Remarkable structural effects on the complexation of actinides with H-phosphonates: a combined experimental and quantum chemical study

Abstract

The structural effects of the carbon chain on the extraction of actinides by organo-phosphorus extractants have been examined experimentally and by computation. Branched butyl H-phosphonates and their linear chain isomer, n-butyl H-phosphonate (DBHP), were synthesised and characterised using IR, NMR and GC-MS techniques. Their physical properties viz. viscosity, density and aqueous solubility have been examined. DBHP, Di-iso-butyl H phosphonate (DiBHP) and Di-sec-butyl H phosphonate (DsBHP) were employed for the extraction of uranium and americium ions from nitric acid. ²³³U (α-tracer) and ²⁴¹Am (γ-tracer) were employed as representative isotopes for the extraction of U and Am, respectively, and their distribution ratios (D) were obtained as a function of nitric acid concentration (0.01–8 M). Branching of the alkyl chain at the secondary carbon atom showed unexpected neutral extractant behaviour for DsBHP which is generally classified as an acidic extractant. The acid-dependent dual extraction mechanisms for the H-phosphonates have been examined both experimentally and through quantum chemical calculations. This dramatic effect can be partly attributed to the hindrance in the formation of the enol tautomer through the strengthening of the PO⋯H hydrogen bonding in the DsBHP extractant. Density functional theory (DFT) based calculations were carried out to understand the complexation behaviour of actinides with the two extractants. Possible structures and binding affinities of actinides with H-phosphonates have been deduced from electronic structure calculations. Finally, the trends in distribution ratios were additionally explored and correlated with experimental observations for both metal ions.