Quantitative experimental monitoring of molecular diffusion in clay with positron emission tomography

Clay plays a prominent role as barrier material in the geosphere. Their small particle sizes cause extremely small pore sizes and induce low permeability and high sorption capacity. Transport of dissolved species by molecular diffusion is less sensitive to the pore size. Heterogeneous structures on centimetre scale could cause heterogeneous effects, like preferential transport zones, which are difficult to assess. Laboratory measurements with diffusion cells yield limited information on heterogeneity, and pore space imaging methods have to consider scale effects. We established positron emission tomography (PET), applying a high-resolution PET-scanner, as spatially resolved quantitative method for direct laboratory observation of the diffusion process of a PET-tracer on the prominent scale of 1 to 100 mm. Although PET is rather insensitive to bulk effects, quantification required significant improvements of the image reconstruction procedure with respect to Compton scatter and attenuation. The experiments were conducted with 22Na and 124I over periods of 100 resp. 25 days. From the images we derived trustable anisotropic diffusion coefficients and, in addition, we identified indications for preferential transport zones. We thus demonstrated the unique potential of the PET imaging modality for geoscientific process monitoring under conditions where other methods fail, taking advantage of the extremely high detection sensitivity that is specific to radiotracer applications.