The function of carboxyls in the structure of humic acids to binding of organic substances

The humic substances, the major and the most important part of soil organic matter, are responsible for the immobilization of organic compounds (e.g. heavy metal ions, organic dyes, surface active agents, etc.) in the soil. Unfortunately, there are a lot of gaps in the knowledge in the complex mechanism of binding of organic charged compounds by humic substances.

The unconventional diffusion and dialysis (transport) techniques have been developed for the purpose of study on the interactions between humic substances and organic charged substances.  In our contribution we are focusing on one fraction of humic substances – humic acids. The binding between humic acids with/without selectively blocking of carboxylic functional groups (methylation by –CH3) with organic charged compounds (e.g. organic dyes, surface active agents) has been studied by diffusion and dialysis approach using the spectroscopic techniques as analytical method. The strong impact of methylation on the positive affinity of humic acids towards organic compounds has been expected. These expectations were not confirmed by diffusion and dialysis techniques. The role of carboxyls in the structure of humic acids is not so essential as was expected and mentioned in the literature. The other effects (e.g. hydrophobic interactions, pi-pi stacking, etc.) are more important in the binding of organic compounds as was expected. The positive binding as well as kinetics of this process is also strongly depending on the physico-chemical circumstances of the system (e.g. pH, ionic strength, temperature, etc.).