Oxidative Leaching of Plutonium from Simulated Hanford Tank‐Waste Sludges

Abstract

The behavior of surface‐sorbed plutonium during leaching of four Hanford tank waste sludge simulants with alkaline permanganate solutions has been investigated. The sludge simulants are representative of materials created as a result of the operation of the Bismuth Phosphate (sludges B and M), PUREX (sludge P), and REDOX (sludge R) processes at Hanford. Leaching with oxidants under alkaline conditions has been proposed as a means of reducing the chromium content of actual sludge samples prior to their vitrification. With identical amounts of plutonium deposited on each sludge sample, the percentage of radiotracer Pu leached from the sludges increased in the order P≥R>B. This order is the reverse of the chromium content of the sludges. The percentage of plutonium leached was independent of its initial oxidation state. The reverse trends of chromium content and plutonium leaching can be understood in terms of Pu re‐adsorption onto the MnO₂ generated as Cr(III) (and other substrates) are oxidized by permanganate/manganate. The effect of chelating agents that are known to be present in tank wastes (EDTA, gluconate, glycolate, oxalate, and citrate) on KMnO₄ reduction/MnO₂ production and redeposition of plutonium was also studied and found to have varied effects on the process of oxidative dissolution/Pu redeposition. Overall, it appears that the MnO₂ produced as a byproduct of oxidation helps to control Pu concentrations in the leachate phase. Oxidation of Pu to the hexavalent state without concomitant production of MnO₂ leads to greater plutonium content in the leachate. The results are discussed, emphasizing the potential impact of oxidative leaching on plutonium mobilization from actual tank‐waste sludge.

Work performed under the auspices of the Environmental Management Science Program (EMSP) of the U.S. Department of Energy at Argonne National Laboratory under contract number W‐31‐109‐ENG‐38

Keywords:

• Oxidative leaching
• tank waste sludge simulants
• plutonium
• organic chelating agents