(99)Tc(VII) Retardation, Reduction, and Redox Rate Scaling in Naturally Reduced Sediments

Area of Science:

• Environmental Science
• Geochemistry
• Nuclear Chemistry

Background:

• Technetium-99 ((99)Tc) is a major groundwater contaminant at the U.S. Department of Energy's Hanford site.
• Pertechnetate (Tc(VII)O4(-)) is the mobile and dominant species of (99)Tc in oxic groundwater.

Purpose of the Study:

• Investigate the retardation, reduction, and rate scaling of Tc(VII) migration in Hanford site sediments.
• Determine the influence of sediment properties, particularly reactive iron (Fe(II)), on Tc(VII) behavior.
• Compare Tc(VII) reduction rates and migration behavior between batch and column experimental setups.

Main Methods:

• Experimental study using batch reactors and diffusion columns with three Ringold formation sediments.
• Chemical analysis to quantify HCl-extractable Fe(II) concentrations.
• X-ray computed tomography and autoradiography to visualize Tc(VII) reduction and transport paths.
• Modeling to understand reaction rate scaling.

Main Results:

• Tc(VII) was reduced in all tested sediments under both batch and column conditions.
• A higher concentration of HCl-extractable Fe(II) correlated with faster Tc(VII) reduction rates.
• Tc(VII) migration was reductively retarded, with retardation degrees linked to reduction rates.
• Observed faster reduction rates in diffusion columns compared to batch reactors, attributed to mineral distribution.
• Spatial imaging confirmed Tc(VII) reduction locations and transport paths.

Conclusions:

• Tc(VII) migration at the Hanford site can be significantly retarded by reductive processes.
• The degree of retardation is dependent on the content and spatial distribution of reactive Fe(II) in the sediments.
• Reaction rates for Tc(VII) reduction can be faster in dynamic transport systems (columns) than in static, well-mixed batch reactors, highlighting the importance of experimental setup for accurate rate determination.