Characterization and thermodynamic study of the protonation of humic acid dissolved in deep groundwater at Horonobe, Hokkaido, Japan

Kimuro, Shingo*   ; Kirishima, Akira*; Akiyama, Daisuke*; Sato, Nobuaki*; Nagao, Seiya*; Saito, Takumi*; Amano, Yuki   ; Miyakawa, Kazuya   

In this study, size exclusion chromatography (SEC), flow-field flow fractionation (Fl-FFF) and calorimetry were applied to the humic acid isolated from the groundwater collected at 350m below the ground level at the Horonobe Underground Research Laboratory, JAEA. First, the characteristic molecular weight distribution of Horonobe humic acids were observed from the SEC analysis. Second, the small hydrodynamic diameter of Horonobe humic acids were observed by the Fl-FFF. Finally, the protonation enthalpies of Horonobe humic acids were determined by the combination of potentiometry and calorimetry. The values of protonation enthalpy of Horonobe humic acids were close to simple organic acids (i.e., benzoic acid and phenol). It was suggested that Horonobe humic acids have lower molecular weight than IHSS surface soil humic acids (Elliot Soil and Waskish Peat) and the simple protonation mechanism, which is not affected by the heterogeneity.