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 MnO 2 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 KMnO 4 reduction/MnO 2 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 MnO 2 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 MnO 2 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.

Comparison of the Ca2+complexing properties of isosaccharinate and gluconate – is gluconate a reliable structural and functional model of isosaccharinate?

“…Additionally, the formation of mononuclear CaLH −1 0 with lower stability was also detected and it is noticeable that all of the complexes formed are neutral. Organic ligands containing -OH and -COO − donor groups (i.e., sugar carboxylates) are known to form stable binary solution complexes with various metal ions (e.g., alkaline earth and transition metals, [1][2][3][4][5][6][7][8][9][10][11] lanthanides, [12][13][14][15] actinides, [16][17][18][19][20][21][22][23][24][25][26] etc. ), and their complexing ability is more pronounced under (hyper)alkaline conditions than under neutral conditions, due to the binding of the metal ion to the alcoholate group(s).…”

Strontium and Cesium Release Mechanisms during Unsaturated Flow through Waste-Weathered Hanford Sediments

“…Of particular concern is leakage of caustic high-level radioactive waste (HLRW) from underground storage tanks into the vadose zone and groundwater aquifer [1][2][3]. Hanford Site cleanup efforts have focused on removing the caustic HLRW from old storage tanks using various mining and water sluicing processes [4][5][6][7]. Of the 149 single shell tanks, approximately 65 to 70 have been diagnosed as having released HLRW to the subsurface either from loss of tank integrity or from pipes used during transfers into and out of the tanks [8].…”

Characterization of Actinides in Simulated Alkaline Tank Waste Sludges and Leachates

“…Our studies of oxidative alkaline leaching of the simulants indicated that low concentrations of permanganate in 3.0 M NaOH (10 -5 to 10 -4 M) had minimal impact on the removal of Cr from sludge samples, that is, a similar amount of Cr was leached to that seen in 3.0 M NaOH contacts. [1] At higher concentrations of permanganate, increasing amounts of chromate were seen in the leachate (Figure 1a). One possible implication of the existence of a threshold is that some disruption of the surface of the sludge simulant is needed to provide adequate access of the oxidant to the reduced Cr(III).…”

“…Four research topics as the reasonable path forward building on our previous results including; 1) to explore the efficacy of oxalic acid and gluconic acid as a leaching solution in which the pH of the leaching is varied between 1 and 14; 2) to investigate the performance of mixtures of HEDPA and phytic acid on aluminum dissolution and actinide speciation across the pH 1 to 14 range; 3) to continue the research on the cleanup of actinides from aluminum nitrate, a hypothetical end product of sludge dissolution; 4) to continue development of meaningful correlations of experimental results leading to discovery of "guiding principles" and a useful descriptive model for sludge performance during sludge scrubbing.…”