Structural differences of Chernozem soil humic acids SEC–PAGE fractions revealed by thermal (TG–DTA) and spectroscopic (DRIFT) analyses

Humic acid extracted from the A horizon of a Chernozem soil and their different molecular size (MS) fractions obtained by SEC–PAGE were investigated using thermal (TG–DTA), spectroscopic (DRIFT) and isotopic (δ13C) analyses. Moreover, the molecular structure of the high temperature stable fraction (residues above 400 °C) was studied by DRIFT spectroscopy. The lowest MS fraction (C + D) was characterized by the highest content of aromatic compounds and by the lowest content of aliphatic and carboxyl groups, carbohydrates and amino acids with respect to the other fractions. Nevertheless, the aromatic structure of A and B fractions seems to be more complex. The δ13C values which indicated the presence of urea traces in the lowest MS fractions (B and C + D) may be due to the purification process.