Hiromichi Ogawa

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.

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 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 behavior of plutonium(IV) onto soils in the presence of humic acid

Influence of humic acid on the sorption of 238Pu onto geologic media using a coastal sand, which does not sorb humic acid, and an ando soil, which sorbs humic acid very well, was examined with respect to molecular sizes of humic acid. Apparent sorption affinity of 238Pu for the coastal sand decreased with increasing humic acid concentration. As to the ando soil, the sorption affinity of 238Pu became largest in the presence of several mg/dm3 of humic acid. The humic acid fraction smaller than 100,000 daltons relates to the complexation with 238Pu and the sorption onto the soils, particularly the humic acid fraction of 30,000-100,000 daltons was dominant. Most 238Pu formed humic compounds even in a low humic acid concentration region; resulting plutonium humates are considered to control the apparent sorption affinity of ~38Pu. It is concluded from above results that a specific molecular size of humic acid controls the complexation ability and the apparent sorption affinity of 238Pu.

Migration of 137Cs Adsorbed on Fine Soil Particles through Soil Layer: Filtration by Unsaturated Sandy Soil Layer

The experiments on the migration of 137Cs adsorbed on fine soil particles in unsaturated sandy soil layers were carried out. The soil layer reduced the concentration of 137Cs in the effluent, but did not retard its migration. The concentration of 137Cs remained in the soil layer increased in proportion to the amount of 137Cs introduced. By the measurement of size distribution of fine soil particles, it was revealed that the concentration of 137Cs in the effluent decreased exponentially with the length of layer in the case of monodispersed fine soil particles. From these results, the first-order irreversible reaction formula was obtained in order to formulate the reaction of 137Cs adsorbed on fine soil particles with soil layer. A good agreement was obtained between the predicted result by polydispersed irreversible model and experimental one.

Migration mechanisms of 237Np and 241Am through loess media

Migration experiments with 237Np(V) and 241Am(III) have been performed using a column system, packed with loess, taken from Shanxi, China. The adsorption mechanism of 237Np and 241Am on the loess was examined by a chemical extraction method. Most of 237Np was adsorbed on the influent edge of the column, and the adsorbtion was mainly controlled by surface complexation. However, the migration of 237Np in loess media could be roughly evaluated by the distribution coefficient. In the case of 241Am, particulate, the 241Am species was formed in the influent solution and moved in the column. The 241Am adsorbed on loess was controlled by irreversible reactions. The migration behavior of particulate 241Am in loess media could be expressed by the filtration theory.

Effect of Flow Field on Colloid Deposition in Filtration Process of Polystyrene Latex Particles through Columns Packed with Glass Beads

The effect of flow field on the deposition rate of colloidal particles onto solid surface in a column packed with spherical collectors was investigated theoretically and experimentally. A new analysis for calculating the colloid deposition efficiency was developed. The analysis includes the effect of the flow field on the deposition of a colloidal particle through the interactive potential barrier between the particle and the solid surface. The deposition behavior of polystyrene latex particles (diameter: 102 nm) in transport process through columns (length: 100 mm) packed with glass beads (diameter: 0.35–0.40 mm) was observed. The colloid deposition efficiencies calculated by the new analysis were comparable with those obtained by the experiments. The result indicates that the effect of the flow field would cause the difference between the flow velocity dependence of the efficiency obtained by a classical analysis and that by the experimental results. The colloid deposition efficiencies were also obtained by estimating the deposited particles onto the glass beads surface suspended in the colloidal suspension from the SEM observation of the glass beads surface. The effect of the heterogeneity of the collector surface was discussed by calculating the colloid deposition efficiency using the evaluated interaction potential between the particle and the heterogeneous collector surface.

Retardation Factor of a Radionuclide for Undisturbed and Disturbed Sandy Soil

The relationship of the retardation factor of a radionuclide for undisturbed soil and that for disturbed soil is investigated. The migration model is based on the assumption that both reactive (dynamic) and nonreactive (stagnant) sites exist in the soil column. The retardation factor for undisturbed soil is represented by the following equation: Rf{sub u} = S{sub 2}/S{sub d}Rf{sub d}, where Rf{sub u} and Rf{sub d} are retardation factors for undisturbed and disturbed soil, respectively, and S{sub u} and S{sub d} are degrees of water saturation in undisturbed and disturbed soils, respectively. The migration experiments for both the undisturbed and disturbed soil columns were carried out using {sup 85}Sr. The average retardation factor for the undisturbed soil is smaller than that for disturbed soil, and the degree of water saturation in the undisturbed soil column is smaller than that in the disturbed soil column. The retardation factor for the undisturbed soil estimated, based on the above equation, is approximately the same as the measured retardation factor for the undisturbed soil.

Deposition behavior of latex particles in filtration process through glass packed column

Most studies on radionuclide migration in groundwater have focused on the transport of dissolved forms of radionuclides. Colloidal particles, however, can play an important role in the migration. The colloids act as a third phase which is neither a liquid nor a solid phase. This phase can increase the amount of actinides that can migrate in a natural aquifer system. The deposition behavior of colloids from a flowing suspension onto a solid collector surface in filtration process was observed by conducting column experiments with polystyrene latex particles and glass beads packed columns. From the observed particle breakthrough curves, single collector efficiencies were calculated. By comparing the single collector efficiency obtained in the column experiment with the existing approximate expression, the effects of flow velocity on the deposition behavior was discussed.