Guoxin Tian

Thermodynamic studies of U(VI) complexation with glutardiamidoxime for sequestration of uranium from seawater

Glutardiamidoxime (H2B), a diamidoxime ligand that has implications in sequestering uranium from seawater, forms strong complexes with UO2(2+). Five U(VI) complexes were identified in 3% NaCl solution. The stability constants and the enthalpies of complexation were measured by potentiometry and microcalorimetry. The competition between glutardiamidoxime and carbonate for complexing U(VI) in 3% NaCl was also studied in comparison with the cyclic glutarimidedioxime ligand (H2A) previously studied.

Extraction of Lanthanide and Actinide Ions from Aqueous Mixtures Using a Carboxylic Acid-Functionalized Porous Aromatic Framework

Porous aromatic frameworks (PAFs) incorporating a high concentration of acid functional groups possess characteristics that are promising for use in separating lanthanide and actinide metal ions, as required in the treatment of radioactive waste. These materials have been shown to be indefinitely stable to concentrated acids and bases, potentially allowing for multiple adsorption/stripping cycles. Additionally, the PAFs combine exceptional features from MOFs and inorganic/activated carbons giving rise to tunable pore surfaces and maximum chemical stability. Herein, we present a study of the adsorption of selected metal ions, Sr2+, Fe3+, Nd3+, and Am3+, from aqueous solutions employing a carbon-based porous aromatic framework, BPP-7 (Berkeley Porous Polymer-7). This material displays high metal loading capacities together with excellent adsorption selectivity for neodymium over strontium based on Langmuir adsorption isotherms and ideal adsorbed solution theory (IAST) calculations. Based in part upon X-ray absorption spectroscopy studies, the stronger adsorption of neodymium is attributed to multiple metal ion and binding site interactions resulting from the densely functionalized and highly interpenetrated structure of BPP-7. Recyclability and combustibility experiments demonstrate that multiple adsorption/stripping cycles can be completed with minimal degradation of the polymer adsorption capacity.

Carbonate–H2O2 leaching for sequestering uranium from seawater

Uranium adsorbed on amidoxime-based polyethylene fiber in simulated seawater can be quantitatively eluted at room temperature using 1 M Na2CO3 containing 0.1 M H2O2. This efficient elution process is probably due to the formation of an extremely stable uranyl-peroxo-carbonato complex in the carbonate solution. After washing with water, the sorbent can be reused with minimal loss of uranium loading capacity. Possible existence of this stable uranyl species in ocean water is also discussed.

Complexation of Uranium(VI) by Gluconate in Acidic Solutions : a Thermodynamic Study with Structural Analysis

Within the pC(H) range of 2.5 to 4.2, gluconate forms three uranyl complexes UO(2)(GH(4))(+), UO(2)(GH(3))(aq), and UO(2)(GH(3))(GH(4))(-), through the following reactions: (1) UO(2)(2+) + GH(4)(-) = UO(2)(GH(4))(+), (2) UO(2)(2+) + GH(4)(-) = UO(2)(GH(3))(aq) + H(+), and (3) UO(2)(2+) + 2GH(4)(-) = UO(2)(GH(3))(GH(4))(-) + H(+). Complexes were inferred from potentiometric, calorimetric, NMR, and EXAFS studies. Correspondingly, the stability constants and enthalpies were determined to be log beta(1) = 2.2 +/- 0.3 and DeltaH(1) = 7.5 +/- 1.3 kJ mol(-1) for reaction (1), log beta(2) = -(0.38 +/- 0.05) and DeltaH(2) = 15.4 +/- 0.3 kJ mol(-1) for reaction (2), and log beta(3) = 1.3 +/- 0.2 and DeltaH(3) = 14.6 +/- 0.3 kJ mol(-1) for reaction (3), at I = 1.0 M NaClO(4) and t = 25 degrees C. The UO(2)(GH(4))(+) complex forms through the bidentate carboxylate binding to U(VI). In the UO(2)(GH(3))(aq) complex, hydroxyl-deprotonated gluconate (GH(3)(2-)) coordinates to U(VI) through the five-membered ring chelation. For the UO(2)(GH(3))(GH(4))(-) complex, multiple coordination modes are suggested. These results are discussed in the context of trivalent and pentavalent actinide complexation by gluconate.

Complexation of U(VI) with dipicolinic acid: thermodynamics and coordination modes.

Complexation of UO2(2+) with dipicolinic acid (DPA) has been investigated in 0.1 M NaClO4. The stability constants (log β1 and log β2) for two successive complexes, UO2L and UO2L2(2-) where L(2-) stands for the deprotonated dipicolinate anion, were determined to be 10.7 ± 0.1 and 16.3 ± 0.1 by spectrophotometry. The enthalpies of complexation (ΔH1 and ΔH2) were measured to be -(6.9 ± 0.2) and -(28.9 ± 0.5) kJ·mol(-1) by microcalorimetry. The entropies of complexation (ΔS1 and ΔS2) were calculated accordingly to be (181 ± 3) and (215 ± 4) J·K(-1)·mol(-1). The strong complexation of UO2(2+) with DPA is driven by positive entropies as well as exothermic enthalpies. The crystal structure of Na2UO2L2(H2O)8(s) shows that, in the 1:2 UO2(2+)/DPA complex, the U atom sits at a center of inversion and the two DPA ligands symmetrically coordinate to UO2(2+) via its equatorial plane in a tridentate mode. The structural information suggests that, due to the conjugated planar structure of DPA with the donor atoms (the pyridine nitrogen and two carboxylate oxygen atoms) arranged at optimal positions to coordinate with UO2(2+), little energy is required for the preorganization of the ligand, resulting in strong UO2(2+)/DPA complexation.

Thermodynamics of the Complexation of Uranium(VI) by oxalate in aqueous solution at 10-70oC

The protonation reactions of oxalate (ox) and the complex formation of uranium(vi) with oxalate in 1.05 mol kg(-1) NaClO(4) were studied at variable temperatures (10-70 degrees C). Three U(vi)/ox complexes (UO(2)ox(j)((2-2j)+) with j = 1, 2, 3) were identified in this temperature range. The formation constants and the molar enthalpies of complexation were determined by spectrophotometry and calorimetry. The complexation of uranium(vi) with oxalate ion is exothermic at lower temperatures (10-40 degrees C) and becomes endothermic at higher temperatures (55-70 degrees C). In spite of this, the free energy of complexation becomes more negative at higher temperatures due to increasingly more positive entropy of complexation that exceeds the increase of the enthalpy of complexation. The thermodynamic parameters at different temperatures, in conjunction with the literature data for other dicarboxylic acids, provide insight into the relative strength of U(vi) complexes with a series of dicarboxylic acids (oxalic, malonic and oxydiacetic) and rationalization for the highest stability of U(vi)/oxalate complexes in the series. The data reported in this study are of importance in predicting the migration of uranium(vi) in geological environments in the case of failure of the engineering barriers, which protect waste repositories.

Hydrolysis of Plutonium(VI) at Variable Temperatures (283–343 K)

The hydrolysis of Pu(VI) was studied at variable temperatures (283-343 K) by potentiometry, microcalorimetry, and spectrophotometry. Three hydrolysis reactions, mPuO(2)(2+) + nH(2)O=(PuO(2))(m)(OH)(n) (2m-n)( +) + nH(+)), in which (n,m)=(1,1), (2,2), and (5,3), were invoked to describe the potentiometric and calorimetric data. The equilibrium constants (*β(n,m)) were determined by potentiometry at 283, 298, 313, 328, and 343 K. As the temperature was increased from 283 to 343 K, *β(1,1), *β(2,2), and *β(5,3), increased by 1, 1.5, and 4 orders of magnitude, respectively. The enhancement of hydrolysis at elevated temperatures is mainly due to the significant increase of the degree of ionization of water as the temperature increases. Measurements by microcalorimetry indicate that the three hydrolysis reactions are all endothermic at 298.15 K, with enthalpies of (35.0±3.4) kJ mol(-1), (65.4±1.0) kJ mol(-1), and (127.7±1.7) kJ mol(-1) for ΔH(1,1), ΔH(2,2), and ΔH(5,3), respectively. The hydrolysis constants at infinite dilution have been obtained with the Specific Ion Interaction approach. The applicability of three approaches for estimating the equilibrium constants at different temperatures, including the constant enthalpy approach, the DQUANT equation, and the Ryzhenko-Bryzgalin model, were evaluated with the data from this work.

Protonation of D-gluconate and its complexation with Np(V) in acidic to nearly neutral solutions

Thermodynamic properties of the protonation of D-gluconic acid (HGH4(aq)) and its complexation with Np(V) have been studied in acidic to nearly neutral solutions at t =25 °C and I=1 M NaClO4 by potentiometry, spectrophotometry and calorimetry. The protonation constant (logKH) and enthalpy (ΔHH) of the carboxylate group are determined to be (3.30±0.10) and −(4.03±0.07) kJ mol-1, respectively. Gluconate forms two Np(V) complexes in nearly neutral solutions. The formation constants and enthalpies of complexation are: log β1 = (1.48±0.03) and ΔH1 = −(7.42±0.13) kJ mol-1 for NpO2(GH4)(aq), log β2=(2.14±0.09) and ΔH2= −(12.08±0.45) kJ mol-1 for NpO2(GH4)2-. The thermodynamic parameters indicate that gluconic acid, like isosaccharinic acid and other α-hydroxycarboxylic acids, is a slightly stronger acid and forms stronger complexes with Np(V) than simple monocarboxylic acids.

Glutarimidedioxime: A Complexing and Reducing Reagent for Plutonium Recovery from Spent Nuclear Fuel Reprocessing

Efficient separation processes for recovering uranium and plutonium from spent nuclear fuel are essential to the development of advanced nuclear fuel cycles. The performance characteristics of a new salt-free complexing and reducing reagent, glutarimidedioxime (H2A), are reported for recovering plutonium in a PUREX process. With a phase ratio of organic to aqueous of up to 10:1, plutonium can be effectively stripped from 30% tributyl phosphate (TBP) in kerosene into 1 M HNO3 with H2A. The complexation-reduction mechanism is illustrated with the combination of UV/Vis absorption spectra and the crystal structure of a Pu(IV) complex with the reagent. The fast stripping rate and the high efficiency for stripping Pu(IV), through the complexation-reduction mechanism, is suitable for use in centrifugal contactors with very short contact/resident times, thereby offering significant advantages over conventional processes.

Thermodynamic and Structural Trends in Hexavalent Actinyl Cations: Complexation of Dipicolinic Acid with NpO22+ and PuO22+ in Comparison with UO22+

The complexation of NpO2(2+) and PuO2(2+) with dipicolinic acid (DPA) has been investigated in 0.1 M NaClO4 by spectrophotometry, microcalorimetry, and single crystal diffractometry. Formation of 1:1 and 1:2 (metal/ligand molar ratio) complexes of DPA with NpO2(2+) and PuO2(2+) were identified and the thermodynamic parameters were determined and compared with those of UO2(2+) . All three hexavalent actinyl cations form strong 1:1 DPA complexes with slightly decreasing but comparable stability constants from UO2(2+) to PuO2(2+), whereas the stability constants of the 1:2 complexes (logβ2) decrease substantially along the series (16.3 for UO2L2(2-), 15.17 for NpO2L2(2-), and 14.17 for PuO2L2(2-) at 25 °C). The enthalpies of complexation for the 1:2 complexes become less exothermic from UO2L2(2-) (-28.9 kJ mol(-1)), through NpO2L2(2-) (-27.2 kJ mol(-1)), and to PuO2L2(2-) (-22.7 kJ mol(-1)). The trends in the thermodynamic parameters are discussed in terms of the effective charge of the cations and the steric constraints in the structures of the complexes. In addition, the features of the absorption spectra, including the wavelength and intensity of the absorption bands, are related to the perturbation of the ligand field and the symmetry of the actinyl complexes.