Evidence of macromolecular structure of humic acids from compost

Humic acids (HA) from compost of domestic-waste origin were characterized by IR and UV-Vis spectra and by elemental analysis. The molecular structure was investigated by treatment with the monocarboxylic acid HCOOH and CH3COOH, dialysis experiments and size-exclusion chromatography (SEC). In contrast to former reports, HCOOH and CH3COOH caused the simple precipitation of HA. SEC experiments gave chromatograms with a broad peak, with an elution volume at the maximum height of the peak very close to that of the globular protein bovine serum albumin, a value slightly above the fractional range of the gel used for SEC tests. The chromatogram profile was found to be independent on HA concentration. The eluted fractions exhibited sharper peaks than the pristine HA sample. It can be concluded that the HA extracted from compost do not consist of relatively small molecules held together by weak intermolecular interactions (as reported in the literature), but rather have a canonical macromolecular structure. The simple approach employed here is proposed as a reference procedure for testing the chemical structure of humic acids of any origin.