Osamu Tochiyama

Dissolution rates of amorphous silica in highly alkaline solution

Cement is an essential materials to construct the subsurface radioactive waste disposal system. However, cementitious materials alter the groundwater pH to highly alkaline condition about 13. To comprehend the effect of such a hyperalkaline condition on the repository surroundings, this study focused on the dissolution rates of amorphous silica at [NaOH]=10-1 mol-dm-3. The used samples were three kinds of pure commercial silica and a natural silica scale which was obtained from inside wall of the hot-water pipe of a geothermal power plant. The observed dissolution rates were interpreted with using the model, which assumed that the particle sizes decrease with the progress of dissolution. Moreover, due to the particle size distribution anticipated in the natural silica scale, this analysis assumed it contained particles with various initial diameters. In the results, (1) all pure silica samples and at least 60wt% of the silica scale showed good agreement of the activation energy of the dissolution in the range of 77 through 88kJ-mol-1 in the highly alkaline solution, (2) these rate constants were of the order of 10-8-10-7 mol-m-2.s-1 at around 310 K and were definitely larger than those already reported for quartz, (3) the specific surface area based on BET method was revealed to be an important factor to give the main difference in the dissolution rates between the synthetic silica and the natural silica.

Complex formation of calcium with humic acid and polyacrylic acid

Summary In order to understand the migration behavior of radionuclides in the underground, it is also important to estimate the effect of the competing cations originally present in the groundwater. In this connection, the complexation of Ca(II) with Aldrich humic acid has been examined. For the study at trace concentrations (∼10−10 M) of Ca(II), the solvent extraction of 45Ca with TTA and TOPO in cyclohexane has been used. At macro concentrations (10−4 M) of Ca(II), the measurement of the free Ca2+ ion concentration with a calcium selective electrode has been conducted. To estimate the polyelectrolyte effect of humic acid separately from its heterogeneous composition effect, polyacrylic acid ([-CH2CH(COOH)-]n) has been selected as a representative of the homogeneous polymeric weak acids and its complexation with Ca(II) has also been examined. The values of logβα have been obtained at pH 5∼7 in 0.1, 0.4 and 1.0 M NaCl, where βα is the apparent formation constants defined by βα=[ML]/([M][R]). In this definition, [ML] and [M] are the concentrations of bound and free Ca2+ respectively, [R] is the concentration of dissociated proton exchanging sites. logβα of humate decreases from 2.19∼2.92 (depending on pH and ionic strength 1.0 < I < 0.4) at pCa= 10 to 1.98∼2.44 at pCa = 4, while the variation of pCa has no appreciable influence on the logβα of polyacrylate (1.36∼3.24 for I=0.1∼1.0). For both humate and polyacrylate, logβα decreases linearly with log[Na+], where [Na+] is the bulk concentration of sodium ion. Their dependences of logβα on ionic strength are stronger than those of logβ of monomeric carboxylates such as oxalate and EDTA, indicating the large electrostatic effect of crowded negative charges on the macromolecules of humic or polyacrylic acid.

Determination of the oxidation states of neptunium at tracer concentrations by adsorption on silica gel and barium sulfate

Abstract The adsorption method for determination of the distribution of neptunium among its oxidation states at tracer concentrations by use of silica gel and barium sulfate as adsorbents is described. In the pH range 6–7, Np(VI) was quantitatively adsorbed on silica gel, while Np(V) remained in the supernatant solution. In acid solutions, the precipitate of barium sulfate adsorbed Np(IV) selectively, leaving Np(V) and Np(VI) in solution. By means of these adsorption methods combined with the use of TBP extractions, the oxidation-state distribution of neptunium in acid or neutral solution in the presence of various redox reagents was determined and shown to be consistent with the expectation from the oxidation potential of the solution.

Luminescence properties of tetravalent uranium in aqueous solution

Summary The luminescence spectra of U4+ in aqueous solutions were observed in the UV-VIS region at ambient and liquid nitrogen temperatures. The excitation spectrum indicates that the luminescence is arising from the deexcitation of a 5f electron at the 1S0 level and no other emissions of U4+ in aqueous solutions were detected for other f–f transitions. All the luminescence peaks were assigned to the transitions from 1S0 to lower 5f levels. To estimate the luminescence lifetime, luminescence decay curves were measured using time-resolved laser-induced fluorescence spectroscopy. At room temperature, the decay curve indicated that the lifetime was shorter than 20 ns. On the other hand, the frozen sample of U4+ in aqueous solution at liquid nitrogen temperature showed the same emission spectrum as at room temperature and its lifetime was 149 ns in H2O system and 198 ns in D2O system. The longer lifetime at liquid nitrogen temperature made it possible to measure the spectrum of U4+ at the concentration as low as 10-6 M. The difference in the anion species (ClO4-, Cl-, SO42-) affected the structure of the emission spectrum to some extent.

Study of the complexes of Np(V) with organic ligands by solvent extraction with TTA and 1,10-phenanthroline

Abstract Complex formation constants of Np(V) with 22 organic ligands, 7 hydroxycarboxylic acids, 4 dicarboxylic acids, 4 aminocarboxylic acids, 3 pyridinecarboxylic acids, 8-hydroxyquinoline-5-sulfonic acid, IDA, NTA and EDTA, have been determined in 1M NaClO 4 at 25°C by using the solvent extraction method with TTA and 1, 10-phenanthroline. The factors influencing the stabilities of Np(V) complexes are discussed in connection with the linear structure of NpO 2 + .

Thermodynamic study on the U(VI) complexation with dicarboxylates by calorimetry

Abstract The thermodynamic quantities (ΔG, ΔH and ΔS) of U(VI) complexation with oxalate, malonate, succinate, glutarate, adipate, phthalate, tartronate and malate, and of Eu(III) complexation with succinate, glutarate and adipate were determined by potentiometric and calorimetric titration techniques. The obtained thermodynamic quantities indicated that these complexation reactions are mainly driven by the entropy changes while the enthalpy changes are nearly zero or preventing the complexation. The thermodynamic quantities of 1:1 U(VI) complexes with 5 dicarboxylates having different carbon chain lengths (−OOC−(CH2)n−COO−) showed an interesting tendency, that is, −ΔH decreased and TΔS increased systematically with increasing the length of the carbon chain from n = 2 to 6. The comparison of TΔS values of 1:1 carboxylate formation with U(VI) and Eu(III) showed that TΔS of U(VI) were always lager than those of Eu(III). The entropies of 1:1 complex formation of U(VI) with hydroxycarboxylates were always smaller than those with carboxylates of the same carbon chain length, while the enthalpies of uranyl hydroxycarboxylates were always larger. This tendency was also observed in the protonation and the complex formation of Eu(III).

Modeling of the complex formation of metal ions with humic acids

Summary Based on the conceptual model of cation-humic interactions considering the heterogeneous composition and polyelectrolyte nature of humic acids, the description of the interaction by the expression, log Kapp= log K - m log [M]+ a log α - b log [Na+], is proposed, where Kapp=[M··(zR)]/([M][R]), [M··(zR)] is the concentration of the cation bound to the humic acid, [M] is the concentration of the free cation with charge z, and [R] is the concentration of the free anionic sites of the humic acid, [R]=(CR-z[M··(zR)])α (CR is a total concentration of proton exchanging sites and α is a degree of dissociation). log K, m, a and b are the constant parameters depending on the cation and the humic acid. The expression is successfully applied to the experimental results on the complex formation (or protonation) of humate and polyacrylate (a homogeneous polymeric weak acid) with H+, NpO2+, Eu3+, Ca2+, Fe2+ and Fe3+ obtained under the condition where hydrolysis of cations and saturation of the reacting sites can be neglected.

Speciation study on complex formation of uranium(VI) with phosphate and fluoride at high temperatures and pressures by time-resolved laser-induced fluorescence spectroscopy

Summary The complex formation of U(VI) with phosphate and with fluoride has been studied at elevated temperatures (20–150 °C) and pressures (0.1–40 MPa) using time-resolved laser-induced fluorescence spectroscopy (TRLFS). Emission spectra and lifetimes of 5×10–5 M uranium(VI) in 0.5 M NaClO4 solutions in the presence and absence of ligand ions were measured as a function of the pH, ligand concentration, temperature and pressure. The emission intensities and lifetimes of uranium(VI) decreased rapidly with increasing temperature. In contrast, a pressure effect on the fluorescence properties was quite small. The fluorescence decay curves could be resolved into several fluorescent species. The temperature dependence of the lifetimes followed the Arrhenius law, and the activation energies were estimated for the phosphate and fluoride complexes. These results were compared with speciation calculations at several temperatures obtained by using the DQUANT equation and thermodynamic data in the literature. From these comparisons, each component of the fluorescence was identified as a complexed species or a free uranyl ion at several temperatures, indicating that the Arrhenius plot of fluorescence lifetime is an effective method for in-situ speciation of uranyl complexation at elevated temperatures.

Determination of the thermodynamic quantities of complexation between Eu(III) and carboxylic acids by microcalorimetry

Potentiometric and microcalorimetric titration techniques were applied to determine the Gibbs free energies and enthalpies of the protonation of some carboxylic acids (acetic, glycolic, malonic and malic acids) and their complexation with Eu(III) in 1.0 M NaClO4 solution at 25 °C, where Eu(III) was used as a chemical analogue of radiologically important Am(III). By using the values of ΔG determined by potentiometric titrations, the results of calorimetric titrations were analyzed to give the values of ΔH and ΔS. To support the discussion on the obtained thermodynamic quantities, the hydration numbers of Eu(III) in the complexes were obtained by TRLFS measurement, where the luminescence lifetime of Eu(III) excited by 394 nm laser was measured. A few to several kJ/mol of enthalpies were determined for the reactions within the uncertainties of ±0.01 to ±0.4. These enthalpy values indicated that the protonation of these carboxylates were entropy-driven, that is, |−TΔS|>>|ΔH| in ΔG=ΔH −TΔS. The complexations of Eu(III) were also shown to be controlled mainly by entropy term. The results of TRLFS measurement revealed that the number of dehydrated water molecule increased with the degree of complexation. A linear relation was found between total entropy change of the system (ΔST) and the net number of released molecules from the central Eu(III) ion. However, the values of ΔST for the hydroxy-carboxylates were smaller than that for simple carboxylates at the same net number of molecules released from Eu(III), suggesting that the dehydration occurred not only in the cation but also in the anion.

Determination of the phenolic-group capacities of humic substances by non-aqueous titration technique

The phenolic-group capacities of five humic substances, such as, the Aldrich humic acid, the humic and fulvic acids extracted from a soil, the humic and fulvic acids extracted from a peat have been precisely determined by the non-aqueous potentiometric titration technique. The titration by KOH in the mixed solvent of DMSO:2-propanol:water=80:19.3:0.7 at [K(+)]=0.02 M enabled to measure the potential change in a wide range of pOH (=-log[OH(-)]), and thus to determine the capacities of phenolic groups which could not be precisely determined in the aqueous titration. The results of the titration revealed that the mean protonation constants of the phenolic groups were nearly the same for all humic substances and close to that of phenol in the same medium, indicating that each phenolic-group in the humic substances is rather isolated and is not electronically affected by other affecting groups in the humic macromolecule.