Molecular behaviors of organic ligands for actinide lanthanide solvent extraction

Abstract

The treatment of used nuclear waste can be achieved through a strategy known as Partitioning andTransmutation. Partitioning can be used to isolate certain elements like U and Pu so that they can be recycled for their unused energy potential. Transmutation can be used to convert minor actinides into shorter lived isotopes and has been primarily targeted towards minor actinides such as Np, Am, and Cm as these have been shown to be the major contributors to radiotoxicity. However, before the benefits of recycling or conversion to more tractable isotopes can be fully realized, the actinides must first be separated from fission product lanthanides. The extraction of U and Pu have been effectively achieved at the industrial scale via the well-known PUREX process but processes that aim to isolate the minor actinides have yet to be developed well enough for industrial implementation. The Actinide Lanthanide SEParation (ALSEP) was recently created to separate Am and Cm from lanthanides and has been a topic of interest in recent years. ALSEP is a simplified solvent extraction that uses the combination of two organic ligands, 2-ethylhexylphosphonic acid mono-(2-ethylhexyl) ester (HEH[EHP]) and N,N,N’N’-tetra(2-ethylhexyl)diglycolamide (T2EHDGA) in n-dodecane combined with an acidic aqueous phase for extraction of trivalent lanthanides and actinides, or an actinide-selective aminopolycarboxylate ligand, for selective stripping of the actinides. Although successful at isolating Am and Cm, many of the molecular-level details such as complexes formed, the nature of the ligands’ interactions, and the chemical mechanism of mass transfer between phases remain poorly understood. Molecular dynamics simulations were employed to help elucidate the underlying chemistries involved with HEH[EHP] and T2EHDGA in the context of ALSEP to aid research in advancing the process for industrial adoption. Charge-modified Generalized Amber Force Fields were used to describe HEH[EHP] and T2EHDGA in n-dodecane, water, or biphasic n-dodecane-water solvents. Fundamental studies were performed on ligand exclusive (HEH[EHP] or T2EHDGA) systems in which Radial Distribution Functions (RDFs) were used to ascertain chemical interactions via coordination number (CN) analyses, vector angles were used to evaluate interfacial orientation, and self-diffusion coefficients were also calculated. From these ligand exclusive studies, expected amphiphilic behaviors were observed and the ligands were primarily oriented in parallel-like fashion with respect to the interface. Biphasic n-dodecane-water solvent systems containing both HEH[EHP] and T2EHDGA were also investigated by spatial distributions, CN analyses, interfacial orientations, and interfacial conformations as a function of increasing nitric acid concentration. These studies revealed that HEH[EHP] and T2EHDGA were unresponsive to the increase in aqueous acidity. HEH[EHP] possessed a unique interfacial behavior while T2EDHGA remained to be more like its bulk iiicounterparts. CN analyses also showed that T2EHDGA in the interfacial region, on average, sat relatively further from the aqueous phase than HEH[EHP]. Finally, preliminary studies were performed to assess the effects of charge distribution on the organophosphorus acid head group. Simulations of T2EHDGA mixtures with either di-(2-ethylhexyl)phosphoric acid (HDEHP) or bis(2-ethylhexyl)phosphinic acid (HD[EHP]) extractants in n-dodecane-water were scrutinized in the context of CNs, interfacial orientation, and interfacial conformation. Organophosphorus derivatives were observed to behave similar to one another while T2EHDGA’s amide carbonyl oxygen favored a perpendicular conformation more when HDEHP was present. These molecular observations of HEH[EHP] and/or T2EHDGA leaves impressions that may help explain extraction and separation mechanisms within the ALSEP process. Moreover, T2EHDGA exclusive studies may also be helpful in aiding our understanding in future kinetic investigations on T2EHDGA extraction systems.