Yoshiaki Sakamoto

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.

Effects of humic acid on the sorption of neptunium(V) on kaolinite

Abstract The sorption coefficient of Np(V), K d , on kaolinite was measured at pH 6 to 11 and in humic acid concentrations of 0 to 40 mg dm −3 at the ionic strength of 0.1 M. The K d value increased with pH both with and without humic acid. The K d value increased slightly with increasing concentration of humic acid in the pH range below 8 and decreased not more than an order of magnitude with increasing concentrations of humic acid in the pH region above 8. Below pH 8, Np(V) sorption is considered to be enhanced by the sorption of humic acid on the kaolinite to form Np(V)-humate complexes. Above 8, it is probable that the desorption of humic acid and formation of Np(V)-humate in solution result in the decrease of K d . The behavior of Np(V) sorption on kaolinite with humic acid is described by a simple model.

Effective diffusivity of the uranyl ion in a granite from Inada, Ibaraki, Japan

The effective diffusivity of uranium(VI) in Inada granite has been determined by through-diffusion. Experiments were performed at room temperature (20–25°C) in a 0.1 mol 1−1 KCl solution where uranium is present predominantly as the poorly sorbing UO22+. An effective diffusivity (De) of (3.6 ± 1.6) × 10−14 m2 s−1 was obtained, close to that for uranine (nonsorbing organic tracer), but one order of magnitude lower than those obtained for Sr2+ and NpO2+, and two orders of magnitude lower than that obtained for I−. According to well established theory, a proportional relationship exists between De and the diffusivity in the bulk of the solution (Dv). The effective diffusivity obtained in granite was not proportional to Dv. This agrees with results obtained for effective diffusivity in a Swedish granite. The ratio DeDv was found to be not constant but increased with De or Dv. This result suggests a limit to the application of the theory.

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.

Sorption of Neptunium on Naturally-Occurring Iron-Containing Minerals

Sorption of neptunium on naturally-occurring hematite (aFe 2 0 3 ) , magnetite (Fe 3 0 4 ) , goethite (α-FeOOH) and biotite (K(Mg,Fe)3AlSi3O10(OH)2) in 0.1 Μ N a N 0 3 solution was studied at 30°C for pHs between 4 and 11. The mass to volume ratio was lg/1. The sorption-desorption reaction was reversible for all the minerals in the pH ranges studied. The dependence of neptunium sorption on pH differed between goethite and the other three minerals; goethite showed a strong sorption at pHs above 6 while the sorption on hematite, magnetite and biotite occurred at pHs above 9. Other oxyhydroxides, lepidocrocite (y-FeOOH) and boehmite (y-AlOOH), have a similar pH dependence of sorption to goethite. The sorption on hematite was similar to that on alumina (α-Α1203).

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.

The migration behavior of Np(V) in sandy soil and granite media in the presence of humic substances

Migration behavior of Np(V) in sandy soil and granite media was studied in the presence of humic substances using a column method. Two kinds of naturally occurring fulvics were isolated from groundwater at the Chalk River Laboratories of AECL (hereafter, CRL-fulvics) and at the Underground Research Laboratory of AECL (hereafter, URL-fulvics). The commercial humics from Aldrich Co. (hereafter, Aldrich-humics) was also used to compare the influence of humics from different origin on the migration behavior of Np(V). The dominant molecular size of these humic substances was under 5,000 daltons for the fulvics, and in the range from 30,000 to 100,000 daltons for Aldrich-humics. An enhanced elution of Np(V) from the sandy soil column under coexistence of CRL-fulvics at 20 mg/L was observed in comparison with the studies in the absence of humic substances. On the other hand, Np(V) under coexistence of Aldrich-humics was retarded. The difference in the effects of the humic substances on the migration behavior of Np(V) may be caused by the difference in the molecular size distribution of the humic substances. That is, the CRL-fulvics, being smaller in size, has lower sorption ability on the sandy soil than the Aldrich-humics, and thereby Np-fulvics complexes may be able to easily migrate through the sandy soil column. In the case of granite column experiments, again an enhanced elution of Np(V) under the coexistence of URL-fulvics was found and this observation was similar to that found with the CRL-fulvics. From these observations, it is likely that the migration behavior of Np(V) is influenced by the presence of humic substances.

Sorption of Europium(III)-Humate Complexes onto a Sandy Soil

The sorption of Eu(III) by a sandy soil was studied by batch experiments at pH 7.5 in the presence of four humic acid (HAs) from different origins. Percentages of Eu sorbed onto the sandy soil were 95% in the absence of HA and 17-25% in the presence of HAs. The HA fraction with molecular weight (MW) of less than 5,000 daltons was remaining in the solution after the Eu-HA sorption experiments. Studies on fluorescence quenching of HAs indicated that the HA fraction was complexed with Eu(III). The sorption of Eu onto the sandy soil was decreased by the formation of complexes with the HAs.

Modeling of Neptunium(V) Sorption Behavior onto Iron-Containing Minerals

Sorption behaviors of neptunium (V) on naturally-occurring magnetite (Fe 3 O 4 ) and goethite (α-FeOOH) in 0.1M NaN0 3 electrolyte solution under aerobic conditions were interpreted using the surface complexation model (SCM). The surface properties of these materials were experimentally investigated by C0 2 -free potentiometric titration, and SCM parameters for the constant capacitance model, such as protonation/deprotonation constants of the surface hydroxyl group, were determined. The number of negatively charged sorption sites of goethite rapidly increased with the increase of the bulk solution pH compared with that of magnetite and this tendency was similar to the pH dependence of neptunium sorption. This implies that the neptunyl cation, NpO 2 + , plays a dominant role in possible sorption reactions. Assuming that the dominant surface complex is XO-NpO 2 , modeling by means of SCM was carried out, and the results were found to agree with experimental data.

The Solubility of Metallic Selenium under Anoxic Conditions

The solubility of metallic selenium was measured in a mixture of 0.1M-NaCl and 0.05M-N 2 H 4 under anoxic conditions (O 2 4 2 at pH between 10 and 13, by UV-Vis absorption spectrometry. The solid phase was identified as Se (cr) by X-ray diffraction. The equilibrium constants of Se(cr) + H + + 2e - = HSe - logK 0 = -6.5±0.5 and 4Se(cr) + 2e - = Se 4 2- logK 0 = -16.8±0.5 were determined. The standard molar free energy of formation of HSe - and Se 4 2- was determined to be (37.1±2.9) and (95.9±2.9) kJ/mol, respectively.