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The aim of this work was study molecular and quantitative analysis of metal ion binding to humic acids (HAs). The object of our study were two various samples of HAs. HAs were isolated from South-Moravian lignite from the mine Mír (locality Mikulčice, Czech Republic) and compost which was obtained from the composting plant in Náměšť n. Oslavou, Czech Republic. Isolation of HAs was performed according to the procedure recommended by the International Humic Substances Society (IHSS). The reactivity (thermodynamics) of humic acids (HAs) and metals was studied by isothermal titration calorimetry (ITC). As model metals were chosen Cu (II) and Pb (II) ions. ITC was observed reaction enthalpy, Gibbs energy, enthropy and the amount of metal bound to a specific humic acid. The HAs were characterized and compared using spectroscopic techniques, including FTIR and liquid-state 13C NMR spectroscopy. Infrared spectroscopy is a useful technique in characterization of structure, functional groups and formation modes of HAs. 13C NMR spectra of HAs were obtained with a Bruker Avance III NMR spectrometer. For data analysis, the spectra were divided into chemical shift regions assigned to the following classes of chemical groups: alkyl C (0–45 ppm), O-alkyl C (45–106 ppm), aromatic C (106–165 ppm), and carbonyl and carboxyl C (165–220 ppm), respectively. Aromaticity (fa), hydrophilicity and hydrophobicity ratio (Hfi/Hfo) and biological activity (BiA) of HAs were calculated from the area of the NMR spectra.
Keywords: humic acids; isothermal titration calorimetry; complexation; metal ions© This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.