Experimental evaluation of the influence of saturation media on the mineralogical and physicochemical stability of bentonites

Abstract Three bentonite buffer materials (Rokle bentonite, FEBEX bentonite, and Mock-Up-CZ mixture composed of 85% Rokle bentonite, 10% silica sand and 5% graphite) interacted with natural water collected from Josef Underground Educational Facility (UEF) Czech Republic, and four different types of artificial groundwater enriched in K + and/or Mg 2 + . The experimental material was prepared in the form of (1) highly compacted samples, (2) dispersion of bentonites (both at temperature of 95 °C), (3) drill hole filling during in situ tests at the Josef UEF (at ordinary temperature), and (4) common batch laboratory experiments (at 20 °C and at 80–90 °C). Partial samples were extracted after 3, 6, 12, and/or 18 months of interaction in the first three set-ups, and after 1 week, 2 weeks, 1 month, 2 months, 3 months, 4 months, 5 months, 7 months, 10 months, and 12 months in the batch experiment. Phase composition and its potential changes were evaluated by X-ray diffraction. In the first three experimental set-ups, no changes were identified. For the batch experiment, formation of illite was detected in FEBEX bentonite saturated with artificial groundwater with K + concentration of 1083 mg/L. By using the same saturation medium, gypsum and/or bassanite formed in the Rokle and FEBEX bentonite. Other media did not affect mineralogical composition. The stability of studied materials was also evaluated by using adsorption and through-diffusion experiments on the basis of migration behaviour of 134 Cs. Dispersion of bentonites at laboratory temperature and bentonite samples from in situ test were used as solid phase, and four types of saturation media (artificial groundwater) were used as liquid phase. The distribution coefficient (Kd) is influenced by high concentration of K + and Mg 2 + in saturation media on retention of Cs + . In Rokle bentonite, the increase concentration of K + and Mg 2 + led to a decrease of Kd. In the case of FEBEX bentonite influence of composition of saturation media was not observed. The diffusion experiments showed good retardation ability of Rokle and FEBEX bentonites. The values of apparent diffusion coefficient ( D a ) of cesium were influenced by composition of saturation media only for Rokle bentonite.