N. Sivaraman

Complexation Behavior of the Tri-n-butyl Phosphate Ligand with Pu(IV) and Zr(IV): A Computational Study.

Tri-n-butyl phosphate (TBP), used as the extractant in nuclear fuel reprocessing, shows superior extraction abilities for Pu(IV) over a large number of fission products including Zr(IV). We have applied density functional theory (DFT) calculations to explain this selectivity by investigating differences in electronic structures of Pu(NO3)4·2TBP and Zr(NO3)4·2TBP complexes. On the basis of our quantum chemical calculations, we have established the lowest energy electronic states for both complexes; the quintet is the ground state for the former, whereas the latter exists in the singlet spin state. The calculated structural parameters for the optimized geometry of the plutonium complex are in agreement with the experimental results. Atoms in Molecules analysis revealed a considerable amount of ionic character to M-O{TBP} and M-O{NO3} bonds. Additionally, we have also investigated the extraction behavior of TBP for metal nitrates and have estimated the extraction energies to be -73.1 and -57.6 kcal/mol for Pu(IV) and Zr(IV), respectively. The large extraction energy of Pu(IV) system is in agreement with the observed selectivity in the extraction of Pu.

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

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 DU(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 DU(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.

Experimental determination and model correlation for the solubilities of trialkyl phosphates in supercritical carbon dioxide

The solubilities of a series of trialkyl phosphates in supercritical carbon dioxide have been investigated. The solubility measurement was carried out using a dynamic flow method. The measurements were performed at 313, 323 and 333 K and in the pressure range of 10 to 25 MPa. The trialkyl phosphates were found to be highly soluble in supercritical carbon dioxide and the solubilities were in the range of 0.01 to 0.1 mol fraction. The solubility of trialkyl phosphates increases with an increase in pressure at constant temperature. A reverse behavior was observed, wherein the solubilities decreased with increase in temperature in the investigated pressure region. At constant temperature and pressure, the solubilities of the trialkyl phosphates decrease with the alkyl chain length. The solubility data was found to be consistent with the Mendez–Teja model. The solubilities were correlated by the Chrastil model and an association model based on the van Laar activity coefficient model with absolute deviations of less than 10%. A key result is that the model parameters based on the association model varied linearly with the carbon number.

Experimental and theoretical studies on extraction of actinides and lanthanides by alicyclic H-phosphonates

Abstract Three different alicyclic substituents H-phosphonates, namely, dicyclopentyl H-phosphonate, dicyclohexyl H-phosphonate and dimenthyl H-phosphonate were synthesized and used for liquid–liquid extraction of actinide elements (U, Am and Th) and lanthanide (Gd) in n-dodecane from nitric acid medium. The physicochemical properties of the extractants, such as density, viscosity, solubility were determined. At lower acidities, these H-phosphonates exhibit higher distribution values and the extraction following cation exchange mechanism through P–OH group of tri-coordinated phosphite form. At higher acidities (2N), the extraction is primarily via solvation mechanism through P=O group of penta-coordinated phosphonate form. Amongst the three H-phosphonates, examined dimenthyl H-phosphonate showed the best results for the actinide extraction. Density functional theory (DFT) calculations were applied to understand the electronic structure of the ligands and the metal complexes. The calculated large complexation energy of UO2(NO3)2·2DMnHP is in agreement with the observed trend in experimental distribution ratio data.

Trends in small angle neutron scattering of actinide–trialkyl phosphate complexes: a molecular insight into third phase formation

The “third phase” formation phenomenon in solvent extraction is due to the aggregation of extracted species and formation of reverse micelles. As Small Angle Neutron Scattering (SANS) is a powerful tool to probe colloidal particles, it is considered as an important technique to study the aggregation behaviour of actinide complexes in solvent extraction systems. The actinide specific trialkyl phosphate (TalP) based extractants, namely, tri-n-butyl phosphate (TBP), tri-iso-butyl phosphate (TiBP), tri-sec-butyl phosphate (TsBP) and tri-sec-amyl phosphate (TsAP) have been examined for the first time using the SANS technique to investigate third phase formation phenomena with some of the actinides. SANS was employed to get insight into third phase formation in the extraction of Th(IV) and U(VI) from 1 M HNO3 by 1.1 M solutions of TalP in deuterated dodecane (n-C12D26, 98 atom% D). Deuterated diluent was used in order to provide contrast during the neutron scattering measurements. Potential energy and the stickiness parameter of reverse micelles formed in the above solvent systems have been quantified as a function of organic metal loading. The data are fitted using Baxters sticky-sphere model. The stickiness parameter, (τ−1) a measure of the attractive interaction between the micelles, as well as the attractive potential energy (U0) was quantified. A clear correlation has been established between the stickiness parameter and the tendency for third phase formation with TalP systems. As U(VI) does not form a third phase with these extractants at 1 M HNO3, comparative studies were carried out with U(VI)–TalP complexes. These studies established lower stickiness and attraction between the micelles with the U(VI) system. The correlation between SANS parameters and third phase formation tendency was extended to a temperature dependence study and these studies established higher third phase limits when the temperature was enhanced, corroborating well with our experimental results. Our studies also revealed the “prediction of third phase formation” before its occurrence for a range of actinide–extractant systems.

Solubility of tri-iso-amyl phosphate in supercritical carbon dioxide and its application to selective extraction of uranium

ABSTRACT The solubility of tri-iso-amyl phosphate (TiAP) in supercritical carbon dioxide (SCCO2) was determined at 313–333 K with pressure ranging from 10 to 25 MPa. The solubility data of TiAP in SCCO2 medium were correlated using four semi-empirical models. Selective extraction of uranium from simulated dissolver solution was demonstrated using SCCO2 modified with TiAP. The extraction efficiency of uranium was found to be 95 ± 5%. The influence of temperature, pressure and acidity on the extraction of uranium was studied. An attempt was made to explain the plausible extraction mechanism.

Third phase formation in the extraction of Th(NO3)4 by Tri-sec-butyl phosphate: a comparison with Tri-n-butyl phosphate

Abstract Earlier studies carried out in our laboratory indicated that Tri-sec-butyl phosphate (TsBP) is a potential extractant for U/Th separation. Also, the third phase formation tendency of TsBP is lower compared to its isomers, Tri-n-butyl-phosphate (TBP) and Tri-iso-butyl phosphate (TiBP). In this context, the extraction and third phase formation behaviour of 1.1 M solutions of TiBP and TsBP in n-dodecane in the extraction of Th(IV) from 1 M HNO3 at 303 K over a wide range of Th concentrations were investigated in the present study and the results are compared with the literature data on TBP system. Concentrations of Th(IV) and HNO3 loaded in the organic phase before third phase formation (biphasic region) as well as in third phase and diluent-rich phase after third phase formation (triphasic region) were measured as a function of equilibrium aqueous phase Th(IV) concentration. The density of loaded organic phase was also measured at various Th(IV) concentrations. The extraction profiles in the biphasic region indicated that extraction of Th(IV) by TBP is higher than that of TiBP which in turn is higher than that of TsBP. Extractant concentration in the diluent-rich phase and third phase was measured for the triphasic region.

Extraction and coordination behavior of diphenyl hydrogen phosphine oxide towards actinides

Abstract Extraction behavior of some selected actinides like U(VI), Th(IV), and Am(III) was investigated with three different H-phosphine oxides, viz. diphenyl hydrogen phosphine oxide (DPhPO), dihexyl hydrogen phosphine oxide (DHePO) and diphenyl phosphite (DPP). The H-phosphine oxides exhibited a dual nature towards the extraction of actinides where the ligand not only extracts the metals by cation exchange but also by coordination with the phosphoryl group at lower and higher acidic concentrations, respectively. Among all ligands employed, DPhPO showed highest extraction with actinides with a substituent dependent trend as follows: DPhPO > DHePO > DPP. This trend emphasizes the importance of substituents around the phosphine oxide towards their extraction of actinides. The coordination behavior of DPhPO was studied by investigating its corresponding complexes with Th(NO3)4 and UO2(NO3)2. The metal complexes of these actinides were characterized using FT-IR, 1H and 31P NMR spectroscopic techniques. Density Functional Theory (DFT) calculations were also performed to understand the electronic and geometric structure of the ligand and the corresponding metal complexes.

Effects of temperature on the extraction of U(VI) and Pu(IV) by tris(2-methylbutyl) phosphate from nitric acid media

Abstract Extraction behavior of tris(2-methylbutyl) phosphate (T2MBP), a higher homologue of tri-n-butyl phosphate (TBP), in the extraction of U(VI) and Pu(IV) from nitric acid media has been evaluated in the present study. Distribution ratios for the extraction of these metal ions by T2MBP were measured as a function of equilibrium aqueous phase nitric acid concentration, extractant concentration and temperature to understand the temperature effects on extraction. Enthalpies of extraction of T2MBP/n-dodecane-U(VI)-HNO3 and T2MBP/n-dodecane-Pu(IV)-HNO3 systems were calculated from the distribution ratio data using “second law” method. The data reveal that the extraction of U(VI) by T2MBP is exothermic in nature, whereas that of Pu(IV) is endothermic.

Dissolution and characterisation studies on U–Zr and U–Pu–Zr alloys in nitric acid medium

Dissolution of metallic alloys, U–Zr and U–Pu–Zr has been investigated in HNO3 media. An electro oxidative dissolution technique was also examined. Explosive nature of metallic alloys during dissolution in nitric acid has been investigated. A method has been developed for the determination of zirconium in the presence of uranium and plutonium using a spectrophotometric technique. The influence of HNO3, uranium and plutonium during the estimation of Zr has been studied. Plutonium interferes in the estimation of Zr and it can be avoided by employing ascorbic acid. The method was employed for the estimation of Zr in samples generated during dissolution of Zr containing alloy fuels.