Characterization of humic acids extracted from sewage sludge during composting and of their Sephadex® gel fractions

The molecular weight of humic acids extracted from sewage sludge at different stages of composting is estimated by separation on Sephadex gel. The two types of fractions obtained are characterized by Fourier Transform Infrared, Ultraviolet-Visible spectroscopy, and the E4/E6 ratio. The fraction of high molecular weight excluded from the gel largely prevails. This fraction consists of polycondensed lignin-like structures bearing peripheral lipidic and peptidic chains. The fraction retained on the gel is characterized by a low and medium molecular weight and is composed of low-condensed lignin constituents with high acidic functional group content. During composting, an increase in aromaticity and polycondensation degree of humic acids, and decrease in aliphatic, peptidic and lipidic compounds are observed. The results obtained show the continuity of the molecular weight gradient from small to very large humic macromolecules, and that humic macromolecules can be regarded as associations of molecular humic sub-units of a similar chemical nature.