Macromolecular changes of humic substances induced by interaction with organic acids

Two different humic materials, one from a forest soil and the other from wormcasts, were used to study the influence of mineral and organic acids on the conformational properties of humic substances. The macromolecular changes were followed by low pressure gel permeation chromatography after titrating humic material to low pHs with acids. All organic acids (mono-, di-, tricarboxylic, and oxy-acids), added to humic solution prior to a gel permeation in an alkaline buffer, were able to shift the totality of absorbance of the humic chromatographic peak from high to low molecular sizes. Mineral acids, phenol, alcohols, dipolar aprotic solvents, could not produce the same shift and gave total absorbance at the column void volume as in the case of humic substances alone. The chromatographic peak shifted back to elution volumes proper of higher size molecules when the humic-organic acid mixture was back-titrated to high pHs before gel permeation. Elution in a much stronger alkaline buffer did not change the overall macromolecular behaviour. These results suggest that humic substances behave as micelles in solution and that hydrophobic bondings play an important role in holding humic molecules together. The organic acids enter the interior of the humic micelle-like aggregates and alter the stereochemical hydrophobic arrangement of the humic material. In alkaline conditions the negative charges developed disrupt the apparent high molecular size configuration and disperse the humic aggregates into small micelles. Such conformational properties of humic substances appear to be a function of pH and of the concentration of organic acid.