The Effect of Diluents and Phase Modifiers on Lanthanide(III) and Actinide(III) Extraction by Diglycolamide Extractants

Solvent extraction of trivalent f-elements by diglycolamide extractants, such as N,N,N’,N’-tetra(n-octyl)diglycolamide (TODGA), is effective for nuclear fuel cycle separations and is also competitive with the best-in-class separations of light lanthanides. Diglycolamides undergo significant aggregation when extracting into alkane diluents, but this aggregation and associated phase splitting can be mitigated by altering the diluent or adding phase modifiers. Changing the diluent leads to important changes in the degree of extraction, selectivity, and speciation of the trivalent f-elements. Addition of a polar phase modifier can inhibit aggregation and third phase formation, but it also affects the extraction parameters, for example extraction efficiency, selectivity, organic phase speciation, and phase splitting limits. These have been studied extensively for diglycolamides in a range of diluents and with the phase modifiers N,N-di(n-hexyl)octanamide (DHOA), tri(n-butyl)phosphate (TBP), and 1-alcohols. The extraction and selectivity of europium(III) and americium(III) by TODGA vary depending upon the diluent used, but n-dodecane remains a top performer. Extractant dependence slopes show that the TODGA:lanthanide(III) ratio in the extracted species decreases with increasing diluent polarity, suggesting changes to organic phase complexation. Studies show that DHOA, TBP, and 1-alcohols are effective at preventing phase splitting, but DHOA and TBP extract more acid, which inhibits metal stripping.