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Diffusion of Iodide in Compacted Bentonite
Author(s) -
Oscarson D. W.,
Hume H. B.,
Sawatsky N. G.,
Cheung S. C. H.
Publication year - 1992
Publication title -
soil science society of america journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.836
H-Index - 168
eISSN - 1435-0661
pISSN - 0361-5995
DOI - 10.2136/sssaj1992.03615995005600050011x
Subject(s) - tortuosity , bentonite , diffusion , retardation factor , porosity , chemistry , analytical chemistry (journal) , effective diffusion coefficient , materials science , mineralogy , thermodynamics , chromatography , geology , composite material , geotechnical engineering , medicine , physics , column chromatography , radiology , magnetic resonance imaging
Diffusion coefficients, D , are critical parameters for predicting migration rates and fluxes of contaminants through dense bentonite‐based barrier materials used in many waste containment strategies. Values of D were determined for I ‐ ( 129 I is a relatively long‐lived radionuclide present in high‐level nuclear fuel waste) in saturated bentonite using both transient and steady‐state techniques. The bentonite was compacted to dry densities, ρ b , ranging from 0.9 to 1.6 Mg m −3 , and saturated with a synthetic groundwater solution having an ionic strength of 0.22. Two different D values were determined: an apparent diffusion coefficient, D a , defined as D 0 τ, and an effective diffusion coefficient, D e , defined as D o τ n e where D o is the diffusion coefficient in pure bulk solution, τ the tortuosity factor, and n e the fraction of the porosity of the saturated clay that is available for diffusion. The value of D a decreased from ≈6 × 10 −10 m 2 s −1 at ρ b ≈ 0.9 Mg m −3 to 1 × 10 −10 m 2 s −1 at 1.6 Mg m −3 . The decrease in D a with increasing ρ b is attributed to a decrease in τ as ρ b increased. The effect of ρ b on D e was even greater: D e decreased from ≈ 6 × 10 −11 m 2 s −1 at ρ b ≈ 0.9 Mg m −3 to 3 × 10 −12 m 2 s −1 at 1.6 Mg m −3 . In addition to tortuosity effects, this decrease is ascribed to a decrease in n e with increasing ρ b . For I ‐ , n e is generally less than n (the total solution‐filled porosity of the clay) because of factors such as anion exclusion. Within the ρ b range examined, the migration time for 129 I to move through the barrier material can be increased somewhat by increasing ρ b ; relative to the long half‐life of 129 I, however, the increased migration time is not significant. On the other hand, there appears to be a critical ρ b value for this clay of ≈1.4 Mg m −3 , beyond which the flux of 129 I from the clay can be markedly decreased. This critical value is probably a function of the specific surface area of the clay inasmuch as it influences the magnitude of the anion‐exclusion volume. By compacting a bentonitic barrier material to a density greater than the critical value, the potential hazard associated with the long‐term disposal of nuclear fuel waste that contains 129 I can be decreased.