Punam Thakur

Sorption of U(VI) species on hydroxyapatite

Abstract The sorption of uranyl (UO22+) cations to hydroxyapatite was studied as a function of the amount of sorbent, ionic strength, U(VI) concentration, pH and temperature. The rate of uranyl sorption on hydroxyapatite decreased with increased uranyl concentrations. The amount sorbed decreased with increased ionic strength and increased with pH to a maximum at 7–8. The sorption data for UO22+ were fitted well by the Freundlich and Dubinin–Radushkevich (D–R) isotherms. The anions Cl−, NO3−, SO42- and CH3COO− decreased the sorption of uranium on hydroxyapatite while S2O32- slightly increased it. The sorbed uranium was desorbed by 0.10 M and 1.00 M solutions of HCl and HNO3. The thermodynamic parameters for the sorption of UO22+ were measured at temperatures of 298, 313, 323 and 333 K. The temperature dependence confirmed an endothermic heat of sorption. The activation energy for the sorption process was calculated to be +2.75±0.02 kJ/mol.

Complexation studies of Cm(III), Am(III), and Eu(III) with linear and cyclic carboxylates and polyaminocarboxylates

Solvent extraction and potentiometric titration methods have been used to measure the stability constants of Cm(III), Am(III), and Eu(III) with both linear and cyclic carboxylates and polyaminocarboxylates in an ionic strength of 0.1 mol L−1 (NaClO4). Luminescence lifetime measurements of Cm(III) and Eu(III) were used to study the change in hydration upon complexation over a range of concentrations and pH values. Aromatic carboxylates, phthalate (1,2 benzene dicarboxylates, PHA), trimesate (1,3,5 benzene tricarboxylates, TSA), pyromellitate (1,2,4,5 tetracarboxylates, PMA), hemimellitate (1,2,3 benzene tricarboxylates, HMA), and trimellitate (1,2,4 benzene tricarboxylates, TMA) form only 1 : 1 complexes, while both 1 : 1 and 1 : 2 complexes were observed with PHA. Their complexation strength follows the order: PHA∼TSA>TMA>PMA>HMA. Carboxylate ligands with adjacent carboxylate groups are bidentate and replace two water molecules upon complexation, while TSA displaces 1.5 water molecules of hydration upon complexation. Only 1 : 1 complexes were observed with the macrocyclic dicarboxylates 1,7-diaza-4,10,13-trioxacyclopentadecane-N,N′-diacetate (K21DA) and 1,10-diaza-4,7,13,16-tetraoxacyclooctadecane-N,N′-diacetate (K22DA); both 1 : 1 and 1 : 2 complexes were observed with methyleneiminodiacetate (MIDA), hydroxyethyleneiminodiacetate (HIDA), benzene-1,2-bis oxyacetate (BDODA), and ethylenediaminediacetate (EDDA), while three complexes (1 : 1, 1 : 2, and 1 : 3) were observed with pyridine 2,6 dicarboxylates (DPA) and chelidamate (CA). The complexes of M-MIDA are tridentate, while that of M-HIDA is tetradentate in both 1 : 1 and 1 : 2 complexes. The M-BDODA and M-EDDA complexes are tetradentate in the 1 : 1 and bidentate in the 1 : 2 complexes. The complexes of M-K22DA are octadentate with one water molecule of hydration, while that of K21DA is heptadentate with two water molecules of hydration. Simple polyaminocarboxylate 1,2 diaminopropanetetraacetate (PDTA) and ethylenediamine N,N′-diacetic-N,N′-dipropionate (ENDADP) like ethylenediaminetetraacetate (EDTA) form only 1 : 1 complexes and their complexes are hexadentate. Polyaminocarboxylates with additional functional groups in the ligand backbone, e.g., ethylenebis(oxyethylenenitrilo) tetraacetate (EGTA), and 1,6 diaminohexanetetraacetate (HDTA) or with additional number of groups in the carboxylate arms diethylenetriamine pentaacetato-monoamide (DTPA-MA), diethylenetriamine pentaacetato-bis-methoxyethylamide (DTPA-BMEA), and diethylenetriamine pentaacetato-bis glucosaamide (DTPA-BGAM) are octadentate with one water molecule of hydration, except N-methyl MS-325 which is heptadentate with two water molecules of hydration and HDTA which is probably dimeric with three water molecules of hydration. Macrocyclic tetraaminocarboxylate, 1,4,7,10-tetraazacyclododecanetetraacetate (DOTA) forms only 1 : 1 complex which is octadentate with one water molecule of hydration. The functionalization of these carboxylates and polycarboxylates affect the complexation ability toward metal cations. The results, in conjunction with previous results on the Eu(III) complexes, provide insight into the relation between ligand steric requirement and the hydration state of the Cm(III) and Eu(III) complexes in solution. The data are discussed in terms of ionic radii of the metal cations, cavity size, basicity, and ligand steric effects upon complexation.

Complexation thermodynamics and structural studies of trivalent actinide and lanthanide complexes with DTPA, MS-325 and HMDTPA

Abstract The protonation constants of DTPA (diethylenetriaminepentaacetic acid) and two derivatives of DTPA, 1-R(4,4-diphenyl cyclohexyl-phosphonyl-methyl diethylenentriaminepentaacetic acid (MS-325) and (R)-hydroxymethyl-diethylenentriaminepentaacetic acid (HMDTPA) were determined by potentiometric titration in 0.1 M NaClO4. The formation of 1 : 1 complexes of Am3+, Cm3+ and Ln3+ cations with these three ligands were investigated by potentiometric titration with competition by ethylenediaminetetraacetic acid (EDTA) and the solvent extraction method in aqueous solutions of I=0.10 M NaClO4. The thermodynamic data of complexation were determined by the temperature dependence of the stability constants and by calorimetry. The complexation is exothermic and becomes weaker with increase in temperature. The complexation strength of these ligands follows the order: DTPA ≈ HMDTPA > MS-325. Eu3+/Cm3+ luminescence, EXAFS (Extended X-ray Absorption Fine Structure) and DFT (Density Functional Theory) calculations suggest that all three ligands are octadentate in the complex. In the complex, M(L)2− (L = DTPA, MS-325 and HMDTPA). The M3+ binds via five carboxylates oxygen atoms, three nitrogen atoms, and the complex contains one water of hydration.

Polymerization and Complexation Behavior of Silicic Acid: A Review

The role of dissolved silica (silicic acid) in the migration of radionuclides has been of interest for several decades. Studies have shown that the interaction of silicic acid with the metal cations is complicated due to a tendency to polymerize which is dependent on pH, ionic strength, temperature, etc. of the system. While polymerization of silicic acid has been studied in detail, uncertainty remains on the effect of these changes on cations in environmental systems. The nature and the role of silicate species in the environmental behavior of cationic radionuclides is the focus of this review. The interaction of metal cations such as U02, Eu, Cm and Cu with polymeric silica is discussed. The effects of different experimental parameters (e.g., pH, ionic strength, temperature, acids, fluoride and hydroxyl anions, organic compounds, etc.) on the polymerization behavior of silicic acid are reviewed. Monomeric silicic acid is most stable at ca. pH 2 and most unstable at pH 7-8. Polymerization increases with increased ionic strength, temperature, and the aging time of the silicic acid solutions. Hydroxyl and fluoride ions catalyze the polymerization of silicic acid. The presence of metal ions, (e.g., aluminium, beryllium, thorium and iron) inhibits the polymerization process through complexation with hydroxyl and fluoride ions. However, the presence of monoand divalent inorganic salts in silicic acid solutions accelerates its polymerization.

Thermodynamic modeling of trivalent Am, Cm and Eu-citrate complexation in concentrated NaClO4 media

Abstract The dissociation constants of citric acid (Cit), and the stability constants of Am3+, Cm3+and Eu3+ with Cit were determined as a function of ionic strength (NaClO4) using potentiometric titration and an extraction technique, respectively. The results have shown the presence of both 1:1 and 1:2 complexes under the experimental conditions. A thermodynamic model was constructed to predict the apparent stability constants at different ionic strengths by applying the Pitzer ionic interaction parameters β(0), β(1), and Cφ which were obtained to fit the experimental data. Thermodynamic stability constants of M(Cit) and M(Cit)23- (where M = Am3+, Cm3+ or Eu3+) were calculated to be log β0101 = 9.91 ± 0.10, log β0102 = 14.47 ± 0.14 for Am3+, log β0101 = 9.53 ± 0.16, log β0102 = 14.46 ± 0.16 for Cm3+ and log β0101 = 9.82 ± 0.14, log β0102 = 13.31 ± 0.12 for Eu3+ as obtained by extrapolation to zero ionic strength.

Np(V)O2+ sorption on hydroxyapatite-effect of calcium and phosphate anions

The sorption of NpO2+ from aqueous solution on hydroxyapatite was studied as a function of the amount of sorbent, initial NpO2+ concentration, ionic strength and pcH. The hydroxyapatite was characterized by SEM, EDS, XRD, FT-IR and ICP-MS measurements. At ionic strengths of 0.10 to 5.00 M NaClO4, the sorption increased with increased pcH to a maximum between pcH 8−8.5, then decreased as the pcH increased. The kinetics of NpO2+ sorption on hydroxyapatite followed Lagergren first order kinetics. The temperature dependence of sorption was small in the range of 273−283 K, but increased more sharply at higher temperatures of 298−333 K. The heat of sorption of NpO2+ was endothermic and the free energy values were exothermic indicating large, positive entropy. The activation energy for the sorption process was calculated to be 29.52±1.2 kJ/mole. The effect of calcium and phosphate on NpO2+ sorption was studied as a function of concentration and pcH.