Facilitated Transport of Charged Colloids in Geologic Media
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FACILITATED TRANSPORT OF CHARGED COLLOIDS IN GEOLOGIC MEDIA
SHINYA NAGASAKI, SATORU TANAKA AND ATSUYUKI SUZUKI The Department of Quantum Engineering and Systems Science, The University of Tokyo, Tokyo 113, JAPAN
ABSTRACT Diffusion of a charged colloidal particle in a two-dimensional simple shear flow was studied by means of Monte Carlo calculations and the effects of a bounding wall and charge of a particle on convective diffusion were elucidated. Taking charge effects into account has a marked effect on the diffusion behavior of the particle, increasing the migration distance. Diffusion of latex colloidal particles in a quartz-powder packed cell was also studied by through-diffusion methods. For the large latex colloidal particles, the effective diffusion coefficients measured are larger than those estimated by Stokes-Einstein equation.
INTRODUCTION Many radionuclides are readily sorbed on immobile solid phases and therefore are considered to be virtually immobile in the geosphere. In soil and aquifer material, many radionuclides bind strongly to mineral components. However, colloids may be mobile in the groundwaters. Since the chemical composition of colloids is expected to be similar to that of surfaces of immobile solid phases, these colloids could sorb radionuclides and stabilize them in the groundwaters. The migration of colloids in the engineered barrier limited by diffusion. In the natural barrier, there is a possibility that the migration of colloids is controlled not only by advection but also by diffusion when the Pe number of groundwater is small. Therefore, it is important to understand the diffusion of radionuclide-sorbing colloids in the multi-barrier system. Failure to account for the diffusion of colloids can lead to underestimates of the distances that radionuclides will migrate. This article calls attention especially to diffusion of a charged colloidal particle near a charged wall. Many theoretical and experimental investigations on colloid diffusion have been carried out [1 - 5], but few studies describe the effect of charges of a colloidal particle and a wall on colloid diffusion. In the present work, we studied the coupling of Brownian displacement and shear-induced convection of a charged spherical colloidal particle in dilute suspension, and examined the influence of charges of the colloidal particle and the bounding wall on the convective diffusion of the colloidal particle in a two-dimensional shear flow by means of Monte Carlo calculations. We also studied experimentally the diffusion of colloidal latex particles in quartz-powder packed columns by through-diffusion methods.
Mat. Res. Soc. Symp. Proc. Vol. 353 01995 Materials Research Society
158
MONTE CARLO CALCULATIONS Diffusion of Non-charged Colloidal Particles In order to verify the validity of our Monte Carlo calculations used in the present work, we calculated the mean-square displacement A2 of colloidal particles undergoing translation diffusion in a two-dimensional shear flow. The mean-square displacement A2 is given as [
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