Abstract
AbstractAnisotropy and compositional and structural heterogeneity in clays are causes of considerable deviations from homogeneous diffusion, in particular in terms of direction-dependent transport rates and preferred transport zones. Conventional diffusion experiments, in which the sample is treated as a homogeneous black box in a concentration gradient, are interminable and insensitive to spatial effects. In contrast, tomographic imaging methods are capable of both reducing the amount of observation time required and revealing space-dependent features of the diffusion process.In the present study, positron-emission-tomography (PET) was applied as the most sensitive quantitative spatiotemporal tomographic modality for direct observation of positron-emitting radiotracers in opaque media at reasonable resolution (1 mm) on a laboratory scale (100 mm).Geoscientific applications of PET, or GeoPET, have revealed anisotropic and heterogeneous effects in diffusion experiments that have been conducted on Opalinus clay samples of different sizes, as well as on other rock types. Applying the Comsol Optimization Module to 2D-image sections of the PET tomograms, effective parameter values were derived, thereby quantifying the anisotropic diffusion.
Highlights
In the present study, positron-emission-tomography (PET) was applied as the most sensitive quantitative spatiotemporal tomographic modality for direct observation of positron-emitting radiotracers in opaque media at reasonable resolution (1 mm) on a laboratory scale (100 mm)
Geoscientific applications of PET, or GeoPET, have revealed anisotropic and heterogeneous effects in diffusion experiments that have been conducted on Opalinus clay samples of different sizes, as well as on other rock types
Diffusion in clays is based on Fickian diffusion (Altmann et al, 2012) of some species in a concentration gradient, restricted by the pore-space geometry of the material and the sorption of the diffusing species on the portion of the internal surface area that comes in contact with the species
Summary
Positron-emission-tomography (PET) was applied as the most sensitive quantitative spatiotemporal tomographic modality for direct observation of positron-emitting radiotracers in opaque media at reasonable resolution (1 mm) on a laboratory scale (100 mm). Geoscientific applications of PET, or GeoPET, have revealed anisotropic and heterogeneous effects in diffusion experiments that have been conducted on Opalinus clay samples of different sizes, as well as on other rock types. Molecular diffusion is the dominant transport mechanism of dissolved species in clays, complemented by species interactions with the large internal surface area along the propagation pathways. Diffusion in clays is based on Fickian diffusion (Altmann et al, 2012) of some species in a concentration gradient, restricted by the pore-space geometry of the material and the sorption of the diffusing species on the portion of the internal surface area that comes in contact with the species. These multi-scale material characteristics cause complex diffusion pathways that are difficult to predict with simple homogeneous models. In the range below the centimetre scale, which is the size of the largest
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