Derivatization and structural studies by spectroscopic methods of humic acids from Leonardite

Humic acids (HA) from Leonardite were methylated with CH3I/Ag2O in a methanol suspension (yield 261±23.6 g kg−1; N=4) and dimethylformamide (DMF) (yield 839±57.6 g kg−1; N=4) solution. Methylated humic acids are soluble in organic solvents and have been examined by IR-FT, 1H and 13C-NMR spectroscopy methods. The OCH3/COOCH3 ratio was calculated by 13C-NMR for HA methylated in methanol suspension and DMF solution obtaining values of 0.38 and 0.82, respectively. Methylated acids molecular weight distribution was measured by Size-Exclusion Chromatography (SEC). The product obtained in methanol suspension has a molecular weight (Mn) of 2918 Da and a dispersion index (D=Mw/Mn) of 1.13. The methylated product prepared in DMF solution is characterized both by higher molecular weight, (Mn=3623 Da) and dispersion index (D=2.10).