On the Origin of Selectivity in Extracting and Separating Lanthanide Ions by Alkyl-Nitrilotriacetamide

Extraction and separation technology forms an essential component of contemporary spent nuclear fuel reprocessing. Comprehending the underlying factors that drive extractant selectivity holds immense importance in crafting and advancing new extractants with optimal and desired performance. However, the majority of previous studies predominantly concentrated on organometallic complexes generated, while overlooked the entirety of the extraction process. In this study, we employ molecular dynamics (MD), extended tight-binding (xTB), and density functional theory (DFT) methods to comprehensively investigate the entire reaction process, with particular attention to overlooked aspects. Our findings reveal that the selectivity preference in the extraction reaction originates from the competition involved in the formation of metal complexes in both the aqueous and organic phases. Addressing the previously neglected aspects is crucial for providing clear guidance to enhance extraction selectivity. In the process, we have developed two driving force models and explained the origin of extraction selectivity. Model I is propelled by distinctions among organometallic complexes, whereas Model II is influenced by their similarities. Little is known about Model II since reported extractants fulfill Model I. Notably, we have discovered that the extraction and separation of La3+/Lu3+ using hexaocactyl-nitrilotriacetamide conforms to Model II, marking the first example and potentially introducing a novel approach to design selective extractants.