Extraction of the uranyl ion with 3-phenyl-4-benzoyl-5-isoxazolone (HPBI) and neutral donors from dilute nitric acid medium

The extraction behaviour of Uranium (VI) from aqueous nitric acid medium employing 3-phenyl-4-benzoyl-5-isoxazolone (HPBI) has been studied in the presence of various amides, e.g, di-2-ethylhexylacetamide (D2EHAA), di-2-ethylhexylpropanamide (D2EHPrA), di-2-ethylhexylisobutylamide (D2EHiBA) and di-2-ethylhexylpivylamide (D2EHPvA) in toluene medium. The extraction constant (log k(ex)) for the binary species UO2 (PBI)(2)(.)nH(2)O was found to be 1.42 +/- 0.14 which is by far the largest amongst the corresponding values known for the other,beta-diketones. The overall equilibrium constant (log K) for the ternary species UO2(PBI)(2)(.)D2EHAA, UO2(PBI)(2)(.)D2EHPrA. UO2(PBI)(2)(.)D2EHiBA and UO2(PBI)(2)(.) D2EHPvA were estimated to be 6.51, 6.21, 6.11 and 5.77 respectively. This trend in equilibrium constants was in accordance with the substitution of the amides as sterically crowded amides show relatively lower equilibrium constant values. The extraction constants follow an inverse correlation with the pK(a) values of the beta-diketonates while the equilibrium constants vary linearly with the size of the amides. The steric factors have been found to play a significant role in the co-ordination of the amides to the uranyl beta-diketones when the carbonyl group is a part of the heterocyclic ring. The distribution ratios of the metal were measured in the temperature range 15 to 52 degreesC using each of the amide donors in combination with HPBI. Calculation of enthalpy and entropy therefrom indicated that the extraction was thermodynamically favoured process and maximum extraction resulted in the case of UO2(PBI)(2)(.)D2EHAA.