Neetika Rawat

Effect of humic acid on sorption of neptunium on hematite colloids

Sorption of neptunium on hematite colloids has been studied at varying pH values (3–10) in absence and presence of humic acid using 239Np as a tracer. The experiments were carried out under aerobic and anaerobic conditions. The results show negligible sorption of Np(V) on hematite colloids at lower pH<5, beyond which it was found to increase sharply reaching ∼90% at pH 10. Presence of humic acid (2mg/L) was found to decrease the sorption of Np(V) on hematite at higher pH values while at lower pH the effect was found to be negligible. The stability constant of Np(V)-humate complex (logβ) was determined to be 4.2±0.2 by solvent extraction method. Sorption of humic acid on hematite as a function of pH was also measured by UV-visible spectrophotometry. Under nitrogen atmosphere and in presence of sodium dithionite, Np(V) was reduced to Np(IV) as evident by the enhanced sorption at all pH values, which was further enhanced in presence of humic acid at lower pH. However at higher pH values, humic acid does not affect the sorption of neptunium under reducing conditions, which could be due to competing reduction process involving ferric ions obtained from dissolved hematite.

Effect of humic acid on sorption of technetium by alumina

Sorption of technetium by alumina has been studied in absence as well as in presence of humic acid using (95)Tc(m) as a tracer. Measurements were carried out at fixed ionic strength (0.1M NaClO(4)) under varying pH (3-10) as well as redox (aerobic and reducing anaerobic) conditions. Under aerobic conditions, negligible sorption of technetium was observed onto alumina both in absence and in presence of humic acid. However, under reducing conditions (simulated with [Sn(II)] = 10(-6)M), presence of humic acid enhanced the sorption of technetium in the low pH region significantly and decreased at higher pH with respect to that in absence of humic acid. Linear additive as well as surface complexation modeling of Tc(IV) sorption in presence of humic acid indicated the predominant role of sorbed humic acid in deciding technetium sorption onto alumina.

Thermodynamics of complexation of lanthanides with 2,6-bis(5,6-diethyl-1,2,4-triazin-3-yl) pyridine

Abstract Solvent extraction studies on separation of trivalent actinides from lanthanides using 2,6-bis(5,6-dialkyl-1,2,4-triazin-3-yl) pyridines have shown promising results with respect to separation factor and efficiency in acidic medium. In order to understand their complexation behavior, the stability constant (log β) of trivalent lanthanides (La, Nd, Eu, Tb, Ho, Tm, Lu) with 2,6-bis(5,6-diethyl-1,2,4-triazin-3-yl)pyridine (ethyl-BTP) have been determined in methanol medium (ionic strength 0.01 M) using spectrophotometric titrations. The stoichiometry of the complexes is found to vary with the ionic size of lanthanide ion. The variation in log β across the lanthanide series is attributed to variation in solvation characteristics of the metal ion. Comparison of log β for Ln(III)–ethyl-BTP complexes with other alkyl derivatives showed increase in the stability with increasing length of the alkyl group due to hydrophobic interaction. In the case of Eu(III), the speciation was also corroborated by time resolved fluorescence spectroscopy. The thermodynamic parameters (Δ G, Δ H, Δ S) for complexation of Eu(III) with ethyl-BTP, were determined by microcalorimetry, which revealed strong metal ion–ligand interaction with the reactions driven mainly by enthalpy.

Distribution coefficients of radionuclides around uranium mining area and effect of different analytical parameters on their determination

Macroscopic batch sorption study has been carried out to measure distribution coefficient (Kd) of uranium, thorium and their daughter products, radium and polonium, on soil samples collected from a proposed uranium mining site in India. Results showed high Kd values for Th(IV) and Po(IV) (log Kd ~4), while lower Kd values were obtained for U(VI) with the site ground water as aqueous medium compared to the synthetic water. Ra(II)–Kd has been found dependent on soluble Ca(II) content of the water samples. Determination of Kd values in an inter-comparison manner involving different estimation methodology produced a consistent data set.

Effect of glass composition on diffusion of cesium in borosilicate glass

Summary Heavy ion Rutherford backscattering spectrometry, with 30 MeV 19F beam, has been used to study the diffusion of cesium in borosilicate glasses of different compositions. The coefficient of cesium diffusion in borosilicate glass was obtained by analyzing the depth profile of cesium in samples annealed at different temperatures in the range of 100-350 °C. The activation energy of diffusion of cesium was found to be in the range of 9-22 kJ/mol, indicating the inter-diffusion between the cesium and sodium as the mechanism of the diffusion process. The activation energy was found to decrease with increasing sodium content of the glass, indicating that the alkali metal ion diffusion is governed by the interaction between the jumping ion and the dipole field around the oxygen ions.

Structural, luminescence, thermodynamic and theoretical studies on mononuclear complexes of Eu(III) with pyridine monocarboxylate-N-oxides in aqueous solution

The mononuclear complexes formed by Eu(III) with three isomeric pyridine monocarboxylate-N-oxides namely picolinic acid-N-oxide (PANO), nicotinic acid-N-oxide (NANO) and isonicotinic acid-N-oxide (IANO) in aqueous solutions were studied by potentiometry, luminescence spectroscopy and isothermal titration calorimetry (ITC) to determine the speciation, coordination, luminescence properties and thermodynamic parameters of the complexes formed during the course of the reaction. More stable six membered chelate complexes with stoichiometry (MLi, i=1-4) are formed by Eu(III) with PANO while non chelating ML and ML2 complexes are formed by NANO and IANO. The stability of Eu(III) complexes follow the order PANO>IANO>NANO. The ITC studies inferred an endothermic and innersphere complex formation of Eu(III)-PANO and Eu(III)-IANO whereas an exothermic and outer-sphere complex formation for Eu(III)-NANO. The luminescence life time data further supported the ITC results. Density functional theoretical calculations were carried out to optimize geometries of the complexes and to estimate the energies, structural parameters (bond distances, bond angles) and charges on individual atoms of the same. Theoretical approximations are found to be in good agreement with the experimental observations.

Selective micellar extraction of ultratrace levels of uranium in aqueous samples by task specific ionic liquid followed by its detection employing TXRF spectrometry

A task specific ionic liquid (TSIL) bearing phosphoramidate group, viz., N-propyl(diphenylphosphoramidate)trimethylammonium bis(trifluoromethanesulfonyl)imide, was synthesized and characterized by 1H NMR, 13C NMR, 31P NMR, and IR spectroscopies, elemental (C H N S) analysis, and electrospray ionization mass spectrometry (ESI-MS). Using this TSIL a cloud point extraction (CPE) or micelle mediated extraction procedure was developed for preconcentration of uranium (U) in environmental aqueous samples. Total reflection X-ray fluorescence spectrometry was utilized to determine the concentration of U in the preconcentrated samples. In order to understand the mechanism of the CPE procedure, complexation study of the TSIL with U was carried out by isothermal calorimetric titration, liquid-liquid extraction, 31P NMR and IR spectroscopies, and ESI-MS. The developed analytical technique resulted in quantitative extraction efficiency of 99.0 ± 0.5% and a preconcentration factor of 99 for U. The linear dynamic range and method detection limit of the procedure were found to be 0.1-1000 ng mL-1 and 0.02 ng mL-1, respectively. The CPE procedure was found to tolerate a higher concentration of commonly available interfering cations and anions, especially the lanthanides. The developed analytical method was validated by determining the concentration of U in a certified reference material, viz., NIST SRM 1640a natural water, which was found to be in good agreement at a 95% confidence limit with the certified value. The method was successfully applied to the U determination in three natural water samples with ≤4% relative standard deviation (1σ).

Complexation of U(VI) with Cucurbit[5]uril: Thermodynamic and Structural investigation in aqueous medium

The assessment of cucurbituril (CBn) for selective removal of actinides from nuclear waste streams requires comprehensive understanding of binding parameters and coordination of these complexes. The present work is the first experimental report on complexation of actinide ion with Cucurbit[5]uril (CB5) in solution. The thermodynamic parameters (ΔG, ΔH and ΔS) for complexation of CB5 with U(VI) in formic acid water medium were determined using microcalorimetry and UV-Vis spectroscopy. The enthalpy and entropy of complexation revealed the partial binding of U(VI) to CB5 portal. The partial binding was confirmed by spectroscopic techniques viz. extended X absorption fine structure spectroscopy (EXAFS), 1H and 13C NMR. The EXAFS χ(r) versus r spectra for U-CB5 complex has been fitted from 1.4 to 3.5 Å with two oxygen shells and a carbon shell. The presence of three carbon atom in secondary shell shows the involvement of only three carbonyl oxygens directly bonding to U(VI) which is in contrast to that calculated from gas phase DFT calculation of unhydrated system. The combined effect of hydration and formic acid encapsulation led to the enhanced stability of partially bound U(VI) to CB5. In the present work the binding of formic acid has also been studied by fluorescence spectroscopy. ESI-MS data shows the unusual stabilization of U(VI) by CB5 in gas phase.

Complexation of thorium with pyridine monocarboxylate-N-oxides: Thermodynamic and computational studies

Abstract The feed wastes and waste water treatment plants are the major sources for the entry of N-oxides into the soils then to aquatic life. The complexation of actinides with potentially stable anthropogenic ligands facilitate the transportation and migration of the actinides from the source confinement. The present study describes the determination of thermodynamic parameters for the complexation of Th(IV) with the three isomeric pyridine monocarboxylates (PCNO) namely picolinic acid-N-oxide (PANO), nicotinic acid-N-oxide (NANO) and isonicotinic acid-N-oxide (IANO). The potentiometric and isothermal calorimetric titrations were carried out to determine the stability and enthalpy of the formations for all the Th(IV)-PCNO complexes. Th-PANO complexes are more stable than Th-NANO and Th-IANO complexes which can be attributed to chelate formation in the former complexes. Formation of all the Th-PCNO complexes are endothermic and are entropy driven. The geometries for all the predicted complexes are optimized the energies, bond distances and charges on individual atoms are obtained using TURBOMOLE software. The theoretical calculation corroborated the experimental determinations.

Complexation thermodynamics of tetraalkyl diglycolamides with trivalent f-elements in ionic liquids: spectroscopic, microcalorimetric and computational studies

The thermodynamics of lanthanide complexation with a series of diglycolamides (DGAs) containing varying alkyl chain lengths (methyl, ethyl, n-butyl and n-hexyl) was studied in [C4mim][Tf2N], a room temperature ionic liquid. The stability constants, enthalpies and entropies of complexation were determined by spectrophotometry and calorimetry. All the DGA ligands formed a 1 : 3 (Nd3+/DGA) complex as the limiting species, and their stability constants increased linearly with increasing alkyl chain length. The stability constants of the Nd3+/DGA complexes are orders of magnitude higher in [C4mim][Tf2N] as compared to those observed in aqueous medium. For all the complexes, the enthalpy of complexation was negative with a positive entropy change, indicating that the complexation was driven by both enthalpy and entropy. The enthalpy changes observed in [C4mim][Tf2N] medium were more exothermic than those in the aqueous medium. Fluorescence lifetime data indicated that the complexation proceeded via the replacement of water molecules from the primary coordination sphere of the metal ion. DFT calculations were performed on the structures of the different Nd3+/tetramethyl DGA (TMDGA) complexes, and the corresponding free energies in both gas and solution phases were calculated. Insights into the structural features by DFT studies confirmed that the nature of Ln3+/DGA complexes formed in [C4mim][Tf2N], n-dodecane, and aqueous medium and that in the crystalline state is identical.