Shinya Nagasaki

Adsorption behavior of IO3− by CO32−- and NO3−-hydrotalcite

Abstract For the improvement of radioactive waste disposal, the materials with high adsorption capacity for IO 3 − should be used. Hydrotalcite-type (HT) compounds are considered potential materials. The adsorption behavior of IO 3 − by hydrotalcite-type compounds with CO 3 − (HTCO 3 ) or NO 3 − (HTNO 3 ) interlayer anions was studied. Adsorption equilibrium was reached more quickly for HTCO 3 than for HTNO 3 . The adsorption isotherm for HTCO 3 consisted of two Langmuir type adsorption processes. The desorption behavior was quite different for HTCO 3 –HTNO 3 . The results suggested that IO 3 − is adsorbed on the external surface of HTCO 3 whereas IO 3 − is exchanged for interlayer NO 3 − .

Sorption and reduction of neptunium(V) on the surface of iron oxides

Summary Sorption and desorption experiments of Np on magnetite and hematite under aerobic and anaerobic conditions were carried out to investigate the possibility of reduction of Np(V) to Np(IV) at pH 4 to 8 within 7 days. The amount of sorbed Np on magnetite under anaerobic conditions was about 2 or 3 times greater than that under aerobic conditions. Furthermore, the results of desorption experiments indicated that the dominant sorption behavior of Np on magnetite under anaerobic conditions was quite different from that under aerobic conditions. The oxidation state of Np sorbed on the iron oxides was determined by extraction technique using 0.5 M TTA in xylene and 2.0 M HNO3 solution from the solid phase after sorption experiment. 90% and 10% of extracted Np was Np(IV) for magnetite system under anaerobic and aerobic conditions, respectively. On the other hand, almost 100% of extracted Np was Np(V) for hematite system under both aerobic and anaerobic conditions. These results indicated that Np(V) was reduced to Np(IV) by Fe(II) in magnetite. Redox reaction between Np(V) and Fe(II) was also studied in homogeneous solution without solid to decide if Fe(II) ions released from magnetite into silution or Fe(II) on the solid cause the reduction. Only 6% of Np(V) was reduced by Fe(II) at pH=4 and 6 even after 7 days. According to this result, it was conjectured that the reduction of Np(V) to Np(IV) takes place not in the liquid phase by Fe(II) ion but on the surface of magnetite.

Silicate Anion Structural Change in Calcium Silicate Hydrate Gel on Dissolution of Hydrated Cement

High pH conditions of aqueous solutions in a radioactive waste repository can be brought about by dissolution of cementitious materials. In order to clarify the mechanisms involved in maintaining this high pH for long time, we investigated the dissolution phenomena of OPC hydrate. In the present research, leaching tests on powdered cement hydrates were conducted by changing the ratio of mass of leaching water to mass of OPC hydrate (liquid/solid ratio) from 10∽2, 000 (wt/wt). Ordinary Portland Cement hydrate was contacted with deionized water and placed in a sealed bottle. After a predetermined period, the solid was separated from the solution. From the results of XRD analysis on the solid phase and the Ca concentration in the aqueous phase, it was confirmed that Ca(OH)2 was preferentially dissolved when the liquid/solid ratio was 10 or 100 (wt/wt), and that C—S—H gel as well as Ca(OH)2 were dissolved when the liquid/solid ratio was 500 (wt/wt) or larger. 29Si-NMR results showed that the silicate anion chain of the C-S-H gel became longer when the liquid/solid ratio was 500 (wt/wt) or greater. This indicates that leaching of OPC hydrate results in a structural change of C—S—H gel.

A model for dissolution of CaO-SiO2-H2O gel at Ca/Si > 1

Abstract Calcium silicate hydrate (CaO-SiO2-H2O) gel is the principal hydration product of Portland cement. Experimental data show that the dissolution of CaO-SiO2-H2O (C-S-H) gel is strongly dependent on Ca/Si (C/S) ratio in the range of C/S > 1. A model for dissolution of C-S-H gel is presented by considering a nonideal mixture of binary solid solutions of Margules type. Guggenheim and Prausnitz equations are applied to represent the activities of model solids as a function of mole fractions. The Gibbs-Duhem equation, incorporating the activities of model solids, is then used to express the conditional solubility products of model solids in terms of C/S ratio. The determination of Guggenheims empirical parameters is performed with geochemical code PHREEQE on experimental data. By these procedures, a dissolution model of C-S-H gel is described. The solubility results predicted by the proposed model are comparable with experimental data.

Application of Parallel Factor Analysis for Time-Resolved Laser Fluorescence Spectroscopy: Implication for Metal Speciation Study

Time-resolved laser fluorescence spectroscopy (TRLFS) is an analytical technique capable of discriminating different chemical species of a fluorescent metal ion such as UO(2)(2+), Cm(3+), and lanthanides. Although TRLFS has been widely used to investigate the speciation of the fluorescent metal ions, extracting quantitative and structural information from multiple TRLFS data measured as a function of chemical and physical parameters is not a simple task. The purpose of this study is to apply parallel factor analysis (PARAFAC) for the interpretation of a series of TRLFS data. PARAFAC is a robust technique because it utilizes the entire information contained in a multiway TRLFS data set. The complexation of Eu(3+) by acetate was studied as a test case for the PARAFAC decomposition. It is shown that three factors are necessary and sufficient to explain the systematic variations in the original data set. The resulting spectra, decay, and relative concentrations of the factors were all in agreement with the fluorescent properties and the complexation behaviors of Eu(3+)-acetate complexes. Based on these results, it was concluded that PARAFAC is a promising data analysis tool for TRLFS used for the speciation studies of fluorescent metal ions.

Reduction rate of neptunium(V) in heterogeneous solution with magnetite

Summary The sorption kinetics of neptunium on magnetite was investigated under both aerobic and anaerobic conditions in 0.1mol L-1 NaNO3 at pH=5.7 to 5.9. It was found that the sorption of neptunium on magnetite reaches an equilibrium in 1h under aerobic conditions, while it takes 10h or more to reach the equilibrium under anaerobic conditions. The difference in sorption kinetics between aerobic and anaerobic conditions took place because sorption kinetics under the anaerobic condition involved the reduction of Np(V) to Np(IV), confirmed by an extraction technique using 0.5mol L-1 TTA in xylene and 2.0mol L-1 HNO3 solution. The reduction rate of Np(V) by Fe(II) in magnetite was calculated in solution with magnetite. It was found that Np(V) was reduced 1000 or more times faster in solution with magnetite than the Np(V) reduction by Fe(II) ions in homogeneous solution. It was revealed that the reduction of Np(V) in solution with magnetite took place on the surface of magnetite.

Surface speciation of Eu3+ adsorbed on kaolinite by time-resolved laser fluorescence spectroscopy (TRLFS) and parallel factor analysis (PARAFAC)

Time-resolved laser fluorescence spectroscopy (TRLFS) is an effective speciation technique for fluorescent metal ions and can be further extended by the parallel factor analysis (PARAFAC). The adsorption of Eu(3+) on kaolinite as well as gibbsite as a reference mineral was investigated by TRLFS together with batch adsorption measurements. The PAFAFAC modeling provided the fluorescence spectra, decay lifetimes, and relative intensity profiles of three Eu(3+) surface complexes with kaolinite; an outer-sphere (factor A) complex and two inner-sphere (factors B and C) complexes. Their intensity profiles qualitatively explained the measured adsorption of Eu(3+). Based on the TRLFS results in varied H(2)O/D(2)O media, it was shown that the outer-sphere complex exhibited more rapid fluorescence decay than Eu(3+) aquo ion, because of the energy transfer to the surface. Factor B was an inner-sphere complex, which became dominant at relatively high pH, high salt concentration and low Eu(3+) concentration. Its spectrum and lifetime were similar to those of Eu(3+) adsorbed on gibbsite, suggesting its occurrence on the edge face of the gibbsite layer of kaolinite. From the comparison with the spectra and lifetimes of crystalline or aqueous Eu(OH)(3), factor C was considered as a poly-nuclear surface complex of Eu(3+) formed at relatively high Eu(3+) concentration.

Affinity of finely dispersed montmorillonite colloidal particles for americium and lanthanides

We studied (i) the affinity of the finely dispersed montmorillonite colloidal particles (<0.45 μm), which originated from Japan, for trace amounts of Am3+ and lanthanide ions (Ln3+: Nd3+, Eu3+, Gd3+), and (ii) the differences between the sorption behavior of Am3+ and Ln3+ onto the colloidal particles and that onto the massive montmorillonite solids. The ion-exchange stoichiometry of sorption reaction of Am3+ and Ln3+ onto the colloidal particles was 1:3 in the low Na+ concentration region. In the high Na+ concentration region, the sorption ratio was constant, and specific for Am3+ and each Ln3+. The influence of Na+ and Ca2+ on the sorption of Ln3+ onto the finely dispersed Na- and Ca-montmorillonite colloidal particles at different NaCl and CaCl2 concentrations was examined. It was found that the coverage of the sorption sites increased with the Ln3+ concentration. The affinity for Ln3+ was discussed by selectivity and a Langmuir-type isotherm.

Evaluation of the complexation behavior between humic acid and UO22+ with fluorescence spectroscopy and its mixture analysis

Summary The fluorescence of humic acid (HA) is quenched due to its complexation with UO22+ ions, leading to the evolutionary quenched intensity profile (quenching profile) as a function of UO22+ concentration. This quenching behavior was investigated using 3-dimensional (3D) and synchronous (SyF) fluorescence spectroscopy. The quenching profile obtained by means of SyF spectroscopy was deconvoluted by SIMPLISMA, which is an interactive self-modeling mixture analysis method. Two sites, which contribute to the fluorescence of HA and whose fluorescence is quenched due to its complexation with UO22+, have been extracted, and the quenching profile was resolved to the fluorescence intensity profiles of these sites and the corresponding spectra. Based on these intensity profiles, the apparent stability constant (log Kapp) and the concentration of site (CL) were evaluated for each site, using nonlinear least-squares regression to the Multisite Ryan-Weber Model.

Sorption and desorption kinetics of Np(V) on magnetite and hematite

Sorption kinetics of Np(V) on magnetite and hematite were investigated, and a sequential desorption method was applied to investigate changes in the chemical form of Np sorbed according to the amount of time they were in contact with the Np solution. It was found that the sorption process consists of fast sorption and slow sorption which reaches equilibrium in 1 h. According to the desorption results, it was conjectured (i) that fast sorption is attributable to sorption on/into the surface and non-crystalline phases of iron oxides for magnetite and hematite in both acidic and alkaline solutions, (ii) that sorption on/into the crystalline phase also contributes to fast sorption for hematite in an alkaline solution, and (iii) that slow sorption represents sorption into the crystalline phase of magnetite in both acidic and alkaline solutions and that of hematite in an acidic solution. From the results of sorption and desorption kinetics, it was concluded that the equilibrium between various chemical forms of sorbed Np was achieved in about 1 week, although the amount of sorbed Np reached an equilibrium in only 1 h.