Articles | Volume 7, issue 4
Research article
18 Aug 2016
Research article |  | 18 Aug 2016

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

Johannes Kulenkampff, Abdelhamid Zakhnini, Marion Gründig, and Johanna Lippmann-Pipke

Abstract. Clay plays a prominent role as barrier material in the geosphere. The small particle sizes cause extremely small pore sizes and induce low permeability and high sorption capacity. Transport of dissolved species by molecular diffusion, driven only by a concentration gradient, is less sensitive to the pore size. Heterogeneous structures on the 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 a spatially resolved quantitative method for direct laboratory observation of the molecular diffusion process of a PET tracer on the prominent scale of 1–100 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 and 25 days, respectively. From the images we derived trustable anisotropic diffusion coefficients and, in addition, we identified indications of 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 typical of radiotracer applications.

Short summary
Clay is the prominent barrier material in the geosphere, but diffusion of dissolved species is possible. Diffusion parameters are commonly determined on small samples, disregarding heterogeneity. With positron emission tomography (PET), we monitored heterogeneous transport patterns on larger samples. From the time dependence of the spatial tracer distribution, we derived reliable anisotropic diffusion coefficients, and found indications of preferential transport zones.