Competitive Reduction of Pertechnetate and Chromate in Radioactive Waste Streams via Zero-Valent Iron

Abstract

To prevent environmental release and exposure, it is essential to immobilize radioactive waste for long-term storage and disposal. One waste constituent, long-lived radionuclide technetium-99 ( 99 Tc), which is primarily produced during the irradiation of uranium fuels in nuclear reactors, represents a significant risk to human health and the environment. Zero-valent iron (ZVI) has shown promise in reducing highly soluble Tc(VII) into a less soluble Tc(IV) form. However, the presence of other redoxreactive elements can compete with Tc in the reduction process, potentially impeding its treatment. Cr(VI) is a hazardous, redoxactive contaminant commonly found at elevated concentrations in legacy waste streams. While simultaneous removal of Tc(VII) and Cr(VI) would be advantageous, the challenge lies in the typically higher concentrations of Cr(VI) compared to Tc(VII), potentially interfering with ZVI's reductive treatment of Tc. This study explores the removal of aqueous Cr(VI) and Tc(VII) by two different ZVI materials under alkaline pH 10 conditions (pH 10). Experiments were conducted across various Cr(VI) concentrations (1−50 mg/L) while maintaining a constant Tc(VII) concentration (1 mg/L) in 0.30 M NaCl solution for 30 days. The results indicate that ZVI can quantitatively reduce simultaneously present Tc(VII) and Cr(VI) when the Fe 0 :(Cr + Tc) molar ratio exceeds 106. However, a decrease in this stoichiometric ratio due to higher Cr(VI) content results in reduced Tc removal, possibly due to Cr(VI) inhibiting ZVI oxidation and saturating the ZVI surface.