Günter Kahr

Bio: Günter Kahr is an academic researcher from École Polytechnique Fédérale de Lausanne. The author has contributed to research in topics: Adsorption & Montmorillonite. The author has an hindex of 8, co-authored 8 publications receiving 264 citations.

Thermal analysis of pyridine-treated sepiolite and palygorskite

Abstract: The thermal behaviour of sepiolite and palygorskite treated with pyridine (Py) was investigated by DTA, TG, DTG and EGA (by MS) under a flow of air and inert gases (N2 or Ar). The thermal analysis showed that the organic base was adsorbed by the clays, penetrating into the clay pores and replacing zeolitic and bound water. Its presence in the pores shifted the dehydroxylation peak to temperatures higher than the corresponding peak in the untreated clays. Under N2 and Ar, thermal desorption of Py from sepiolite was observed at 260 and 650°C whereas for palygorskite only traces of Py were detected in the evolved gases. In addition to desorption, reactions of pyrolysis and condensation to coke were detected by the evolution of ammonia, methane and hydrogen, respectively. In air, oxidation of the organic matter led to the appearance of exothermic peaks. The profile of the DTA curve and the temperatures of the most intense exothermic peaks were determined by the rate of the carbon oxidation.

Clay mineralogical investigations related to nuclear waste disposal

Abstract: Mineralogical and geotechnical investigations on the possible use of compacted bentonite as a buffer material in nuclear waste repositories are reported. The swelling capacity is highly dependent on the density of the compacted bentonite. Swelling pressures >30 MPa were measured for dry densities of ∼2.0 g/cm 3 . Added iron or magnetite powder up to 20 wt% had no influence on the swelling capacity. Compacted mixtures of 20 wt% ground set cement and bentonite showed higher swelling pressures but lower swelling strain capability than compacted bentonite alone. Steam lowered the swelling pressure of compacted bentonite to ∼60% of the original value. The influence was, however, reversible by ultrasonic treatment. The thermal conductivity of saturated compacted bentonite at a density of 2.0-2.1 g/cm 3 is ∼1.35-1.45 W/m°K. The volumetric heat capacity ranges from 3.1 x 10 6 to 3.4 x 10 6 J/m 3 °K. The saturated hydraulic conductivity of the compacted bentonite is <10 -12 mis. The apparent diffusion coefficients for various ions in compacted bentonite for water contents in the range of 20 to 25 wt% are: K + : 5 x 10 -11 , Cs + : 6 x 10 -12 , Sr 2+ : 3 x 10 -11 , UO 2+ 2 : <10 -13 , Th 4+ : <10 -13 , Fe 2+ : 4 x 10 -11 , Fe 3+ : 4 x 10 -11 , Cl - : 1 x 10 -10 , and I - : 1 x 10 -10 m 2 /s. The 'breakthrough time' for an apparent diffusion coefficient of 10 -11 m 2 /s in compacted bentonite 1 m thick was estimated to be ∼3000 years. The mineralogical longevity was investigated on natural K-bentonites from Kinnekulle, Sweden, and Montana, USA. Although these materials have undergone considerable changes during diagenesis and contain various amounts of mixed-layer illite-smectite, they still have a substantial swelling and adsorption capacity. The investigations demonstrate that although the properties of bentonite are negatively influenced to a certain extent by heat, hot steam, iron and cement, compacted bentonite is still the best choice to act as a buffer material in a nuclear waste repository.

Swelling pressures of compacted bentonites from diffuse double layer theory

Abstract: The swelling pressures of several compacted bentonites (MX80, Febex, and Montigel) proposed for use as barrier materials in storing high-level radioactive waste in many countries were determined fr...