Molecular characterization of reactive deep underground dissolved organic matters for metal ion using fluorescence spectroscopy and high-resolution mass spectrometry

蛍光分光法および高分解能質量分析法を用いた地下深部の反応性溶存有機物の金属イオンに対する分子特性調査

佐藤 颯人*; 戸田 賀奈子*; 別部 光里*; 天野 由記   ; 宮川 和也   ; 斉藤 拓巳*

Sato, Hayato*; Toda, Kanako*; Beppu, Hikari*; Amano, Yuki; Miyakawa, Kazuya; Saito, Takumi*

Geological disposal is considered to be a feasible option for the final disposal of high-level radioactive wastes. To evaluate the safety of geological disposal, it is necessary to know the reactions of radionuclides with geological media and their migration behaviors in deep underground environments. In this research, we used Eu as a chemical analogue of trivalent actinide elements, and evaluated the molecular characteristics of dissolved organic matter (DOM) components bound to Eu by relating the binding properties of DOM and Eu obtained by fluorescence spectroscopy to the results of high-resolution mass spectrometry. In fluorescence spectroscopy, quenching experiments with Eu were carried out and the different fluorescent components contributing to the binding reaction with Eu were extracted, using Parallel Factor Analysis(PARAFAC). In high-resolution mass spectrometry, mass spectra of DOM in deep groundwater were obtained using Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) and the chemical formula of organic molecules were assigned to the obtained mass-to-charge ratios. PARAFAC analysis of the Excitation-Emission Matrix (EEMs) from the quenching experiments showed that four fluorescent components can be extracted, each of which binds to Eu differently. Among them, comp3, which was suggested to be formed by decomposition of terrestrial organic matters, was revealed to have strong binding ability for Eu. By rank correlation analysis between results of PARAFAC of the EEMs and FT-ICR-MS, this component turned out to have low H/C and O/C values and rich in double bonds, and contain a relatively large amount of phytochemicals such as lignin and tannin. Comp1, organic matters of marine origin, was turned out to have weak binding ability for Eu, high H/C and O/C, low double bonds, and contain a large amount of lipids.