Characterization of humic acid-like material isolated from the humin fraction of a topsoil

A humic acid-like material (HALM) was obtained by base extraction of a demineralized humin. The objective of this study was to investigate how HALM differs from the humic acids (HAs) associated with the same soil by systematically characterizing the chemical, structural, and molecular properties of both HALM and HA fractions. The methods used for characterization included elemental analysis, high performance size exclusion chromatography, solid state 13 C-nuclear magnetic resonance spectroscopy, pyrolysis-gas chromatography-mass spectrometry, and tetramethyl ammonium hydroxide pyrolysis-gas chromatography-mass spectrometry. The results showed that the HALM fraction constituted 8.8 % of the total organic carbon of the original soil and that it had O/C and H/C atomic ratios of 0.32 and 0.96, respectively, compared with 0.46 and 0.89 for the HA. The high performance size exclusion chromatography chromatograms indicated that both HA and HALM had two subunits of macromolecules with average molecular weight values of 3.38 to 3.47 and 40.1 to 45.8 kDa for the smaller and the larger size subunits, respectively, and that the relative contents of the larger size subunits was 26.9 %, and 41.4 % for HA and HALM, respectively. The CP-magic-angle-spinning C-13-nuclear magnetic resonance spectroscopy and pyrolysis data indicated that compared with HA, HALM was characterized by lower contents of oxygen-containing groups and higher contents of aliphatic carbons. The study suggests that HALM may have undergone a greater degree of biogeochemical alterations and that its larger apparent molecular weight and higher H/C and lower O/C atomic ratios may result from hydrophobic aliphatic constituents that were incorporated to HALM under water-flooded conditions.