Category: Research

Adsorption of Eu3+ on mesoporous silica

We analyzed the adsorption states of europium aqua ions (Eu3+) on mesoporous silicas with different pore distributions in comparison with nonporous silica using time-resolved laser-induced fluorescence spectroscopy. The parallel factor analysis was used to differentiate the contribution of different chemical species of Eu3+ to the fluorescence signal and determine the influence of the pore size on each chemical species. The results show that Eu3+ mainly form outer-sphere complexes with silica below pH 6, where Eu3+ adsorption is low. Within nano-sized pores, distortion of the hydration structure and a decrease in the hydration number were suggested in this pH range. As the concentration of the silicate ions derived from the dissolution of silica increases with increasing pH, Eu3+ form the silica/Eu3+/silicate ternary surface complexes. Within nano-sized pores, the concentration of silicate ions decreases due to the overlap of the electric double layer, which inhibits the formation of the ternary surface complex. Furthermore, at high pH, Eu3+ multinuclear complexes formed only on the mesoporous silica surface. This adsorption behavior specific to nano-sized pores could not be concluded by macroscopic adsorption experiments alone because the amount of Eu3+ adsorbed per unit surface area did not differ between the mesoporous and nonporous silicas. Consideration of the silicate complexes should be indispensable in future studies on the adsorption of lanthanide ions using mesoporous silica. Murota, K., Aoyagi, N., Mei, H., Saito, […]

Adsorption phenomena in confined space

by Murota & Saito Water in confinement becomes more structured than bulk water, and its properties, such as the dielectric constant, change. It remains unclear, however, how the interfacial reactions in confinement, such as the adsorption of ions on the surfaces of small pores, differ from those in larger spaces. We focused on the deprotonation reaction of hydroxyl groups, a fundamental surface reaction, and investigated the dependence of the surface charge density on pore size by determining the surface charge densities of six types of mesoporous silicas with micropores and mesopores at different ionic strengths and pH levels from batch titration tests. The surface complexation model assuming a potential distribution based on the Poisson-Boltzmann equation in cylindrical coordinates was fitted to the obtained surface charge densities to relate the electrostatics near the surface to the surface reaction. The results showed that the absolute values of the surface charge densities decreased with decreasing pore diameter due to the overlap of the electrical double layers. Furthermore, the capacitance of the Stern layer optimized by fitting decreased with decreasing pore diameter, especially in pores smaller than 4 nm in diameter, which suggested that the dielectric constants of water decreased near the surfaces of small pores. Murota, K.*, Saito, T., “Pore size effects on surface charges and interfacial electrostatics of mesoporous silicas”, Phys. Chem. […]

Desorption mechanisms of cesium from illite and vermiculite

Background Vast areas of the eastern part of Japan have been contaminated by a short-lived radioisotope of cesium (Cs), 137Cs, whose half-life is 30.1 years, from the accident of the Fukushima Dai-ichi Nuclear Power Plant in 2011. Most of contaminated soils in residential areas have been stripped and gathered for future disposal. On the other hand, air dose rates in forests are still high. Understanding the behaviors of Cs+ in soils is thus crucial for the safety assessments of the disposal of contaminated soil in future and the evaluation of long-term exposure of residents living near forest areas. It is known that Cs+ is strongly sorbed to micaceous minerals. However, the desorption of Cs+ at a trace sorption level with time in the presence of different salt ions is not well understood. In this study, we conducted long-term sorption and desorption experiments of Cs+ with illite and vermiculite, which are micaceous minerals, at room temperature to study the effects of sorption time and co-existing cations on the desorption. Method A small amount of Cs+ (50 nM Cs+ spiked with 900 Bq 137Cs) was sorbed to the illite and vermiculite in the presence of 1 mM K+ or Ca2+, or 1 mM K+ and 100 mM Ca2+ over 8 weeks, which was then desorbed in the presence of Prussian blue (PB) […]

Evaluation of labile fraction of radioactive Cs by diffusive gradients in thin films (DGT)

Background A large area of Fukushima Prefecture was contaminated by radioactive nuclides, radioactive cesium in particular, released by the accident of the Fukushima Daiichi nuclear power plants in Mar., 2011. Most of residential area has been decontaminated by now; nevertheless, a large part of forests Environmental behaviors of radioactive Cs is crucial to predict long-term changes of its distribution in soil or river environments. It is known that Cs is selectively bound to micaceous minerals in nature and inert for chemical exchange reactions. Nevertheless, dynamics of labile fractions of Cs in a given environment if any must play an important role for its environmental behaviors, as it directly contributes to the mobility and bioavailability of Cs. In this study, we perform in-situ evaluation of the labile Cs in soil and aqueous environments of Fukushima Prefecture by diffusive gradients in thin films (DGT), which is a technique designed for the lability measurement of trace elements in nature. Research contents Two types of DGT devices, one for aqueous environments and the other for soil environments, were developed and their applicability for environments have been tested by laboratory experiments. With the developed devices we have revealed that the labile fractions of radioactive Cs In river environments exceeds the dissolved ones, suggesting that the desorption of Cs from suspended particles modulates its behaviors in […]

Research equipment

ICP-MS (Inductively Coupled Plasma Mass Spectrometry): Agilent 7500 cx ICP-AES (Inductively Coupled Plasma Atomic Emission Spectrometry): Shimadzu ICPS-7510 Field-flow fractionation: Postnova Analytics AF2000 FOCUS UV/Vis detector Fluorescence detector RI detector Automated titrator Wallingford Titrator Metrohm Titrand UV/Vis spectrometer: Shimadzu UV-2700 Fluorescence spectrometer: JASCO FP-8300 Infrared spectrometer: JASCO FTIR-6600 MCT detector ATR unit (diamond, single reflection) Diffuse reflection unit Ion chromatography: Metrohm IC930 CO2 suppressor Chemical suppressor TOC: Shimadzu TOC-V CSH Thermal analysis (TG/DTA) Centrifuge Glovebox Laser spectroscopy systems

Long-term desorption of radioactive Cs from soilsLong-term desorption of radioactive Cs from soils

Introduction Vast areas of north-eastern regions of Japan has been contaminated by radionuclides released from the accident of the TEPCO’s Fukushima Dai-ichi nuclear power plants. In particular, radioactive cesium, namely 137Cs, cause many problems due to its relatively long half-life (30.1 years), including decontamination and the management of contaminated soil wastes. To settle such problems, we need to know long-term behaviors of radioactive Cs in soils. Cesium is reported to be strongly fixed micaceous to so-called frayed edge sites (FES) in minerals in soils by interlayer collapse. Nevertheless,the long-term desorption behaviors is of great importance, which is not yet fully understood in terms of the mechanisms, partly because of its slowness and also of the existence of re-sorpton. In this research, we performed the long-term desorption experiments of 137Cs and 133Cs from the real contaminated soils in the presence of varying concentrations of competing cations. The resorption of desorbed Cs was diminished by adding cation-exchange resin as an adsorbent. Experimental The contaminated soils collected in Fukushima prefecture after 40 days since the accident were used. Soils were suspended in dilute KCl solution together with a dialysis bag containing cation exchange resins (Dowex 50w-X8，K+ form).The dialysis bags were exchange 10 times in 3 months with new ones. 133Cs and 137Cs released from the soils and captured by the resigns were quantified […]

Speciation of radioactive Cs in Fukushima soils

Radioactive cesium (<sup>137</sup>Cs) released from the accidence of the Fukushima Dai-ichi nuclear power plant contaminated areas of the eastern part of Japan, particularly Fukushima prefecture and has affected the life of local residents in many ways. During the release events, 137Cs (b.p. 678˚C) was emitted to the atmosphere as CsI in aerosols. After a couple of years soil is likely the most significant reservoir for it. As <sup>137</sup>Cs emits 652 keV γ-ray, <sup>137</sup>Cs in top-layers of soils may contribute to external exposure of local residents and biospheres. It can be absorbed by plants from contaminated soils, which may lead to internal exposure. Thus, It is of great importance to understand behaviors of <sup>137</sup>Cs. In this research, we’ve evaluated the geochemical and grain-size distribution of <sup>137</sup>Cs in contaminated soils sampled in Fukushima Prefecture after the accident. The obtained results have been also compared with those of stable cesium (<sup>133</sup>Cs) to infer the variation of the geochemical distribution in the near future. <sup>137</sup>Cs are most extracted to the acid-digestible factions and the extraction residues. It is also found that <sup>137</sup>Cs is associated with relatively small silt and clay  grains, which are rich in micaceous clay minerals and kaolins according to X-ray diffraction. We’ve found positive correlation between <sup>137</sup>Cs content in each fraction and the amount of micaceous minerals, suggesting crucial roles of the clay minerals on […]

Speciation analyses by laser fluorescence spectroscopy

Time resolved laser fluorescence spectroscopy (TRLFS) is the analytical technique which observes de-excitation processes of excited fluorophore created by short-pulsed laser. TRLFS data of a fluorescent metal ion includes shape of emission spectrum its temporal decay These information reflects the number of allowed transitions and their probabilities competitive non-fluorescent de-excitation pathways Thus, the spectral shape will change when a fluorescence metal coordinates with a ligand and changes its energy levels and/or associated transition probabilities due to variation of the symmetry around it. Similarly, fluorescence decay lifetime will vary when a ligand around the central metal ion is displaced by another ligand, as de-excitation pathways change. We can use these changes in TRLFS data to probe reactions with a fluorescent metal ion and estimate its structural information. As good approximation, For instance, Eu3+, a chemical homologue of trivalent actinides such as Am3+ and Cm3+, has the energy diagram as in Figure 1 and the emission spectrum as in Figure 2. Its decay lifetime is around 110 μsec. Three major peaks in Figure 2 corresponds to the transitions from the 5D0 states to the 7F1，7F2，and 7F4 states. Upon complexation with acetate ligand, the spectrum changes in Figure 3. Particularly, the peak around 620 nm due to so-called hyper-sensitive transition, 5D0 → 7F2, is significantly enhanced. An overtone of OH vibration of water molecule provides an efficient pathway for excited Eu3+; the […]

Modeling the interaction of metal ions with mineral surfaces and natural organic matters

In order to estimate to understand environmental behaviors of radionuclides and toxic metal ions, we need to construct and extrapolate reaction models, which can describe changes of their speciation, from the results of detailed reaction studies and molecular-scale structural information. Such mechanistic models could help us to  improve reliability of safety assessment of nuclear disposal and to develop efficient decontamination strategies. In our group, we are pursuing model development of sorption/binding interactions of metal ions and mineral surfaces or metal ions and natural organic matters. Possible effects of the interaction between mineral surfaces and natural organic matters on metal uptake are also of our great interests. Identifying adsorption structures by X-ray absorption spectroscopy and density function theory simulation X-ray absorption spectroscopy, especially extended X-ray absorption fine structure (EXAFS) analysis, is a power analytical tool, which can be applicable for structural determination around a central metal ion in amorphous samples and sorption or solution samples that lack long-range orders. The applicability of EXAFS can be further imporved, if it is combined with molecular-scale simulations such as density functional theory calculation. In this research we’ve identified possible adsorption structures of hexavalent uranium (uranyl, UO22+) on gibbsite (α-Al(OH)3) surface, which is a representative Al-containing mineral and a model for edge sites of alumino-silicate clay minerals. Hattori, T.*, Saito, T., Ishida, K., Scheinost, A. C., Tsuneda, T., Nagasaki, […]

Comparative study on deep groundwater colloids

Mobility of elements in soil and aquatic environments are regulated not only by complexation with simple ligands with low molecular masses, but also by formation of or sorption to nano-sized inorganic or organic colloids or macromolecules. In this research we have determined and compared the size distribution and elemental composition of various elements in granitic and sedimentary deep groundwater by flow-field flow fraction (Fl-FFF) hyphenated with ICP-MS. Fl-FFF is a non-invasive chromatographic size fractionation technique. Thanks to the selectivity on both size and elemental composition of colloids, we have successfully analyzed heterogeneous natural colloids in the deep groundwater. This research was conducted under collaboration with Radioactive Waste Processing and Disposal Research Department, JAEA. This research was partly supported by the Ministry of Economy, Trade and 15 Industry (METI), Japan, and “Grant-in-Aid for Young Scientists (B)” (Grant No. 25820446), the Japan Society for the Promotion of Science. Saito, T.*, Hamamoto, T., Mizuno, T., Iwatsuki, T., Tanaka, T., “Comparative study of granitic and sedimentary groundwater colloids by flow field flow fractionation coupled with ICP-MS”, J. Anal. At. Spectrom. 30, 1229-1236 (2015).