Using porous iron composite (PIC) material to immobilize rhenium as an analogue for technetium

Abstract

Technetium (⁹⁹Tc), a uranium-235 (²³⁵U) and plutonium-239 (²³⁹Pu) fission product, is a primary risk driver in low level radioactive liquid waste at U.S. Department of Energy sites. Previous studies have shown success in using Zero Valent Iron (ZVI) to chemically reduce and immobilize redox sensitive groundwater contaminants. Batch and column experiments were performed to assess the ability of a novel porous iron composite material (PIC) to immobilize Tc(VII) in comparison with two commercial Fe oxide sorbents and reagent grade ZVI in the presence and absence of NO₃^-, a competing oxidized species that is often found in high concentrations in liquid nuclear waste. Perrhenate (ReO₄^-) was used as a non-radioactive chemical analogue for pertechnetate (TcO₄^-) under both oxic and anoxic test conditions. The PIC powder was the most effective at immobilizing Re(VII) under all batch test conditions. The presence of nitrate (NO₃^-) slowed the removal of ReO₄^- from solution, presumably through chemical reduction and precipitation. Even so, the PIC and ZVI were effective at removing both Re(VII) and NO₃^- completely from solution. Nitrate was reduced to NH₃ with very little nitrite (NO₂^-) buildup during equilibration. Significant Re immobilization was observed in the column tests containing PIC sorbent, even though inlet solutions were in equilibrium with O₂. The presence of NO₃^- hastened Re breakthrough, while NO₃^- reduction to NH₃ was observed. The results suggest that PIC and ZVI would be the most effective at the removal of TcO₄^- from contaminated groundwater sites.

Keywords: Column experiment; Nitrate; Permeable reactive barriers; Rhenium; Technetium; Zero-valent iron.