Experimental and theoretical studies on extraction behavior of di-n-alkyl phosphine oxides towards actinidesElectronic supplementary information (ESI) available. See DOI: 10.1039/c5ra19090h

Di- n -alkyl phosphine oxides (DAPOs) with different alkyl chain length viz. C 6 , C 7 , C 8 and C 10 and their corresponding Th( iv ) and La( iii ) complexes were synthesized and characterized using FT-IR, 1 H, 31 P{ 1 H}, and 13 C 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. The extraction behavior of U( vi ) & Am( iii ) with di- n -alkyl phosphine oxides (DAPOs) was investigated. DAPO behaves like an acidic extractant at low acidity and at higher acidity they extracted via cation exchange mechanism.