Pore Water Chemistry of the Febex Bentonite
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[7], [8]). In all of them, the modelling of the pore water is posed as a means to obtain the representative pore water at initial conditions of each argillaceous material (for a given water content, porosity and density), instead of using the chemical compositions of the pore water obtained in tests at high solid to liquid ratios (squeezing and seep water). In this work, a methodology has been followed, combining the next tools: 1) different pore water extraction methods, 2) physico-chemical and mineralogical bentonite characterization and 3) geochemical modelling to: • Create a database with the main physical, mineralogical and geochemical parameters of the bentonite. • Evaluate the water-bentonite interaction processes controlling the physico-chemical parameters and the chemistry of the system. • Obtain the best possible estimation of the bentonite pore water composition at initial conditions of water content in a repository, with the available tools. MATERIALS AND METHODS The tests were performed with bentonite from Cortijo de Archidona deposit (Almería, SE Spain) which has been selected by ENRESA (Empresa Nacional de Residuos Radiactivos) as a suitable material for the sealing and backfilling of HLRW repositories. This bentonite has been used in the FEBEX project during the in situ (Grimsel, Switzerland) and mock-up (Madrid, Spain) tests. The dry density of the samples was determined by the mercury displacement method. The gravimetric water content is defined as the ratio between the weight of water lost after heating the sample at 110ºC for 24 hours and the weight of the dried clay, expressed as percentage. The soluble salts were analyzed in an aqueous extract solution. Crushed rock samples were placed in contact with de-ionized water at several solid to liquid ratios and allowed to react for 2 days. After phase separation by centrifuging, the supernatant solutions were analyzed. The Chapman displacement method has been used to determine the exchangeable cations by means of successive washing with ammonium acetate 1N at pH=7.0, after washing the soluble salts [9]. To determine the total exchange capacity, the exchange sites of the sample are saturated with sodium by means of successive washing with sodium acetate 1N at pH=8.2. The adsorbed sodium is displaced by successive extractions with ammonium acetate 1N at pH = 7.0 [10]. The method applied to obtain pore water is the compression technique (squeezing). A pore water squeezer has been designed to allow a one-dimensional compression of the sample [11] (Figure 1). The design is similar to that developed by Peters et al. [12] and Entwisle & Reeder [13]. The press is a hydraulic compression machine. The stainless steel compaction chamber permits an uniaxial compaction of the sample up to 200 MPa. The filtration system allows the extraction of interstitial water by drainage at the top and at the bottom of the sample. This system is composed of a 0.5 µm stainless steel porous disk in contact with the sample and the liquid is collected through polyamide tubes in a
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