Nanoparticles as X-ray CT Imaging Contrast Agents in Saturated Porous Media

Parallels exist between medical imaging techniques, such as X-ray computed tomography (CT) or magnetic resonance imaging, and geophysical methods used to analyze the subsurface, such as ground penetrating radar, electromagnetic induction, magnetic susceptibility, or nuclear magnetic resonance. These methods measure a property (or properties) of electromagnetic radiation as it travels through a porous medium, which in turn allows information about the area of interest (i.e., a patient’s body or subsurface feature) to be gathered. The use of nanoparticles (NPs) as imaging contrast agents for electromagnetic detection methods is well established in the medical industry but has not been part of the toolbox used to characterize contaminated sites. \nX-Ray CT measures differences in X-ray attenuation between two or more materials. The density and the effective atomic number of the material the X-rays are passing through influence attenuation. Thus, in theory, NPs comprised of elements with higher atomic numbers than geologic materials and water should attenuate X-rays more strongly and, therefore, should be distinguishable from the background material. The objective of this research was to evaluate the ability of X-ray CT to monitor the transport of engineered NPs in saturated geologic porous media at a typical column scale. The research findings serve to demonstrate the use of NPs with X-ray CT as a potentially valuable tool to assess hydrodynamic behavior at the bench-scale, and as a proof-of-concept for the use of NPs as imaging contrast agents for field scale EM geophysical techniques. \nA 240 kV, GE Phoenix v|tome|x m compact micro CT system was used with three column designs: glass column (15.24 cm long, 2.54 cm inner diameter), acrylic column (10.16 cm long and 0.9 cm internal diameter), and acrylic cuvettes (4.5 cm long, 1 cm by 1 cm cross section). Columns were packed with 150-212 µm acid-washed glass beads and saturated with Milli-Q water. This study assessed the use of two NP solutions (Fe3O4 and bismuth ferrite) as X-ray CT contrast agents for use in saturated porous media columns in comparison to a known contrast agent, NaI. \n\tThe minimum concentration of NaI needed to attenuate the X-ray beam enough to be distinguished in processed X-ray CT images was found to be 15 gI/L in the saturated glass column and acrylic cuvettes. Qualitative comparisons of the relative X-ray attenuation between NaI and the NP solutions were made by scanning two stacked cuvettes, one containing a 15 gI/L NaI solution and the other containing the respective NP solutions. Results showed that the X-ray attenuation due to Fe3O4 NPs was inadequate when used at concentrations < 70 g/L. At this concentration, the Fe3O4 NP suspension was too viscous for transport through the column, and thus ineffective as X-ray CT imaging contrast agents in a saturated glass bead porous medium. The bismuth ferrite NP solution had a higher relative X-ray attenuation than the 15 gI/L solution. These bismuth ferrite NPs were injected at a concentration of ~7.8 gBi/L into a saturated glass column, and the monitoring of NP transport was successful. \nThis study marks the first use of engineered NPs as X-ray CT imaging contrast agents in a saturated representative geologic porous medium at the column scale. NPs offer the potential benefit of target-specific binding to impacted soil media in comparison to traditional X-ray CT contrast agents such as NaI, thus opening exciting opportunities for future geological X-ray CT studies. As X-ray CT is an electromagnetic imaging technique, the work presented here serves as a proof-of-concept for the use of NPs as imaging contrast agents for electromagnetic geophysical methods.

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