Experimental and theoretical studies on extraction behavior of di-n-alkyl phosphine oxides towards actinides

Abstract

Di-n-alkyl phosphine oxides (DAPOs) with different alkyl chain length viz. C₆, C₇, C₈ and C₁₀ and their corresponding Th(IV) and La(III) complexes were synthesized and characterized using FT-IR, ¹H, ³¹P{¹H}, and ¹³C NMR. The extraction behavior of U(VI) and Am(III) with DAPOs was investigated and it largely depends on nitric acid concentration. These DAPOs showed the highest distribution coefficients with U(VI) among U(VI) and Am(III). The D_U(VI) values of DAPOs are higher at lower acidities i.e. 0.1 M nitric acid concentration. Among the four tested DAPOs for the extraction of uranium with a molar concentration of 0.025 M, DDPO bearing the longest alkyl chains showed the highest D_U(VI) values of 84.6 and their distribution coefficients increased with an increase in alkyl chain length in the tested DAPOs. Here we have proposed a mechanism for the extraction of actinides using di-n-alkyl phosphine oxides, which was further supported by theoretical calculations. At lower acidity, DAPO behaves like an acidic extractant and extracts the metal ion via a cation exchange mechanism. On the contrary, at higher acidity, the metal ions are extracted via a solvation mechanism through phosphoryl group coordination. Density functional theory (DFT) calculations support a bimolecular mechanism for the tautomerism reaction in the DHePO (di-n-hexyl phosphine oxide) ligand, where both pentavalent (tetracoordinate) and trivalent phosphorus are in equilibrium. An activation barrier of ∼27.3 kcal mol⁻¹ is estimated with respect to the reactant complex at the B3LYP/def2-TZVP level. The length of the alkyl chain in di-n-alkyl phosphine oxides (DAPO) also plays a significant role in actinide extraction at lower acidity.