Evaluation of the remaining spent extraction solvent in vermiculite after leaching tests via PIXE analysis

Abstract

The spent extraction solvent, primarily comprising tributyl phosphate (TBP) and normal dodecane, is waste product generated by the solvent extraction of plutonium and uranium to develop the plutonium–uranium redox-extraction process. The spent solvent should be appropriately treated for disposal or storage. One treatment option for storing the spent solvent is the adsorption of the liquid into vermiculite. The organic liquid is trapped between the vermiculite layers. Our previous experiments on the adsorption of spent solvent into vermiculite have shown that some parts of the loaded solvent gradually leaked from the vermiculite. Therefore, the elution behavior of the loaded solvent into organic diluents, such as hexane or acetone, were evaluated to investigate the adsorption mechanism and capacity. Some loaded solvents easily leaked into the diluent, whereas some remained inside the particle, even after the leaching test. The difference in behavior might be attributed to different adsorption mechanisms, and the amount of the remaining solvent is the adsorption capacity for safely storing the solvent. This study evaluated the vermiculite adsorption capacity via the remaining solvent after washing with diluents, such as ethanol or distilled water. The remaining solvent was analyzed using particle-induced X-ray emission (PIXE) on phosphorous (P) contained in the TBP. PIXE analysis quantitatively analyzed the trace amounts of residual TBP, and n-dodecane and TBP were suggested to be adsorbed to vermiculite by different mechanisms.