| Abstract: | Humic substances (HS) produced during the aerobic decomposition of polysaccharides still need to be recognized as such and characterized. Humic (HA) and fulvic (FA) acids extracted at different time intervals during composting of cotton residues from carding, where no decomposition of lignin occurred, were investigated by scanning electron microscopy (SEM) size exclusion chromatography and infrared spectroscopy. Throughout the experiment, fulvic acids possessed a larger number of carboxyl groups but a lower number of weak acidic groups (negatively charged acid groups at pH 11) than humic acids. the number of carboxyl groups increased with time in both fulvic and humic acids, while that of phenolic groups decreased. Infrared spectra showed that the ratio between carboxyl and methyl groups decreased from 8.5 to 4 in FA after 18 days. This change corresponded with the disappearance of flat sheet structures observed by SEM at pH 6. Our results suggest that the shape of humic molecules as observed by SEM reflects the strength of hydrophilic/hydrophobic interactions with the solvent molecules. About 80% of FA molecules had a molecular weight lower than 3500 as deduced from column calibration with HS standards obtained by ultrafiltration. About 40% of humic acids were composed of small molecules; only 20% HA had a molecular weight larger than 45000. Polysaccharide-derived HS may not be limited to the high molecular weight poorly transformed fraction containing bonded polysaccharide structures, but may also consist of low molecular weight components with no structural similarity to the original material. |
