Do nitrate ions preferentially bind to Ln/An ion in nuclear waste treatment? – Answers from DFT calculations

Abstract

Stripping of radioactive minor actinides from lanthanides is an important process in the treatment of spent nuclear fuel (SNF). Almost in all the partitioning processes, the SNF is first introduced into the nitric acid medium before suitable reagents are added to selectively remove actinides. Therefore, in the present investigation an attempt has been made to understand the nature of interactions between the M3+ (M3+ = La, Nd, Pm, Sm, Eu, Gd, Ac, U, Np, Pu, Am, and Cm) and nitrate ions [(NO3)3]−3, metal–oxygen bond and stabilizing forces present in M(NO3)3 complexes using relativistic density functional theory calculations. Also, complexes with higher oxidation states [U(VI), Np(V), Pu(IV, VI)] along with nitrate ions are investigated. Optimized geometries of M(NO3)3 reveal that these metal ions readily form complexes with nitrate ions. Chemical reactivity is addressed from frontier molecular orbital analysis while the nature of metal–oxygen bonds is examined using Mayer and Nalewajski – Mrozek bond order analyses. Mulliken population analysis portrays the donor–acceptor interactions in these M(NO3)3 complexes. Results show that the ionic interactions persist in these complexes and QTAIM analysis endorses the ionic character of these complexes. Energy decomposition analysis gives an idea about the various energy components that stabilize the complexes. The water solvent environment does vary the various parameters involved in the complexation process. Overall, a comprehensive quantum chemical study is taken up to understand the nature of interactions present between M3+ and [(NO3)3]−3 ions to improve our understanding of the partitioning processes.