The supramolecular structure of humic substances
The scientific understanding of the molecular size and shape of humic substances (HS) is critically reviewed. The traditional view that HS are polymers in soil is not substantiated by any direct evidence but is assumed only on the basis of laboratory experiments with model molecules and unwarranted results produced by incorrectly applying either analytical procedures or mathematical treatments developed for purified and undisputed biopolymers. A large body of evidence shows an alternative understanding of the conformational nature of HS, which should be regarded as supramolecular associations of self-assembling heterogeneous and relatively small molecules deriving from the degradation and decomposition of dead biological material. A major aspect of the humic supramolecular conformation is that it is stabilized predominantly by weak dispersive forces instead of covalent linkages. Hydrophobic (van der Waals, pi-pi, CH-pi) and hydrogen bonds are responsible for the apparent large molecular size of HS, the former becoming more important with the increase of pH. This innovative understanding of the nature of HS implies a further development of the science and technology for the control of the chemistry and reactivity of natural organic matter in the soil and the environment.