Category: Research

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).

Natural organic matters in deep groundwater

Natural organic matters (NOMs) such as humic substances (HSs) are known to play important roles in mobilities and bio-availabiliities of actinide elements and heavy metals. Most of researches on NOMs have been performed with NOMs isolated from surface environments such as soil and river water. The properties and roles of NOMs in deep groundwater has been largely unknown. In this research we have investigated the physiochemical and ion binding properties of HSs isolated from sedimentary groundwater collected at a borehole at the -150 m experimental stage of the Horonobe underground research laboratory, JAEA. It was the first attempt to clarify details of ion binding to deep groundwater HSs over a wide range of conditions. Horonobe HSs consist of relatively small organic molecules rich in aliphatic C and S, which are rather distinctive, compared with structures and compositions of HSs with surface origins. Although the densities of carboxylic groups, which are supposedly responsible for metal ion binding in HSs, are similar to or even larger than those of surface HSs, the binding of copper ion (Cu2+) is smaller for Horonobe HSs. This can be explained by mono-dentate coordination of the carboxylic groups which are sparsely distributed on aliphatic chains of the HSs. Extended X-ray absorption fine structure analyses have revealed involvement of S in binding of Cu2+ to Horonobe HSs at relatively low pH, […]