related to the different states and location wetting experiments, the FEBEX montmorillonite ex of water. Moreover, these domains also corre pands or swells in the c-dimension to an extent sponded to the dooi spacings obtained from XRD equivalent to a maximum of three monolayers of measured at each water activity, aw. Domain A is water in the interlayer space (Figure 4). However, observed at the lowest values of aw (0.032-0.069) this expansion, from two-monolayers of water to and corresponds to adsorption on the external sur three-monolayers, is through a continuous of 2°0 faces of the stacks, edges and edge surface sites. distances. Thus, the d(001) values are average val This domain corresponds to the original spacing of ues from a lattice that contains a random alterna the montmorillonite in the dry state with virtually no tion of successive discrete hydrates in bentonite with water in the interlayer space (closed state, dooi — water contents from 14% to 1 00%. For this reason, 9.6-1 0 A). From an analysis of the data, adsorption the d(001) peak can be decomposed in different on external surfaces is —60.36 mg/g (see Table 6). Hire: monolayers rum intensity minimum of 2nd derivatin Two moot layers Water Content (%) Figure 4. c-spacing of the FEBEX bentonite at different water contents. 29 FEBEXII project. THG Laboratory Experiments 0.99 0.97 °-85 ' 0.2 0.115 P/Po >.47 pF 3 ML log w.c. (mg/g) 3 monolayers 2 ML .069 P/Po —□— Isotherm 2 monolayers 0 ML Figure 5. FHH water sorption plot obtained for FEBEX bentonite describing the different states and locations of water. Thus, the letters h, B, C, D, E and F correspond to domains where water retention is due to different hydration sites and mechanisms, the basal spacings d(001) are also shown on the right (filled circles). Table 6 Quantification of the amount of water, surface area, energy and number of water molecules per ion in each domain (location of water) by means of Dubinin s equations. h2o c-spacing aw mg/g S„ (m2/g) E (kJ/mol) molecules/ion Domain A -9 .8 % 0.032-0.069 60.36 213.89 10.0285 3.3 Domain B 10.3-12.1 A 0.069-0.1 102.38 362.83 6.3448 5.6 Domain C 15.2 A 0.2-0.58 183.42 650.01 3.5521 10.8 Domain D 15.6 A 0.58-0.9/ 274.16 971.58 1.5400 14.9 Domain E 18.8 A 0.97-0.99 378.99 1 343.8 0.5502 20.6 As a consequence, according to Figure 5 and Table B (aw = 0.069-0.1) corresponds to the adsorption 6, the water adsorbed on external surface has a value of water for filling a monolayer of water in the inter between 3-6%. This quantity, —5%, is interpreted as layer space. Water is adsorbed around the interlayer the free water volume and will be used for model cations and the c-spacing increases from 9.6 to ling of pore water in the as received bentonite (5 ml 12.1 A. The amount of interlayer water in the com of H2O in 95 g of dry bentonite, i.e., a solid to liq plete one layer hydrate is 102.4 mg/g, which theo uid ratio of —19 kg/L dry FEBEX bentonite). Domain retically covers a surface area of 363 m2/g. Domain 30 3. Porewater in the clay bam C (aw = 0.2-0.58) corresponds to the solvating of ter and the capillary condensation in intraaggrega interlayer cations (second solvation shell) in the two- tes or inter-aggregates begins to occur (Domain F). layer hydrate (dooi — 1 5.2 A). The amount of inter The amount of water uptake in each domain was layer water in the two-monolayer is 183.4 mg/g, quantified (Table 6) by means of Dubinin-Radush- which theoretically covers a surface area of 650 kevish equations (Kraehenbuehl ef a/., 1987). The m2/g. Domain D (aw = 0.58-0.97) corresponds to amount of water adsorbed in each domain as well the filling of holes between solvated cations (free as the number of water molecules per ion solvating space in the siloxane cavities with no interlayer cat the cations corresponding to the closed state, one- ions) and to multilayer adsorption on exterior sur layer hydrate, two-layer hydrate and three-layer hy faces. drate (3.3, 5.6, 1 0.0-14.9 and 20.6, respectively), are shown. According to this model, the changes in In this domain, the spacings between the unit layers structure by interlayer swelling can be observed. It is remain constant (dooi — 15.8 A) but the system con worthwhile noting that the FEBEX bentonite is in Do tinues adsorbing water. Due to the fact that mul main C at laboratory or initial conditions (RH = 50 tilayer adsorption on exterior surfaces could occur si ±10%) and, for this reason, the amount of external multaneously with interlayer adsorption, it is difficult water present in the system corresponds to the ad to determine how much water is present in the com sorbed water in Domain A. Thus, as received ben pleted two-layer complex between the unit layers. tonite contains 52.6 ml of water per kilogram of dry clay. The total amount of adsorbed water obtained for this domain is 274.2 mg/g, which theoretically cov Knowing the total external surface determined by ni ers a surface area of 972 m2/g. Domain E (aw = trogen adsorption/desorption isotherms and the val 0.97-0.99) corresponds to the filling of the three- ues of the water vapour adsorption isotherms, the layer hydrate (dooi = 1 8.8 A) and the total amount amounts of external and internal water were calcu of water adsorbed is 379 mg/g. At this point, aw lated according to the Cases ef al.'s model (1992). >0.99, the interlayer space is further filled with wa The results are shown in Figure 6. Q external Q internal Total adsorbed water Figure 6. Water adsorption isotherm (1/ adsorbed) for the FEBEX bentonite, expressed as the amount of water adsorbed on the exernal surface (Q external) and interlamellar space (Q internal). 31 FEBEXII project. THG Laboratory Experiments As main conclusion, different types of water in the These values can be compared with other methods FEBEX bentonite powder were determined, their used to determine the types and amounts of water in state, location and amount. These results are shown FEBEX bentonite, as shown in Table 8 . It is worthy in Table 7. (Fernandez and Rivas, 2003b). noticing that the amount of water adsorbed at inter nal and external positions has been calculated for According to bibliographic data, the more stable powdered bentonite samples. However, the dry den state in smectites is two layers of water, i.e. the one sity of the samples must be taken into account for that corresponds to the formed by the cation solva determining these values in compacted material. tion shells (first and second) and the filling of the The amount of internal water depends on the dry den free siloxane cavities. The total amount of internal sity because the free expansion is limited due geomet or interlayer water in this state is around 21 -27%. ric reasons (Fernandez and Rivas, 2003, 2003b). Table 7 Types of water in granulated FEBEX bentonite. Type Value Energy Subdivision Method a of water (%) (kJ/mol) Structural TG 4.3 ± 0.6 water — — — — Monolayer of water 10.2 6.34 10-12 0.06-0.1 Two layers of water (2nd solvation shell) 18.3 3.55 15.2 0.2-0.58 Interlayer FHH +Dubinin water Filling water of free siloxane 21.4-27.4<‘) 1.54 15.8 0.58-0.97 cavities + (external water) Three layers of water 31.9-37.9C1 0.55 18.8 0.97-0.99 External External FHH +Dubinin 3-6 10.3 -9.8 0.03-0.06 water ^ The values 27.4% and 37.9% can be over-estimated, due to accumulated values could be obtained with this method. For this reason, the value obtained in these two states has been calculated by sustracting the amount of external water. Thus, the real values for two layers and three layers would be: two layers, 214% and three layers: 319%. Table 8 Amounts of adsorbed water in each state as a function of the model used, in (g/g). Methods External water Total internal water Monolayer Two layers Three layers BET Method — — 0.082 — — t-plot Method 0.056 0.311 — — — FHH and Dubinin 0.03-0.06 — 0.102 0.183-0.274 0.379 Laird' s Model 0.038 0.322 0.002-0.075 o Cases ' s Model fk,OH20 = 10.6A) 0.132 0.239 0.358 0.003-0.105 o — fk 01,20 = 14.8 A) Touret ' s Model 0.038-0.115 (f(M=6)) 0.292 (f(M=6)) f(a„ a^o) and f(M) indicate values as a function of the activity of water, water surface considered and number of lamellae apilkd to form a guasicrystal, respectively. 32 3. Porewater in the clay bam Chloride and sulphate inventories fect of crushing. However, it seems that the sulphate concentrations (M) measured by PS I do not follow The chloride and sulphate inventories were determined the same tendency than those obtained by CIEMAT in both CIEMAT and PS I laboratories. The results of at the different solid to liquid ratios and particle size. PS I laboratory will be also described in this section. The differences obtained between both laboratories In order to calculate the bentonite porewater com can only be attributed to the heterogeneity of the position, the chloride and sulphate inventories and FEBEX bentonite sample. The chloride and sulphate the fractional cation occupancies need to be deter inventory data are summarised in Table 10. These mined for the as received material. values can be also used for modelling purposes. Aqueous extracts experiments at different solid to liquid ratios (1:1, 1:2, 1:4, 1:8, 1:16) were per CEC, exchangeable cations, fractional occupancies formed in oxic atmosphere with the FEBEX bentonite ground to less than 63 /cm. This experiments were Also in this case, the determinations were carried out in both CIEMAT and PS I laboratories. The re made to check the Cl", SO4” inventory obtained in FEBEX I phase with FEBEX bentonite ground to 5 sults of PS I will be shown in this section. mm (Fernandez ef a/., 2001). The capacity of a clay mineral to interact chemically The results are shown in Table 9 and Figure 7. The with water and solutes is due to the electrical charge sulphate concentration does not depend on the ef on their surface. The amount of charge and its dis- Table 9 Aqueous extracts with distilled water at different solid to liquid ratios (interaction time 48 hours) of the FEBEX bentonite ground to < 63/cm. 1:1 1:2 1:4 1:8 1:10 1:16 20g/20ml 20g/40ml 10g/40ml 5g/40ml 4g/40ml 2.5g/40ml pH 8.10 ±0.00 8.20 ± 0.00 8.30 ± 0.00 8.8 ± 0.00 8.45 ± 0.21 8.80 ± 0.42 Cond. (wS/cm) 4186 2259 1200 719 641 491 E.N (%) -1./3 -0.48 -2.37 -0.93 2.20 2.75 Br" (mg/L) 1.55 ±0.0/ < 1 < 1 < 1 < 1 < 1 Cl' (mg/L) 764.5 ±24.75 366.5±3.54 175.0±2.83 78.5±0.71 60.0± 1.41 39.5±0.71 S042" (mg/L) 832.0 ±52.3 419.5 ± 10.61 200.5±3.54 100.5±0.71 77.0 ± 1.41 46.5 ± 0.71 NO^ (mg/L) 26.5 ± 12.02 6.65 ± 1.06 2.7 ± 2.40 2.35 ± 0.21 1.70 ±0.14 1.35 ±0.21 HCO" (mg/L) 1*1 244.0 ±7.07 221.5±0.71 209.5±2.12 182.0±0.00 171.5±0.71 155.5±0.71 Al3+ (mg/L) <0.5 0.06 ± 0.00 0.1 ± 0.01 0.16 ±0.08 1.13 ±0.81 0.93 ± 0.25 Ca2 + (mg/L) 42.5 ± 10.61 14.5 ±0.71 4.55 ± 0.07 1.75 ±0.21 1.45 ±0.07 1.20 ±0.00 Mg2 + (mg/L) 31.5 ± 10.61 9.1 ±0.14 2.75 ± 0.07 1.05 ±0.07 1.35 ±0.21 0.95 ± 0.01 Na+ (mg/L) 820.0 ±1.41 471.0 ± 12.73 262.0±0.0 159±4.24 138.5 ±2.12 106.0 ±0.00 K+ (mg/L) 16.00 ±0.0 9.5 ± 0.0 5.5 ± 0.14 3.4 ± 0.0 3.0 ± 0.0 2.50 ±0.14 Sr2 + (mg/L) 1.09 ±0.16 0.30 ± 0.00 0.1 ± 0.00 <0.05 <0.05 <0.05 (*) Total alkalinity expresed as HC03 content. 33 FEBEXII project. THG Laboratory Experiments 2.5E+01 Cl EMAT < 63 micros 2.0E+01 Cl EMAT 5 mm 1.5E+01 1.0E+01 5.0E+00 0.0E+00 Solid to liquid ratio 2.0E-02 1.8E-02 PSI <63 micros 1.6E-02 C— Cl EMAT < 63 micros A Cl EMAT 5 mm 1.4E-02 1.2E-02 1.0E-02 6.0E-03 4.0E-03 2.0E-03 0.0E+00 Solid to liquid ratio Figure 7. Chloride and sulphate concentrations measured in different aqueous extracts with FEBEX bentonite ground to o particle size of < 63 fim and 5 mm. Table 10 Cl" and SO*- inventories of FEBEX bentonite i s , Cl _ E _ Particle size > (mmol kg ') 5 mm 21.85 ±3.96 10.26 ±0.68 < 63/cm 22.69 ±3.14 9.87 ± 0.81 34 3. Porewater in the clay bam tribution is centred in two types of functional groups, are feasible methods for the determination of the which give rise to different reactivities: exchangeable cations. However, the CsNOg met hod is preferable due to its simplicity and the high a) ditrigonal siloxane cavities in tetrahedral layers, selectivity of the Cs. It is very important to avoid in which the exchange reactions are given. aqueous extracts with high S:L ratios. A 1:8 solid to b) aluminol and silanol groups at quasicrystals ed liquid ratio seems to be the more suitable. ge sites, which will give rise the surface comple- xation reactions. The total cation exchange capacity determined for FEBEX bentonite was determined by means of the In order to model the chemical composition of the NaAcO/NH4AcO method at pH = 8.2 and it is pore water is necessary to know the cation exchange 102 ± 4 meq/1 OOg. properties of the FEBEX clay, as the cation occupan cies and the selectivity coefficients of the exchange Selectivity coefficients determination reactions. It was observed that the method used in FEBEX-I, for the determination of exchangeable cat The fractional cation occupancies were used to ions, strongly perturbed the system due to the re gether with the concentrations of Na, K, Mg and Co moval of pore water salts by washing with distilled measured in each of the aqueous extraction experi water prior to the washing with ammonium acetate. ments to calculate four sets of selectivity coefficients The "in situ" Na and Co occupancies derived were with respect to sodium, according to the Gaines lower and higher, respectively, than the expected and Thomas (1953) convention. The procedure fol due to the calcite dissolution. In FEB EX-11, different lowed is the same given in Bradbury and Baeyens methods were applied to determine the cation ex (2002b). The average values are given in Table 13 change capacity, the concentration of the ex together with the corresponding cation exchange re changeable cations and the selectivity coefficients actions. The cation occupancies, cation exchange for exchange reactions: capacities and selectivity coefficients found in the current studies, the ones obtained in FEBEX I and a) Extractions with NH4CI, 1M, in an alcoholic those obtained by PS I from their laboratory results (mixture metanol/water or acetone/water) or are compared. aqueous solution. The soluble salts were previ ously washed. The cation occupancies, CEC value and selectivity coefficients for the FEBEX bentonite obtained in this b) Extractions with NH4CI, 1 M. The soluble salts study will be used in the porewater chemistry calcu were not previously removed. lations. The modelling of the porewater, that will be c) Extractions with 0.5 M CsN03. This method described in the Section 3.2 was a task performed was used by PS I and CIEMAT. Cs acts as a high jointly with PS I (Fernandez ef a/., 2004). selective cation, and displaces all exchange able cations from the montmorillonite if its con centration is high enough. Two solutions were used; in one of them the pH was fixed to 7. 3.2 FEBEX bentonite porewater The soluble salts were not previously removed. chemistry modelling The exchangeable cations determined for FEBEX bentonite by means of different methods are shown 3.2.1 Introduction in Table 1 1 and Table 12. From the results obtained it can be concluded that, The work described in Section 3.2 has been carried if it is known that the material contain traces of sulp out by B. Baeyens and M. Bradbury (PSI). hates and carbonates, it is better not to use a met A procedure based on physico-chemical characteri hod in which the salts are previously washed, for the sation and geochemical modelling (Bradbury and determination of exchange cations. Also, it is prefe Baeyens, 2003) will be applied to calculate the po rable to use an aqueous solution at pH 7 or 8.2. rewater chemistry in compacted FEBEX bentonite. A On other hand, alcoholic solutions imply a change brief description of the model concepts are given to of the real solution, i.e., aqueous solutions, which gether with the porewater chemistry calculations. affect the exchange reactions. These calculations take into consideration factors The methods with NH4NO3 1 M and CsNOg 0.5 M such as the industrial preparation of the "as recei at pH 7 without any previous soluble salts washing ved" bentonite powder, montmorillonite swelling, 35 EEBEXII project. THG Laboratory Experiments Table 11 Summary of the results obtained for the exchangeable cations by using different methods. The results are compared with that obtained by PSI laboratory. No K Mg Ca Sr Sections S:L ratio meq/lOOg meq/lOOg meq/lOOg meq/lOOg meq/lOOg meq/lOOg NH4Ac O pH 7.0 aqueous 25 ±2 2.5 ± 0.3 32 ±2 42 ±3 0.38 ± 0.01 102 ±2 salts: 1:4 aqueous extract NH4NH3 pH 8.5 ethanol/water salts: 23.61 ± 1.76 1.75 ±0.21 29.94 ± 2.02 29.86 ± 2.34 0.29 ± 0.02 85 ±6 Acetone/water 50% NH4NH3 pH 8.5 ethanol/water salts: 23.62 ± 0.00 1.68 ±0.10 30.66 ± 1.01 30.57 ± 0.00 0.27 ± 0.00 86.8 ± 1.1 Acetone/water 60% NH4NH3 pH 8.5 ethanol/water salts: 23.99 ± 1.23 1.53 ±0.10 30.65 ± 1.01 31.51 ±0.67 0.28 ± 0.01 88.0 ± 3.1 Ethanol/water 60% NH4NH3 pH 8.5 ethanol/water salts: 1:4 21.50 ±0.52 1.75 ±0.00 32.07 ± 1.02 31.73 ±0.35 0.33 ± 0.00 87.4 ± 0.9 aqueous extract NH4NH3 pH 8.5 etanol/agua 23.48 ± 0.71 1.42 ±0.05 30.29 ± 0.51 31.73 ± 1.67 0.31 ± 0.01 87.2 ± 3.0 salts: aqueous extract 11 NH4CI pH 8.5 ethanol/water 27.33 ± 5.29 2.05 ± 0.21 37.77 ± 7.08 39.02 ± 6.01 0.37 ± 0.06 107 ± 1.8 salts: Acetone/water 50% NH4CI pH 8.5 ethanol/water 28.57 ± 3.53 2.05 ± 0.21 38.48 ± 2.04 38.78 ± 5.01 0.35 ± 0.05 108.2 ± 10.8 salts: Acetone/water 60% NH4CI pH 8.5 ethanol/water 27.33 ± 0.01 1.83 ±0.10 34.20 ± 2.00 37.13 ±0.65 0.32 ± 0.01 100.8 ±2.7 salts: Ethanol/water 60% NH4CI pH 8.5 ethanol/water 27.94 ± 0.89 2.04 ± 0.21 33.47 ± 1.00 36.88 ± 0.99 0.33 ± 0.01 100.7 ±0.9 salts: 1:4 aqueous extract NH4CI pH 8.5 ethanol/water 25.78 ± 1.32 1.94 ±0.15 32.06 ± 2.02 38.19 ±2.49 0.33 ± 0.02 98.3 ± 6.0 salts: 11 aqueous extract NH4NO3 aqueous 23.91 1.32 32.37 36.51 0.32 94.42 NH4NO3 aqueous 22.25 1.30 32.34 36.49 0.32 93.26 CsN03 aqueous (CIEMAT) 27.28 2.86 28.76 36.44 0.32 95.66 CsN03 aqueous, pH 7 (PSI) 26.95 2.29 33.15 33.10 0.43 95.92 Table 12 Exchangeable cations by the CsN03 0.5 M at pH 7 and at different solid to liquid ratios (CIEMAT), in meq/lOOg. S:L No K Mg Ca Sr Ba 2 Cations 1:4 31.38 ± 0.00 2.81 ± 0.00 28.54 ± 0.69 36.46 ± 0.00 0.29 ± 0.02 0.004 ± 2.94E-4 99.5 ± 0.7 1:8 29.77 ± 0.29 3.12 ± 0.00 28.68 ± 0.27 37.68 ± 0.33 0.26 ±0.00 0.006 ± 3.86E-4 99.5 ± 0.2 1:10 29.97 ± 0.35 3.19 ±0.21 25.10 ± 0.40 31.48 ± 8.71 0.21 ± 0.00 0.007 ± 1.19E-4 89.9 ± 7.7 1:16 30.40 ±1.12 3.45 ± 0.17 26.55 ± 0.74 42.13 ±1.21 0.22 ±0.01 0.012 ± 2.67E-3 102.8 ±0.8 36 3. Porewater in the clay bam Table 13 Summary of the Log of selectivity coefficients calculated for the exchange reactions. FEBEX 1 Exchange Reaction CIEMAT PSI (Isotherms) Na-mont. + Ko K-mont + Na 0.9299 1.0253 1.0186 2Na-mont. + Ca oCa-mont + 2Na 1.1038 1.1072 1.2753 2Na-mont. + Mg oMg-mont + 2Na 1.0246 1.0294 1.0982 2Na-mont. + Sro Sr-mont + 2Na^ 1.1030 1.1072 1.2753 *The selectivity coefficient for Sr has been taken os the same than Kc(Ca-Na). semi-permeable membrane effects, very low "free when the stack separation is large enough, and also water" volumes and the highly effective buffering as films surrounding the other component mineral characteristics of the exchangeable cations and the grains in the bentonite. The term bentonite pore wa amphoteric edge sites in the compacted material. ter is used here to refer to this "free-water". Most of the physico-chemical characterisation results, No experimental data exist as to the magnitude of required for the modeling, for FEBEX bentonite have this free-water volume as a function of the initial already been determined and are summarized in bentonite dry density. In the interlayer spaces, and Table 1 0, Table 1 1 and Table 1 3. regions where the individual montmorillonite stacks are in close proximity, double layer overlap will occur FEBEX bentonite is composed mainly of montmori- and anion exclusion effects will take place. However, llonite, and consequently the properties are essen Cl anions do move relatively readily through com tially determined by this clay mineral. Montmorillo- pacted bentonite since diffusion rates have been nites have large surface areas (—750 m2 g'1) and measured in "through-diffusion" tests (e.g. Muurinen high cation exchange capacities (— 1 eq. kg'1). Wa ef a/., 1987). The hypothesis put forward here is ter, and other polar molecules can enter between that the pore volume associated with the transport the unit montmorillonite layers causing expansion in of chloride (and other anions) is the "free-water" the c-direction (interlayer swelling). volume, and that this is the pore water in a com The swelling accompanying the saturation of highly pacted bentonite. The chloride accessible porosity compacted bentonite kept a constant volume can measured in through-diffusion experiments in com be sufficient to fill virtually all of the initially present pacted bentonites is usually only a small fraction of macro porosity and convert it into predominantly in the original dry density porosity. The Spanish refer ence dry density for FEBEX bentonite is 1.65 g cm'3. terlayer space. This, in turn, has a decisive influence on the types and distribution of water in the com Only a limited number of through-diffusion tests pacted material. Most of the water taken up will re with chloride have been carried out up to the pres side in the interlayer space between the individual ent time. The best estimate available for the chloride montmorillonite units, "interlayer water". The rest accessible porosity at this density is — 3 vol. % can be described as "external water" and this can (Garaa-Gutierrez, Section 5.1.1 ) implying an ef be viewed as being of two types. Part will be in the fective solid to liquid (S:L) ratio in the compacted electrical double layers associated with the external material of — 33 kg to one liter. These values will surfaces of the clay stacks, "double layer water". be used in the model calculations. Where the stack surfaces are close together the double layers will overlap and in such cases the in 3.2.1.1 Ion pool in compacted bentonite ter-stack external surface spacings will be of the same order as that between the interlayers. The re A further hypothesis in the approach proposed is maining fraction of water can be regarded as that highly compacted bentonite can function as an "free-water". The free-water may exist as intercon efficient semi-permeable membrane (Horseman ef nected thin films on the outside of the clay stacks, a/., 1996). This implies that the saturation of com 37 FEBEXII project. THG Laboratory Experiments pacted bentonite involves predominantly the move 3.2.2 Porewater chemistry calculations ment of water molecules and not solute molecules. Thus, to a first approximation, the composition of in FEBEX bentonite the external saturating aqueous phase should be a Although the physico-chemical characterisation data second order effect, which has little influence on the are extensive, they are not sufficient to fully define initial compacted bentonite pore water composition the bentonite-water system and two further quanti (see for example Dixon, 2000). ties need to be fixed in order to calculate a unique aqueous chemistry. For bentonite systems, it turns Even if highly compacted bentonite were to function out that the two most readily quantifiable ones are as a less efficient semi-permeable membrane than the pH (or Pco2) and the chloride concentration. proposed, there are such large quantities of mont- morillonite present (~ 90 wt% in FEBEX bentonite) compared to the low volumes of free-water (see 3.2.2.1 Porewater chemistry in “as-received ” above) that the montmorillonite, together with the FEBEX bentonite other solid phases, will determine the composition In the following, the moist bentonite powder is first of the pore water simply because the ion capacities considered so that the state of the amphotheric of the solids are massively greater than those in the =SOH sites can be defined. It is vital to know this aqueous phase. The high exchange capacity of the initial state since these sites are considered to deter montmorillonite component acts as a powerful buf mine the pH of the pore water in the closed com fer for the composition of the pore water. pacted bentonite system as will be described below. There is a second category of reactive sites associ Once the raw bentonite is removed from the deposit ated with montmorillonite which are perceived as and throughout the time it is stored on the surface, it being surface hydroxyl groups (=SOH) situated is exposed to atmospheric conditions. At some point, along the edges of the clay platelets ("edge" or the raw wetted bentonite is processed further on a "broken bond" sites). These sites have a capacity of production line in which the clay sequentially passes ~ 1 0% of the CEC and can protonate and deproto- through slicers, roll crushers, rotary dryers, mill cy nate so that the concentrations of neutral, protona- clones, sieves etc. before being stored again, in ted and de-protonated edge sites changes as a granulated form, prior to transport. The water con function of pH, Bradbury and Baeyens (1997). Using tent of the granulated FEBEX bentonite at this stage a similar argument to the one given above, the has been reduced to about 14 wt%. The prepara hydroxyl groups can function as a powerful pH tion procedure has conditioned the granulated ben buffer. The =SOH site types, capacities and proto tonite to be in equilibrium with air. The cation occu lysis constants are given in Table 14 and are taken pancy data presented in Table 1 1 results from this to be the same as those for SWy-1 montmorillonite. atmosphere conditioning. However, probably most Table 14 Site types, site capacities, and protolysis constants (Taken from Bradbury and Baeyens, 1997). Site types Site capacities = SW]0H 4.0x1 O'2 mol kg'1 = SWi0H 4.0x1 O'2 mol kg'1 Surface complexation reaction log Kim = SW]0H + H+ 4.5 = SW]0Ho=SW]0~ + H+ -7.9 = sWion+n + o=sWion; 6.0 = Swl0Ho=Swl0~ + H+ -10.5 38 3. Porewater in the clay bam significant effect of all, is that the pH in the moist late (Pco2 = 10"3'5 bar), a Cl" concentration needs bentonite powder will be that determined by carbon to be specified. The Cl" inventory of the FEBEX pow ate/sulphate mineral equilibration at air Pco2 he. der of 21.85 x 1 O'3 mol kg'1 is known, Table 1 0, as 10"3'5 bar. The consequence is that the ampho is the quantity of "free-water" associated with the as teric hydroxyl groups at the edges of montmorillo- received powder i.e. ~ 5 wt % (see above), yielding nite platelets will also have been conditioned to a a Cl' concentration of 0.42 M and an effective S:L state reflecting this equilibrium. Further, there is no ratio of —19 kg L'1. The above information allows a reason to believe that any subsequent compaction unique water chemistry be calculated for the solu of the powdered bentonite will alter the charge state tion/air equilibrated bentonite powder system (see of these sites. Bradbury and Baeyens 2003). Such calculations were carried out using the geochemical code MINSORB The hypothesis put forward here is that the bentonite together with the Nagra/PSI thermodynamic data base pore water in the compacted material will be buf compilation from Hummel etal. (2002). The chem fered to a pH reflecting the protonation/depro- istry of the water calculated to be in equilibrium with tonation state of the =SOH sites because the masses the FEBEX bentonite powder is given in Table 15. of montmorillonite are so large and the volumes of free-water so small. In the calculations it is assumed The initial cation loadings and the state of the =SOH that there is saturation with respect to calcite, gyp sites on the FEBEX bentonite powder in an open sys sum, celestite and quartz. The concentrations of K, tem in equilibrium with air are given Table 1 6. Mg and Co are determined over the cation ex change equations and selectivity coefficients given in Table 1 3. 3.2.2.2 Porewater chemistry in saturated FEBEX bentonite at initial dry density A further constraint on the system is the mineral in ventories e g. calcite, gypsum. Clearly, the quantity of 1.65 g cm-3 of any mineral phase dissolved must be less than In the compacted system the loadings and condition the quantity originally present. of the =SOH sites are taken to be the same as in In order to calculate a unique pH for the solution in the granulated. However, the compacted system is equilibrium with the air equilibrated bentonite granu treated as being closed, which is a reasonable as- Table 15 Water compositions (in M) in equilibrium with the "as received" FEBEX bentonite powder. log P[02 (bar) -3.50 pH 7.44 I.S. (M) 0.66 No 3.3x1 O'1 K 2.6X10-3 Mg 8.1x1 O'2 Co 6.8 xl0-2 Sr 6.1x1 O'4 Cl 4.2x1 O'1 so4 1.9x10-2 Fnorg. 3.1x1 O'4 Si 1.8 xl0'4 39 FEBEXII project. THG Laboratory Experiments Table 16 Initial cation loadings and state of the amphotheric = SOH sites in FEBEX bentonite powder in equilibrium with air. Exchangeable cations and = SOH sites Concentration (mol kg ') Na-mont. 2.74x10"' K-mont. 229 xlO"2 Mg-mont. 1.62x10'' Ca-mont. 1.66x10"' Sr-mont. 1.47x10-" = SW]0H 3.0x1 O'2 = SW]0Hj 3.4x1 O'5 = f 0- l.OxlO'2 = SW10H 3.9 x!0"2 = SW10H+ 1.4x10-" = Sm0~ 3.3x1 O'5 The charge on the = SOHj and = SO~ sites are compensated by outer sphere complexes which have not been included in the modelling. sumption for the calculation of the initial pore water 3.3 Physicochemical properties composition. As stated previously, the implication is then that the solid phases will determine the chemis of bentonite: effect try of the pore water because of the very high S:L ra of the exchangeable cations tio and the absence of air. A mass balance approach to modelling the pore water chemistry in compacted bentonite was adopted. 3.3.1 Introduction Such an approach automatically takes into account The work described in Section 3.3 has been carried the buffering effects of the exchangeable cations out by E. Caballero, C. Jimenez de Cisneros and J. and the amphoteric =SOH sites. From the values Linares (CSIC). given in Table 15, the mass balance inventories can readily be calculated at any chosen initial dry den Many studies have been carried out on the hydration sity. The solubility limiting phases, cation exchange of clay minerals uninterruptedly for more than fifty reactions and selectivity coefficients were taken to years. This gives an idea of the complexity and the be the same as those used before. difficulty of the subject in question. Among the most interesting works are those by Norrish (1973), Mac- The pore water chemistry for the Spanish reference Ewan and Wilson (1980), Low (1982), Sposito and dry density for FEBEX bentonite of 1.65 g-crm"3 was Frost (1982), Sposito (1984), Newman (1987), Gu- calculated assuming that the chloride accessible po ven (1992), Frost ef al. (1998), Laird efal. (1999), rosity at this density is 3 vol. % (Garaa-Gutierrez, etc. Section 5.1.1). This value, together with the chloride inventory, yields a chloride concentration of 0.73 M. The clay-water system has four components: clay particles, water, cations (counter-ions) and anions Definition of the system in the above manner allows (co-ions). The properties of the system depend on the a unique pore water chemistry to be calculated at different proportions of each component and on the the reference dry density of 1.65 g-crm'3 (Table 1 7). interactions between them. The pore water is calculated to be a Na-Ca-Mg chloride type with a high ionic strength, 0.9 M and Bentonite is formed by montmorillonite particles, a pFH of —7.4. Porewater chemistries at other initial normally in quantities far above 90%. It is accompa FEBEX bentonite dry densities can readily be calcu nied by small amounts of accessory minerals, spe lated if the corresponding chloride accessible poros cially quartz and feldspars, and trace amounts of ot ity values are available. her minerals. From a colloidal point of view, ben- 40 3. Porewater in the clay bam Table 17 Initial porewater chemistry (in M) in a re-saturated FEBEX bentonite having an initial dry density of 1650 kgm" 3. pH 7.44 I.S. (M) 0.9 No 3.9x10"' K 3.0 xlO"3 Mg 9.7x10-2 Co 8 .2x10-2 Sr 7.3x1 O'4 Cl 7.3x10"' so4 1.7x10-2 Qnorg. 2.4 xlO"4 Si 1.8 xl0"4 tonite can therefore be considered as an essentially layer is extremely important when it comes to hydra smectitic material. tion phenomena. The density of the negative charge of smectites is usually around 1 5 / 41 FEBEXII project. THG Laboratory Experiments lecules associated most strongly with the cation in magnesium) bentonite. In this way, the properties of creases when the charge increases, and they may the system can change. As it is well known most of even form a second hydration sphere (Newman, the properties of bentonite vary with the type of ex 1987). changeable interlayer cation. In order to find out the implications of this exchange process, samples of The hydration process of clay sheets is very com pure calcium or sodium bentonite are studied, so as plex. In principle, three types of hydration can be some sodium-calcium intermediate ones. As cal distinguished (Guven, 1992): cium and magnesium behave very similarly as far as a) Interlayer. It is limited to interlayer spaces and hydration is concerned, calcium is chosen as the associated with the hydration of the exchange representative of divalent cations. Sodium was cho able cations. sen as the monovalent one, which is the main cat ion in granitic percolation waters. Potassium hardly b) Continuous or osmotic. Besides interlayer spaces, exists in bentonite and in percolation waters. The this includes external surfaces. This is the case aim of this study is to clarify some aspects related to of montmorillonite. the mechanical and geochemical behaviour of ben c) Capillary condensation of free water, which tonite barrier in radioactive waste repositories. fills up the micropores existing inter- and intra The methods of studying hydration at low water con aggregates. tents use numerous techniques, especially Differen Interlayer hydration includes the hydration of ions, tial Thermal Analysis, Thermogravimetric Analysis, the polarisation of water molecules by cations, inte Infrared Spectroscopy, Neutron Dispersion, Electro raction with the silicate surfaces, variations in water nic Spin Resonance, etc. The thermogravimetric activity in the system. The size and the morphology method of preparing the water adsorption isotherms of clay particles and spatial distribution between is still the simplest method, the most readily availa particles (fabric) have also important controbutions ble and the one that supplies sufficiently acceptable (Guven, 1992). results. This method will be used in this study. Interlayer hydration complexes are more stable and better organised at low water contents due to the high energy of hydration of interlayer cations. When 3.3.2 Effect of the exchange complex the amount of water increases, aggregates of water on the bentonite hydration properties molecules are formed, joined by a hydrogen bond to other water molecules. The ordered structure of Studies of water adsorption in montmorillonites pro water loses its order as the osmotic swelling increa vide evidence about the structure of the clay-water ses. In the case of sodium montmorillonite, the in complex. This system strongly depends on the ex terlayer spacing increases inversely to the square changeable cation, as well as on the clay/water root of the saline concentration (Norrish, 1954). proportion. The aim of this study is to establish the degree of influence of the exchange complex on the There are numerous studies on hydration of homo adsorption water of bentonite. Its purpose is to de ionic smectites. However, work carried out on bi duce the consequences due to possible variations in ionic smectites is practically non-existent. The inter the exchangeable cations during the interaction of est in carrying out this type of studies, besides filling bentonite with granitic interstitial solutions, as was a gap in scientific literature, lies in answering a se described above. ries of questions raised during the FEBEX-I and II Projects. In these projects, it was proved that during the interaction of granitic waters with the bentonite 3.3.2.1 Material and experimental methods barrier, a process of ionic exchange occurs and leads to the preferential adsorption of divalent ions In order to carry out this study, we have worked with by the bentonite. Calcium, or magnesium, are prefer the fraction <20/ 42 3. Porewater in the clay bam 50Na; 25Ca/75Na, in accordance with Laird (1999), tend to be similar, regardless of the sodium-calcium by mixing pure sodium and calcium bentonite sus content. pensions in the appropriate quantities. As previously indicated, this water adsorbed by the To study the hydration of the samples, each one was bentonite may be filling layers in the interlayer space previously dried at 300°C overnight. Later, they by solvating exchangeable cations, filling pores by were subjected to controlled saturation conditions in capillary condensation or it may be adsorbed on the chambers with atmospheres of different relative hu external surfaces of the particles. midity and at a constant temperature of 20°C, until the equilibrium was achieved. The method followed The total surface area of the smectite (total of basal is essentially the one described by Dios ef al. faces) was determined from the crystallographic pa (1997). The different vapour pressures were: P/Po rameters "a" and "b" obtained from the (400) and = 0.05; P/Po = 0.078; P/Po = 0.15; P/Po = (060) reflections of the of X-ray diffraction disori 0.32; P/Po - 0.45; P/Po - 0.63; P/Po - 0.8; ented powder diagram of bentonite. For a "formula P/Po = 1, obtained from saturated solutions of dif weight" of 751 g, a value for the total "swelling" ferent salts or sulphuric acid. After equilibrium had area of 750 m2/g was obtained. In addition, the been reached, which was verified by weight control, theoretical amount of water that can form a mono- the water vapour adsorption isotherms were calcu layer of the smectite was estimated. A value of 104 lated for every degree of saturation defined. mg F^O/g was obtained, taking into account the number of water molecules that can occupy the to 3.3.2.2 Experimental results and discussion tal surface area and the corresponding weight of water. The external area of the bentonite was also Figure 8 shows that all the samples gradually ad calculated, 81 m2/g, assuming that the quantity of sorb water along the entire range of relative pres adsorbed water (or area covered) by the sodium sures. The pure calcium samples are those which bentonite for the value of P/Po = 0.05 belongs ex tend to capture most water, although when the pro clusively to the water adsorbed in the external sur portion of water adsorbed increases all the samples face area, as discussed further on. P/Po Figure 8. Adsorption isotherms of powdered FEBEX clay. 43 EEBEXII project. THG Laboratory Experiments On the other hand, the number of milligrams of wa spacing changes in stages, unlike the gradual and ter per monolayer, which enter the interlayer, obser continuous adsorption of water, which is shown in ved experimentally, can also be calculated from Figure 8 . Three stages can be differentiated, for both data obtained on the adsorption isotherms, using the calcium and the sodium samples. The sodium the well-known BET equation: sample has an interlayer spacing value of 10.15 A, for P/Po = 0.05; this value is very similar to that of P/X(Po-P) = 1/XmC + (C-l)/XmC*P/Po E. 3.3.1 dehydrated bentonite, so the water, at this relative humidity, has to be adsorbed on the external sur where X is the amount of water adsorbed in g/g of face, as there is not enough energy to open the bentonite at pressure P and temperature T; Po is the interlayer, hydrate the cations and create a mono- pressure of saturated vapour at the same tempera layer of water. For P/Po = 0.078, the basal spacing ture, Xm is the amount of water associated with the changes to 12.5 A, remaining constant until a rela monolayer. Finally, C is a constant related to the tive humidity of P/Po = 0.61 is reached and giving adsorption heat of the first water layer and the li rise to the formation of the first monolayer of water. quefaction heat of the water, according to the ex In other words, the interlayer spacing does not in pression: C = el Ea"Ee )'/RT; and which gives an idea of crease in this long range, but, however, the content the energy required to form the monolayer. in adsorbed water does increase, as shown in Fig Table 1 8 shows the values determined of Xm and C, ure 8 , because water is filling the voids in the obtained from the linear regression of P/X(Po-P) ver interlayer space. For P/Po = 0.8 the basal spacing sus P/Po for the samples. We can see in the table increases again and it continues until P/Po= 1, so that the amount of water per monolayer depends on that at the end there are two more monolayers. the amount of exchangeable Ca. On the other hand, Therefore, in total three monolayers of water will the values of the energetic parameter C show that it have been formed from the initial dehydrated sam is easier to form the hydration complex in the cal ple until the point of maximum hydration. cium samples than in the sodium ones, because the The Ca samples, just like the Na ones, show three equilibrium energy in the former is greater. In other stages of hydration, as indicated by the variation of words, in principle and in energy terms, the calcium the basal spacing. In these samples, unlike the pre samples have more capacity to retain water. vious ones, a small amount of water is enough to Figure 9 shows the results obtained for the basal open the interlayer, since this cation has more spacing d(A) obtained by X-ray diffraction for every hydration energy, as stated above. We can therefore one of the hydrated samples. (We have to point out establish that at low vapour pressures (P/Po=0.05), that the diffraction diagrams were done in minimum we obtain a basal spacing of 14 A, which is main time so that the hydration status of each samples tained until P/Po = 0.078. This behaviour shows that were not modified by exchange with the environ monolayers and bilayers of water must have formed. mental humidity). With regard to the degree of The value of 14 A must be interpreted as an mi hydration, it can be seen that in this case, the basal xed-layer formed by calcium smectite sheets in mo- Table 18 Values determined of Xm and C, obtained using the BET equation for each sample. Samples Xm C Natural 83.33 152.1 100%Ca 99.7 337.7 75%Ca/25%Na 80 333.2 50%Ca/50%Na 69 228.2 25%Ca/75%Na 66.7 19.5 100%Na 61 16.18 44 3. Porewater in the clay bam -0— Ca —Na A Natural X 50Cq/50Na X 75Cq/25Na 0 25Cq/75Na P/Po Figure 9. Results obtained for the basal spacing d (A) obtained by X-ray diffraction for the hydrated samples. nolayer (12.5 A) and others in bilayer (15.5 A). For ing rise therefore to random interstratifications of one P/Po = 0.15, the spacing increases and remains and two layers of water. Finally, for relative constant until P/Po = 0.8, and increases again to humidities over P/Po = 0.8, the water molecules com P/Po = 1. These values indicate that a stable bilayer plete the second layer of water and form a trilayer has formed first and finally, a hydrated complex of complex of water molecules. three layers is formed for maximum hydration. As far as the Na is concerned, it is surrounded by a This behaviour can be explained bearing in mind that smaller number of water molecules per cation and Ca, a bivalent cation, has more polarising power an energetic balance below that of the Ca. That is than Na and tends to be surrounded by a larger because the formation of the water bilayer in so number of molecules of water than the latter. Figure dium bentonite only occurs at high hydration values. 1 0 shows the values calculated for N, the number of Up to a value of 0.6 of P/Po, the interlayer spacing water molecules associated with each cation. It can does not vary, but the amount of water adsorbed be seen that Na ion is surrounded, on average, by does. This is indicative of the fact that the monola 3.4 water molecules, while Ca is surrounded by 1 1, yer is not only completely filled, but also that the ex and that in the samples with a mixture of both cat cess of water adsorbed must fill pores and external ions, the number of water molecules depends on the surfaces, as it cannot enter the interlayer space and sodium/calcium ratio. Therefore, when the water form the bilayer complex. The latter occurs for va molecules come into contact with a sample of ben lues of P/Po of one. tonite or calcium smectite, the hydration process of the divalent cation is so energetic that it not only sep Going back again to Table 1 7, we can already con arates the sheets to form a monolayer complex with clude that Xm value of the BET equation does not the cation, but also, practically at the same time, represent the milligrams of water associated with ev other molecules form hydration complexes in bilayer. ery monolayer, but rather the water directly associ This does not necessarily mean that the first ated with the exchangeable cations in monolayer. monolayer has filled up completely, but that a sec This means that there is still empty space to fill up ond sheet of water has formed in addition, even giv among cation groups and water molecules. As this 45 EEBEXII project. THG Laboratory Experiments %Co Figure 10. Values calculated for H, the number of water molecules associated with each cation. situation depends on the ionic potential of the rising power of Ca. For the formation of the bilayer, interlayer cation, it can be seen that for the Ca, the there are no differences between the different sam theoretical value of the monolayer calculated is al ples, and values of around 200 mg of water are ob most reached (1 04 mg h^O/g bentonite). However, tained to form the bilayer (c). for the Na the value is much less, 83 mg H20/g In summary, Ca, as it is a bivalent cation with a bentonite. high polarising power, is surrounded by a larger On the other hand, the adsorption equation of Du number of water molecules than Na, and it has binin and Astakhov: more swelling energy. Therefore, the swelling of the calcium bentonite occurs with a small amount of W = W. exp{-(RTLn(Po/P)/EM E.3.3.2 water, the water occupies the interlayer in a double layer: first surrounding the cations and then occup allows the amount of water adsorbed by mono- and ying the free spaces among the groups of water mo bilayer to be calculated directly. In the equation, W lecules co-ordinated to the cations. The trilayer is the volume of water adsorbed at temperature T complex only forms at very high hydration. Therefo and at the relative pressure P/Po; Wo is the maxi re, all the adsorbed water is occupying the interlayer mum possible volume of adsorbed water; n is a pa spaces. rameter of the system; and E is the characteristic ad sorption energy of the system. In Table 19 we can Na is a monovalent cation with a smaller number of see that the values obtained for the monolayer (b) co-ordinated water molecules than Ca, and it has are greater than those determined with the BET less swelling energy to form the monolayer, so the equation and similar to the value determined previ adsorbed water will fill the interlayer, will condense ously from crystallographic parameters (approxi capillarily in the micropores and it can also be ad mately 1 OOmg^O/monolayer). sorbed on the external surface. In any case, the most calcic samples show an excess In accordance with other studies for pure end mem of adsorbed water molecules in monolayer. A stac bers (Newman, 1987), when the water/bentonite king phenomenon must occur due to the high pola ratio increases, the calcium sample does not increa- 46 3. Porewater in the clay bam Table 19 Values obtained for the monolayer and bilayer of water from the adsorption equation of Dubinin and Astakhov. Samples Xm "b" (monolayer) "c" (bilayer) Natural 83.33 136 221 100%Ca 99.7 159 221 75%Ca/25%Na 80 128 222 50%Ca/50%Na 69 115 224 25%Ca/75%Na 66.7 101 207 100%Na 61 95 223 ses its interlayer spacing (only three layers of water). topic composition of the hydrogen of water was de The sodium sample can swell freely (osmotic swe termined by reduction with Zn at 550°C (Tanweer, lling) until the formation of a gel or a suspension 1988). The reproducibility of the isotopic analyses with the individual sheets completely separated (No- of oxygen is » ±0,2 %o and of hydrogen » ± 1 %o. rrish, 1854, 1973). Traditionally, the extraction of the different types of In conclusion, FEBEX bentonite, when it is hydrated waters, hydration and structural, in clay minerals has and becomes more calcic (due to the interaction been carried out by different researchers using dif with the granitic waters, as in the Grimsel case), can ferent methodologies. The most used was heating eliminate its free water (intro- or inter-aggregates) to the clays at 1 1 0°C to determine the hydration water enter the interlayer and form a bilayer, as our cal and then at 1000°C to obtain structural water, cium sample did. The trilayer can only be formed at Linares ef al. (1993); Caballero et al. (1985, 1991, values of P/Po = 1, and the inter- or inter-aggre 1992). However, some authors consider this heating gate spaces may start to fill again. All these facts to be insufficient, Fripiat ef a/. (1960); Savin and prove that the hydration process of bentonite is diffi Epstein (1970); Cuadros ef al., (1994). From stud cult and very complicated. ies of DTA and chemical determinations, these last authors concluded that the best treatment to ensure the largest quantity of hydration water extracted 3.3.3 Isotopic study of adsorption water without producing dehydroxylation is heating it at in bentonite. A preliminary study 300°C overnight. Vacuum distillation for the extrac tion of hydration water is used by Jusserand, (1 980), The aim of this study is to deduce the mechanism of Saxena and Dressie (1984), Araguas-Araguas (1995). hydration of bentonite. Other objectives are the Revesz and Woods (1990) use an azeotropic distil knowledge of the different types of waters, the mobi lation with toluene and Turner and Gaelitis (1988) lity of interstitial solutions and the amount of useful extract the hydrogen from the water for isotopic water, free water, present for the transport of solutes analysis after a microdistillation with zinc. Walker ef in the bentonite barrier system. It is expected that al. (1994) compare the different methods. the isotopic analysis of adsorbed waters shows new insight on the smectite-water system. 3.3.3.2 Extraction of interstitial water 3.3.3.1 Material and Experimental Methods The water adsorbed for each bentonite at different The methodology used to prepare the samples is the relative humidities was extracted for isotopic analysis same described in the section 3.3.2.1. from an aliquot of a saturated sample. The isotopic composition of the oxygen of water was The technique of vacuum extraction used was adap determined by equilibrium with CO? at 25°C in ac ted for this project and it is based on a modification cordance with Epstein and Mayeda (1953). The iso of that described by Araguas-Araguas (1 995), which 47 FEBEXII project. THG Laboratory Experiments consists in a vacuum extraction during heating the termined by DTA, probably due to a possible sample at 11 0°. The water collected is analysed iso- hydration of the sample in the process of introduction topically (d18 O and d2H). Since the starting water into the vacuum system. This table also shows the has a known isotopic composition, the value ob value of hydration water determined gravimetrically tained after extraction will indicate if there was isoto to carry out the adsorption isotherms, and it is also pic fractionation during the process. This possible quite coincident, as Fig. 11 shows, where both pa fractionation can be due to incomplete water extrac rameters are shown. We can observe that they fit to a tion (most negative isotopic values) or to the mixture straight line, whose correlation coefficient has a value with other waters existing in the clay. These water of r2 = 0,9191. Therefore, we can conclude that the molecules would not have been extracted in the technique adapted for the extraction of the hydration process prior to hydration and could be considered waters is perfectly reliable, suitable and ideal for this as immobile waters, but they could be fractionated type of studies, since it allows all the hydration water with the marked water, introduced during the hy to be extracted with maximum performance and with dration of the clay. out kinetic isotopic fractionations. As an example, the interstitial waters extracted from 3.3.3.3 Results and Discussion S-2 bentonite, which was extracted from the same The extraction processes of interstitial water in cla site as the FEBEX bentonite 5 years before, were yey materials are complex. The waters extracted by analysed isotopically. The extraction of this water means of vacuum distillation show an important was carried out to check the adaptation of the ex problem: if the extraction process is not completed, traction technique. The data are shown in Table 20. an impoverishment of the heavy isotopes of water The average value obtained was d18 0 = -0.83%o extracted takes place and an isotopic fractionations and d2 H = -31 %o. These values could be the aver can occur. This is the reason because it is funda age isotopic value of interstitial waters of bentonite mental to control the performance of the process. in the Serrata de Nijar area in the Cabo de Goto To do this, the % of water extracted was determined region. by means of weight control of each sample before Once the vacuum extraction technique had been and after extraction. In addition, in an aliquot of adapted, water was extracted from the Ca-hydrated every one, this percentage of hydration water was samples and then analysed isotopically, both for determined by means of Differential Thermal Analy d18 0 and d2 H. Table 22 and Figure 12 show the sis, in order to check whether both percentages are isotopic values obtained for the homoionic calcium equivalent. samples and for some relative humidities of P/Po = Table 20 shows the data for the samples correspond 0.05; 0.078; 0.15; 0.32; 0.45; 0.8; 1. Similarly, ing to different P/Po of relative humidity for the Figure 12 shows values of the world meteoric water homoionic calcium samples. It can be seen that, line (WML), as well as the isotopic value of the wa bearing in mind the experimental error of every one ter from which the hydration solutions were pre of the experimental techniques, the values can be pared (d18 O = -8,2%o). It can be seen that the val considered similar even though those obtained by the ues obtained tend to be in line with the same slope vacuum technique are slightly higher than those de that the meteoric waters have. (There are two sam- Table 20 Data of hydration water of calcium bentonite. DTA (400-500°C) mg H20/g P/P. % Vacuum extracted H20 % Loss of H20 bentonite 0.0/8 11.82 10.57 102.61 0.15 13.94 14.99 127.67 0.32 14.69 13.12 140.55 0.63 16.14 15.57 182.79 48 3. Porewater in the clay bam bentonite Figure 11. Correlation between % of water vacuum extracted and the mg of water /g of bentonite determined gravimetrically. Table 21 Isotopic values of interstitial water extracted in the S2 bentonite from the Cortijo de Archidona in Cabo de Goto (Allmeria). Sample cS,8 0 <52H Sample (5,8 0 <52H S-2/1 1.5 -24 S-2/15 -0.5 -21 S-2/2 -0,9 -31 S-2/16 0.7 -30 S-2/3 -2.4 -36 S-2/17 -1 -30 S-2/4 -4 -31 S-2/18 -3.1 -31 S-2/5 1.3 -32 S-2/19 -0.2 -23 S-2/6 -1.8 -38 S-2/20 0.4 -19 S-2/7 -1.3 -32 S-2/21 0.6 -30 S-2/8 -0.5 -30 S-2/22 -0.63 -32.5 S-2/9 -0.3 -26 S-2/23 -4.19 -40.4 S-2/10 0.4 -34 S-2/24 -1.46 -31 S-2/11 -1 -32 S-2/25 0 -33.5 S-2/12 0.2 -31 S-2/26 -3.36 -30 S-2/13 0 -33 S-2/27 -2.3 -31.8 S-2/14 1.5 -34 49 EEBEXII project. THG Laboratory Experiments Table 22 Isotopic values obtained for the homoionic calcium samples and for some relative humidities of P/Po= 0.05; 0.078; 0.15; 0.32; 0.45; 0.8; 1. Sample P/Po 6,8 0 <52H Co-100 0.05 -11.4 -96 0.078 -9.92 -86.2 0.15 -9.13 -78 0.32 -1.91 -50.6 0.45 -6.06 -51.7 0.8 -4.76 -82.4 1 -7.66 -62.7 40 O Co-100 20 □ WML p 0 A Starting water -20 c p -40 A -60 V < V : L -12 -10 Figure 12. Values of 5,s 0 vs. d 2 Hof the waters extracted in the homoionic calcium samples. Included is the meteoric water line (WML), as well os the isotopic value of the starting water. pies, at 0.32 and 0.8 P/ Po, with incorrect values. value of the vapour entering and then condensing in There is not explanation for this behaviour in this the bentonite. The fractionation equations used were moment). (Majzoub, 1 971), for lsO: On the other hand, the isotopic fractionation value of the vapour in equilibrium with every hydration 7 0 007na^ - 7,737(70^ T-2) - 0/756(7(7 T') - system was determined, in other words, the isotopic -2,0667 E.3.3.3 50 3. Porewater in the clay bam and for 2 H : terstitial solutions coming from higher relative humi dity or those near saturation. The different reactivity 7 0 007na^ - 24.844(7(7 - 76.248(7(7 T-') + should be taken into account in studies on cation + 52.67 E.3.3.4 and solute transport of these solutions in bentonite. All these results are very preliminary, and it is neces The isotopic fractionation value obtained was a™v sary to continue studying and corroborating these (20°C)= 1.0096 and a°_v = 1,0852 and the values aspects with a larger number of solutions marked -17,77%oand 62 Hy - -138,27%o. isotopicaIly to be a complete picture of the clay-wa ter system. Nevertheless, bearing in mind that this vapour con denses again as water enter into the bentonite, the isotopic value of the water extracted as it is in equi librium with that vapour will have an identical isoto 3.4 Experiences obtained pic value to the starting water. If we observe the val in previous studies ues determined (Figure 12), only the water extracted in the sample at maximum saturation is similar to with MX-80 clay the value of the original water. For the rest, the val ues were more negative. The isotopic values of the 3.4.1 Introduction water extracted are lower than those of the starting water. This fact leads us to believe that perhaps, as The work described in Section 3.4 has been carried the hydration water vapour comes from saturated out by A. Muurinen (VTT). solutions of different salts, the saline effect could The pore water chemistry is strongly related to the modify the isotopic value of the vapour phase, mak microstructure of bentonite. The electrical double lay ing it more negative. Flowever, different authors ers forming on the montmorillonite surfaces of ben (Stewart and Friedman, 1975; Truesdell, 1974; Ka- tonite cause a non-homogenous distribution of the kiuchi, 1990; O'Neil and Truesdell, 1991; Florita ef ions in the bentonite pores (van Olphen 1977). In a/., 1993a) state, in studies on liquid-vapour isoto pores of different sizes, this distribution varies and it pic fractionation in saline solutions of different con is even difficult to define what the term "pore water centrations, that the effect of salts on the isotopic composition" means. The review report by Sacchi ef fractionation can be considered negligible at tem a/. (2000) provides a synthesis of available pore wa peratures below 1 00°C. ter extraction studies. Despite the many difficulties re Therefore, the most negative isotopic values of oxy lated to the pore water studies, such studies seem to gen and hydrogen found in the saturated samples at be one way to improve the understanding of ben lower relative humidity could be due to the different tonite. This chapter summarizes the methods devel kinetics (diffusion rates) of light isotopes versus heavy oped in the Finnish nuclear waste programme for ones. Lighter isotopes, due to their low mass, have pore water extraction from bentonite and for analys more energy, which makes them more unstable and ing the chemical parameters of interest (Muurinen therefore more reactive than heavy isotopes. In any and Lehikoinen 1999a, 1999b, Muurinen 2001, isotopic exchange reaction, light isotopes will ex Muurinen 2003a). The methods were used in the change before the heavy isotopes do, as they have studies of the bentonite samples taken from the FEBEX more reactivity, and therefore more diffusion veloc "in situ" test (Muurinen 2003b). ity, until equilibrium is reached (O'Neil, 1986; Ji menez, ef a/., 1999). For samples at low relative humidities, the water vapour in the interlayer of the 3.4.2 Equipment used for squeezing bentonite will come from the lightest atoms of oxy of bentonite porewaters gen and hydrogen. The squeezing apparatus developed to separate The relative proportion between the isotopes of lsO pore water from compacted bentonite is shown in and 2FI in the water of bentonite samples, depen Figure 13. It consists of a pressing apparatus that is ding on the different relative humidity, may affect used to create the necessary long-term compression the ions diffusion in the interstitial solutions. In the and the compaction cell where the pore water is case of low relative humidity, there could be a hig separated with a sinter from the bentonite and col her diffusion velocity because they are formed by lected in a syringe. The need to be able to perform molecules with light isotopes. The inverse is for in several parallel squeezings in anaerobic conditions 51 EEBEXII project. THG Laboratory Experiments m rrn Hydraulic cylinder Frame Figure 13. Pressing apparatus and compaction cell for squeezing porewater from bentonite. led us to use a small equipment and sample size. water. Most of the components can be determined The diameter of the sample used in the studies is 20 from a small pore water sample by diluting the sam mm and the height 1 0 to 30 mm. The height of the ple. Some parameters, like pH and Eh, presume the frame is about 50 cm. The constant long-term force use of a non-diluted sample, however. Table 1 sum is maintained with a strong spring. marizes the selected methods and the evaluated de termination limit in the initial pore water sample as The pressure is increased manually stepwise up to suming that the size of the pore water sample is at about 100 MPa, which typically takes 1 to 2 weeks. least 1.5 ml. The procedure for bentonite pore wa Figure 14 shows an example of the increasing of the ter analysis consists of the steps seen in Figure 15. pressure and the volume of the squeezed pore water. In order to avoid changes in the pore water sample 3.4.3.1 Conditions and preparative steps during squeezing all the metal parts in contact with the pore water were prepared of titanium. All the To avoid oxidation of the redox-sensitive compo joints of the cell are sealed with o-rings and the nents, the squeezing and handling of the pore water syringe was fixed with glue to the cell in order to samples have to be carried out in an anaerobic glo avoid the leakage of gases. The test with the squee ve-box where the oxygen concentration is < 2 ppm. zing cell showed that reactions between sulphide The pH and Eh electrodes have to be calibrated and the titanium cell cannot be completely avoided, and tested with suitable test solutions in the anaer but the reaction is slow enough to allow squeezing obic glove-box to ensure their quick use. The elec without major changes. trodes, filling solutions and cells have to be kept in the glove-box beforehand to remove oxygen from 3.4.3 Analysis methods for porewaters them. All the reagents to be used for S(-ll) and Fe(ll) analyses have to be prepared and made oxy The analysis methods have been selected and tested gen-free. The methods then have to be tested. If in Muurinen (2001) for the most interesting chemi possible, the size of the bentonite sample should be cal components and parameters in bentonite pore selected so that the volume of the squeezed pore 52 3. Porewater in the clay bam Pressure Time (h) Figure 14. Pressure and pore water volume os o function of time during pore water squeezing. The dry density of the sample was 1.5 g/cm3 (Muurinen 2001). water sample is > 1.5 ml. At the end of squeezing, 3.4.3.3 Ultrafiltering the sample is moved from the syringe into a centrif ugal tube of 2.5 ml. The sample is centrifuged for 5 The rest of the sample in the centrifugal tube is mo minutes at 1 0 000 rpm to separate possible particu ved into a centrifuge filter tube (Whatmann, Vectra Spin™ Micro, MWCO 12 k). The sample is filtered late material. in order to remove traces of colloidal bentonite. The ultrafiltered sample is used in the analyses below. 3.4.3.2 pH and Eh measurement 3.4.3.4 ICP-AES and 1C analyses A sample of 0.3 ml is moved from the centrifugal tube to the pH/Eh measurement cell with a magne The sample of 0.3 ml from the ultrafiltering is dilu tic stirrer on the bottom. The pH is measured with a ted to 5 ml and stored for ICP-AES and 1C analyses. micro glass combination electrode (Orion Model Further dilutions are made according to the needs 98-03) calibrated beforehand. To avoid changing of the analysis method. of the sample, the measurement should be carried out within a few minutes. 3.4.3.5 Titration of HC03" The pH electrode is replaced by an Eh electrode A sample of 0.3 ml from ultrafiltering is diluted with (Microelectrodes Inc., Model MI-800-71 0). The cell CC>2-free water to 5 ml and titrated to pH 4. HCO)) is carefully closed and the Eh readings recorded un is determined on the basis of the Gran curve, as til the electrode is stabilised. Normally this takes seen in Figure 1 7. (Appello and Postma 1 993). from a few hours to one day (Figure 16). The S(-ll), Fe(ll) and Fetot of the sample will be determined 3.4.3.6 Determination of S(-ll) when the Eh measurement is completed in order to see the changes in the redox components during the A sample of 0.2 ml is taken from the centrifugal measurement. tube and diluted to 3 ml with oxygen-free water. The 53 EEBEXII project. THG Laboratory Experiments Conditions and preparative steps - Assurance of low-oxygen conditions - Calibration of pH electrode - Testing of Eh electrode - Testing of S(-ll) analyse Squeezing of bentonite pore water Centrifugation of water pH (micro electrode) Eh (micro electrode) 0.6 ml, dilution 0.2 ml, dilution 0.3 ml, dilution 0.3 ml, dilution Figure 15. Procedure of the pore wo ter analysis. reagents are added into the sample according the sample is then taken out of the glovebox. All of the standard method (SFS 3038). The sample is taken iron in the sample is reduced for Fe(ll) with out of the glovebox and measured in the spectrop tioglycolic acid and determined as Fe(ll) (Dimmock hotometer. etal. 1979, Ruotsalainen etal. 1994). 3.4.3.7 Determination of Fe(ll) 3.4.4 Experiences of pore water studies A sample of 0.2 ml is taken from the centrifugal tube and diluted to 3 ml with oxygen-free water. The ferro- The performed interaction tests with compacted zine reagent is added into the sample, according to bentonite and solutions of different compositions the method by Dimmock et al. (1979), and Ruotsa- have shown that most of the components of interest lainen et al. (1994). The sample is taken out of the like Na+, K+ Ca2+, Mg2+, Si"+, CM, SO);, HCO; glovebox and measured in a spectrophotometer. and pFH can be determined without significant diffi culties from squeezed porewater samples. S'2, Fe 2 + 3.4.3.8 Determination of Fe tot and Fetot have appeared to be difficult to find in the porewaters, however. In addition, the Eh values have A sample of 0.2 ml is taken from the centrifugal been quite high owing to the missing redox compo tube and diluted to 3 ml with oxygen-free water. The nents. It seems that, in addition to oxidation, also 54 3. Porewafer in the clay bam *—S(-ll) Time (h) Figure 16. Measured Fh and S(-ll) concentration as a function of time. The sample size ro 0.3 ml (Muurinen 2001). A x A x 0 0.5 1 1.5 2 2.5 Volume of HCI (ml) Figure 17. Gran functions vs. added HCI volume for different HCOi concentrations in the sample. The initial HCCF concentrations (mg/L) are given in the legend. A sample of 0.3 ml ro diluted to 5 mi with de-ionized water before titration (Muurinen 2001). 55 EEBEXII project. THG Laboratory Experiments Table 23 Summary of the analytical methods for bentonite pore water. The size of the initial pore water sample is assumed to be at least 1.5 ml (Muurinen 2001). Analyzed Water type Determination limit Uncertainty Method component and sample data in the initial sample (mg/L) (%) Na+ ICP-AES Diluted pw. sample 4 ±10% K+ ICP-AES Diluted pw. sample 14 ±25% Ca2+ ICP-AES Diluted pw. sample 4 ±10% Mg2+ ICP-AES Diluted pw. sample 4 ±10% Si ICP-AES Diluted pw. sample 4 ±10% cr 1C Diluted pw. sample 2 ±10% SO2- 1C Diluted pw. sample 2 ±10% S(-ll) Spectroscopy Diluted pw. sample 0.1 ±20% Fe(H) Ferrozine Diluted pw. sample 0.1 ±20% Fe, ot Ferrozine Diluted pw. sample 0.1 ±20% HCO- Titration Diluted pw. sample 1.0 ±20% pH Micro electrode Non-diluted pw. sample Not determined ±0.1 pH units Eh Micro electrode Non-diluted pw. sample Not determined ±30 mV precipitation may decrease the concentration of those 3 M were used as the external solution for saturation. components below the determination level (Muurinen The concentrations in the porewater were determined 2001). by the direct analysis of the squeezed porewaters and by dispersing the sample in deionized water. Valu The studies have shown that the density of bentonite able information could be obtained by both meth clearly affects the concentration of the porewater in ods. Figure 19 and Figure 20 present the concen equilibration studies. The obtained concentrations trations in the porewater determined by squeezing are lower at higher densities, which is typical for ex and by dispersing methods, respectively. The model clusion caused by the overlapping electrical double ling curves obtained by coupling the microstructure layers. Typical for the successive porewater fractions of the clay and the Donnan model have been pre is also a decrease in the concentration as a function sented in the figures, too. of increasing density (Figure 18). Reporting the den sity interval during squeezing is thus most important In Figure 1 9 the curve represents the average con for interpreting and comparing the results. centration in the porewater while in Figure 20 the In the study by Muurinen et al. (2002) the concen concentrations in the interlamellar and large pores trations caused by the external solution into the have been presented separately. The concentrations porewater were studied with compacted bentonite in the squeezed samples are typically somewhat (MX-80), from which easily dissolving components higher than the average concentrations determined had been removed in order to ensure that the ions by the dispersing method. Modelling results suggest in the porewater came from the external solution. that the squeezed water comes both from the large The dry densities of the samples varied from 700 pores and from the interlamellar pores and the rela kg/m3 to 1 700 kg/m3 and NaCI solutions of 0.1 to tive amounts depend on the density of the sample. 56 3. Porewater in the clay bam □ Ew. □ Pw. 0.6-0.9 □ Pw. 0.9-1.1 □ Pw. 1.1 -1.4 □ Pw.1.4-1.5 n | 40 |i I r-^______1—_ |_TH~h n.rhm pH No K Co Mg Cl SO, HCO, Figure 18. Equilibrium concentrations of different ions and pH in external water (Ew.) and porewater (Pw.) fractions at different squeezing densities indicated in the legend. The densities for porewater increase from left to right. Interaction experiment with MX-80 and fresh water at bentonite dry density 0.6 Mg/m? and bentonite/water ratio 0.5 Mg/m? (Muurinen and Lehikoinen 1999b). .0E+01 .OE+OO Diy density (kg/m !) Figure 19. Chloride concentrations in the porewater as a function of the dry density of the sample and the concentration of the saturation solution. The concentrations were determined by the dispersion method. The solid and open points mean two sample types used in the experiments. The modelling curves are based on coupling of the microstructure and the Donnan model. 57 FEBEXII project. THG Laboratory Experiments .OE+Ol .0E+00 Dry density (kg/m 3) Figure 20. The chloride concentrations in the squeezed porewater fractions (points) os o function of the dry density of the sample and the concentration of the saturation solution, the modelling curves are based on coupling of the microstructure and the Donnan model. The dotted lines (G-) represent the calculated concentrations in the large pores and the solid lines (IL-) in the interlamellar pores. 3.5 Pore water obtained The variation of the height of the sample was mea sured by means of an automatic displacement sen by squeezing as a function sor built-in to the hydraulic press. During the test, of the applied pressure each pressure step was prolonged as much as to obtain all the extractable water. 3.5.1 Introduction The work described in Section 3.5 has been carried 3.5.2 Hydrodynamic results out by A. M. Fernandez and P. Rivas (CIEMAT). The water started to be expelled from a pressure of A study of the variation of the porewater chemical 10 MPa. The variation of the water volume extracted composition of the FEBEX bentonite as a function of versus the squeezing time is shown in Figure 21. A the applied squeezing pressure was performed in total amount of 65.4 ml were collected after 222 order to analyse: days. The final squeezing pressure was of 100 MPa. a) the possible ion ultrafiltration through the clay, or The variations of the water content (w) as a function of the squeezing pressure (P) and the dry density (pd) b) water dilution to the liberation of internal or of the sample for each pressure step are shown in interlaminar water. Figure 22 and Figure 23, respectively. The variation A FEBEX bentonite sample, 800 g, was remoulded of the water content of bentonite as a function of its with water to acquire an initial water content of dry density during the squeezing test is plotted in 30.9%. After stabilisation and homogenisation of Figure 24. In this figure, the theoretical water con the solid and liquid phases, 3 months, a squeezing tents needed to fully saturate the bentonite for each test was performed, in which the pressure was in dry density are also plotted. The water contents creased by steps from 25 MPa to 1 00 MPa. reached in the squeezing test are similar to the theo- 58 3. Porewater in the clay bam 70.0 90 o m 80 60.0 o o o o — 70 : 50.0 ♦ — 60 ♦ < 40.0 o o 3 u~ 50 tio o 30.0 0 40 o oo o o — 30 20.0 - - O O 030C 20 1 Acc. volumen (cm 1) 10.0 ° P(MPa) — 10 0.0 1 1 j 0.0 25.0 50.0 75.0 100.0 125.0 150.0 Squeezing time (days) Figure 21. Variation of the water extracted as a function of squeezing time. 34.0 - 32.0 < 30.0 - 28.0 - § 26.0 - 24.0 - i > 22.0 T 20.0 ______i______i______i______i______i______i______10 20 30 40 50 60 70 Pressure (g/cm 1) Figure 22. Variation of the bentonite water content as a function of the squeezing pressure. 59 EEBEXII project. THG Laboratory Experiments p, max = 0.1 min = 0.0036 P A Minimum density o Maximum density Pressure (MPa) Figure 23. Variation of the maximum and minimum bentonite dry density (pi) os a function of the squeezing pressure (P). ■. ■ Measured in squeezing tests □ Theoretical for saturation 320.17 e w — 306.82 e Dry density (g/cr Figure 24. Variation of the bentonite water content (w) as a function of the dry density (pi) of the sample. 60 3. Porewater in the clay bam retical water contents at saturation for each dry den eliminate the most of the clay water have been cal sity. This implies that the water collected in each culated by means of the study of the variation of the pressure step is the water excess that exists between dry density with the pressure and from the relation the saturation water content at an initial and a final between dry density and water content (Figure 23, dry density, reached when the pressure increases. Table 24 and Figure 24). Pressures of about 1 00, 1 55 and 292 MPa would be needed to consolidate The squeezing pressures to be applied in order to the sample at 1.8 , 2.0 and 2.5 g/cm3, de-saturat- obtain a dry density of about 2.5 g/cm3 and to ing the bentonite mass to water content values of Table 24 Dry densities obtained for each squeezing pressure step. Pressure Maximum dry density Water content (MPa) (g/cm 3) (%)(') 10 1.47 31.7 20 1.53 28.8 30 1.58 26.6 40 1.61 25.4 50 1.65 23.9 60 1.66 23.5 70 1.71 21.7 80 1.72 21.4 Table 25 Extrapolated squeezing pressures to be applied for obtaining dry densities of 1.8 to 2.7 g/cm 3 and water content for each dry density. Theoretical water Dry density Maximum pressure Minimum pressure Water content P) content at saturation 01 (g/cm 3) (MPa) (MPa) (%) (%) 1.8 101 96 18.5 18.8 1.9 128 124 15.6 16.1 2.0 155 152 13.0 13.8 2.5 292 291 3.0 6.3 2.7 346 346 0 4.6 W w W =(7/p,J- (T/yT (3> The maximum limit is at the grain density of the bentonite (2.70 g/cm). 61 FEBEXII project. THG Laboratory Experiments 1 8 .8 , 13.8 and 3-6.3 %, respectively. A squeezing ing pressure above 70 MPa (Figure 27). These wa pressure of 346 MPa is needed to consolidate the ters are undersaturated in gypsum and sample up to reaching the maximum limit of dry oversaturated in calcite and dolomite. density, i.e., the grain density of the bentonite sam ple (ys=2.70 g/cm3). It is interesting to note that the In this expansive FEBEX clay (Ca-Mg-Na montmori- llonite), most of the water is internal water, i.e., lo theoretical water content at saturation for each dry cated at interlayer positions, and there is only a little density fits to the experimental water content ob tained up to a dry density of 2.0 g/cm3. Above this proportion of external water (Fernandez, 2003; Fer nandez and Rivas, 2003b). Thus, the decrease of value, some discrepancies are found. the conentration could indicate: 3.5.3 Analytical results i) an ion ultrafiltration through the clay, or ii) a dilution of the ionic concentrations due to The variation of the chemical composition of the some part of the internal or interlaminar water pore water as a function of the applied squeezing is being extracted when the pressure increases. pressure is shown in Figure 25 and Figure 26. The According to previous results of the variation of in concentration of Cl, SO4”, Na and Co in each step ternal/external water as a function of the dry density decreases as a function of the pressure. This behav (Fernandez, 2003; Fernandez and Rivas, 2003b), iour is not similar to that obtained with consolidated the greater part of the water in the bentonite is inter clay rocks composed by non-expansive clays, such nal and, when the dry density increases, the amount as Opalinus Clay rock material, in which a constant of water in the pore space decreases (internal + ex concentration is obtained up to pressures of 200 ternal). For this reason, it is very likely that during MPa (Pearson etal., 2003). the squeezing process the interlaminar water come The bicarbonate concentration increases indicating out, diluting the chemical composition of the ben some carbonate dissolution process from a squeez tonite pore water. Squeezing Pressure (MPa) Figure 25. Variation of the ionic contents as a function of the squeezing pressure. 62 3. Porewater in the clay bam Squeezing Pressure (MPa) Figure 26. Sulphate, magnesium and calcium content as a function of the squeezing pressure. 5—R—0—;—9 Squeezing Pressure (MPa) Figure 21. HC03 content and pH as a function of the squeezing pressure. 63 4. Effect of interlayer cations on the rheological properties of bentonite 4. Effect of interlayer cations on the rheological properties of bentonite 4. Effect of interlayer cations on the rheological properties of bentonite The work described in Chapter 4 has been carried is a measure of the decrease of effective vis out by F.J. Huertas, F. Huertas and J. Linares (CSIC). cosity with shear rate. □ Herschel-Bulkley's model: This model describe a fluid that has an initial yield stress (plastic 4.1 Viscosity of clay suspensions flow) at low shear rates, and afterwards present pseudo-plastic behaviour at higher shear rates. The flow behaviour of any system is described in terms The Herschel-Bulkley equation (Herschel and of the relationship between shear stress (t, Pa) and Buckley, 1 926) describe the consistency curve: shear rate (y, s-1). The shear rate is defined as the change of shear strain per unit time, and the shear r = ry + Kyn (E.4.1.4) stress as the tangential force applied per unit area. The viscosity [t), Pa-s) is defined as the shear stress to shear rate ratio of a suspension (Guven, 1992): 4.2 Materials and methods t] = t / y (E.4.1.1) The effect of the exchange complex on the rheologi cal properties of FEBEX bentonite has been studied The viscosity is a measure of the resistance to flow in several bentonite suspensions. The suspensions of the fluid and can be derived from the measure were prepared by dispersion in ultrapure water the ment of the shear stress at a specific shear rate. The <20 / □ Bingham's model: This model, derived from Bingham's theory of plastic flow (Bingham, 1922), 4.3 Results postulates that a finite stress must be applied to Flow curves initiate flow and at greater stress the flow will be Newtonian. The flow curve is described by The flow curves of the different suspensions prepa the following equation: red from all the smectite sample were obtained and used to calculate the value of the viscosity as a func r = rB + VPi' y (E.4.1.2) tion of the shear rate. where r6 is the Bingham yield stress and t]pl Sodium-calcium smectites. The flow curves of the four stands for the plastic viscosity. sodium-calcium smectites are plotted in Figures 29- 32 The following aspects can be remarked: □ Potential model: Very dilute clay suspensions behave as pseudo-plastic fluids, which can be □ All the sodium and calcium smectite suspen describe by the power-law equation: sions behave as a non-Newtonian fluids. r=Kyn (E.4.1.3) □ The viscosity (or the shear stress) increases with the solid concentration in the suspension. The where K is the consistency coefficient and n increase is steeper in sodium samples that in corresponds to the flow-behaviour index, which calcium smectites. 67 EEBEXII project. THG Laboratory Experiments Table 26 Composition of the exchange complex of the smectites and smectite concentration in the suspension used for the study of their rheological properties. Smectite % No % Ca Suspension Solid wt% Natural ~ 25 ~ 75* 15-33 Na-100 100 0 8 .6-12 No-50 50 50 10-17 No-33 33 6 7 8-17 No-0 0 100 25-35 * Ca+Mg Figure 28. Viscosimeter Fann-VG 35S. □ The addition of Na to the interlayer space pro □ At high shear rates (> 160 s_1), the shear stress duce an increase in the shear stress and, in and shear rate are not proportional. The vis consequence, in the viscosity of the suspension. cosity decreases as the shear rate increases. □ All the flow curves show a yield value, which These results indicate that the studied smectite sus increases with the content of interlayered Na. pensions behave as a plastic fluid at low shear rates The yield value can be estimated from the and as a pseudo-plastic fluid at higher shear rates. shear stress at 4.8 y 9.6 s~'. The flow curves seem to match Herschel-Bulkley's 4. Effect of interlayer cations on the rheological properties of bentonite Shear rate (s-l) Figure 29. Flow curves of three suspensions of smectite Na-0 (pure calcic sample) (25,29.5 and 35 wt% smectite). Lines correspond to the fitting of the experimental values to the potential model (see text). □ 12.5 Shear rate (s- Figure 30. Flow curves of five suspension of smectite Na-33 (8,12.5,14,15 and 17 wt% smectite). Lines correspond to the fitting of the experimental values to the potential model (see text). 69 FFBFXII project. THG Laboratory Experiments Shear rate (s- Figure 31. Flow curves of four suspension of smectite Ha-50 (10,12, 15 and 17 wt% smectite). Lines correspond to the fitting oftbe experimental values to the potential model (see text). Shear rate (s-1) Figure 32. Flow curves of three suspension of smectite Na-100 (8.6,10 and 12 wt% smectite). Lines correspond to the fitting of the experimental values to the potential model (see text). 70 4. Effect of interlayer cations on the rheological properties of bentonite model (Eq. 4.1.4). Furthermore, if the yield value is 4.3.1 Fitting of the flow curves low compared to the share stress, the flow curves can be fitted to the potential model for pseudoplas All the flow curves obtained in this study were fitted tic fluids (Eq. 4.1.3). to a model for pseudopastic fluids (Eq. 4.1.3). The fitted parameters, flow-behaviour index (n) and con Natural sample. The behaviour of the suspensions sistency coefficient (K), are gathered in Table 27. of the natural smectite (Figure 33) resembles that of The predicted curves computed with these values the Na-Ca smectite suspensions. Assuming that Mg are plotted as lines in Figures 29-33. It can be ob and Co interlayered has similar properties, the natu served that the experimental results of the Na-Ca ral sample with 25 % of Na should be intermediate smectite suspensions match the model, and dilute between Na-0 y Na-33 samples. However, the yield suspensions behave as pseudoplastic fluids. How value of the natural smectite, circa 35 Pa for a sus ever, some suspensions of the natural sample devi pension 32.7 %, is far from being similar to the val ate from the computed model at low shear rates. ues of 5 Pa obtained for Na-0 and Na-33 smectite suspensions. Furthermore, there exists an important difference in the value of shear stress between the 4.3.2 Apparent viscosity suspension 32.7 wt% and the other suspensions of the natural smectite (Figure 33). Although interla The viscosity of a suspension is defined as the slope yered Mg and Co have exhibit a similar behaviour of the flow curve. In order to compare the results in ion exchange experiments, they might be consid obtained for the different suspensions, apparent vis ered non-equivalent cations for the rheological pro cosities at a shear rate of 960 s'1. were given in Ta perties of bentonite. However, all the experimental ble 27. These values were also plotted in Figure 34, data were treated in the same form and all the flow as well as the flow curves computed according to curves were fitted to the same model in order to get Equations 4.1.1 and 4.1.3. It can be observed that comparable results. the increase in solid concentration induce an increase 0 200 400 600 800 1000 Shear rate (s-1) Figure 33. Consistency curves of natural smectite (fraction < 20fim) suspensions (17.5,20,22.5,25,28 and 32.7 smectite wtZ). lines correspond to the fitting of the experimental values to the potential model (see text). 71 EEBEXII project. THG Laboratory Experiments Table 27 Viscosity, flow-behaviour index (n) and consistency coefficient (K) (Eq. 3) of the studied smectite suspensions. Sample Smectite wt% t) (mPa-s)* n log K 25 3.5 0.700 -1.577 No-0 29.7 4 0.644 -1.339 35 5.75 0.579 -0.999 8 6 0.776 -1.557 12.5 15 0.676 -0.857 Na-33 14 21 0.659 -0.659 15 44 0.531 0.042 17 62 0.476 0.358 10 11 0.648 -0.917 12 23.5 0.566 -0.338 No-50 15 80 0.366 0.791 17 119 0.352 1.001 8.6 11.5 0.751 -1.209 Na-100 10 23.3 0.593 -0.425 12 69 0.447 0.487 15 4.5 — — 17.5 6.5 0.807 -1.619 20 9 0.825 -1.519 Natural 22.5 12 0.689 -1.007 25 15 0.669 -0.846 28 20 0.458 -0.097 32.7 56 0.201 1.123 * Apparent viscosity at 960 si in the viscosity of the suspension, which is strongly af value depends on the concentration of interlayered fected by the composition of the exchange complex. No. A low content of this element controls the rheo In the solid concentration exceed a certain value, logical behaviour of the suspensions, as can derived the viscosity increases steeply due to the formation of from the comparison of flow curves for samples a gel structure within the suspension. Such a critical Na-33, natural (25 % Na) and Na-0. 72 4. Effect of interlayer cations on the rheological properties of bentonite □ No-50 A No-33 A No-0 Bentonite % Figure 34. Apparent viscosity of bentonite suspension at 960 s~F Symbols correspond to experimental values and lines to calculate viscosity using parameters in Fable 2 with equations 1 and 3. No-bentonite % Figure 35. Estimation of tbe total concentration of bentonite necessary to prepare a suspension with a viscosity of 15 mPos, as a function of tbe percentage of No-bentonite in tbe solid (a mixture of No and Co-smectite). 73 FEBEXII project. THG Laboratory Experiments 4.4 Discussion and conclusions smectite should be raised up to 55 wt% in the case of pure Ca-bentonite in order to keep constant the viscosity of the suspension. The results of this study show the rheological behav iour of the smectite suspension is affected by the The increase in viscosity produces a decrease in sus composition of the smectite exchange complex. Us pension fluency, that is, the suspension capacity to ing data from Table 27 we have computed the con flow and its contribution to the mass transport. Con centration of bentonite suspension to produce an cerning the nuclear waste repositories, the laboratory apparent viscosity of 15 mPa s (at 960 s-1) as a tests performed during FEBEX I show that bentonite at function of the concentration of Na-bentonite. Fig the bentonite/granite interface should take up Co ure 35 shows that only 1 0 % of a pure Na-bentonite from the percolating solutions and becomes progres suspension can produce a viscosity of 1 5 mPa s and sively enriched in Ca. According to the results of this this viscosity is maintained if the interlayered Na study, under conditions of high input of groundwater ions are at least a third of the exchangeable cations. to the interface, the increase in interlayered Ca may If the Na-smectite concentration in the mixture is re contribute to suspend the smectite in the percolating duced even below 33% the total concentration of solutions that could fill and seal fractures. 74 5. Geochemical processes at the smectite-solution interface: Dissolution, transport and precipitation 5. Geochemical processes at the smectite-solution interface: Dissolution, transport and precipitation 5. Geochemical processes at the smectite-solution interface: Dissolution, transport and precipitation The work described in Chapter 5 has been carried glass or plagioclase may dissolve preferentially to out by F. J. Huertas, M. L. Rozalen, S. Garaa-Palma the smectite. On the other hand, the conditions wit and J. Linares (CSIC). hin the barrier are quite similar to that of smectite formation in natural environments, and the dissolu tion product of the accessories may produce more smectite. These topics should be considered in de 5.1 Introduction tail, estimating which are the prevailing processes as a function of temperature, pH, solution composi The bentonite used as an engineer barrier in the nu tion, abundance and specific surface area of the mi clear waste repositories may undergo several pro nerals across the bentonite barrier. cesses derived from its interaction with pore water, from the hosting geological formation or from the In conclusion, the objectives of this set of laboratory bentonite itself. The interaction of such solutions with tests are the following: the smectite and the bentonite accessory phases (vol □ to determine the dissolution rates of smectite canic glass, plagioclase, quartz, etc.) may induce and accessory minerals, as a function of pH chemical and mineralogical changes, which may and temperature; modify the physico-chemical properties of the bar rier. Dissolution of the smectite and the accessory □ to estimate the kinetic parameters of the disso phases under barrier conditions is a process that lution reactions; should be included in any predictive model. Al □ to evaluate their contribution to release and though the hydraulic properties do not allow the flux transport of silica and other mayor elements (Al, necessary to produce and maintain a massive disso Fe). lution initially, this process may start, spread and in crease through fractures and discontinuities. Fur thermore, this reaction can be locally accelerated in 5.2 Materials areas surrounding zones that contain some material that may produce conditions especially favourable for dissolution, such as concrete barriers or plugs, Bentonite acidic fronts, or accumulation of organic matter or FEBEX bentonite was extensively characterised dur microorganisms. ing FEBEX I preliminary phase. Mineralogical analy The smectite and accessory phases dissolution reac sis indicates a yielding of 92 % in smectite, and the tions release into solution (pore waters) different ele presence of volcanic glass and accessory minerals ments. Alkaline and alkaline-earth elements are so (plagioclase, K-feldspar, biotite, cristobalite, amphi- luble under the geochemical conditions found within boles, pyroxenes and zeolites). The interlayer cations the bentonite, and thus they can be easily transpor in the natural smectite are No, Co and Mg (ENRESA, ted. The situation for other major elements, as Si, Al 2000). or Fe, is completely different. Aluminium and iron In order to proceed with the separation of pure min behave as immobile elements, except if they form erals, bentonite suspensions were prepared by dis complexes with organic matter by quelation (this persing 20 g of solid in 1 L of distilled water, using elements are also soluble in acidic and alkaline an ultrasonic probe to improve dispersion. The sus conditions, which are not considered in FEBEX II fra pension was sieved to 37 / 77 EEBEXII project. THG Laboratory Experiments the beaker corresponds to the 0.5 - 4 / Mineralogical analyses were performed by X-ray dif tals of pure minerals were hand-picked under mi fraction (XRD), differential thermal analysis and ther croscope. The solid was analysed for major ele mogravimetry (DTA-TG). Based on these results, the ments by XRF (Table 30). The structural formula samples is composed > 99 % of a mixed layer illi- deduced from the chemical data was the following: te/smectite containing 10-15 % of illitic layers. The No0.676^0.057Cdo 23gAf, TZ^Og structural formula is the following: The specific surface area, measured by BET method, K-o.88 (AI2.54 Fe3+045 A/lg, ,2) (Si7 9] Al0 09) O2o (Oti)4 was of 0.65 m2 s-1. The uncertainty associated to Only 0.76 ions K+ per formula unit are exchangea this low vale is in the order of ± 1 5%. ble, in agreement with the presence of a 10-15 % of illitic layers. Volcanic glass The specific surface area was measured by BET methods, using nitrogen adsorption isotherms. The Dissolution of volcanic glass is likely an important value was of 111 m2 s-1 (uncertainty ±10%), that source of major elements. It may dissolve faster than corresponds to the smectite external surface. BET crystalline particles. In addition, tuffaceous materials surface area is commonly used to normalize dissolu exhibit large surface area, which contributes to a tion rates. faster specific dissolution rate. In consequence, vol canic glass should be included in the dissolution tests. Accessory phases XRD and DTA-TG analyses reveal the presence of a Accessory minerals were separate by sequential small amount of volcanic glass in the bentonite. Pre sieving of the solid fraction (> 37 /cm). The follow liminary observations under microscope showed that Table 28 Chemical analysis of K-Sm (0.5-4/cm fraction). Si02 AI2O3 Fe 203 MgO MnO CaO Na20 k2o Ti02 P205 LOI Sum 56.07 15.87 4.20 5.33 0.02 0.06 0.00 4.91 0.13 0.00 12.65 99.24 Oxides in weight %; LOI: loss on ignition. 78 5. Geochemical processes at the smectite-solution interface: Dissolution, transport and precipitation particles of volcanic glass with different textures The following stoichiometric formula was calculated were present in several density fractions. Most of based on one Si atom: these particles included crystals of minerals or con tains a great amount of smectite. Si, Al0 jp Fe0019 Ca00]7 Na0 084 K0 060 Ti0 00] (Oti)029s Oz.jOd In order to get a volcanic glass sample, free of min erals (especially smectite), specific samples of amor phous (under XRD) material were collected at the bentonite outcrop. This material was crashed, ultra- 5.3 Methods sonically cleaned in distilled water, ground in agate mortar and oven-dried at 60°C. The final powder Experimental set-up was used as volcanic glass for dissolution experi ments. It was completely amorphous by XRD and The flow-through experiments allow us to measure DTA-TG, with no traces of smectite. It was analysed the dissolution rate under fixed saturation state con by XRF (Table 31) and BET (1.51 m2 g”1, ±15%). ditions by modifying the flow rate, initial powder Table 29 Mineralogical analysis of the granulometric fractions collected by sieving from the FEBEX bentonite. Fraction (wm) Phil. Qtz. KF. Plug. Calc. Zeal. Amp. Pyrox. wt% > 400 16 19 6 30 23 6 — — 2.79 400 - 250 14 24 Tr. 26 13 3 3 17 1.03 250 - 177 9 24 2 58 Tr. 7 Tr. — 0.45 177-105 11 10 2 62 — 4 Tr. 10 1.27 105-50 30 27 — 31 6 5 1 — 0.91 50-37 54 12 Tr. 29 2 2 Tr. — 0.31 Total > 37 6.67 Ft phyllosilicates (including micas), Qtz.: cuorlz, KF: K-feldspar, Flag.: plagioclase, Calc.: calcite, leal: zeolites, Anf.: amphiboles, Pyrox.: pyroxenes; Tr.: traces. wt%: weight percent of the fraction in the bulk sample. Table 30 Chemical analysis of plagioclase. Si02 AI2O3 Fe 203 MgO MnO CaO Na20 K20 Ti02 P 20s Sr LOI Sum 61.76 23.36 0.31 0.00 0.002 4.94 7.77 0.90 0.07 0.010 422.3 0.094 99.25 Oxides, in weight %; Sr, en ppm; LOI: loss on ignition. Table 31 Chemical analysis of volcanic glass. Si02 ai2o3 Fe 203 MgO MnO CaO Na20 K20 Ti02 P 20s LOI Sum 72.54 11.7 1.89 0.02 0.03 1.15 3.14 3.39 0.09 0 6.06 100.01 Oxides in weight %; LOI: loss on ignition. 79 EEBEXII project. THG Laboratory Experiments mass, and composition of the input solutions. The reached, dissolution rates were evaluated. The ex dissolution experiments were carried out in flow periments will last until the steady state is reached through reactors designed and constructed for this (constant concentration of dissolved species for at aim (Figure 36). A peristaltic pump, that controls the least five consecutive data points). The duration of flow rate, injects the input solution in the bottom every dissolution test depends on the experimental chamber, where the solution is homogenised with a conditions. Dissolution is expected to proceed under magnetic stirrer, before reaching the reaction cham far from equilibrium conditions. ber (top, 23 ml in volume). The solid sample is con fined in the reaction chamber by membrane filters. The dissolution conditions were the combinations of The reactors are immersed in a water bath to con the following variables: trol the reaction temperature. □ pH: Solutions of constant pH, between 4-10. At intervals of approximately one pH unit. In any run, the flow rate and the input pH were held constant for a long enough time so that steady state □ Temperature: 25, 50, and 70°C, controlled by conditions were closely achieved. Achievement of water baths. steady-state conditions was verified by a series of □ Flow rate: 0.02 - 0.06 ml min ^. constant Si and Al output concentrations that differ by less that 5%. After steady-state conditions were □ Solid/solution ratio: 2 - 6 g drrr3. 0 V=23 ml 1.2 urn Magnetic bar Figure 36. a) Sketch of a flow-through cell for dissolution tests, b) General experiment set-up of the whole dissolution system. 80 5. Geochemical processes at the smectite-solution interface: Dissolution, transport and precipitation □ Input solution composition: The input solutions where A is the pre-exponential factor, Ea, is the ap only contain the reagents necessary to control parent activation energy (Id moM), R is the gas the pH (nitric acid, acetic acid, potassium ace constant, and T is the temperature (K). tate, potassium carbonate and potassium bi carbonate) and ionic strength (potassium ni trate 0.01 or 0.05 mol L-1). No Si, Al, Fe or 5.4 Results and discussion Mg was added. All the reagents were super- pure grade. Aliquots of —20 ml of output solution were periodi Smectite cally (24 h) collected during the duration of the ex The variation in composition of output solutions in periment. The pH was measured immediately, and three of the dissolution tests as a function of time is then the solutions were acidified to pH approxima shown in Figure 37 as representative ones of the ex tely 3 to analyse them for Si (molybdate blue com perimental set-up. The experimental conditions in all plex), Al (lumogallion fluorescent complex) and Fe the experiments are shown in Table 32. Small fluc (ICP-OE). tuations of the pH out with respect to pHi np due to the dissolution reaction are within the uncertainty of the pH measurement. High Al and Si concentrations are Kinetic calculations observed at the onset of most of the experiments. Afterwards, Al and Si concentrations decrease until Based on a simple mass balance equation, the dis a steady state is approached (Figure 37). Dissolution solution rate (mol m-2s-1), R, in a well-mixed flow rates were calculated based on Al and Si steady state through experiment is obtained from the expression: concentrations using Equation 2, after correction of mass of solid lost during dissolution (Table 33). The viR = ^(CiPui ~ Ci,iJ E.5.1.1 uncertainty associated with the measure is ±15% or 0.13 log units. Most of the rates complied in Table 33 correspond to stoichiometric dissolution (%, and where Cp„p and Cj;0Ut are the concentrations of Rai are equal), except those between brackets. component /' in the input and the output solution, re spectively (mol rrf3), Vj is the stoichiometric coeffi Previous studies have shown that for many minerals cient of / in the dissolution reaction, f is time (s), A is the dissolution rate with certain pH ranges is pro the reactive surface area (m2), and q is the fluid vol portional to a fractional power of the hydrogen ion ume flux through the system (m3 s-1). Note that in activity: our formalism, the rate is defined to be negative for R = ka^ E.5.1.4 dissolution and positive for precipitation. The disso lution rate in such an experiment may be readily ob where k is the rate coefficient and n, the order of the tained if steady state is reached, i.e., if the composi reaction with respect to the activity of protons. Fig tion of the output solution reaches a constant value. ure 3 plots log dissolution rate vs. pH at 25, 50, In this case, the dissolution rate is balanced by the and 70°C. Additional data, at pH higher that 1 1 difference between input and output solutions: and those from Zysset and Schindler (1 996) at 25°C normalised for a surface area of 1 1 0 m2 g-1, have been included for comparison. Minimum dissolution = E.5.1.2 rate occurs at pH 6.2 approximately, and it in creases with both increasing and decreasing pH For most of the experiments, the error in the calcu within the range studied (pH 4-10). The calculated lated rate ranged between 12 and 20% and it is do reaction orders at 25 and 50°C are the following: minated by the uncertainty of the BET surface area a pH < 6.2, 25°C R = 10“1293 - 021pH measurement (±10% for smectite, and ±15% for □ pH 6.2 - 11,25°C R= i o~14 86 + 0.099pH glass and plagioclase). a pH < 6.2, 50°C R= 10“u93 - 0.33pH The temperature dependence of the dissolution rate □ pH 6.2- 11,50°C R = 10“14 94 +0.14pH generally follows the Arrhenius law: There are not enough data to calculate them at R = Aexp(-E a / RT) E.5.1.3 70°C. It is observed an increase of the reaction or- 81 EEBEXII project. THG Laboratory Experiments Sm-25-10 Time(h) Sm-50-5b 0 400 800 1200 1600 lime (h) Figure 37. Variation of pH, Si and Al concentration in the output solution as a function of time, for different experimental conditions. (See fable 30). 82 5. Geochemical processes at the smectite-solution interface: Dissolution, transport and precipitation Table 32 Experimental conditions of the smectite dissolution tests. Temp. Initial mass Flow Time Series pHjnp (°C) (g) (ml min') (days) Sm-25-4-1 25 0.0884 3.98 0.020 101 Sm-25-4-ll 25 0.0884 3.98 0.020 101 Sm-25-4b-l 25 0.0900 3.97 0.021 113 Sm-25-4b-ll 25 0.0900 3.97 0.021 113 Sm-25-5b 25 0.0903 5.00 0.020 95 Sm-25-5c 25 0.0908 5.15 0.020 103 Sm-25-9b-l 25 0.0902 9.24 0.020 89 Sm-25-9b-ll 25 0.0902 9.24 0.020 89 Sm-25-10 25 0.0901 10.15 0.020 91 Sm-50-5 50 0.0902 5.26 0.021 95 Sm-50-5b 50 0.0902 5.16 0.020 108 Sm-50-7-1 50 0.0903 7.09 0.020 115 Sm-50-7-ll 50 0.0903 7.09 0.020 115 Sm-50-9b 50 0.0903 9.53 0.020 112 Sm-70-4 70 0.0903 3.98 0.020 77 Sm-70-5-ll 70 0.2251 5.00 0.061 95 Sm-70-9 70 0.0903 7.00 0.020 76 der with temperature, as previously observed in ot value, in the order of 1 CT14 mol m"2s-1. In addition, her phyllosilicates (i.e., Carroll and Walther, 1990). the effect of temperature on dissolution rate, mea sured as apparent activation energy, is minimal in Apparent activation energies in the pH range 4 to the pH range 7-8.5 (Table 32). The dissolution rates 1 0 were calculated using Equation 4.1.4, estimating estimated at 70°C range from 1CT13'7 to 1CT13'5 dissolution rates when experimental values were not mol m"2s-1, at pH 7 and 8.5 respectively. We may available using equations in Figure 38. The calcu derive that these conditions provide a suitable envi lated values are given in Table 34. The minimum ronment to preserve the chemical stability of the value was calculated at pH 7, increasing with both bentonite. Such a situation can only be locally mod increasing and decreasing pH. The v-shape of the ified by proximity of materials that may induce pH dependence of the activation energy is plotted in strong changes in pH, as alkaline plumes produced Figure 39. by concrete or acid front, or by accumulation of or The pH of the pore waters in the barrier and in the ganic matter or microorganism that may contribute interface with the granite host rock are in the range to increase dissolution by the release of organic ac 7 to 8.5. As it was expected, under these conditions, ids (ligand promoted mechanism) or increasing the smectite dissolution rate is close to the minimum acidity. 83 EEBEXII project. THG Laboratory Experiments Table 33 Average values of pH, Si and Al concentration of the smectite output solutions in the steady-state region, and smectite dissolution rates derived from Si and Al concentrations. The uncertainty associated with the rate is ±15%, or 0.13 log units. $i0ut AIqu I log yfsi log Ai Series pH out {fimo\ L™1) (amol L-1) (mol nrV) (mol nrV) Sm-25-4-1 4.01 5.70 0.49 -13.62 (-14.20) Sm-25-4-ll 4.53 4.58 0.20 -13.71 (-14.58) Sm-25-4b-l 3.90 4.05 1.10 -13.73 -13.86 Sm-25-4b-ll 4.10 2.85 0.81 -13.92 -13.94 Sm-25-5b 622 1.47 0.38 -14.21 -14.31 Sm-25-5c 5.69 1.77 0.81 -14.12 -14.38 Sm-25-9b-l 8.40 11.26 0.80 — -13.94 Sm-25-9b-ll 8.73 2.67 0.63 -13.94 -14.09 Sm-25-10 9.46 111 1.10 -13.91 -13.84 Sm-50-5 6.61 2.08 — -14.05 — Sm-50-5b 5.84 2.19 0.68 -14.02 -14.03 Sm-50-7-1 7.30 4.82 0.21 -13.69 — Sm-50-7-ll 7.17 5.57 0.26 -13.63 — Sm-50-9b 8.58 4.52 1.96 -13.71 -13.60 Sm-70-4 4.00 7.68 2.67 -13.47 -13.45 Sm-70-5-ll 5.04 7.08 2.69 -13.44 -13.38 Sm-70-9 8.33 21.36 5.68 -13.45 -13.54 Table 34 Apparent activation energies in the pH range 4 to 10. pH E„ kJ moT1 4 39.36 5 30.71 6 22.05 7 18.63 8 22.14 9 25.65 10 29.16 84 5. Geochemical processes at the smectite-solution interface: Dissolution, transport and precipitation -12 a Si SOT 0.42 pH -1 0.14 pH-14.94 Figure 38. pH dependence of the log of the smectite dissolution rate at o) 25 and b) 50 and 70* C. Fates for pH values higher than 11 were included for comparison, as well as those from Zysset and Schindler (Z&S 1996) at 25°C. Reaction orders were calculated for rates at 25 and 5CPC, according to Eg. 5.1.4. Volcanic glass tainty of the pH measurement. High Al and Si con centrations are observed at the onset of most of the The variation in composition of output solutions in experiments. two of the glass dissolution tests as a function of time is shown in Figure 40 as representative ones of Afterwards, Al and Si concentrations decrease until the experimental set-up. The experimental condi a steady state is approached (Figure 40). Dissolu tions in all the experiments are shown in Table 35. tion rates were calculated based on Al and Si steady Glass dissolution tests behave as smectite ones. state concentrations using Equation 2, after correc Small fluctuations of the pH out with respect to pHi np tion of mass of solid lost during dissolution (Table due to the dissolution reaction are within the uncer 36). The uncertainty associated with the measure is 85 EEBEXII project. THG Laboratory Experiments 2 4 6 8 10 12 14 pH Figure 39. Variation of the apparent activation energy with the pH. Values for pH >11 were included for comparison. Table 35 Experimental conditions of the volcanic glass dissolution tests. Temp. mass Flow Time Series Pffjnp (°C) (g) (ml min') (days) GI-25-4 25 0.0898 3.97 0.020 82 GI-25-5-1 25 0.0900 5.16 0.020 82 GI-25-5-II 25 0.0900 5.16 0.020 82 GI-25-10 25 0.0903 8.92 0.020 36 GI-50-4 50 0.0900 3.97 0.021 96 GI-50-/-I 50 0.0904 7.09 0.020 112 GI-50-7-II 50 0.0904 7.09 0.020 112 GI-50-9 50 0.0900 8.00 0.020 90 ± 1 5% or O.l 3 log units. Since no structural formula Figure 41 plots log dissolution rate vs. pH at 25 and can be defined (the proposed formula is simple a 50°C. At 25°C there is a minimum of dissolution stoichiometric relationship), the rates are computed rate at a pH value of approximately 7-7.5. How per mole of Si. The rates complied in Table 35 cor ever, the dissolution rates at 50°C seem to follow a respond to stoichiometric dissolution. single trend. The reaction orders are also inconsis- 86 5. Geochemical processes at the smectite-solution interface: Dissolution, transport and precipitation 0 500 1000 1500 2000 Time (h) Figure 40. Variation of pH, Si and Al concentration in the output solution of volcanic glass dissolution tests as a function of time. tent each other: at 25°C n equals 0.39 for pH > 8, solution rate is expected (approximately 0.2-0.3 whereas at 50°C n is 0.36. Within the uncertainty orders of magnitude). However, the data available of the value, both orders are the same, which is indicate an increase of dissolution rate of one or very improbable as apparent activation energy in der of magnitude from 25 to 50°C, which is twice silicates exhibits a minimum close the minimum of that found in the smectite. In consequence, glass dissolution rate and increases with both increasing dissolution is apparently more sensitive to tempera and decreasing pH. This behaviour is not clear, be ture than smectite dissolution. ing necessary additional data to derive any de The glass used for these tests consisted on small pendence of rate on pH and temperature. spheres (1 mm size) of compact texture, as also re According to Table 36 and Figure 41, within the pH vealed by the low specific surface area (1.51 m2 interval between 7 and 8.5 a small variation of dis g-1). These were infrequent specimens, which were 87 EEBEXII project. THG Laboratory Experiments Table 36 Average values of pH, Si and Al concentration of the volcanic glass output solutions in the steady-state region, and volcanic glass dissolution rates derived from Si and Al concentrations. The uncertainty associated with the rate is ±15%, or 0.13 log units. $i0ut AIqu I log yfsi log Ai Series pH out (jumol L-') (jumol L-') (mol Si nrV1) (mol Si nrV1) GI-25-4 4.08 1.25 0.26 -11.51 -11.46 GI-25-5-1 5.33 0.92 0.14 -11.64 -11.74 GI-25-5-II 5.60 0.94 0.14 -11.54 -11.67 GI-25-10 8.01 0.78 — -11.74 — GI-50-4 4.14 2.65 0.45 -12.07 -12.35 GI-50-/-I 7.35 6.69 1.04 -10.80 -10.86 GI-50-7-II 7.22 6.31 0.88 -10.82 -10.94 GI-50-9-1 8.49 9.33 1.96 -10.63 -10.60 GI-50-9-II 8.47 8.36 1.62 -10.68 -10.67 4 6 8 10 12 14 pH Figure 41. pH dependence of the log of the volcanic gloss dissolution rate at 25 and 5CEC. Bates for pH values higher than 11 were included for comparison. 5. Geochemical processes at the smectite-solution interface: Dissolution, transport and precipitation chosen because they permitted a right control of min Plagioclase eralogy (no minerals), surface area, and granulo metry. However, the glassy material in the Cortijo de The behaviour of plagioclase during dissolution tests Archidona outcrop is a rhyolitic tuff of similar com is similar to that observed in smectite and glass. The position to the spheres (de la Fuente, 1999). We variation in composition of output solutions in a do not have a value of the specific surface area of plagioclase dissolution test as a function of time is the tuff. In fact it is very difficult to get an accurate shown in Figure 34 as representative one of the ex value of surface area since the vesicular texture of perimental set-up. The experimental conditions in all the tuff is modified during the dissolution process the experiments are shown in Table 37. Dissolution (some cavities initially inaccessible are open to so rates were calculated based on Al and Si steady lutions during reaction). If we assume for the tuff a state concentrations using Equation 2, after correc value of surface area two orders of magnitude tion of mass of solid lost during dissolution (Table greater than for the compacted glass, the corre 38). The uncertainty associated with the measure is sponding dissolution rates would also be two or ± 15% or 0.1 3 log units. It should be noted that dis ders of magnitude greater. These estimations will solution reaction was frequently non-stoichiometric. be labelled tuff dissolution rates. We will go back This behaviour should be attributed to dishomoge- to this point later. neity and zonation of the bulk mineral, and the Table 37 Experimental conditions of the plagioclase dissolution tests. Temp. Initial mass Flow Time Series pHjnp (°C) (g) (ml min') (days) Ab-25-7 25 0.0903 7.09 0.019 73 Ab-25-10 25 0.2702 10.27 0.022 64 Ab-50-4 50 0.2701 4.00 0.019 62 Ab-50-10-1 50 0.2701 10.27 0.020 93 Ab-50-1 0-11 50 0.2701 10.27 0.020 93 Ab-70-9 70 0.1816 9.58 0.020 98 Table 38 Average values of pH, Si and Al concentration of the volcanic glass output solutions in the steady-state region, and volcanic glass dissolution rates derived from Si and Al concentrations. The uncertainty associated with the rate is ±15%, or 0.13 log units. AIqu I log A)i log Aii Series PH out $i0ut (A/mol L-1) {jJmo\ L"1) (mol nr A-') (mol nWs-') Ab-25-7 7.45 2.62 0.23 -11.29 -11.99 Ab-25-10 9.10 3.78 0.31 -11.55 -12.28 Ab-50-10-1 9.44 8.02 2.52 -11.24 -11.39 Ab-50-1 0-11 8.82 3.96 0.75 -11.56 -11.93 Ab-70-9 826 4.59 1.40 -11.31 -11.48 FEBEXII project. THG Laboratory Experiments Figure 42. Variation of pH, Si and At concentration in the output solution of plagioclase dissolution test as a function of time. faster dissolution of the calcic plagioclase than of outside this interval, corresponding to geochemical alkaline ones, introducing an additional scattering conditions not considered in the scope of FEBEX II in the dissolution data. Project. The most important effect of temperature is observed in the glass and it is very small in The plagioclase dissolution rates available are con plagioclase. Only for smectite was possible to cal centrated in the pH region between 7 and 9.5 (Fig culate the variation of apparent activation energy ure 43). Only a few differences in rate are observed with the pH of the solution. varying pH or temperature. It is suppose that they are around the pH value for minimum dissolution The dissolution rates for the three solids are not rate. straight forward comparable, especially those of the glass. In order to evaluate the contribution of each In this range, plagioclase dissolves two or three order phase to the dissolution of the bentonite, it is better of magnitude faster than smectite, computed based to express the dissolution rates in (g mineral) (kg on molar concentration, which agrees with previous bentonite) _1year _1, including the correction for their studies. abundance in the bulk bentonite. According to the The experiments performed have allowed us to mea previous discussion, we may get a tentative estima sure the dissolution rates of smectite, volcanic glass tion only for a pH value (approximately 7.5) at 25 and plagioclase. Smectite is the main component of and 50°C. The corrected rates are given in Table bentonite (92 % in FEBEX bentonite), whereas pla 39 (Figure 44). gioclase (3 %) and volcanic glass (tuff, estimated First, we would like to stress that these data corres 1-2 %) correspond to the most abundant and solu pond to dissolution rates constant, measured in ble accessory phases. conditions of enough flux of solution to be far from The results collected in Tables 6, 9 and 1 1 indicate equilibrium. However, these parameters are neces that the change in dissolution rates within the pH sary to evaluate the effective dissolution rate under range from 7 to 8.5 is small, at constant tempera barrier conditions of pH, flow, solution composition, ture. Important variations are observed or expected temperature, etc. The hydraulic properties do not 90 5. Geochemical processes at the smectite-solution interface: Dissolution, transport and precipitation 10 12 pH Figure 43. pH dependence of the log of the piogiociose dissolution rate at 25,50, and 7 CPC allow the flux necessary to produce and maintain a conditions within the barrier are quite similar to massive dissolution initially, but this process might that of smectite formation in natural environ start, spread and increase through fractures and dis ments, and the dissolution product of the ac continuities. cessories may produce additional smectite. Tuff dissolution preserves smectite from degrada We may derive some conclusions, assuming that the tion. Only 1.5% of volcanic tuff may control dissolution rate of the bulk bentonite is equal to the the rate of overall dissolution process. Small contribution of these phases, which is almost exact, additions of natural tuff to the bentonite used because other accessory phases dissolve slowly and to build the block may improve the chemical have small surface area (i.e., quartz) or are really stability without important modification of other trace minerals (i.e., pyroxenes, amphiboles): physical properties. □ Bentonite dissolution rate increases four times, when temperature rises from 25 to 50°C. Finally, we would like to remind that the geochemi □ The contribution of plagioclase dissolution to cal conditions in the barrier provide a suitable envi the overall rate seems to be negligible. It al ronment to preserve the chemical stability of the lows us to asses that the smectite and the tuff bentonite. Such a situation can only be locally mod govern the process. ified by proximity of materials that may induce strong changes in pH, as alkaline plumes produced □ At 25°C, the smectite contributes with the 70 % by concrete or acid front, or by accumulation of or of the overall rate. ganic matter or microorganisms that may contribute □ In turn, at 50°C tuff dissolution have increased to increase dissolution by the release of organic ac 9 times and represents two thirds of the total, ids (ligand promoted mechanism) or increasing the whereas smectite dissolution rate is only dou acidity. Some of these situations have been rejected ble. It is a very important fact, because the and others are under study. 91 EEBEXII project. THG Laboratory Experiments Table 39 Estimated dissolution rates of bentonite main components, at pH 7.5, and relative contribution of each one to the bulk value. log rate Wt Mol. Wt. rate Contribution 25° C Surface Area mol nrV % g moH g kg-bent-y % Sm -14.11 110 92 767.9 19.0 70 Tuff -11.87 150 1.5 82.84 7.9 29 Ab -11.29 0.65 3 266.3 0.4 1.3 Total 27.3 log rate Wt Mol. Wt. rate Contribution 50° C Surface Area mol nrV % g mol ' g kg-bent-y % Sm -13.8 110 92 767.9 38.8 35 Tuff -10.92 150 1.5 82.84 70.7 64 Ab -11.56 0.65 3 266.3 0.45 0.4 Total 110.0 Sm = smectite Tuff = volcanic Tuff Ab:= plagioclase Figure 44. Estimated dissolution rates (g solid per kg bentonite and year) of smectite (Sm), volcanic tuff (Tuff) and plagioclase (Ab) at 25 and 50° C. 92 6. Radionuclide sorption and migration in bentonite 6. Radionuclide sorption and migration in bentonite 6. Radionuclide sorption and migration in bentonite Chapter 6 is divided in two different sections. The first the geochemistry of the system (pore water che section deals with diffusion experiments on FEBEX mistry), and the temperature. The effective diffusion compacted clay. The work described in this section coefficient (De ) of nuclides in bentonite depends on was carried out by M. G. Gutierrez, M. Mingarro, T. the diffusing species, particularly on its charge. The Missana (CIEMAT) and J. L. Cormenzana (Empre- apparent diffusion coefficient (Da) also depends on sarios Agrupados). The second section deals with the sorption characteristics of the bentonite clay. By batch sorption experiments carried out on purified means of through-diffusion and in-diffusion experi clays. The results obtained on the purified FEBEX ments both diffusion coefficients can be obtained. clay were obtained by T. Missana and M. G. Gu Another important parameter to be determined is tierrez (Section 6.2.1, CIEMAT), whereas the results the diffusion-accessible porosity ( 95 EEBEXII project. THG Laboratory Experiments TR-2700 Packard apparatus, y-emitters were mea solved species, from one side to the other of the sured with a Packard AutoGamma Cobra appara clay sample, are important. Therefore, for each ion tus. In the case of Nd, a non radioactive isotope an "accessible porosity" has to be considered. The was used and its content was analysed by ICP-MS. effective diffusion coefficient, De, defined by: When it was necessary to extract the tracer from the (E.6.1.2) clay, the plug was sliced and the solid of each slice was re-suspended in water and the tracer activity where (p is the diffusion-accessible porosity, can be measured after centrifugation of the supernatant. experimentally obtained when the steady state is Through-diffusion experiments, with variable (TDV) reached. In the case of sorbing elements, reaching and constant (TDC) concentration in the inlet reser the steady state can be very time consuming, so an voir, were performed with 50-mm diameter samples apparent diffusion coefficient, Da, can be calculated and different thicknesses (5.3, 8.3, 10 and 20 mm). from the diffusion profile into the sample. Thus, Da TDV were performed for uranium for the estimation takes into account implicitly the retardation of the of both effective and apparent diffusion coefficients. solute due to the interactions with the porous mate Four samples of 5.3 mm thickness at a density of rial, and is defined by: 1.65 g/cm3 were used in this case. In-diffusion (ID) experiments with constant concentration were ca Da = (E.6.1.3) rried out with 38-mm diameter samples and 50-mm thickness. Saturation experiments were performed where Rf is the retardation factor. If lineal sorption is with 50-mm diameter samples having a thickness of considered, the sorption isotherm can be described either 20 mm (HTO) or 5.3 or 8.3 mm (Cl"). by the simple relation S—Kd -C, where S is the tra Europium and neodymium are elements that are cer concentration on the solid phase; C, the tracer usually strongly adsorbed in laboratory materials. In concentration in the liquid phase; and K* the distri particular in most of the configurations described bution coefficient. In this case, Rf is: before these elements were adsorbed in the diffu R, = l + ^% (E.6.1.4) sion cell. Therefore another type of diffusion experi ments was performed, trying to avoid the contact of the tracer with anything but the clay, introducing the where pd is the dry density of the material. Accord tracer directly in the bentonite sample. These experi ing to these definitions, the following equation can ments will be named as Back-to-Back diffusion ex be finally written: periments (BB). D.-^- D„ ^'DP In all the experiments, the bentonite samples were (E.6.1.5) a saturated with deionised water, requiring approxi 1 + A (t) + Pd 'Kd mately 2.5 months. Complete water saturation of the samples was verified by a final, stable weight at the end of the experiment. where a= (p + pd ■ Kd is a dimensionless parameter called the capacity factor. For non-sorbing solutes, such as HTO and 36CI", Kd = 0, so that the capacity 6.1.3 Theoretical description factor is equal to the accessible porosity, (p. Equa tion E.5 shows the relation between the different dif In a porous medium, such as a compacted clay, the fusion coefficients (Da and De) to be determined. diffusion process is different from the diffusion in free water, since it is affected by the length of the The total porosity of the sample, e, is the ratio of the diffusion path, tortuosity (r), the form of the pores, pore volume to the total volume of a representative and constrictivity [d). Then, the pore diffusion coeffi sample of the medium. For a medium with a dry cient, Dp , is related to the diffusion coefficient in free density, pd, and solid density, ps, the total porosity water, Dw, by: can be obtained by: (E.6.1.1) e= 1- —z (E.6.1.6) Ps In a diffusion experiment, only the pores that are The accessible porosity, (p, represents the fraction of connected and contribute to the transport of the dis- the total porosity used for solute transport. 96 6. Radionuclide sorption and migration in bentonite The one-dimensional diffusion equation for a po + '^{C2i-G2{De,a,tj))2 E. 6.1.8 rous medium, according to Pick's second law, is ex pressed by: where Cj, and 0% are, respectively, the experimen (E. 6.1.7) tal values of the concentrations in inlet and outlet reservoirs at instant f,. The couple of values (De , a) where C is the concentration of the tracer in the that minimise function are taken as De and a. For porewater, f is the diffusing time, Da is the apparent non-sorbing species, such as HTO and Cl", the val diffusion coefficient, and x is the distance from the ues of a are quite small, and for small values of a source. the function (De ,a) is insensitive to changes in a, although very sensitive to changes in De . As a conse As mentioned before, equation E. 6.1.5 shows the quence, the values of a estimated with this method relationship between De , Da, and (f>. If two parame are not considered and only the De value will be taken ters are measured, the third one can be calculated, from the TDV experiments. as has been done in this article for Cl" (De and (j> were measured and Da was calculated). When all TDV experiments were carried out for HTO in 1 - three parameters can be measured and they satisfy and 2-cm thick clay plugs compacted at the dry equation E. 6.1.5, the consistency of the values ob densities of 1.1, 1.3, 1.5, 1.65, and 1.7 g/cm3. In tained is proved, as was done in this work for HTO. the case of 36CI" the plugs were 0.53 and 0.83 cm thick and compacted at the densities of 1.0, 1.2, 1.4, 1.6, and 1.65 g/cm3. 6.1.4 Experimental methods TDV experiments were carried out also for uranium, Through-diffusion experiments, TD as mentioned before, but the treatment of the expe rimental data, in this case was different and, as will In TD experiments, a stainless-steel diffusion cell is con be detailed in the results section the diffusion coeffi nected to two reservoirs (inlet and outlet) where the cients were estimated by different methods. solution is continuously stirred. The cell contains a ring with the compacted bentonite placed between two In TDC experiments, the expression of the cumulati sinters. After the saturation of the clay sample with wa ve amount of solute that has passed to outlet reser ter, the water in the inlet reservoir is spiked with the voir (Q) as a function of the time is (Crank, 1975; tracer. Two different configurations are used: the first Bourke ef a/., 1 993): one is the variable concentration (TDV) case, using a Q - A ■ L ■ C0 inlet and outlet reservoirs of 1 00 ml. In this configu 6 ration, the concentrations in both reservoirs change with time and they are periodically measured. The 2a exp E. 6.1.9 second one is the constant concentration gradient L2a (TDC) case, in which a large (1 L) inlet reservoir and a very small (20 ml) outlet reservoir are used. The where A is the cross-sectional area, L is the thickness outlet reservoir is periodically changed in order to of the sample, and Co is the tracer concentration in keep the concentration close to zero. In order to the inlet reservoir. The other parameters in E.6.1.9 measure diffusion parameters, the TDC method re were already defined: De is the effective diffusion quires that steady-state conditions are reached. coefficient, and a is the capacity factor, which is In the TDV configuration, the effective diffusion co equivalent to the accessible porosity for non-sorbing efficient, De , is estimated from the theoretical time tracers. For long times, the steady-state condition is evolution of the concentrations in the reservoirs for reached, the exponential tends to zero, and the different values of De and a using a computer code. equation becomes: The code provides the concentration in inlet reser a Q - A ■ L ■ C0 E. 6.1.10 voir Gi (De ,a,f) and outlet reservoir G2(De ,a,f) for 6 each sampling instant t. The difference between ex perimental and theoretical concentrations is quanti The effective diffusion coefficient can be calculated fied using the function 97 EEBEXII project. THG Laboratory Experiments at, 1993). These experiments were performed only tion in the saturation solution is equal to the tracer with 36CI in a clay plug of 1.65 g/cm3 and for cells concentration in the pore water of the bentonite of 1 cm thickness. plug, and the following equation can be used: In-Diffusion experiments, ID E.6.1.12 In ID experiments, one or both sides of the water sa where Ar is the activity in the bentonite slice; V, the turated bentonite cell were contacted with a large volume of the slice; C r , the concentration in the res volume of tracer solution. After a given time, the dif ervoir; and (p, the accessible porosity. Each slice fusion cell was disassembled, the bentonite plug cut provides a value of the accessible porosity, which into slices approximately 1 -mm thick, and the acti allows calculating a mean value and its error from a vity in each slice measured to obtain a concentra single experiment. SAT experiments have been per tion profile in the bentonite plug. formed for HTO, at all the clay densities considered, and for 36CI" at the clay density of 1.0 and 1.2 g/cm3. Since the concentration in the reservoir remains prac tically constant during the experiment and the ben Concentration Profile Experiments, PRO tonite plug is long enough, the concentration profile within the plug can be fit by the following analytical In TDV experiments with chloride and high clay den solution (valid for a constant concentration in the sities, the final concentrations in the inlet and outlet boundary and semi-infinite medium) (Crank, 1975; reservoirs are usually quite different. However, since Eriksen and Jacobsson, 1984; Idemitsu et at, 2000): tracer concentration in the outlet reservoir is signifi cant, it can be ensured that the concentration profile in the clay plug pore water is linear. Pore water con = erfc E. 6.1.11 centration at any point in the sample can be calcu lated interpolating between concentrations at the ex where C is the tracer concentration in the bentonite tremes (inlet and outlet reservoirs concentrations). plug pore water, Co the constant concentration in As a consequence, for each slice the average tracer the reservoir, Da the apparent diffusion coefficient, x concentration in the pore water can be predicted, the distance, and f the diffusion time. and using E.6.1.12 the accessible porosity can be For several values of Da, the theoretical concentra calculated. Each slice provides a value of the acces tion profiles in the plug are obtained using E. 6.1.11. sible porosity, which allows calculating a mean va The comparison of the experimental profile and the lue and its error from a single experiment. In these theoretical profiles, for different Da values, allows samples, 5 or 6 slices (approximately 1 -mm thick) identifying the value of Da for which the fitting is the could be obtained. best. ID experiments provide the Da values. These PRO experiments have been performed for 36CI at a experiments were done only for HTO, using 5-cm clay dry density of 1.40, 1.60 and 1.65 g/cm3. thick bentonite plugs of different dry densities. Backto back diffusion, BB Saturation Experiments, SAT Europium and neodymium were introduced within the SAT experiments are performed introducing the dif bentonite sample trying to avoid the contact with the fusion cell into a bath; in these experiments a con trace with anything but the clay. A filter paper (in the stant concentration profile in the clay plug has to be case of Eu) or a slice of traced bentonite contained obtained. In order to ensure that the concentration the tracer (Nd) was sandwiched between two satu profile into the plug is effectively constant, the con rated bentonite clay plugs. In this configuration, the centration in the saturation solution is periodically symmetrical diffusion profiles are determined at the monitored until a constant concentration is mea end of the experiment after slicing the bentonite sam sured. After the experiment, the plug is sliced and ple. The apparent diffusion coefficient for this ar each slice is weighed to determine its volume. Then rangement is evaluated from the following equation: the clay is transferred to a centrifuge tube and a vol 2 \ ume of water added. The tubes are kept in conti C_ 1 exp - E.6.1.13 nuous stirring during three days and, after this pe M 2A^JtzDJ 4DJ ; riod, the samples are centrifuged (14000 rpm, 30 min) and the activity in the supernatant is measured. where C is the concentration of the diffusing substance When the experiment finishes, the tracer concentra in the porous medium, M is the total amount of tracer, 98 6. Radionuclide sorption and migration in bentonite A is the cross-sectional area, Da the apparent diffusion Results and discussion: conservative tracers coefficient, t the time and x the diffusion distance. By (HTO and Cl ) taking the logarithm of both sides of the previous All the results obtained with the different methods will equation, a linear expression is obtained, the slope of be summarised in Table 40 (HTO) and Table 41 (Cl"), which gives the apparent diffusion coefficient. and the methodology used for obtaining each value By the fit of the experimental concentration profile of will also be specified. The results in the same row C/M versus x is also possible to obtain the Da. correspond to the same experimental cell. Table 40 Summary of the experimental results for HTO at different clay density. The experimental method used is indicated. The last column shows the theoretical porosity at the given density. Dry density D=(m2/s) D„ (m2/s) f(%) c(%) (g/cm 3) TDV ID SAT 65.4 63.0 1.00 64.5 63.0 1.92-10' 10 — 59.3 1.10 2.01-10'10 — 59.3 3.3-10"10 59.3 58.3 55.7 1.20 56.6 55.7 1.36-10'10 — 51.9 1.30 1.56-10"10 — 51.9 — 3.1-10"'° 51.9 51.9 48.1 1.40 51.0 48.1 8.80-10' 10 — 44.4 1.50 8.93-10' 10 — 44.4 — 2.6-10"10 44.4 43.3 40.7 1.60 42.8 40.7 5.8±0.2-10' 11 — 38.9 5.8±0.2-10' 11 — 38.9 1.65 — 1.7-1 O'10 38.9 — 1.6-1 O'10 38.9 — 1.6-1 O'10 38.9 5.76-10"11 — 37.0 1.70 4.57-10"11 — 37.0 — 2.6-10"10 37.0 99 EEBEXII project. THG Laboratory Experiments Table 41 Summary of the experimental results for Cl" at different clay density. The experimental method used is indicated. Dry density D„ (m2/s) D=(m2/s) 2.72-10"11 (TDV) 18.0 (SAT) 1.51-10"10 1.0 3.68-10 "11 (TDV) 15.3 (SAT) 2.41-10'10 1.08-10" n (TDV) 10.5 (SAT) 1.03-10'10 1.2 2.52-10"11 (TDV) 15.2 (SAT) 1.66-1 O'10 2.9-10 "12 (TDV) 5.9 ±1.8 (PRO) 4.9-1 O'11 1.4 1.7-10"12 (TDV) 3.4 ±1.4 (PRO) 5.0-10"11 1.6 8.7-1 0"13 (TDV) 2.4± 1.1 (PRO) 3.6-10"" 1.1-1012 (TDC) 2.2 (TDC) 5.0-10"" 7.7-10"13 (TDC) 2.3 (TDC) 3.3-10"" 1.65 — 2.8±0.4 (PRO) — — 2.5±0.3 (PRO) — TO V: Through-diffusion with variable concentration SAT: Saturation experiments TOC: Through-diffusion with constant cocentration gradient Through-diffusion Experiments In-Diffusion Experiments An example of the experimental results, obtained by Figure 47 shows the experimental results of in-diffu means of through-diffusion experiments with vari sion experiments with FITO and the fit obtained for able concentration (TDV), that allowed De estima different clay dry densities from 1.1 to 1.7 g/cm3. tions are shown in Figure 45. Figure 45a shows the Figure 47a shows the experimental concentration results with FITO and Figure 45b shows the results profile in the sample for a dry density of 1.65 g/cm3 with 36CI\ The figures show the experimental results and the theoretical profiles for three values of Da. of only one cell, for each clay density, but the results The experimental values fit very well to the theoreti obtained in different cells with the same dry density cal profile for a mean Da = 1.7-1 O'10 m2/s, and all were always very similar. The upper experimental the experimental points can be described with a points correspond to the concentration evolution in small variation in Da (± 0.1 -1 O'10). Figure 47b the inlet reservoir, and the lower experimental points shows the experimental results and the best fit theo correspond to concentration evolution in the outlet retical curves for the other clay densities. The agree reservoir. The methodology explained in the previ ment between experimental and theoretical profiles ous section (E.6.1.10) was applied at all the TDV is very good. The summary of the Da values for experiments and the results are included in Table 40 FITO, obtained with ID experiments, are shown in and Table 41. Table 40. No ID experiments were performed for Figure 46 shows an example of the experimental results 36CI", and all the Da(CI") values shown in this article of TDC experiments with Cl" (clay density 1,65g/cm3) have been calculated from De and cf> values using E. where the lineal fit in the steady-state region and the 6.1.5. time lag value are indicated. Accessible porosity and effective diffusion coefficient can be obtained Saturation and Concentration Profiles Experiments from this fitting line. Using E. 6.1.5, the apparent diffusion coefficient can be calculated. The results of The results of the accessible porosity obtained for this experiment are summarised in Table 41. FITO and Cl" by means of SAT and PRO experiments 100 6. Radionuclide sorption and migration in bentonite 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 0 20 40 60 100 Time (days) Time (days) Figure 45. Concentration evolution for inlet and outlet reservoirs obtained in ID 1/ experiments in clay plugs at different densities: (o) HIO, (b) 36 Ch are summarised in Table 40 and Table 41. The ex ent diffusion coefficients, all the previous men perimental results show that for HTO the accessible tioned parameters plotted as a function of the clay porosity is always approximately the same as the to dry density. tal porosity. In the case of Cl" the values are signifi cantly smaller. As can be seen in the Figures the effective/apparent Figure 48, Figure 49 and Figure 50 show the sum diffusion coefficients and the accessible porosity mary of all the results obtained in the different ex show an exponential decrease when the clay density periments for FITO and 36CI". Figure 48 shows the increases. The decrease is significantly more pro effective diffusion coefficients, Figure 49 shows the nounced in the case of Cl". Therefore for each pa accessible porosity and Figure 50 shows the appar rameter, the experimental values can be adjusted 101 FEBEXII project. THG Laboratory Experiments Time (hours) Figure 46. TDC experiment with 3iQ; day dry density 1.65 g/cm3. The lineal fit of the steady-state region and the time tag are indicated. using exponential functions of the form D0fe = It is usually accepted that anionic exclusion decrea A'e" B/3d, where A and B are constants and pu is the ses when the pore water ionic strength increases. bentonite dry density. Accessible porosity, apparent The electrostatic effect may be higher at lower clay and effective diffusion coefficients in FEBEX benton density because the overlapping of the double la ite at any dry density can be therefore easily esti yers of the pore surfaces does not occur, but at hig mated by interpolation. her clay density the ionic strength effect is expected to be less significant. However, the effect of pore Results in Figure 49 clearly show that the measured water composition on anionic exclusion will be stu accessible porosity for HTO is roughly equal to total died in next future works. porosity in FEBEX bentonite at different dry densities. Since total porosity is easy to calculate (E.6.1.6), if Figure 50 shows the values of Da(HTO) measured in one of the diffusion coefficients (effective or appar in-diffusion experiments, and the exponential func ent) is measured the other coefficient can be calcu tion that best fits these experimental results for HTO. lated using E. 6.1.5 and giving to (j> the value of the Using equation E.5.1.5 with the exponential func total porosity [e). tions shown in Figure 48 and Figure 49 0(HTO) and De (HTO), an alternative exponential function Chloride ions have a very different behaviour than for Da(HTO) can be created. Figure 50 shows that HTO in FEBEX bentonite. In fact, even at low densi both exponential functions are very similar, giving ties only a fraction of total porosity is accessible for confidence that the parameter values measured for chloride. If dry density increases, the accessible po HTO are correct and that a simple diffusion model in rosity for chloride decreases rapidly, and at the den pore water fully explains experimental results for HTO. sity of 1.65 g/cm3 its value is between 2% and 3%, while the total porosity is close to 40%. These results Results and discussion: non conservative show that a significant anionic exclusion occurs and tracers (U and Eu) that its effects are more pronounced when the dry density increases. As a consequence, only a small The diffusion coefficients of uranium were estimated fraction of the total porosity is accessible for chloride. by different methods. By one hand, the comparison 102 6. Radionuclide sorption and migration in bentonite □ Experimental results ■ ■ ■ ■ Fit D, ------Fit D„ - - Fit D, Distance (mm) r 0.4 Distance (mm) Figure 47. ID experiments for HTO. (a) The method used for the evaluation of D„ and its error are shown (clay dry density of 1.65 g/crrF and experimental time t=i90 h); (b) Experimental results and best fit obtained, for all other experimental densities. of the experimental evolution of the of the tracer tion in the IN (with points) and OUT (continuous concentration in the inlet reservoir with theoretical lines) deposits for different De/Da ratios, can be per curves, for a system with the same geometry, allows formed as shown in Figure 51. to estimate the ratio De/(Da)]/2. The fact that the As can be seen in the Figure, for a given De/Da ra tracer does not reach the outlet deposit at the end tio the relative concentration C/Co can be estimated of the experiment allows making a conservative esti as a function of time. (In this case, L2/Da units). mation for the Da. Secondly, by the analysis of the concentration profile within the bentonite clay, an For example if De/Da = 1 and the concentration independent estimation of Da can be obtained. A measured in the IN deposit is CIN = 0.7 -Cq the calculation of the theoretical concentration evolu concentration in the OUT deposit has to be COUT = 103 EEBEXII project. THG Laboratory Experiments Figure 48. Effective diffusion coefficients obtained with different methods for HIO and 36 Cl os o function of clay density. The squares correspond to values obtained by IDV experiments. The triangles correspond to values obtained by TDC experiments. The exponential fits are included as continuous lines. Dry density (g/cm 3) Figure 4 9. Accessible porosities obtained with different methods for HTO and Cl as a function of clay density. The squares correspond to values obtained by SAT experiments; up triangles, to values obtained by PRO experiments; and down triangles, to values obtained by the time lag technique. The continuous lines correspond to the exponential fits of the experimental values and the dotted line corresponds to the total porosity of the clay (E.6. 1.6). 104 6. Radionuclide sorption and migration in bentonite Dry density (g/cnT) Figure 50. Apparent diffusion coefficients obtained with different methods for HIO and Q~ as a function of clay density. The squares correspond to values obtained by in-diffusion experiments with HTO; and triangles, to values calculated using E. 2 from the De andrp for Ci~. The continuous lines correspond to the exponential fits of the experimental values and the dotted line is obtained by the ratio of De andrp fits obtained in the previous figures. D,/D,= 10 Time (units: LZ/DJ Figure 51. Reservoirs concentration evolution for different De/Da ratios. 105 EEBEXII project. THG Laboratory Experiments 0.25'C qN. In the case of De /Da = 10, for COUT = After 279 days the experimental C,N is between 72 - 0,006'C qN. From our experimental result (see Figure 79% and the COUT between 0.2 - 0.3%, this implies 52) the ratio De/Da must be 1 0 < De/Da < 1 00. that Da(U) < 1 E-l 3 m2/s. Since the concentration in reservoir OUT is practi From the experimental results and the previous cal cally zero during all the experimental time, we can culations Table 42 can be constructed. use the following equation (Put and Flenrion, 1 992), Table 42 Concentrations expected in reservoirs IN and OUT according with the De and Da values. 139 days 279 days D„ (m2/s) 0= (m2/s) [IN [OUT [IN [OUT MO'10 94% 7.5% 8.4% 8.3% IE-12 l-10"n 53.8% 12.9% 43.1% 23.5% 3-10"n 15.2% 2.2% 11.0% 5.7% 3E-13 3-10'12 69.7% 13% 61.1% 6.3% l-10"n 218% 0.03% 18.3% 0.7% 1 E-l 3 MO'12 80.4% 0.008% 74.0% 0.3% —□—C-25 -o — C-26 v C-28 □—□. 10 20 10 120 130 140 Time (days) Figure 52. Fitting of concentration evolution in the IN reservoir. 106 6. Radionuclide sorption and migration in bentonite for describing the evolution of tracer concentration The capacity factors obtained from the experimental in the IN deposit: result are in the range of 1 0 and 35, according with 2 these values the Kj for uranium is in the range of 6 - 21 ml/g, in agreement with results obtained in C'N(f)= Q exp f batch experiments (ENRESA, 2000). Finally, an example of results obtained with Eu is • erfc E. 6.1.14 shown in Figure 53. The modelling of both Nd and Eu data is still ongoing. From the comparison of the calculations of CIN/Co vs time for differents De / yfcT values and the exper imental results, Figure 52, a range for between 3 6.2 Batch sorption experiments and 7-10"6 (being the diffusion coefficients ex pressed in m2/s) is found. in purified clay From the concentration profile into the clay plug ac 6.2.1 Experiments with FEBEX clay cording with Cormenzana et al, (2002) the Da(U) is between 4 and 10-10"14 m2/s. From these results It is necessary to understand transport processes in for compacted FEBEX bentonite at 1,65 g/cm3 it compacted bentonite and to acquire predictive ca can be concluded that: pability. A new approach based on the mechanistic a) if D0(U) = 4-10"14 m2/s, then De (U) = 0.6 - description of sorption is absolutely needed in order 1.4-10-12 mVs and 15< D,(U)/D.(U)< 35 to be able to extrapolate laboratory results to differ ent environments. For this reason, sets of experi b) if D0(U) = 1-10-13 m2/s, then De (U) = 1.0- ments were planned out with the objective to study 2.2-1012 rrf/s and 10< D,(U)/D.(U)< 22 the mechanisms of sorption of Cs and U onto ben In conclusion, Da(U) = 4-1 0“14 m2/s and De(U) = tonite. For the understanding of the sorption mecha 0.6-2.2-10^ m2/s. nisms, it is necessary to work with a simplified and '“Europium 500000 450000 350000 J 250000 150000 0 -MDr<3%X3o6oO(^^ Distance (mm) Figure 53. Example of the results obtained with Eu in the 00 configuration. 107 FEBEXII project. THG Laboratory Experiments very well characterised system. Furthermore, the ef results in the compacted system in connection with fects of the most important physico-chemical pa bentonite porewater studies. In addition, the param rameters such as pH and ionic strengtEi, have to be eters coming from these experiments will be used to studied independently. Thus, batch sorption experi explain batch sorption experiments results obtained ments are being performed using the bentonite which with the "as-received" bentonite during the phase I was purified and converted into an homo-ionised of the project and to validate the sorption models (Na) - form using the method proposed by Baeyens used. and Bradbury (1 995). Sorption experiments with the homo-ionised clay were Since the main expected sorption mechanisms clays proposed by the two involved organisations (CIEMAT are cationic exchange and surface complexation, and RSI), for the FEBEX II. The data obtained for the different types of experimental data have to be gen same radionuclides from the different laboratories erated, in order to identify and quantify to what ex have to be compared taking into account that sorp tent these two mechanisms occur. One type of expe tion values were measured under similar conditions rimental data represents the sorption as a function but on different purified clays (SWyl or Milos for RSI of pH (sorption edges ) at constant ionic strength, and FEBEX for CIEMAT). whereas a second type represents the sorption as a function of the concentration of the radionuclide at Characterisation of the purified material a constant pH (sorption isotherms) . The objective of these experiments is to obtain parameters such as The FEBEX bentonite was purified using a standard surface complexation constants and selectivity coeffi method proposed by Baeyens and Bradbury (1995). cients. These parameters are required to describe A comprehensive characterisation of the FEBEX clay the experimental sorption results using a (quasi)-me- after purification has been carried out at the first chanistic model. The determination of these param stage of the project. Figure 54 shows the SEM pic eters for the most important radionuclides is "per ture of the purified and Na-homo-ionised clay with se" an important result. Furthermore they could be the EDX spectra obtained in the associated micro very useful for the modelling of sorption/migration analysis. Figure 54. SEM picture of the Ha-homoionised FEBEX clay and EDX spectra associated. 108 6. Radionuclide sorption and migration in bentonite The picture shows the typical features of a smecti certainly responsible for this increase, the short and te-like material. No significant impurities or precipi less severe acid treatment seems more appropriate tates have been observed by SEM/EDX. The X -ray for the determination of the intrinsic inventory of the diffraction analysis indicated an aluminic dioctahe- elements. Note that the content of Cs, the first ele dral smectite, and no significant mineral impurities ment under investigation, is below the detection limit have been found. (<0.2/ Table 43 Element extracted from the clay by means of acid treatment. Short treatment Large treatment Element pH —1.78,1 hour pH = 0.7, lday Cs < 0.2 y Eu < 0.5/tg/L < 0.5/cg/L La 0.5/tg/L 5.2/cg/L Nd 1.3/cg/L 8.9/ Se < 1 Ag/L < 1 A gA Th < 0.5/tg/L < 0.5/cg/L Zr < 0.5/tg/L < 0.5/cg/L Co < 0.05 mg/L < 0.05 mg/L Sr < 0.05 mg/L < 0.05 mg/L U Zn < 0.05 mg/L < 0.05 mg/L Al < = 0.05 mg/L 1.7 mg/L 109 FEBEXII project. JHG Laboratory Experiments ferent sorption sites. The first site has high affinity On the other hand, sorption of Cs is strongly depend and low capacity (~3'10"8 eq/g) and the second ent on the ionic strength, as can be seen in the iso one with much higher capacity (—1-1 O'3 eq/g ~ therms (Figure 57) indicating the formation of outer CEC ) but lower affinity. The density of the low den sphere complexes (ion exchange). sity sites can be determined from the isotherms, be cause it is in fact expected that the inflexion point co Considering the kinetic experiments, the adsorption incide with the saturation of these sites (Figure 55). of cesium onto the clay seems to involve a rapid ex change reaction (hours) and a slower component Since two sites were identified, it was considered (days) that seems an anomalous behaviour for ion- useful to analyse the kinetic of sorption using two exchange reactions. This slow process, can be evi different Cs concentrations (corresponding to these denced only when very low tracer concentrations two sites). It is interesting to notice that the kinetic be are used (< 1 -1 O'9 M) due to the very low density of haviour of Cs sorption in these two regions was dif these sites (~3-1 0"8 eq/g) compared to the benton ferent. It is worth noticing that at "high" cesium con ite cation exchange capacity. We interpret these two centration (low affinity) the uptake was very rapid processes, in connection with the high affinity and (hours or less) whereas at "low" cesium concentration low affinity sites, as adsorption on planar sites (rapid (high affinity) the uptake was significantly slower, and process) and cesium diffusion to less available but the sorption process was completed within days. This highly selective sites (slow process). The existence of result shows that sorption in the two different sites "low" and "high" affinity sites for cesium sorption in might be controlled by different mechanisms. clays is usually explained considering that cesium Therefore, the pH dependence of sorption was stud can be exchanged with hydrated cations in basal/ ied at two different concentrations in order to see interlayer sites (low affinity sites) and can sorb, in a whether sorption in the low density sites could be at highly selective way in frayed edge sites, FES, (high tributed to a surface complexation process. This was affinity sites) (Zachara ef a/., 2002). FES sites are finally discarded since it was always observed that not present in expanding clays like smectite but de Cs sorption is independent on pH (Figure 56). velop in weathered micas and illite. a l= 0.1 M Ml T A* A' jr A ** A A' A A A A*’ -12 -10 -8 -6 -4 Log(CsJ (Mol/I) Figure 55. Sorption isotherms experimental determination of the "strong" site density. 110 6. Radionuclide sorption and migration in bentonite [Cs] - 2.3E-06 M pH © r A F A? I ^A L I t ■ T^t tT y AnuT 4 A T 1 -LL 4 4. -TF r -r -p n T TT 1 7 - A T A n A I =EL M = 3.02E-08 M 2 3 4 5 6 7 8 9 10 11 12 pH Figure 56. Sorption edges obtained in FEBEX bentonite, a) "bigb" Cs concentration; b) "low" Cs concentration. As mentioned before, illite was never detected as a Sorption of cesium on FEBEX bentonite will be there "pure" phase by XRD analysis but mineralogical stud fore interpreted with a 2-site model (2 "generic" sites) ies showed that the FEBEX bentonite smectite phase is considering ion-exchange both in the "high" affinity actually made up by illite-smectite mixed layer with a and the "low" affinity; 2-sites models were already 10-15 % of illite layers (Cuadros and Linares, 1996; successfully used to model cesium sorption on clays Huertas ef a/., 2000). The existence of smectite-illite data (Zachara ef a/., 2002). mixed layer may possibly lead to the existence of FES-like sorption sites in FEBEX bentonite, however Modelling: Two ionic exchange-sites model the illite in the illite-smectite mixed layer may not be have exactly in the same manner than a pure illite As mentioned before, two different types of ion ex phase. change sites are introduced, namely T1 and T2 (Ta- 111 FEBEXII project. JH6 Laboratory Experiments M NaCIO, I = 0.1 M I = 0.01 M I = 0.001 M LogfCsJ (Mol/I) Including the effect of LogfCsJ (Mol/I) Figure 57. Cs sorption isotherms on FEBEX Na-clay. a) Simulations with the parameters of fable 45. b) Simulation with the parameters of fable 46 (includes the possible effect of Kin solution). ble 45). The density of the first type of sorption site If A (No) and B (Cs) are the exchanging cations and is equal to the CEC of the clay, whereas the second following the Gaines and Thomas convention: type of exchange sites will correspond to the satura tion value seen in sorption isotherms. 6115 The selectivity coefficient (JaKc) and the correspond where za,b represent the valence of the cation A or ing _ value can be estimated as detailed in Brad B; BKd exch the distribution coefficient due to ionic ex bury and Baeyens (1994). The ^aKex is the parame change (supposing that ionic exchange dominates), ter used by the calculation code to define the ionic )'A,b the solution activity coefficients of A and B and exchange reactions. (A) the concentration of the cation A in solution. 112 6. Radionuclide sorption and migration in bentonite Table 44 Determination of the selectivity coefficients from data of Figure 57. Type 1 site 1 S LogKd IE-03 4.40 1.39 1.39 0.01 3.70 1.69 1.69 0.1 2.78 1.77 1.77 1 1.8 1.79 1.79 Mean 7 66 7 66 Chess 1.65 1.65 Type 2 site 1 S LogKd LoglL, LoglK, IE-03 4.99 6.65 6.65 0.01 4.34 7.00 7.00 0.1 3.89 7.55 7.55 1 3.13 7.79 7.79 Mean 7.24 7.24 Chess 7.5 7.5 Table 45 Ionic exchange constants for Cs in FEBEX Na-montmorillonite. Ionic Exchange (Type 1) Density XT1 = CEC x%) = -Na[+] + XT1(Na) + Cs[+] Log K = 1.65 Ionic exchange (Type 2) Density XT2 = 3.1 1 O Vc mol/m2 x%) - -Na[+] + XT2(Na) + Cs[+] Log K = 7.5 In addition: These parameters can be calculated directly from the sorption curves, so they are not actually fitting param eters. However, they were adjusted by fitting the exper 6.1.16 imental curves by means of the Chess code (Van der ZB Lee and De Windt, 1 999). It has to be remarked that the mean calculated value is slightly different from where S is the sorbent concentration. the one that best fit the curves with the code. 113 FEBEXII project. THG Laboratory Experiments The possible influence of the ionic exchange can be to improve the modelling of each curve (above all evaluated directly from the sorption curves (Figure those at the lower ionic strengths) varying the potas 57). At high equilibrium Cs concentrations the de sium concentration. The quantity of potassium nee pendence of sorption on the ionic strength is per ded in each case will be compared with the experi fectly in agreement with a ionic exchange process. mental variation in the potassium determinations. The logarithm of selectivity coefficients calculated from Figure 57b shows the modelling of isotherms with the different sorption curves at high Cs concentra the two-sites exchange model including the effect of tion are: for I = 1 M 1.8, for 1 = 0.1 M 1.69, for I = potassium. The values used for the modelling are 0.01 1.6 and for 1 = 0.001 M 1.49 (Table 44). The included in Table 46. mean value that will be used in further calculations is Log cNsaKex = Log cNsaKc = 1.65. For the low Cs con Summary of sorption experiments with U centration the value selected as selectivity coefficient is Log n qKc = 7.5 (Table 44). The following experiments were carried out: sorption edges (pH 2 to 11) at different ionic strengths In Table 45 the parameters used for the 2-site (1 ■ 1 0"1 to 1 -1 O'3 M) and different solid to liquid ra model shown in Figure 57a are summarised. tios, the uranium concentration was [U] = 4.4-1 0'7 With this model, the sorption at the lowest ionic M; sorption isotherms ([U]: MO"8 to 3-1 O'3) at dif strengths (1-1 0"2 and MO"3 M) is over - predicted. ferent ionic strengths (1 -1 0"1 to M0"3 M) and two It is clear that at very low Cs concentration, and pHs (4 and 7). Kinetic experiments (pH 4 and pH 7) above all when sorption is very high, the magnitude at uranium concentration [U] = 4.4-10'7 M were of the experimental errors can be enhanced. Howe also carried out. All the experiments were performed ver the deviations observed in the prediction from in anoxic condition under N? atmosphere. This set the experimental data, above all at the low ionic of experiments was intended to be enough to start strengths, can be due to the competition of ions like the modelling. Figure 58 to Figure 60 show the re K+ that can be exist in solution. sults obtained. Effectively, in some sample, a non-negligible quan Figure 58 shows the sorption edges obtained at dif tity of potassium ions have been found that may af ferent solid to liquid ratios in the Na-clay, condi fect the sorption of cesium in the high affinity sites tioned at two different ionic strength (left = M0"1 (up to 2 mg/L). Therefore another modelling at M and right M0"3 M). Sorption depends on the pH tempt was carried out considering the possible effect and a maximum is observed around pH 7. In gen of potassium in solution. The quantity of potassium eral, there is no evidence that the solid to liquid ra was fixed to 2-1 0"6 mol/L for all the samples at the tio is affecting sorption, but a certain difference is different ionic strengths. A sensitivity analysis will be observed at the lower ionic strength and a very low done in the next future to see whether it is possible pHs. This point will be verified with new tests. Table 46 Two exchange-sites model: Ionic exchange constants for Cs in FEBEX Na-montmorillonite. With potassium contribution 114 6. Radionuclide sorption and migration in bentonite I = 0.1 M NaCIO,: Dependence on S/L: o VL 3.759/1 VL 6.799/1 a $/L 0.899/1 pH I = 0.001 M NaCIO,: Dependence on S/L: [U]=4.4E-07 M 5.0 4.5 4.0 3.5 3.0 2.5 2.0 3 4 5 6 7 9 10 pH Figure 58. Sorption edges of uranium in Na-clay. Effect of the solid to liquid ratio: 0) 1=0.1 N\ and b) 1=0.001 N\. Figure 59 shows the sorption edges obtained at dif Figure 60 shows the isotherms of uranium obtained ferent ionic strength in the Na-clay (~ lg/L). Two at two different pH (7 and 4) and ionic strength regions can be clearly distinguished: the first one, at 1 -1 O'1 M. It can be observed that sorption is not lin acidic pH, where sorption strongly depends on the eal over the entire range of concentration used. ionic strength and a second one, at basic pH where sorption does not depend on the ionic strength. In the lower part of the figure, where the data are These results indicate that two mechanisms are ta expressed as Log[U]ads vs. Log[U]eq the straight lines king place. Ionic exchange is predominant at acid that fit the experimental points have a slope less pH whereas surface complexation seems to be the than one confirming the non linearity. A similar be main sorption mechanism at high pH. haviour was observed at ionic strength MO"2 M. 115 EEBEXII project. THG Laboratory Experiments S/L ~1 g/L Dependence on Ionic strong lit: 4.4E-07 ndence on o 1=0.05 M NaCIO, □ 1=0.1 M & 1=0.001 M v 1=0.001 M 1=0.01 M pH Figure 5 9. Sorption edges of uranium in Na-clay. Effect of the ionic strength. Modelling: surface acidity constants The first simulation of titration data was made con sidering a classical diffuse double layer model (DM) It is clear from the experimental results that the mo which includes an electrostatic correction. delling has to be carried out considering both ionic exchange and surface complexation reaction at the The data used for the simulations are included in clay surface. In order to model surface complexa Table 47, and the simulation is included in Figure tion reactions it is important to determine the acid - 61a. In this initial simulation, the total surface area base properties of the SOH groups. of the clay 725 m2/g was used for calculating the density of the SOFi sites. The estimation of the SOFi In a previous work, potentiometric acid / base titra site density come from the apparent end-point of the tions were used to determine the surface acidity con titration, that corresponds approximately to 40 - 70 stants of the surface functional groups of the clay mmol/Kg. The value of 0.08 /rniol/m 2 was therefore edge sites. They have been obtained, using a "fast" fixed for the simulations and as non-adjustable pa titration method. More details of the experimental rameter. Considering that the CEC is 1 03 meq/lOOg methods can be found in Missana et al (1999). Ti (1.4 / Table 47 Fitting parameters used for titration data in Figure 61a. SOH density — 0.08 /imol//m2 and surface area 725 m2/g. Site LogK SO" -7.9 S0H+ oo 116 6. Radionuclide sorption and migration in bentonite 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 ■4.0 -4.5 5.0 ■5.5 ■6.0 ■6.5 7.0 ■7.5 8.0 7 ■6 5 ■4 Log(Uj (mol/l) Figure 60. Sorption isotherms of uranium at pH 7 and 4.1=0.1 N\ Bradbury and Baeyens (1995) proposed the elimina surface area is used (33 m2/g) a successful model tion of the electrostatic term for the modelling of titra ling could be found, as shown in Figure 61b. The tion curves (and therefore for the surface complexa- acidity constant used for the modelling are summa tion modelling) of the homoionic Wyoming montmo- rised in Table 48. rillonite. Thus, following this method, a second sim Following the results obtained in the previous para ulation of titration data was made considering a graphs, two possible models can describe the titra non-electrostatic model (NEM). tion curves and both could be applied, in principle, It was impossible to simulate the titration curves with for the modelling of the surface complexation reac a NEM using the value of the total surface area. tions. At present, the model that is being tested is However, if instead of total surface area, the BET the NEM. 117 FEBEXII project. THG Laboratory Experiments pH pH Figure 61. curves of FEBEX Na-montmorillonite: (a) Simulation with o DL model (parameters in fable 41); (b) Simulation with a HE model (parameters in fable 48). Table 48 Fitting parameters used for titration data in Figure 61b. SOH density — 1.81 y Site LogK SO" -8.4 S0H+ 5.3 118 6. Radionuclide sorption and migration in bentonite 6.2.2 Experiments with MX-80 clay dology as described in the previous report. With this selectivity coefficient the sorption of Cs on Ca- montmorillonite could only be quantitatively descri Cs sorption modeling on montmorillonite be the isotherms in the concentration range from 1 O'4 to 1 O'2 M. Background Cs sorption model Cs sorption edges and isotherms were measured on a pure montmorillonite clay (SWy-1) and these A caesium sorption model appropriate for argilla results have already been presented in earlier re ceous rock systems has been developed by Brad ports. The Cs solid liquid distribution coefficients bury and Baeyens (1999). This model is based on (log Rd) as function of pH on Na-montmorillonite the proposition that the uptake of Cs in argillaceous at 0.1 M and 0.01 M NaCIC>4 indicated that there rocks is dominated by the illite clay mineral compo is almost no influence of pH, but a clear effect of nent. The uptake of Cs on illite at low concentra the Na background electrolyte concentration was tions is particularly efficient and dominant due to the observed. This uptake behaviour is characteristic of presence of high affinity sites. Such sites are not ge cation exchange. The Cs sorption isotherm on Na- nerally associated with montmorillonite. However, montmorillonite was linear in the Cs equilibrium there is mineralogical evidence (Baeyens and Brad concentration range between 1 O'5 and 1 O'8 M. At bury, 1997) that the SWy-1 montmorillonite con higher concentrations the Cs sorption decreased tains ~1 wt% illite. In the following, a similar ap due to site saturation effects, which is characteristic proach to this model was applied to the SWy-1 for a Langmuir type sorption behaviour. From the montmorillonite system. A brief outline of the model sorption edge and isotherm data on Na-SWy-1 a concept and associated data is given below. selectivity coefficient for Cs-Na exchange of 15.1 was obtained. Additional measurements at lower Cs The first assumption of the model is that Cs sorption equilibrium concentrations below 1 0"8 M showed a in SWy-1 is governed, in addition to the montmori non-linear behaviour indicating that an additional llonite component, by cation exchange reactions on more selective site is taking part in the uptake of the illite mineral component. For the purposes of Cs. calculating Cs sorption in the SWy-1 system an illite is defined which has a cation exchange capacity Cs isotherms were also measured in the homo-ionic (CEC) equal to 0.20 eq. kg'1. Ca-SWy-1 at pH = 7 in the equilibrium concentra tion range from 1 O'2 to 1 O'9 M with background Secondly, the uptake of Cs on illite is envisaged as electrolytes of 0.005 and 0.1 M Ca(NC>3)2, These taking place on three site types each having differ data showed a more pronounced non-linear sorp ent site capacities and affinities. These are denoted tion behaviour which can only be explained by a as frayed edge (FES), type II and planar sites. The model in which at least two sorption sites for Cs are relations between these site capacities are expressed considered. In analogy to the Na-SWy-1, a selecti in Table 49 as fixed percentages of the illite CEC. vity coefficient for Cs-Ca exchange of 28.8 was de These capacities were scaled to the illite content in rived for the montmorillonite using the same metho the SWy-1 system and fixed. Table 49 Site types and distributions for illite. Site types Site capacities Frayed edge sites 0.25% of the CEC* Type II sites 20% of the CEC Planar sites -80% of the CEC "CEC=cation exchange capacityin eg. kg]. 119 EEBEXII project. THG Laboratory Experiments Thirdly, selectivity coefficients for Cs-Na were taken seen that the predicted isotherms correspond very from Bradbury and Baeyens (1999) directly. The se well to those measured for the montmorillonites. lectivity coefficients for Cs-Ca were obtained from the best fit to the non-linear part of the isotherms. A summary of the selectivity coefficients used in the U(VI) sorption measurements on conditioned modeling for both the No- and Ca-SWy-1 isotherms Na- SWy-1 are presented in Table 50. The selectivity of Cs-Na and Cs-Ca on the planar sites of illite are taken to Sorption measurements were carried out on a pure be the same as the Kc values on the planar sites of montmorillonite clay (SWy-1). The purification and montmorillonite. conditioning procedures to obtain the homo-ionic Na-montmorillonite have been described in detail Finally, with all model parameters fixed, the compu by Baeyens and Bradbury (1 997). ter code MINSORB was used to calculate the Cs sorption in No- and Ca-SWy-1 systems. U(VI) edges Modelling results Sorption edge measurements (sorption at trace con The Cs isotherms were calculated for SWy-1, with centration determined as a function of pH at a fixed the site capacity values and selectivity coefficients for ionic strength) were carried out under inert atmosp montmorillonite (see above), together with those for here conditions in 40 ml polyallomer centrifuge tu illite given in Tables 50 and 51. The site capacity bes in the presence of 2 x 1 O'3 M of an appropriate values for illite were scaled according to the 1 wt % buffer. After labeling with 233U, and shaking end- fraction of illite in the SWy-1 system. The measured over-end for 1 week, the samples were centrifuged Cs data for No- and Ca-SWy-1 are given in Figures at 95000 g (max.) for one hour before returning 62 and 63 respectively, together with the curves them to the glove box for sampling of the superna predicted by the model (continuous lines). It can be tant solutions and pH measurements. -9 -8 -7 -6 -5 -4 -3 -2 log Cs equilibrium cone. (M) Figure 62. Cs sorption isotherm on homo-ionic Na-montmorillonite (SWy-1) at 0.01 MN0CIO4. Si ratio = 1 gt]. pH = 7.0. The continuous line is the modelling result. 120 6. Radionuclide sorption and migration in bentonite Table 50 Site capacities and selectivity coefficients for the illite. (CEC — 0.2 equiv. kg' 1). Site types FES Type II sites Planar sites Site Capacities: (eq. kg'1) 5x10'4 4x10'2 1.6 xlO'1 looX 7.0 3.6 1.18 looX 13.8 9.5 1.46 log Cs equilibrium cone. (M) 0,1 MCa S’ -9 -8 -7 -6 -5 -4 -3 -2 log Cs equilibrium cone. (M) Figure 63. Cs sorption isotherm on homo-ionic Co-montmorillonite (SWy-1) at (a) 0.005 N\ Ca(N03)2 and (b) 0.1 N\ Ca(N03)2. S:L ratio = 1.65 gt1. pH = 7.0. The continuous lines are model results. 121 EEBEXII project. THG Laboratory Experiments The U solid liquid distribution coefficients (log Rj) as U(VI) isotherm function of pH on Na-montmorillonite at 0.1 M and For the sorption isotherm determination, a series of 0.01 M NaCIC>4 are shown in Figure 64. U02(NC>3)2 solutions covering the concentration range required was made up at the pH = 5 in a buffered 0. The data in this figure show that there is a clear in 1 M NaCIOj background electrolyte and labelled with 233U. A similar procedure to that described above was fluence of pH which is indicative for a surface com- plexation mechanism. Further, an effect of the Na then followed. The U sorption isotherm is shown in background electrolyte concentration can be ob Figure 65. The sorption is linear in the U equilibrium concentration range between 1 O'5 and 1 O'8 M. At served at pH < 5. This uptake behaviour is charac higher concentrations the U sorption is non-linear. teristic of cation exchange. pH Figure 64. Usorption edge on homo-ionic Na-montmorillonite (SWy-1) at 01.01 and 0.1 M N0CIO4. Total U concentration = ~107 M. S:L ratio = 1.4 g L'1 122 6. Radionuclide sorption and migration in bentonite log [U(VI) equilibrium cone.] (M) Figure 65. U sorption isotherm on homo-ionic Na-montmorillonite (SWfl) at 0.1 M NaCI04. pH = 5.0; S:L ratio = 1.3 g t’ 123 7. Conclusions 7. Conclusions 7. Conclusions The principal objectives proposed for the THG in clay and bi-ionic Ca/Na smectites with different Ca/ vestigations during FEBEX II have been satisfactorily Na ratios. Ca is a bivalent cation, with a high polar fulfilled. The main conclusion obtained from the THG ising power and it is surrounded by a larger number laboratory experiments will be summarised below. of water molecules than Na. In addition it has more swelling energy. Thus, the swelling of the calcium bentonite occurs with a small amount of water. The 7.1 Porewater in the clay barrier water occupies the interlayer in a double layer, first surrounding the cations and then occupying the free A methodology to obtain the porewater composition spaces among the groups of water molecules co of the FEBEX bentonite based on the characterisa ordinated to the cations. The trilayer complex only tion of the solid phase, the determination of the forms at very high hydration. Therefore, all the water physico-chemical properties of the montmorillonite adsorbed is occupying the interlayer spaces. Na is a component and geochemical modelling was pro monovalent cation, surrounded with a smaller num posed, taking into account that the montmorillonite, ber of co-ordinated water molecules than Ca. It together with the other mineral phases present (car shows a less swelling energy to form the monolayer, bonates, sulphates, pyrite, and organic matter...), thus the water adsorbed fills the interlayer, con will determine the composition of the porewater. denses capillarily in the micropores and it can also be adsorbed on the external surface. Water vapor adsoption/desorption isotherms, together with c-lattice spacing determinations, were used to As the water/bentonite ratio increases, the calcium identify the different states and location of water. It sample does not modify its interlayer spacing (only was found that most of the water in the as received three layers of water), while the sodium sample can bentonite resides in the interlayer space. Measure swell freely (osmotic swelling) until it forms a gel or ments indicated that about 0.053 L/kg may be re a suspension with the individual separated sheets. If garded as external water, implying a chloride con as it hydrates, FEBEX bentonite, become more calcic centration of 0.42 M, since the chloride inventory in (due to interaction with the granitic waters in Grim- the FEBEX bentonite is —22 mmol/kg. The pH of the sel), it could eliminate its free water (intro- or inter system is fixed by equilibrium with the atmosphere ( aggregates) to enter the interlayer and form a bila PCOj = 10'3'5 bar) and saturation with the carbonate yer, as observed in Ca-clay samples. The trilayer will phases present. be formed at values of P/Po = 1, and the intro- or inter-aggregate spaces may will start to fill again. Considering the above mentioned premises, the The isotopic analysis of interstitial waters extracted porewater in equilibrium with the as received FEBEX from the bentonite indicated d18 0 = -0.83%o and bentonite powder was calculated to be a Na-Ca- <32 H = -31 %o. These values could be approximate Mg chloride type with a high ionic strength, 0.66 M, of the average isotopic value of interstitial waters of and a pH of — 7.4. bentonite in the Serrata de Nijar area in the Cabo The volume of external wafer in the compacted ben de Goto region. tonite was taken as the chloride accessible porosity The preliminary results of d18 0 y d2 H determined in obtained from Cl" through-diffusion tests. Therefore the interstitial waters extracted in the bentonite sam the amount of external water in compacted bentonite was 0.03 L/kg at a dry density ofpd = 1650 kg/m3. ples saturated at different P/Po show that for high relative humidity (P/Po > 0.32) the isotopic values The corresponding chloride concentration is thus are similar to those of the hydration water (isotopi- —0.73 M. The porewater of compacted FEBEX ben cally marked). In that case, the water did not suffer tonite at pd =1.65 g/cm3 was calculated to be a any exchange process with other types of water pres Na-Ca-Mg chloride type with a high ionic strength, ent in the clay. This means that the water extracted 0.90 M, and a pH of — 7.4. from the samples with high saturation degree corre The FEBEX smectite stacks at ambient conditions (RH sponds to the mobile water that fill the interlaminar = 50-60 %) have two monolayer of water (dooi — spaces and this would indicate the mixing with water 15.2 A). In wetting experiments, the FEBEX montmo of other type (structural) does not exist. rillonite expands in the c-dimension to acquire a ma On the contrary, for lower relative humidity, the iso ximum of three monolayers of water (dooi = 19.0 A). topic values are lower than those of the isotopically The hydration behaviour of the natural FEBEX clay marked hydration water. This can be due to the fol was compared to that of the homoionic Co or Na- lowing reasons: 127 FEBEXII project. THG Laboratory Experiments 1) The extracted water interchanged with other 7.2 Effect of the interlayer cations water within the clay with lower isotopic values. 2) The water that enters in the interlayer has on the rheological properties more negative values because these isotopes of bentonite are more reactive and show faster diffusion. Results showed that the rheological behaviour of the Since the number of isotopic analysis was limited and smectite suspension is strongly affected by the com they have been applied only to the samples homo- position of the smectite exchange complex. The ap ionised in Ca, the two hypothesis cannot be con parent viscosity of the bentonite suspensions in firmed. It is strongly recommended to continue this creases with the increase of Na as exchangeable study and to perform an higher number of analysis. cation. The increase in viscosity produces a de Regarding the squeezing extraction methods to ob crease in suspension fluency, that is, the suspension tain the porewater, the studies have shown that most capacity to flow and its contribution to the mass of the components of interest like Na+, K+ Ca2+, transport. Due to the contribution of groundwater, Mg2+, Si4+, Cl", SO4, HCO3 and pH can be deter the bentonite at the bentonite/granite interface will mined without significant difficulties from squeezed take up Ca from the solution and become progres porewater samples. S'2, Fe 2+ and Fe tot have ap sively enriched in Ca. Therefore, according to the peared to be difficult to find in the porewaters, how results presented in this report, under conditions of ever. Additionally, it was found that the density of high input of groundwater to the interface, the in bentonite affects the concentration of the porewater. crease in interlayered Ca may contribute to suspend These concentrations are lower at higher densities, the smectite in the percolating solutions that could which is typical for exclusion caused by the overlap fill and seal fractures. ping electrical double layers. Typical for the succes sive porewater fractions is also a decrease in the concentration as a function of increasing density. 7.3 Geochemical processes Reporting the density interval during squeezing is thus most important for interpreting and comparing at the solution/bentonite interface the results. The dissolution rates of smectite, volcanic glass and The modelling of the data obtained by squeezing plagioclase were measured. Smectite is the main MX-80 bentonite samples at different densities satu component of bentonite (92 % in FEBEX bentonite), rated with external solution of different ionic strengths whereas plagioclase (3 %) and volcanic glass (tuff, suggested that the squeezed water comes both from estimated 1-2 %) correspond to the most abundant the large pores and from the interlamellar pores and soluble accessory phases. The change in disso and the relative amounts depend on the density of lution rates within the pH range from 7 to 8.5 is the sample. small, at constant temperature. Important variations are expected outside this interval, however, these The variation of the chemical composition of the geochemical conditions are not relevant for the pore water as a function of the squeezing showed a scope of FEBEX II Project. The most important effect decrease in Cl", SO4” Na+ and Ca2+ concentration of temperature is observed in the glass but it is very as the pressure increased. The slight bicarbonate small in plagioclase. Only for smectite it was possi concentration increase with the pressure indicates ble calculating the variation of apparent activation some carbonate dissolution process. These waters energy with the pH of the solution. are undersaturated in gypsum and oversaturated in calcite and dolomite. The water dilution when the The contribution of each mineral phase to the disso pressure increases indicates that some interlaminar lution of the bentonite was estimated (pH approxi water is being extracted. As previously indicated, the mately 7.5 and at 25 and 50°C). It can be assumed greater part of the water in the bentonite is internal that the dissolution rate of the bulk bentonite is and, when the dry density decrease, the amount of equal to the contribution of these phases, which is water (internal + external ) in the pore space de almost exact, because other accessory phases dis creases. For this reason, it is very likely that during solve slowly and have small surface areas (i.e., the squeezing process the interlaminar water come quartz) or are really trace minerals (i.e., pyroxenes, out, diluting the concentration of the pore water of amphiboles). With this assumption it can be con the bentonite. cluded that the bentonite dissolution rate increases 128 7. Conclusions four times, when temperature rises from 25 to 50°C. and apparent diffusion coefficients for conservative The contribution of plagioclase dissolution to the tracers (De — Da - a). The consistency of the trans overall rate seems to be negligible, thus the smectite port parameters measured using different methods and the tuff govern the process. At 25°C, the confirms that these methods are appropriate and smectite contributes with the 70 % of the overall the values obtained are correct. The experimental rate. In turn, at 50°C tuff dissolution have increased values can be adjusted using exponential functions 9 times and represents two thirds of the total, of the form Da/e — A where A and B are con whereas smectite dissolution rate is only double. It is stants and pd is the bentonite dry density. Thus ac a very important fact, because the conditions within cessible porosity, apparent and effective diffusion the barrier are quite similar to that of smectite for coefficients in FEBEX bentonite at any dry density mation in natural environments, and the dissolution can be therefore easily estimated by interpolation. product of the accessories may produce additional Diffusion experiments with non conservative tracers smectite. Tuff dissolution preserves smectite from (U, Eu and Nd) allowed to complete the diffusion degradation. Only 1.5% of volcanic tuff may control database needed for the modelling of transport pro the rate of overall dissolution process. Small addi cesses of the "in situ" and "mock-up" tracer tests. tions of natural tuff to the bentonite used to build the block may improve the chemical stability without The sorption of cesium both in the FEBEX and the important modification of other physical properties. SWy-1 homoionised clay showed a non linear be haviour. Since cesium can be present in solution only as the species Cs+, the non linear sorption in 7.4 Sorption and migration dicates the existence of at least two different sorp tion sites. The first sorption site is of "high" affinity in bentonite and low capacity whereas the second type of sorp tion site is of "low" affinity and high capacity (~ the Accessible porosities for HTO (for all clay densities) CEC of the clay). Considering the kinetic experi and 36CI" (for clay densities up to 1.2 g/cm3) were ments, carried out in the FEBEX montmorillonite, the directly obtained by means of saturation experi adsorption of cesium seems to involve a rapid ex ments. In these experiments the water-saturated change reaction (hours) and a slower component bentonite samples were introduced into a reservoir (days), which seems an anomalous behaviour for with the tracer until equilibrium concentration was ion-exchange reactions. This slow process, can be reached. The measurement of the tracer activity in evidenced only when very low tracer concentrations the clay pore water allowed a direct measurement are used (< 1 -1 O'9 M) due to the very low density of of the accessible porosity. Alternative estimations of the high affinity sites (~3'1 0"8 eq/g) compared to accessible porosity were done in through-diffusion the bentonite cation exchange capacity. In connec experiments with constant concentration and con tion with the high affinity and low affinity sites con centration profile experiments, which provided re cept, the sorption process can be described in two dundant, although less precise, methods for its esti steps: the adsorption on the planar sites (rapid pro mation. Similar values of accessible porosities were cess) and the diffusion to less available but highly obtained with the three methods. selective sites (slow process). The existence of "low" Results showed that, in FEBEX bentonite the accessi and "high" affinity sites for cesium sorption in clays ble porosity for HTO agrees very well with total po is usually explained considering that cesium can be rosity, which implies that all the pores in compacted exchanged with hydrated cations in basal/interlayer bentonite are available for diffusion of neutral spe sites (low affinity sites) and can sorb, in a highly se cies. Different results were obtained in the case of lective way in frayed edge sites, FES, (high affinity Cl", for which the accessible porosity is significantly sites). FES sites are not present in expanding clays smaller than total porosity, even at the lower densi like smectite but develop in weathered micas and ties. (At 1.65 g/cm3, the porosity accessible to Cl is illite (Zachara et al., 2002). —3%). Results for chloride clearly show that FEBEX The SWy-1 montmorillonite contains ~1 wt% illite bentonite displays a significant anionic exclusion. The and therefore a cesium sorption model on the SWy-1 effect of pore water composition on anionic exclusion was developed based in a previous work (Bradbury is an interesting issue to be studied in the future. and Baeyens, 1999). This model is based on the For HTO, the values of Da/ De and (j> measured in proposition that the uptake of Cs in argillaceous dependently satisfy the relation between effective rocks is dominated by the illite clay mineral compo- 129 FEBEXII project. THG Laboratory Experiments nent due to its FES-sites. It successfully modelled the the illite in the illite-smectite mixed layer may not sorption results both in the No and in the Ca-clay behave exactly in the same manner than a pure system. The first assumption of the model is that Cs illite phase. This may be the main reason for which sorption in SWy-1 is governed, in addition to the the modelling of the Cs sorption data considering montmorillonite component, by cation exchange re the above -mentioned model (considering a 10% actions on the illite mineral component. For the pur content of illite in the FEBEX bentonite) was not suc poses of calculating Cs sorption in the SWy-1 the cessful. cation exchange capacity of the illite was assumed equal to 0.20 eq/kg. The uptake of Cs on illite is Therefore sorption of cesium on FEBEX bentonite envisaged as taking place on three site types each was interpreted with a 2-site model (2 "generic" sites) having different site capacities and affinities. These considering ion-exchange both in the "high" affinity are denoted as frayed edge (FES), type II and planar and the "low" affinity. Also this model reproduced sites. These capacities of the sites were scaled to the satisfactorily the experimental data. illite content in the SWy-1 system and fixed. Thirdly, Sorption experiments of uranium both in the FEBEX selectivity coefficients for Cs-Na were taken from and the SWy-1 homoionised clay showed that at Bradbury and Baeyens (1999) directly. acidic pH, sorption depends on the ionic strength On the other hand, illite was never detected as a and at basic pH where sorption does not depend on "pure" phase by XRD in the FEBEX clay but mineral- the ionic strength. These results indicate ionic ex ogical studies showed that the smectite phase is ac change is the mechanism predominant at acid pH tually made up by illite-smectite mixed layer with a whereas surface complexation has to be the main 10-15 % of illite layers (Cuadros and Linares, 1 996; sorption mechanism at high pH. Therefore the mod Huertas et a/., 2000). The existence of smectite-illite elling of sorption data has to take into account both mixed layer may possibly lead to the existence of mechanisms to describe correctly the uranium sorp FES-like sorption sites in FEBEX bentonite, however tion behaviour. 130 8. References 8. References 8. References Appello. C. A. J. and Postma, D. 1993. Geochemistry, Caballero, E., Reyes, E., Delgado, A., Huertas, F., groundwater and pollution. Rotterdam: A. A. Balkema, Linares, J., 1992. The formation of bentonite: mass 536 p. ISBN 90 5410 1059. balance effects. Applied Clay Science, 6, 265-276. Araguas-Araguas, L, Roza ns ky, K., Louvat, D. 1995. Iso Caballero, E., Reyes, E., Huertas, F., Linares, J., Pozzuoli, tope effects accompanying vacuum extraction of soil A. 1991. Early-stage smectites from pyroclastic rocks water for stable isotope analices. Jour, of hydrology, of Almeria (Spain). Chemical Geology, 89, 353-358. 168, 159-171. Caballero, E., Reyes, E., Linares, J., Huertas, F., 1985. 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SKB 92-37, 28 pp. Truesdell, A. H., 1974. Oxygen isotope activities and Wieland E., Wanner H., Albinsson Y., Wersin P., Karnland concentrations in aqueous salt solutions at elevated O., 1994. A surface chemical model of the bento temperatures: consequences for isotope geochemis nite-water interface and its implications for mode try. Earth Planet. Sci. Lett., 23, 387-396. lling the near field chemistry in a repository for spent Turner, J.V., Gaelitis, V., 1988. Single-step method for hy fuel. SKB 94-26. drogen isotope ratio measurement of water in po Wollast, R., 1990. Rate and mechanism of dissolution of rous media. Anal. Chem., 60, 1244-1246. carbonates in the system CaC03-MgC03. In: Aquatic Van der Lee J. and De Windt L. 1999. CHESS tutorial and Chemical Kinetics. Ed. W. Stumm. Wiley Interscience. cookbook, Technical Report LHM/RD/99/05. Yu, J-W., Neretnieks, I. 1997. Diffusion and sorption pro Van Olphen, 1977. An introduction to Clay Colloid Che perties of radionuclides in compacted bentonite. mistry for clay technologies, geologists and soil SKB, Technical Report 97-12. scientists. John Wiley, N.Y., 31 8 pp. Zachara J. M., Smith S. C., Liu C., McKinley J. P., Seme Villar, M.V., 2002. Thermo-hydro-mechanical characteri R. J., and Gassman P. L. 2002. Sorption of Cs+ to sation of a bentonite from Cabo de Gata. ENRESA micaceous subsurface sediments from the Hanford Publicacion Tecnica 04/2002. site, USA. Geoch. Cosmoch. Acta 66(2), 193-211. 137 Titulos publicados PUBLICACIONES TECNICAS 1991 Publicaciones no periodicas 1996 o; Bewszdw so# ms maos mwfmcos Beaooaoos seG/zwooB/awoew. zwcommu Z772. 01 DESARROLLO DE UN PROGRAMA INFORMATICO PARA EL dSeSOBAW/eW/D cow a dawcmw/ewzo oe Beszo//os a/acz/yos. BBaezws #aas oe z+o ew a oesndw oe Beszo/zos ao/aez/yos. oe a oeeac/dw oe eocos aw/soBes oe cow/maw/es o^seosos. 02 zzewszdw so# ms Aiooems wz/AifB/cos Beaooaoo eouos/y// 02 awu BeBOBeoe /mysm /esrBBOom cowceBW/wosawsw cays cow a ^aacmw/ewro oe Beszo//os jMOMCTTyos. mxoz. caBow/zesmoew/Dw/ms). 1994 03 M/BMc/owes^/ cowocM/ew/o oe a ewx//odw Bue/mU/za os BBe/mAByso///3z//zysmo/Koe//ZMWz//)Wozom//woeB y /M/eoAwo/ewzw ew a Bew^soa zof/oa o/mze /os 00s d/z/sios me cowozzzows memo zwd su/we BeBos/roBy. mowesoemsd B#B oe/ esmo/o oe zayeBnwos 01 MODELO CONCEPTUAL DE EUNCIONAMIENTO DE LOS ECOSISTEMAS yesBe/eozem. 04 GEOESTA DISTICA PARA EL ANALISIS DE PIESGOS. Una introduccion EN EL ENTORNO DE IA FABRICA DE URANIO DEANDUJAR. abOmwAKtzkaaapaamSdKa. 04 )WA"ooos oeoesao&zzcos m a zw/eoac/dw oe zweomc/dw. 02 coBBoszowoeawoaazeuazeB/ameoBawzszeBaBBuazzows os szm^czowKswdBzzasyamzsoeyzm'os^socMoos zwBocxsueeoBuazzows. os eszz/ozooemwGewao^Bewrowmeszzi 3z//ao//zoBozezMM/ we/aoB/r oes4Bowm 03 STOCHASTIC MODELING OE GROUNDWATER TRAVEL TIMES os zmwezez% Aiemooo/OGZKm moms oe szze saeczzow 05 ALTERACION HIDROTERMAL DE IAS BENTONITAS DE ALMERIA. 04 THE DISPOSAL OE HIGH LEVEL RA DIOACTIVE WASTE IN ARGILIA CEOUS HOST eoB aozaczzve mze o/seosu * BocKa/eeoz&azzows BOCKS. AWko&m o/pamnwZm% coasdmak am/ gmZogZcd assesmanf 07 may. //w cdozoo /%a zmaBM/s/s oe /wceBzzo//)W3Be pm%#& Y SENSIBILIDAD. Manuales. os a oes/e oe azBoz* y a Bew^s/za efB/a oesoe /wee -/20.000 ms 1992 Publicaciones no periodicas Z/aSM e/ BBESeWZE. ZsosZnadgladdc patogoogmZk pdeaZBaymaZWi 01 SIA IE OF THE ART REPORT: DISPOSAL OF RADIACM WASTE IN DEEP os ecomoA a /osszszewasac/zjzzcos a e/ ewroz/wo oe a cm/. a oeBBoae BBom yom^we / sco/oma/ smozes m/aceo//SBOzwMzzows 07 uuacmwEwzoGeo/dGzcoBBoe/zwoooeBesB/zosao/aczzyos a oeBBoae BBom you# & z/yoBooeocz/eiw/szBy. 02 ESTUDIO DE IA MIL TRACION A TRA1VES DE IA COBERTERA DE IA FUA. Oe d/M aCZZy/aO (40% ComopAK ^aSmmm do mAraodd. a oeBBoae BBom yom^we #/. aooamBymazzow zeszsm zw os saw/s/z BABzzczazxzw zw me zwzeBazxza/ zwzaa/ BBQ/az: 03 zzwaaoes udw/es mi aaczeB/zaczdw z/zoBOGeoo/z6wza SITU TRACER TEST. 04 m CTERIIACION DE ESMECT1TAS MAGNESICAS DE LA CUENCA DE MADRID 07 exmewcMS meuwmes oe maezdw oe aozow/zc/aoos a oeBBoae BBom you# zy. //yoBOGeomoza/^ooea/wom COMO MATERIALES DE SELLADO. Ensayos de alteracion hidrotermal. cowM/emes GBaw/zzcos. a oeBBOcu, esB# CODE DEVELOPMENT. os so//m/zyszms oe /mzuw ozoxzoe uwoeB me cowozzzows ;o esm/moeoeseo//m/os/soedacosoeseBES/MOMC//MS eweezeo zw^ su/we BCBOsz/OBy. zw zz NATURALES EN UNAMBIENTE GRANITICO: PLUM DE EL BERROCAL 1997 os Bewszdw oe jwfmoos oeoe/^zcosm/ao/es^ esmozo c/oaoo;. Y CARACTERIZACION DE EMPLAIAMIENTOS PARA ALMACENAMIENTO ;; Beuodw me mLweaos oeoeA/cos e ////meo/do/cos. 0; cowszoeaodw oa CAwao ae/mwa#!/ ew a ernaodw oe zzeszo/zos a/aezm oe^ea ^czzwao a GZMWzzos sues //oamciSadaAwaAaa. DE LA SEGURIDAD. ESCENARIOS CLIMATICOSAIARGO PIAIO EN IA PENINSUIA ymz/as. /2 o/s^o y cowsmocodw oe u cooeBm/w ;w/////mi oe/ o/oue zg&za. o/ coazczmsoeo/szBzo/zczdwmBeaozow/zc/aoos. DE ESTERILES DE LA FABRICA DE URANIO DE ANDUJAR. 02 mooo/OG& oe ewuaczdw oe B/esoo skw/co ew se&wewzos os cowzBzo/zzzow oy emeu m me Bacozw aya-s exeBC/se. DEE ALIA. op oeswmooeowmaooefesuseWwoesuems Publicaciones no periodicas 03 DETERMINACION DE RA DIONUCLEIDOS PRESENTES EN EL INVENTARIO cowzmaoos. M/aodwu^Ba oe Buomes. DE REFERENCIA DEL CENTRO DE ALMACENAMIENTO DE EL CABRIL. SeGWWOOBMWkO /77/-/77S. /WR/mmi/ /773. ; o eszz/ozo oa cdozeo eesw mi am) a/m /wa wzaczdw a ax 04 ALMACENAMIENTO DEFINITIVO DE RESIDUOS DE RADIA CTIVIDAD ALTA. /1 IA EVALUACION DE IA SEGURIDAD DE LOS SISTEMAS DE ALMACENAMIENTO C(McfMZDdi%yconpw&nm&a/mpop/arodb/mc^ oe Bcszo/zos a/aczzwzs. //zz/zaezdw oeufeooos BBoaoz/zszas. 1995 im/m/mCZA 12 METODOLOGIA CANADIENSE DE EVALUACION DE IA SEGURIDAD DE LOS os mooo/OG& oeM/s/s oe a ozoseea a a emaodw dWMcewAwmos oe Bcszo/zos a/aezzyos 01 DETERMINACION DEL MODULO DE ELASTICIDAD DE FORMACIONES DEALMACENAMIENTOS GEOLOGICOS PROEUNDOS DE RESIDUOS RADIACTIVOS DE ALTA ACTIVIDAD ESPEGFICA. / 3 oesaz/rzdw oe a erase oe oaeos imeB. m/aogsBBoa/w/MS: 02 UO LEACHING AND RADIONUCLIDE RELEASE MODELLING UNDER HIGH AND 06 EVALUACION DEL CAWBOB/AW/eW/D Y DE IA SEGURIDAD DE UN Publicaciones no periodicas aw/ow/c s/Bmm som//owm ox/o^//ow cowo///ows. d/mewAwmo seo/doco BBoa/woo ew om/zo. Z777 03 me&wo//yoB&wecm/ou c/M/MC/e/aa/Tow oe mesm/sw 07 s/wzes/s zecroeszazzGB^aa oa worn z/es/^EO. yom^wew / Bowewaasezweoazes, Z733-Z77Z. REFERENCE CLAY MATERIAL FOR ENGINEERED BARRIER FOR GRANITE AND seo/wooewwoebo, Z77;-Z77s. zouosc z/y/z/. 03 #/ joBaas oe z+o y zecwomoAs oe oesz%)w oe Beszo//os cay w/iyBeBos/roBy.- /mw/DBym sawa aocm asawo. RADIACTIVOS. Posters descriptivos de los proyectos de l+D y evnluacion secowoBeseaBC/zawooeyemBwewrBaAC ;77Z-;77S, yomue/ w oocmim/Es^as/s/Eaas/s/ewcMGeo/Aw/oia/o/s^o de la seguridadalaigo pirn. aOMBO/a. Cmopfa * m/wandd. 07 eeoex ezaa BBeoeeacwm ZAFOBSie oes/wzes/s. 1993 os oe/eBmic/dw oe a eWi ac//ai//a/M a as Boas sum 10 METODOLOGIA DE GENERACION DE ESCENARIOS PARA IA EVALUACION FUERTEMENTEIRRADIADAS MEDIANTE TECNICAS DE TERMOLUMINISCENCIA. DEL COMPORTAMIENTO DE LOS ALMACENAMIENTOS DE RESIDUOS o; zwyeszzaczdw oe owzowzzas couo ua /ernes oe se/aoo ^)3codi^a/a/id6Bd9n^osWcKdbmsdbosfDAdkwd9d/aad/^^ aowcz/yos. PARA ALMACENAMIENTO DE RESIDUOS RADIACTIVOS DE ALTA ACTIVIDAD. 06 PREDICCION DE FENOMENOS DE TRANSPORTE EN CAMPO PROXIMO ZZ ^a//ECemBy2. CS%?pm7AoM]W5adbagum/m/dbaa IONA DE CABO DE GATA, ALMERIA. y /EM WO. /n&mcdSn oa Aw dmacaaaawafaapa/AxddaaMj^faAdkwda/iaAyaMAadWm/. 02 TEMPERA TURA DISTRIBUTION IN A HYPOTHETICAL SPENT NUCLEAR FUEL 07 ^acrosBeacmooscowaBBO/Kc/dw^/WG/ao///MW/ea BeBoszroByzwasurooue. DESMANTELAMIENTO YCLAUSURA DE LA FABRICA DEANDUJAR. 03 MMoacow/ew/ooewm^ewm/^c/owess)iem.su#DdSn 1998 03 mys/s oe as seweanow^wam/sws/w //woeBGBoowo aooicm daZmacaaaawakdemsdtwfmAKZ/MK m/e Beeos//OB/es /Pogmo /^%c). 0; eeoex BBeoBeazzoww szm s/zmiBy Beazz/z: 04 SPANISH PARTICIPA TION IN THE HA W PROJECT. Laboiatoiy Investigations on 07 ewayos oe rnme/dw oe eiw/soBes oea a/Bos oe aBa m Ganwna Widdm eZAdrZn BodtSd/t 02 BeBBOBmce ^ssesswewr oe^ oeeB seomoza/ BeBos/zoz/y ;o 2* eaw oe w. oesmoaos^aooo/dacos: acwo/doro^ IN GRANITE. March 1997. os aaczezazaczdw y w/zaezdwzwo/zszzau oeua/ezauesaBcimsos /ws/B/wmas y M/)WfB/cos ew a oesndw oe Bes/o//os aoMcwos. COMO BARRERA DE INGENIERIA. 03 eeoex o/sefo ezwu y^owwe oe/ ewayo iwszm' ew GBasec ;; BBoyecro ^awcew#m seo/daco BBoa/woo. ase 2. 06 CHEMISTRY OE URANIUM IN BRINES RELA TED TO THE SPENT FUEL DISPOSAL 04 eeoex oew/owmi- omAC BBoaeaoes y aoBzaczdw oe omo/zes /2 zwaa/TBeBosz/OByw. /ws/m ms//a//ow oe me /owo-zeAW saawo sysM os eeoezc oew/ow/za OB/o/AC BBOBeB/ism e^3B/azzow oe o/oos. AS COMPONENT OE DAM CONSTRUCTION (DAM PROJECT). o/ sw/zaezdw zfawza oa a/uacmwmo ew a/eB^wss. os zeBcezws #aas oe z+o y zecwomoAs oe seszzdw oe Beszo//os WaoMmca/iWafm:C«W#C os BBOGBauas comeuewzmzs mi a M/s/s eszooiszzco aOMCZ/yOS. 24-27 Woweo^r^ Z777. KxbmaaZ oa zawsBOB/e oe aozow/zc/aoos. 07 zeBcezws./OBaas oe z+o y zecwomoAs oe eeszzow oe Beszo//os Publicaciones no periodicas 07 BBOGBamBaae/okc/zmoeBezawaoz/za/zesoeomo/ze aOMCZ/yOS. 24-27 Woweo^r^ Z777. Ib/unwaZZ Z 0 METHODS AND RESULTS OE THE INVESTIGA TION OE THE /eBceBBawoew/77S-/777. 03 )Wow//aczdwysa//aczdwoeaiBBezMsaB/aBes THERMOMECHANICAL BEAVIOUR OF ROCK SALT WITH REGARD TO THE PINAL seouwo^s joBwias oe w. ew a sesz/dw oe Bes/o//os aowc/M/s 07 eeoex BBeoeeaz/owu mez&wo//yoBOvwecawa/ czz/w Aiooea/wo o/seosu oe z/zG/weya aozaczzve mas. mm/y// OE THE "IN SITU" TEST. Titulos publicados 7o ffBfx 72x500572477(m 777f72wo77yoxo4(f67(4W7(4( mw 4(ooa((W6 00 m(#*/B77G4(7()wy0B4XX0a07f(W0(()6760724X4 (4 Gf577()W 07 (472467f72W467()W 0577700)67(4, 4(7*7240)67(4, 6fOO(A(4 y OF THE "MOCK UP" TEST. Of XB0(05X4O(4677/O5 7777-2003.Xf/75K)W2000. 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(40(77BH77. o/ /mmicA oa 6754(4 r a 557770705 of wowmoG& 04 60W57(m770W50W00557B(f57Wa7f(4W0 777677 (f/f(W457f 04 EVOLUCION PSLEOSMBIENTSL DE IS 4(7740 5(777 DE IS PENINSUISIBERICS. a fcosKTfm 4(4«wo5 577x00505. 4(4W4Gf4(fWr00770W5. 47%7(4(7()W4 (4 fWK(746f()W OB 604(72077744(7^50 Of (05775720575077705 oo m of 7w/5577G464)w y of54xxoao motdmmi (4 GB# Of 77B707705 7240(4677/05. 05 IS PECHBLENDS DE LS MINS FE (67(7040 RODRIGO, SALAMANCA), Of XB707705 7240(4677/05 7 W2003. 067770X5 7 727. COMO SNSLOGO NSTURSL DEL COMPORTSMIENTO DEL COMBUSTIBLE 05 THE ROLE OF COLLOIDS IN THE RSDIONUCLIDE TRSNSPORT IN S DEEP 07 55772477672456 0704(0(56(7(477 (4 7246f4UZ4(7()^72W(f72IZ4(7()W 6457400. Proyecto Matrix I. GEOLOGICSL REP0SI8T0RY. Participation ofCIEMSTin the CRRproject. DEAMINOSCIDOS COMO HERRSMIENTS GEOCRONOLOGICS 05 / _/077M4(M5 Of 7W/B7764(7()W y 0B477770B0 756*70)6760 W GB77()W y/mmwwAm 06 TESTING SND VSLIDSTION OF NUMERICSL MODELS OF GROUNDWSTER FLOW, SOLUTE TRSNSPORTSND CHEMICSL RESCTIONS IN FRSCTURED GRSNITES:S DE RESIDUOS RSDISCTIVOS. Resumenes de ponencias. 7 0 64757(75 6(4y 72X07565 6&Mkicm o/*7 7K% of Wmwof wammW 0(74W7774 77/f 57W0y Of 77# 77y07706f0(067(4( 4W0 77y0770(77f4W(4( 07 / _/077M4(M5 Of 7W/B7764(7()W y 0B477770B0 756*70)6760 ^ GB77()W clay as barrier in radioactive waste repositories. STSGE I: VERIFICSTION IMPSCT PRODUCED. DE RESIDUOS RSDISCTIVOS. Sinopsis de posteres. fXfKKB 07 7/J077W4045 Of 7W/E5776467()W y 055477720(70 7f(m)G760W G557A 00 /_/077M4(M5 Of 7W/B7764(7()4( 0B477770B0 75(WOO)60O y 7 7 64757(75 6(4y 72X07565 6&Mkicm omf 7K% of Wmwof wammW DE RESIDUOS RSDISCTIVOS. Volumen I. DEMOSTRSCION EN GESTION DE RESIDUOS RSDISCTIVOS. 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Of X55707705 7240(4677/05. /oWm 7/. 07 MO6054X0(7fO(()67605f7W0(757X!4(B7247244(4(46^^ (% (4(7246T0 Of (4 4(7M4 724C0WB (4(04(4 0(6f77B) 300775 (4546045 72X05(7*705; B777070 Of 727f2454XQ(7fO(()G7(454(f767(45. 5072f72F76(4(B y 50B757772<(W545; 4(OOfOa67()W mOGfOOOW 00 ff OfX 72XQ#65 72057iW0X754( 4^4(55(5' 6077X0570W 577705. Febex II Project THG Laboratory Experiments PUBLICACION TECNICA 09/2004 Para mas information, dirigirse a: ejj/tiLi Direction de Comunicacion Cl Emilio Vargas, 7 28043 MADRID http://www.enresa.es Septiembre 2004