Caesium and strontium diffusion through sodium montmorillonite at elevated temperature

Abstract

Counter ionic migration rates of dilute Cs' and Sr2+ against sodium in a Namontmorillonite gel are measured using a radially perfused diffusion cell. Enhanced or surface diffusion for caesium is observed both at ambient and elevated temperature. To quantify these surface diffusivities the ion-exchange properties of the diffusing species are required. Thus, exchange isotherms for caesium against sodium on montmorillonite at 22 and 90°C are given for lo-', lo-', and M background NaCl concentrations. Caesium counterion surface diffusivities at 22 and 90°C are 2.2 x and 8.0 x cm2 s-', respectively. These values are found to be essentially independent of background ionic strength over the range to lo-' M NaCI. Experimental evidence of enhanced transport through Na-montmorillonite for the divalent cation Sr2+ is also confirmed with a surface diffusivity of 2 x cm2 s-' at 22°C and in lo-' M NaCl. D E F I N I T I O N O F S Y M B O L S =gel cross-sectional area, cm2 =tank concentration, M =initial tank concentration, M =total gel concentration, M =interstitial gel concentration, M =inlet concentration to the diffusion cell, M =outlet concentration from the diffusion cell, M =overall diffusion coefficient, (D,/r2) (1 +(I -E) KDJED,), cm2 s -l =effective diffusion coefficient, D / a , cm2 s-' =molecular diffusion coefficient, cm2 s-' =radially perfused cell overall mass transfer coefficient, cm s-' =dimensionless ion-exchange isotherm slope =absolute permeability, m2 =length of gel sample, cm =volumetric flowrate, cm3 s-' = Laplace space variable, s =time, s =time of experiment, s =fluid volume of external tank, cm3 =cell dead volume, cm3 =axial position inside gel, cm =dimensionless axial position in gel, x / L = sorption retardation coefficient, (1 + (1 -E)K/E) = 4~~a D A l V k~, =porosity =clay density, g cm-3 = tortuosity ' 53

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

“…There was no evidence of surface diffusion of Cs +, which agrees with the results of Cho et al (1993a). Jensen and Radke (1988) concluded that surface diffusion was the dominant mechanism of Cs + and Sr 2+ transport in a Namontmorillonite gel with a porosity of 0.93; they reported surface diffusion coefficients of about 200 ~mZ/s for both diffusants at 22~ and 800 #m2/s for Cs + at 90~ In a bentonite compacted to a dry density of about 1.8 Mg/m 3, Muurinen et al (1985) obtained diffusion data for Cs + and Sr 2+ that are consistent with surface diffusion, i.e., diffusion coefficients did not decrease as sorption increased. They reported surface diffusion coefficients of about 8 ~tm2/s for Cs + and 0.5 /~mE/s for Sr 2+ .…”

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.…”

Molecular Simulation of Interlayer Structure and Dynamics in 12.4 Å Cs-Smectite Hydrates

“…Both natural and engineered attenuation strategies for these radionuclides make use of their adsorption and sequestration by the interlayer region of smectite clay minerals (1,2,(4)(5)(6)(7). In the typical engineered scenario for nuclear waste containment (2), compacted smectite-based backfill material is used as an environmental buffer separating waste containers from a host rock formation.…”