Determination of Effective Diffusion Parameters in Compacted Kaolinite

Clay minerals are main components of many soils, sediments, pelitic rocks, as well as fracture filling material in crystalline rocks. Furthermore, clays are used in geo-engineering, particularly to design hydraulic and geochemical barriers in contaminated sites, landfills or underground repositories for toxic or nuclear wastes. Due to the low permeability of clay molecular diffusion is the main transport mechanism of dissolved or colloidal substances at natural hydraulic gradients. This process has to be studied to assess the long-term behavior of geoengineered barrier systems.
In order to determine effective transport parameters in kaolinite diffusion experiments with a conservative tracer were performed. Kaolinite from Hirschau (Germany) was filled in a diffusion cell (cross sectional area 78.6 cm2, layer thickness 1.8 cm), compacted to a dry density of 1.1 g cm-3 and fixed between two filter plates. The through-diffusion of tritiated water (HTO) was observed applying a tracer reservoir with a starting activity of 4·105 Bq in 100 mL. The tracer activities of the high and the low concentration side were measured in intervals by liquid scintillation counting.
An analytical solution of the one dimensional transport equation was developed to determine the diffusion coefficient and the effective porosity. Two variable boundary conditions accounted for the changing concentrations in the transient phase.
A diffusion coefficient of De = 3.2·10-10 m2s-1 and an effective porosity of e = 0.6 were determined from the measured concentration versus time. These results are compared with stationary diffusion experiments using reactive tracers (Cu2+, Zn2+ Pb2+, AsO43-) and natural sealing material (loess loam) and kaolinite (Hamad, 2003). For instance, in loess loam De was found to be 2.47, 3.04, and 4.14·10-10 m2s-1 for Cu, Pb, and Zn, respectively. These results indicate that the diffusion parameters in the natural and the engineered barrier material are in the same order of magnitude.