Measurements of Eh and pH in Compacted MX-80 Bentonite
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0985-NN13-09
Measurements of Eh and pH in Compacted MX-80 Bentonite Torbjörn Carlsson and Arto Muurinen VTT Technical Research Centre of Finland, Otakaari 3A, Espoo, FI-02044 VTT, Finland
ABSTRACT The low-content free water and high swelling pressure in compacted bentonite, planned to be used as a buffer in nuclear waste repositories, create adverse conditions for direct measurements of the chemical conditions. This paper presents laboratory results from online measurements with Eh and pH electrodes in water-saturated compacted MX-80 bentonite. The Eh was measured with Au and Pt wires as electrodes, while the pH was determined with IrOx electrodes. The latter were prepared in accordance with the method by Yao et al. [1]. The measurements were carried out in two types of cells: ‘squeezing cells’ and ‘diffusion cells’. The squeezing cell excludes almost completely all chemical interactions between the sample and the surrounding environment outside the cell. The diffusion cell, on the other hand, contains a sample that stays in contact with an external solution and therefore allows following of the physico-chemical interaction between the sample and the external solution. The measuring electrodes were positioned inside the cell in the compacted bentonite, while the reference electrode was positioned outside the cell. INTRODUCTION One topic of concern in the evaluation of the safety of a nuclear waste disposal is the chemical evolution of the repository. Two important parameters that should be evaluated in the bentonite buffer are pH and Eh. Until now, their values have been studied, for example in batch experiments, by squeezing porewater from bentonite or by geochemical modelling [2-5]. The development of methods for direct measurement of pH and Eh in bentonite has been reported in [6]. In the bentonite buffer, the pH is probably determined by calcite and the hydroxyl groups at the edge sites of montmorillonite. The situation is less clear in the case of the Eh. One assumption is that it is determined by the chemical reaction between the dissolved oxygen in the clay porewater and the redox-active minerals like pyrite and iron oxides. MX-80 is a physical mixture of several components. The main component is montmorillonite clay (65-80 weight %), an expanding 2:1 clay mineral with a number of advantageous properties that serve its function as an effective buffer material. Other components are quartz (15%), feldspar (5-8%), carbonate (1%), sulfate (0.8%), phosphate (0.1%), fluoride (0.1%), sulfide (0.1-0.2%), organic matter (0.3%) and N-compounds (0.01%) [7]. The specific density of MX-80 is about 2.75 g/cm3 and the specific area of the montmorillonite component is ~750 m2/g. This means that a watersaturated buffer bentonite with a dry density of 1.5 g/cm3 includes water of about 45% of the volume. The water is organized between the montmorillonite layers as interlamellar water and in larger pores as external water. The external water consists of the water in the electrical double-
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