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

• Published in: Solid earth (Göttingen) Vol. 7; no. 4; pp. 1207 - 1215
• Main Authors: Kulenkampff, Johannes, Zakhnini, Abdelhamid, Gründig, Marion, Lippmann-Pipke, Johanna
• Format: Journal Article
• Language: English
• Published: Gottingen Copernicus GmbH 18.08.2016; Copernicus Publications
• Subjects: Attenuation; Biomedical research; Clay; Clay (material); Coefficients; Concentration gradient; Diffusion; Diffusion cells; Diffusion coefficients; Earth science; Elastic scattering; Experiments; Geometry; Geosphere; Heterogeneity; Image processing; Image reconstruction; Imaging; Imaging techniques; Membrane permeability; Methods; Molecular diffusion; Monitoring; Permeability; Pore size; Porosity; Positron emission; Positron emission tomography; Quantitative analysis; Radioactive tracers; Scanners; Simulation; Sodium 22; Species diffusion; Tomography; Tracers; Transport
• ISSN: 1869-9529; 1869-9510; 1869-9529
• DOI: 10.5194/se-7-1207-2016

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 1-100mm. 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 super(22) Na and super(124) I 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.