Extraction of tetravalent neptunium isoxazolonates as their TOPO adducts

The extraction behaviour of Np(IV) by 3-phenyl-4-benzoyl-5-isoxazolone (HPBI) in the presence of tri-n-octyl phosphine oxide (TOPO) was investigated from nitric acid medium. The speciation studies, carried out by the slope analysis method, yielded binary species of the type Np(PBI)(4) when only HPBI was used as the extractant and ternary species of the type Np(PBI)4(.)TOPO when both HPBI and TOPO were used as the extractants. The two-phase extraction constant for the binary species (log k(ex)) was calculated to be 10.11 +/- 0.03, while that for the ternary species was found to be 17.24 +/- 0.05. Thermodynamic parameters for the binary and ternary extraction systems were also determined. Both the binary and ternary extraction systems were found to be highly spontaneous processes with predominant enthalpy stabilization. The very high negative entropy factor suggested contribution of unfavourable steric factors.