Effect of pH on Supramolecular Assemblies in Fulvic Acid Solutions

The effect of alkalinization of fulvic acid (FA) solutions on an increase in their optical density has been studied. To explain this phenomenon, FA particles existing in solutions at different pH have been studied by scanning electron microscopy (SEM) and scanning tunneling microscopy (STM). It has been found that an increase in pH results in a noticeable decrease in the size of the supramolecular assemblies of FA molecules from ~200 to ~100 nm. An STM study of FA samples demonstrates that FAs exist in solutions as supramolecular assemblies of several hundred nanometers in size formed by 10–20 nm FA particle molecules. The observed phenomenon has been explained using the existing ideas about the supramolecular fractal cluster organization of humic substances. The explanation states that the upper layer of F-clusters degrades to give FA particle molecules, while the cluster size is seen to decrease. The FA particle molecules cannot be detected by SEM due to their small size, but they are readily visualized by STM.