Density functional theory studies on the interaction of amyl phosphates with zirconium nitrate

Apart from tri-iso-amyl phosphate (TiAP), which has been used in fast reactor fuel reprocessing, several studies have focused on various derivatives of amyl phosphates for their potential application in the extraction of U and Pu. For effective decontamination of heavy metals, it is crucial that the uptake of fission products be minimized by target ligands. In this study, we explore the primary interaction, i.e., the molecular level interaction of Zr [in the form of Zr(NO₃)₄], a troublesome fission product, with four amyl phosphates using computational chemistry techniques. The four ligands studied are tri-sec-amyl-phosphate (TsAP), tris (2-methylbutyl) phosphate (T2MBP), tri-iso-amyl-phosphate (TiAP), and tri-n-amyl-phosphate (TAP). Calculations indicate that, for the single-step 1:2 complex formation, all four ligands exhibit almost the same binding affinity towards zirconium nitrate, with a marginally higher propensity for TsAP and TAP. The stepwise formation of 1:2 complex was also studied starting from Zr(NO₃)₄.2H₂O, in which the formation of the 1:2 complex was attributed via the respective 1:1 complexes, i.e., Zr(NO₃)₄·H₂O·L. Here the 1:1 complexes of TsAP, TiAP and TAP ligands were found to be more stable compared to the T2MBP complex and the higher stability is attributed to the presence of intra-molecular hydrogen bond present between the H atom of water and the O atom of the nitrate. Interestingly, it was observed that the zirconium nitrate moiety is highly sensitive to the external ligand environment, as evident from the very high strain or deformation energy of the metal nitrate upon complexation.

Graphical abstract

Zirconium Amyl Phosphate Complexes: The interaction of zirconium nitrate [Zr(NO₃)₄] with four amyl phosphates (TsAP, T2MBP, TiAP, and TAP) indicates a higher affinity for the TsAP and TAP ligands. The study also highlights the sensitivity of zirconium nitrate to external ligand environments, as evidenced by significant deformation energy upon complexation.