Experimental evidence and quantum chemical insights into extraction and third phase aggregation trends in Ce(IV) organophosphates

Abstract

“Third phase” formation is an aggregation phenomenon that arises due to the mutual attraction of the polar cores of reverse micelles in a non-polar medium. A major concern in the solvent extraction of actinides during processing of nuclear fuels is third phase formation at the aqueous-organic interface. When the metal loaded in the organic phase exceeds a threshold known as the limiting organic concentration (LOC) the organic phase splits into a metal-extractant rich third phase and a metal poor diluent rich phase; hampering the entire process. Pu(IV) behaviour towards third phase formation is crucial for its extraction and processing as a fissile material. In this light, in a preliminary study on the third phase behaviour of Ce(IV), which is chemically similar to Pu(IV) and normally considered as a surrogate of Pu(IV) has been carried out in the present work. Tri-sec-butyl phosphate (TsBP) and tri-n-butyl phosphate (TBP) have been evaluated for the extraction efficacy and their third phase thresholds. Superior extraction as well as resistance to third phase formation has been shown by TsBP as compared to the conventionally used TBP extractant. Apart from experimental studies, quantum chemical calculations using density functional theory (DFT) have also been carried out. The solvent corrected energetics of complex formation for these extraction systems with Ce(IV) have been computed and are in good agreement with experimental observations. It is known that Ce(III) is experimentally not extracted by organophosphates. In addition, Ce(III) complexation with TBP has been theoretically investigated and resulted in unfavourable binding. Furthermore, our electronic structure calculations have percolated into the experimentalist’s realm of third phase formation in metal-extractant systems, and shed new light on the reasons behind the lower tendency of Ce-TsBP complexes to form third phase.