Technetium Retention and Remobilization Potential after Prolonged Reaction in Fe(III)- and Sulfate-Reducing Model...

Area of Science:

• Environmental Science
• Geochemistry
• Radiochemistry

Background:

• Technetium-99 (99Tc) is a mobile, long-lived radioactive fission product.
• Subsurface mobility of 99Tc is controlled by redox conditions, with soluble Tc(VII)O4- under oxic conditions and poorly soluble Tc(IV) phases under reducing conditions.
• Previous studies reported TcO2-like solids and Tc(IV)-sulfides, but the stability of Tc(IV)-sulfides remains unclear.

Purpose of the Study:

• To investigate the speciation and stability of 99Tc in flowing sediment columns under simulated subsurface conditions.
• To determine the long-term fate and mobility of 99Tc under varying redox conditions, including Fe(III)- and sulfate-reducing environments.

Main Methods:

• Flowing sediment column experiments using Sellafield subsurface sediment.
• Addition of 99TcO4- and incubation under reducing conditions for ~1 year.
• Analysis of Tc speciation using X-ray absorption spectroscopy and sequential extractions.
• Simulated oxic groundwater pumping to assess remobilization.

Main Results:

• >90% of added 99TcO4- was retained under both Fe(III)- and sulfate-reducing conditions.
• TcO2-like phases dominated Fe(III)-reducing columns, while Tc(IV)-sulfides dominated sulfate-reducing columns.
• Tc in sulfidic sediments was more recalcitrant than in Fe(III)-reducing systems.
• Remobilization during oxic pumping was higher from Fe(III)-reducing columns (~50%) compared to sulfidic columns (~25%).
• Tc(IV)-sulfides gradually oxidized to TcO2 phases under oxic conditions.

Conclusions:

• Tc(IV)-sulfides can enhance 99Tc retention under sustained sulfate reduction.
• TcO2 phases are likely the dominant control on 99Tc mobility during long-term redox cycling and oxidation events.
• Findings provide crucial data for modeling 99Tc fate at contaminated sites and in radioactive waste disposal.