Molecular characterization and ion binding properties of deep underground dissolved organic matters using fluorescence spectroscopy and high-resolution mass spectrometry

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

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

Geological disposal is a feasible option for the final disposal of high-level radioactive wastes. To evaluate its post-closure safety, it is necessary to know the reactions of radionuclides with geological media and their migration behaviors in deep underground environments. Among various biogeochemical reactions and processes, dissolved organic matters (DOM), which are ubiquitous in groundwater, can change the charges and sizes of radionuclides by forming complexes, adsorb on mineral surface, and catalyze redox reactions, thereby modifying the mobility of radionuclides and their interaction with rock surfaces. A main difficulty in understanding such DOM impacts on radionuclide migration is its diversity. The diversity of DOM originates from mixing of different endmembers in a given environment, being modulated by biotic or abiotic activities altering DOMs. In this research, we have tackled this by extensive sampling of groundwater from various boreholes at different depths in an underground research laboratory and extraction of DOM from them with solid-phase extraction, which were studied by fluorescence spectroscopy and high-resolution mass spectrometry. Integrated analysis of the two analytical techniques have revealed DOM components with different reactivity with Eu and UO together with their molecular characteristics. It was revealed that relatively strong reactivity of DOM with terrestrial origin against Eu and UO as well as weak one of DOM with marine origin could explain variability of DOM impact of radionuclide migration as a function of depth or the degree of mixing of between recharged meteoric water and connate seawater.