Elemental and spectroscopic characterization of humic acids fractionated by gel permeation chromatography

Humic acids were extracted from one year aged farmyard manure and domestic waste composts and from carob tree fresh litter. Their elution pattern and particle weight distribution were investigated by successive gel chromatography using three grades of sephadex gel: G25, G75 and G100. The fractions obtained were characterized by elemental analysis, E4/E6 ratio, Fourier transform infrared and fluorescence spectroscopy. Compounds apparently of high molecular weight dominated in composted farmyard manure and domestic waste, nitrogen-rich, humic acids, whereas the compounds of apparently medium molecular weight were more important in the litter's, less nitrogen-rich, humic acids. Besides, the increase in molecular weight was accompanied by an important increase in the amount of nitrogen-rich components and, to a lesser extent, polysaccharides and aliphatic long-chain components. Polycondensation of humic substances did not develop significantly with increasing particle weight but the confrontation of all the results agrees with the coexistence of both more or less highly humified and non-humified compounds in the large particle weight fraction. At the same time a striking similarity in either elution patterns, quantitative and qualitative composition was observed among all the fractions of the same humic acid. The structure of humic acids is discussed on the basis of these results.