Extraction of Trivalent Lanthanides with Oxa-Diamides in Supercritical Fluid Carbon Dioxide

This paper describes the results of the extraction of trivalent lanthanides Ln(III) by supercritical fluid CO 2 (SF-CO 2 ) containing N, N, N ‘, N ‘-tetrabutyl-3-oxapentanediamide (TBODA), one of a group of neutral oxa-diamide ligands that have the potential to replace traditional organophosphorus extractants and make the SF-CO 2 extraction process more environmentally benign. According to this study, TBODA dissolves in neat SF-CO 2 easily, but the solubility of Ln(III)/TBODA complexes in neat SF-CO 2 is low and little Ln(III) can be extracted in the SF-CO 2 phase. Addition of small amounts of acetone as a modifier in SF-CO 2 can significantly improve the extraction of Ln(III) from either aqueous nitric acid solutions or oxide solids. The low solubility of Ln(III)/TBODA complexes in neat SF-CO 2 and the improved solubility of Ln(III)/TBODA complexes in acetone-modified SF-CO 2 are discussed in terms of the structure of Ln(III)/TBODA complexes and its effect on solute−solvent interactions.