Examination of Technetium Transport Through Soils Under Contrasting Redox Conditions: Batch and Column Work

Abstract:

Experiments were performed under various reducing conditions to evaluate the transport behavior of technetium-99 (⁹⁹Tc) in the presence of sandy clay loam soil from the Savannah River Site (SRS) and goethite, magnetite, and iron sulfide, which were selected for their increasing reducing potential. The experiments were conducted to investigate how redox reaction equilibria and rates affect the overall mobility of ⁹⁹Tc as it transitions between the mobile Tc(VII) and immobile Tc(IV). Under oxygen-rich conditions, batch sorption isotherms measured for TcO₄^- across the concentration range 0.5 to 50 μg/L were linear with distribution coefficients (K_d) of 0.78 mL/g or lower, with decreasing sorption for goethite, magnetite, and iron sulfide, respectively. Addition of Na₂S resulted in a marked increase in apparent ⁹⁹Tc sorption to the solid phase, with K_d of 43 mL/g, 35 mL/g, and 29 mL/g, following the same mineral trend as previously. The increased K_d values are possibly due to reduction of Tc(VII) to Tc(IV), resulting in the formation of TcO₂(s). SRS soil batch sorption isotherms measured for TcO₄^- across the same concentration range were also linear, with K_d of 0.7 mL/g for unadjusted pH, 5.1 mL/g for pH of around 6, and 6.7 mL/g for pH of around 4. Kinetic batch sorption tests showed less than 10% ⁹⁹Tc sorption in an oxidizing environment and greater than 95% sorption in a reducing environment, with both reactions occurring on the order of minutes. In contrast, desorption experiments initiated by transferring the samples from a reducing environment (0.1% H₂(g)/99.9% N₂(g)) to atmospheric conditions resulted in a slow desorption step on the order of days. Column experiments conducted with the SRS sands indicate a retardation factor of 1.17 for ⁹⁹Tc under oxygen rich conditions. Additional column experiments are being conducted to evaluate ⁹⁹Tc transport dependencies on transitions between oxygen rich and poor conditions.