Analysis of the porewater chemical composition of a Spanish compacted bentonite used in an engineered barrier

Abstract

Compacted bentonites are being considered in many countries as a backfill material in high-level radioactive waste disposal concepts. A knowledge of the porewater chemistry in the clay barrier is essential since the porewater composition influences the release and transport of the radionuclides. However, quantification of the water chemistry in compacted bentonite under repository conditions is difficult. The methodology followed to obtain the porewater composition of the FEBEX bentonite is described in this paper. It is based on the characterisation of the solid phase, determination of the physico-chemical properties of the montmorillonite component and geochemical modelling. The FEBEX bentonite has a high cation exchange capacity (∼1 eq/kg), high surface area (∼725 m 2/g total surface area and 62 m 2/g external surface area) and accessory minerals such as carbonates, sulphates, pyrite, etc.; and organic matter. The chloride inventory in the FEBEX bentonite is ∼22 mmol/kg. The montmorillonite, together with the other mineral phases present, will determine the composition of the porewater. However, in order to calculate a unique aqueous chemistry, two further quantities are required, the chloride concentration and the pH. Water vapour adsoption/desorption isotherms, together with c-lattice spacing determinations, were used to identify the different states and location of water. Most of the water in the as received bentonite resides in the interlayer space. However, the measurements indicate that about 0.053 l/kg may be regarded as free water, implying a chloride concentration of 0.42 M. The pH of the system is fixed by equilibrium with the atmosphere ( P CO 2 =10 −3.5 bar) and saturation with the carbonate phases present. The porewater calculated to be in equilibrium with the as received FEBEX bentonite powder is a Na–Ca–Mg chloride type with a high ionic strength, 0.66 M, and a pH of ∼7.4. Likewise, in order to calculate the porewater composition of compacted re-saturated bentonite, the volume of free water is required. This value is taken to be the chloride accessible porosity obtained from Cl − through-diffusion tests (due to anion exclusion, Cl − anions can not move through the interlayer and overlapping double layer regions). The amount of free water in compacted bentonite determined in this manner was 0.03 l/kg at a dry density of ρ d=1650 kg/m 3. The corresponding chloride concentration is thus ∼0.73 M. Arguments are presented that the initial pH is fixed in the compacted material by the high buffering capacity afforded by the amphoteric edge sites, SOH sites, of the montmorillonite. The pH of the porewater depends directly on the speciation of these sites, i.e., the proportions of sites present as SOH, SOH 2 +, SO −, and this is fixed in the powdered source material through equilibration with air (compaction will not alter the state of the SOH sites). The porewater of compacted FEBEX bentonite at ρ d=1650 kg/m 3 was calculated to be a Na–Ca–Mg chloride type with a high ionic strength, 0.90 M, and a pH of ∼7.4.