Secondary structure of humic acids.: Can micelle-like conformation be proved by aqueous size exclusion chromatography?

Understanding the chemical and secondary structures of humic substances is essential to predict their interactions with organic and inorganic pollutants in the environment. Humic substances are suggested by some to form micelles under certain conditions. Disruption of the hypothesized micelle-like structure by the addition of organic acids was reported earlier based on observed retention shift in size exclusion chromatographic analyses. In this work the effects of added organic and inorganic acids on the retention shift are studied in humic acid samples. Parameters such as eluent pH, salt concentration, and buffer capacity were examined simultaneously with the addition of organic and inorganic acids to the samples. The results suggest that the retention shift was not the consequence of micellar disaggregation but secondary (mainly hydrophobic) interactions caused by the pH change on the stationary phase were responsible. Therefore, micellelike structure of humic substances cannot be confirmed solely on the basis of the change in retention volumes. For the characterization of humic substances by size exclusion chromatography secondary interactions (ionic and hydrophobic) should also be considered.