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

Abstract

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.

Hexavalent Uranium Diffusion into Soils from Concentrated Acidic and Alkaline Solutions

“…Surface diffusion has been reported for Cs + in smectite clays (17,18), and Sr 2+ in smectites and granite (17)(18)(19). On the other hand, the results of Oscarson (15) indicate that surface diffusion has a relatively minor contribution for Sr 2+ , Ca 2+ , and Na + transport in smectite. To our knowledge, no studies have been published that evaluate the significance of U(VI) surface diffusion in soils.…”

Ionic Diffusivity, Electrical Conductivity, Membrane and Thermoelectric Potentials in Colloids and Granular Porous Media: A Unified Model

“…These properties are of interest in chemistry of colloids and fields such as material, geophysical, and environmental sciences (3)(4)(5)(6)(7). In this paper, the transport of ions in a representative elementary volume (REV) of a granular porous material is investigated.…”

A Technical Problem in the Through-Diffusion Experiments for Compacted Bentonite

“…Consequently, the surface diffusion hypothesis can explain the observed salinity dependence of D e . However, Oscarson 3) and Choi and Oscarson 9) reported that there was no positive relationship between K d and D e for a different type of bentonite media. As they also used the through diffusion system with circulation, there are probably other explanations for the observed differences.…”

“…2(a)), tracer and tracer-free solutions were circulated individually in the porous plastic filter on each side of the specimen. The flow rate controlled by the tube pump was several cm 3 per h. A membrane filter (pore size 0.22 µm, ca. 0.1 mm in thickness) was placed between the porous filter and bentonite to restrict the leaching of bentonite during the experiment.…”

“…Several groups have reported that the D e for cations such as Sr and Cs decreased with increasing salinity, and the obtained D e value was greater than expected on the basis of the pore-water diffusion model. [1][2][3] The surface diffusion hypothesis invoked to explain the above diffusion behavior was a key issue in describing mass transport of the sorbing species in bentonite. [1][2][3] In a previous study, 4) we found a different dependence of D e on salinity from that reported previously 1,2) as shown in Fig.…”