An experimental and modeling study was conducted to investigate Tc(VII) migration, reduction, and redox rate scaling in Fe(II)-containing sediments from a natural redox transition zone at US Department of Energy’s Hanford site, where <sup>99</sup>Tc is a major contaminant in unconfined aquifer and vadoze zone. Diffusion experiments revealed that the Tc(VII) migration was reductively retarded at variable degrees dependent on grain size composition and Fe(II) mineralogy in the sediments. Batch experiments with pristine and ground sediments revealed that the sand/gravel size fractions in the sediments were more reactive toward Tc(VII) reduction than the silt/clay size fractions. Modeling indicated that the rates of Tc(VII) reduction in the diffusion systems were, unexpectedly, much faster than those derived from the batch reactors. X-ray Computed Tomography (XCT) analysis, auto-radiographic imaging, tracer Br diffusion, and Mössbauer spectroscopy measurements were performed to provide insights into these observations. The unusual scaling behavior of Tc reduction rate was attributed to the combined effect of physical and mineralogical heterogeneity that led to the preferential association of reactive, coarser grains with faster diffusion paths in the sediments. The new findings of this study implied that the apparent rates of geochemical reactions may decrease or increase with scale dependent on physical and chemical heterogeneity and their combination.

Tc migration, reduction, and redox rate scaling </b>in Fe(II)-containing sediments from a natural redox transition zone at Hanford site

249th National Meeting (2015)

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