Understanding and utilizing DOTA coordination chemistry for trivalent lanthanide/actinide separation

Abstract

This study reports the results of an investigation centered on increasing the understanding of the coordination chemistry of the tetraaza macrocyclic ligand 1,4,7,10-tetraazacyclododecane-N,N',N",N'"-tetraacetic (DOTA) acid with trivalent f-elements La3+-Lu3+ (excluding Pm3+) and Am3+. This work includes results of kinetic studies exploring the mechanism of complexation of Ln(III) ions by DOTA. Through the studies completed, a more complete understanding of the mechanism is presented with an alternative theory regarding the rate determining step. The results presented emphasize the intramolecular rearrangement of the DOTA donor atoms as the most likely process accounting for the slow complexation kinetics. These results also provide greater insight into adjustments of conditions that could increase the rate of complexation. Through these studies, it is clear that the most effective technique that can be employed to improve the rate of complexation is elevation of the reaction temperature. Utilizing the understanding gained from a lanthanide series studies, a series of kinetic studies for a solvent extraction system combining the cation exchanging extractant Cyanex 272 with DOTA using radiotracer 152,154Eu3+, 147Nd3+, and 241Am3+ to monitor the rate of progress of the phase transfer reaction were completed. These results indicate that DOTA prefers to form complexes with the actinides over the lanthanides, confirming the theoretical prediction. Finally, reported in this work is a study done to determine the reversibility of the AmDOTA- complex and the ability to break the DOTA "cage". It was observed that at pH 1.0, 60°C, it will take between 15 hours and 6 days for the complex to fully dissociate. Suggestions on improving this condition are provided. The overall impact of this work on the broader scientific community is an increased understanding of DOTA chemistry with the f-elements. This result is also the first report characterizing the interactions of DOTA with a trivalent actinide (Am3+). The results from the lanthanide series study contribute to the understanding of the Ln-DOTA complexation mechanism. The solvent extraction studies indicate that the predicted selectivity of DOTA for An3+ exists. The overall conclusion to this work is that DOTA can be considered a separations relevant ligand for trivalent lanthanide/actinide separation.