Use of 3D X-ray Computed Microtomography to Observe the Development of Sediment Structure and Colloidal Zirconia Deposits at the Pore Scale

We are using X-ray Micro-Tomography (XMT) to study in situ sediment structure using the facilities of the DuPont-Northwestern-Dow Collaborative Access Team (DND-CAT), Advanced Photon Source (APS), Argonne National Laboratory. Images of a sediment sample are taken at a number of different angles as the incident x-ray beam passes through it, and a three-dimensional view of the interior of the sample is then reconstructed from these maps using Computed Tomography (CT). These 3D images allow us to observe individual sediment grains as well as the pore structure with near-micron-scale resolution. We are also using difference tomography to resolve the distribution of zirconium in sediment cores. We performed experiments in a recirculating flume to observe the depositional structures resulting from both bed sediment transport and the stream-subsurface exchange of colloidal zirconia. A series of sediment cores were obtained from the streambed at the end of each experiment and transported to the APS for analysis. The tomographic reconstructions reveal the heterogeneous distribution of sediment grains resulting from sorting of sediments with different transport properties (e.g., because of differences in particle size or density). The 3D maps of deposited zirconia show the development of zirconia clusters within the pore space, and analysis of the distribution of zirconium along the length of the column reveals larger-scale deposition patterns. Zirconia accumulated to the greatest extent near the bed surface and generally decreased with depth in the bed. Relatively high concentrations of zirconia were also found in deeper layers of the bed, which could reflect either preferential deposition of zirconia colloids in these layers or the organization of zirconium-rich sand grains within the sediments. The structure of the zirconia deposits is related to the non-uniform packing of sediment grains as well as the pore water flow pattern. Future analysis will focus on the spatial correlation of pore structure, pore water flows, and zirconia deposits.