2. Origin and Chemistry of Humic Acids

Biological Cycle of Natural Compounds

Figure 1: Biological Cycle of Natural Compounds
Together with chlorophyll in plants and animal haemin (see Figure 1), humic acids constitute the third component in nature's biological cycle.

The biological centre or main fraction of these natural humic compounds is humic acid.

Humic acids are derived from the amorphous part of dark brown to brown-coloured substances in soil, lignite, peat and brown coal which is produced during the humification of decaying organic materials.

Their chemical structure has not been clearly defined yet. Approximate models describe humic acids as three dimensional macro molecules with a molar mass between 25000 and 100000.

Structural Model
Figure 2: Structural Model of Humic Acids (Kickuth, 1972)
Their basic structure always has two distinctive features:
  • There is a central core with a high aromaticity and strong cross linkage
  • The peripheral functional groups are linked by bridge binding.

Polyionic structures emerge with, e.g., carbon acid ester, phenolic hydroxyl, carbonyl and carboxyl groups. Also amino and sulfhydryl groups as well as chinoide and flavonoide structures occur. In particular, humic acids from plant products (also brown coal humic acids) show flavonoide structures (figetin, quercetin, flavone, xanthine) in addition to their basic aromatic-phenolic structure.