Diffusion of 1-naphthol and naphthalene through clay materials: Measurement of apparent exclusion of solute from the pore space

Diffusion is fundamental in controlling movement of organic contaminants in clay textured materials. Diffusion coefficients for 1-naphthol and naphthalene were measured for a weathered shale based on the breakthrough time and on the steady state flux of solute through the clay barrier. Steady state solute flux through the clay barrier was 9 to 35 times lower than that predicted based on breakthrough diffusion coefficients within the clay barrier. Analysis of napthalene concentration in the pore solution of a non-sorbing medium (montmorillonite clay) showed that the solute concentration in the clay pores was only 40% of that in the free solution suggesting that the clay barrier was restricting the entry of the solute into the clay pore space. Results suggest that the clay can act to exclude the diffusing species from the pore space, reducing the effective solute concentration gradient within the clay cell. This will act to further enhance the ability of clay materials to limit contaminant flux.