Dissolution of Oxalate Precipitate and Destruction of Oxalate Ion by Hydrogen Peroxide in Nitric Acid Solution

摘要

This study aims at developing an oxalate precipitation process, which is applicable to a partitioning of long--lived radionuclides from the high-level radioactive liquid waste. In order to achieve this, a study for decomposition reaction of oxalic acid by hydrogen peroxide was first carried out. The decomposition rates of H_2O_2 and oxalic acid increased with an increase of nitric acid concentration, and especially those decomposition rates steeply increased at more than 2 M HNO_3. Based on this result, the decomposition kinetics of H_2O_2 and oxalic acid were suggested in this work. Then, the dissolution of oxalate precipitate and the destruction of oxalate ion in the solution were examined. Oxalate precipitates were prepared by adding oxalic acid into a simulated radioactive waste containing 8 metallic elements. The precipitates obtained thereby were dissolved in various nitric acid concentrations and reacted with H_2O_2 at 90deg. C. When the oxalates were completely dissolved, most of the oxalates were decomposed by adding H_2O_2, but in a slurry state the decomposition yield of the oxalate decreased with an increase of the slurry density in the solution. Such phenomenon was considered to be due to a catalytic decomposition of H_2O_2 on a solid surface of oxalate and the decomposition mechanism was explained by a charge transfer from a surface of oxalate solid to H_2O_2. producing OH radicals which can destruct H_2O_2 explosively. Accordingly, the experimental condition for the decomposition of the oxalate precipitates was found to be most favorable at 3 M HNO_3 under the initial concentrations of 0.2 M oxalate and 1 M H_2O_2. At 3 M HNO_3, oxa1ate precipitates could be safely and completely dissolved, and almost decomposed. Additionally, it was observed that the presence of ferric ion in the solution largely affects the decomposition rate of H_2O_2. This could be explained by a chain reaction of hydrogen peroxide with ferric ion in the solution.