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Technischer Bericht NTB 91-28

Diffusion von Eisen- und Jodid-Ionen in hochverdichtetem Bentonit

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The diffusion coefficients of iron(II) and iodide ions in the Na-bentonite MX-80 and Ca-bentonite Montigel were determined as non-stationary apparent diffusion coefficients under dynamic conditions using special diffusion apparatus.

For dry densities around 16 kN/m3 or saturation water contents of 25 %, the apparent diffusion coefficients of iron(II) ions at ambient temperature are in the order of 8 × 10-13 m2/s to 7 × 10-12 m2/s. An exchange of free cations in the bentonites, i.e. Ca ions rather than Na ions in the MX-80 and Na ions rather than Ca ions in the Montigel, had no effect on the diffusion coefficients of the iron(II) ions. For dry densities of around 17 kN/m3 or water contents of around 20 %, the apparent diffusion coefficients of iodide ions are 2 × 10-11 m2/s to 4 × 10-11 m2/s at ambient temperature and 3 × 10-11 m2/s to 5 × 10-11 m2/s at 150°C. These values are around an order of magnitude smaller than those determined by KAHR et al. (1985) using free-standing, compacted, non­saturated bentonite samples. At ambient temperature, diffusion coefficients of 1 × 10-10 m2/s to 4 × 10-10 m2/s were determined for iodide ions in such experiments.

The apparent diffusion coefficients for iron(II) and iron(III) ions were also measured for free-standing, compacted, non-saturated bentonite samples (MX-80 and Montigel). With water contents of around 17 %, the diffusion coefficients for iron(II) ions at ambient temperature are 3 × 10-11 m2/s to 4 × 10-11 m2/s; with water contents of around 27 %, the figure is 6 × 10-11 m2/s. These values are thus around an order of magnitude above those determined using the diffusion apparatus. With water contents of around 28 %, the diffusion coefficients for iron(lII) ions are 8 × 10-11 m2/s.

The differences between the results for diffusion coefficients for the two measuring methods (diffusion apparatus versus free-standing samples) were a maximum of one order of magnitude for both iron(II) and iodide ions. The larger values for diffusion coefficients were determined for the free-standing samples.

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