Diffusion of Iodide in Compacted Bentonite

Oscarson, D. W.; Hume, Harold B.; Sawatsky, N.; Cheung, S.C.H.

doi:10.2136/sssaj1992.03615995005600050011x

Cited by 58 publications

(22 citation statements)

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“…Berry and Bond (1992) reported a significantly lower value of,I' from I-(0.012) than from HTO (0.096) experiments in London clay. In Avonlea bentonite at p 1.3 Mg/m 3, both the geometry factor and effective porosity are about 0.1 (Oscarson et al 1992); since 't~ is the product of the geometry factor and porosity, xI, would be about 0.01 in this clay, similar to the value reported by Berry and Bond (1992). If a q value of 0.01 is used in Eq.…”

Section: Total Intrinsic Diffusion Coefficientssupporting

confidence: 72%

Surface Diffusion: Is It an Important Transport Mechanism in Compacted Clays?

Oscarson

1994

Clays and clay miner.

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Abstract--Surface diffusion, or migration within the electrical double layer next to mineral surfaces, is often invoked as a significant contributor to the overall diffusion coefficient in compacted clays, particularly where model predictions underestimate measured diffusion coefficients. The potential for surface diffusion of Sr 2 § Ca 2+ and Na + on three clays compacted to dry bulk densities of 1.25 and 1.60 Mg/m 3 was examined. The clays were a bentonite, an illite/smectite, and a glacial lake clay (composed mainly of smectite, illite, kaolinite and quartz). The clays were saturated with a Na-Ca-Cl-dominated synthetic groundwater solution with an effective ionic strength of 220 mol/m 3. Total intrinsic diffusion coefficients for the cations were determined from their steady-state flux through compacted clays, and apparent diffusion coefficients were obtained from the time-lag technique. Models of diffusive transport in compacted clays, based only on diffusion in the pore solution, adequately described the diffusion data for all clays and diffusants, and there was no need to invoke other transport mechanisms, like surface diffusion. The data indicate that surface diffusion is not a significant transport mechanism in compacted clays at least to a clay density of 1.60 Mg/m 3.

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Section: Total Intrinsic Diffusion Coefficientssupporting

confidence: 72%

Surface Diffusion: Is It an Important Transport Mechanism in Compacted Clays?

Oscarson

1994

Clays and clay miner.

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“…Typical concentration profiles for I with each of the four clays are shown in Fig. 2, the shape of these profiles is consistent with molecular diffusion as given by Fick's second law (Oscarson et al, 1992;Robin et al, 1987). All measured D values, Din, are given in Table 2.…”

Section: Resultssupporting

confidence: 64%

“…As ne < n, the use of n rather than n e in eqn. 2 means D c is somewhat overestimated, but at most by less than a factor of 5 (Oscarson et al, 1992). Because of the properties of the clays, such as the specific surface area and particle size, the overestimation would probably be greatest for bentonite and lowest for kaolinite.…”

Section: Discussionmentioning

confidence: 97%

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Comparison of measured and calculated diffusion coefficients for iodide in compacted clays

1995

International Journal of Rock Mechanics and Mining Sciences &am

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A B S T R A C T: An understanding of the behaviour of contaminants in compacted clays is important in assessing the effectiveness of clay-based barrier materials used in many waste containment strategies. Here the diffusion an d sorption behaviour of I-with selected compacted clays is examined (129I is a relatively longlived radioisotope present in high-level nuclear fuel waste.) Diffusion coefficients, D, and distribution coefficients, Ka, were measured for I with four clays: bentonite, Lake Agassiz clay (a glacial lake clay composed mainly of smectite, illite, kaolinite, and quartz), interstratified illite-smectite, and kaolinite. For the diffusion experiments the clays were compacted to a dry bulk density, p, of -1.2 Mg m -3. The mean measured D values, Din, were as follows: bentonite, 310 ~tm 2 s-l; Lake Agassiz clay, 0.62 ~m 2 s 1; illitesmectite, 190~tm2s a; and kaolinite, 74~tm2s l. The measured values were compared with those calculated, De, from the following model: D = Do~an/(n + pKd), where D O is the diffusion coefficient in pure bulk water, ra the apparent tortuosity factor, and n the solution-filled porosity of the clay. Except for the Lake Agassiz clay, where Dm was about an order of magnitude lower than De, the Dm and D~ values agreed within a factor of about two. The Lake Agassiz clay has a markedly higher organic carbon content than the other three clays, and this could affect ra, which may be overestimated in the model.

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“…The porosity we have termed accessible porosity would be equal to the sum of ne and ns. From through-diffusion experiments with I-and Cs + in Avonlea bentonite at p ~ 1.35 Mg/m 3, n, has been estimated to be about 0.1-or 20% ofn (Oscarson et al 1992;Cho et a! 1993).…”

Section: Discussionmentioning

confidence: 99%

Sorption of Cesium on Compacted Bentonite

Oscarson

1994

Clays and clay miner.

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Abstract--Sorption parameters are important components of models used to predict mass transport through dense or compacted earthen materials. These parameters are, however, generally determined in batch tests with loose, unconsolidated materials. Here we directly measure, using a specially designed cell, the extent ofCs + sorption on bentonite compacted to a series of densities ranging from 0.50 to 1,50 Mg/m 3, and compare the results with those obtained from batch tests with loose bentonite. The clay was saturated with a Na-Ca-Cl-dominated solution with an effective ionic strength of 220 mol/m 3. The sorption data were expressed as distribution coefficients, I~. Over the clay density range examined, Ko values for Cs + with compacted clay are about one-half to one-third the value of those with loose clay. The lower sorption on compacted clay is attributed to small and occluded pores that Cs + cannot enter; thus it cannot access the entire volume, or all the sorption sites, of compacted clay. The data suggest that reasonable estimates of K~ with compacted clay can be obtained by scaling down the I~ values measured on loose clay by a factor na/n, where n a is the accessible porosity and n the total porosity of compacted clay.

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Diffusion of Iodide in Compacted Bentonite

Cited by 58 publications

References 0 publications

Surface Diffusion: Is It an Important Transport Mechanism in Compacted Clays?

Surface Diffusion: Is It an Important Transport Mechanism in Compacted Clays?

Comparison of measured and calculated diffusion coefficients for iodide in compacted clays

Sorption of Cesium on Compacted Bentonite

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