Impact of an Anionic Surfactant Addition on Solubility of Humic Acid in Acid-Alkaline Solutions

The solubility of humic acid (HA) is a very important factor in the study of its interaction with metal ions and radionuclides. The solubility of HA as a function of hydrogen ion concentration was studied. It has been found that chemical behaviour of HA is similar to other polyelectrolytes. Solubility depends on pH, ionic strength, and many other factors. Unmonotonous character of the solubility curve in relationship with pH may be connected with conformation changes of HA saturated solutions. The results show that the addition of anionic surfactant to HA saturated solution can influence the solubility curve of HA. A large fraction of soil organic carbon and waterdissolved organic carbon is constituted by humic sub­ stances. Humic substances are complex, heterogeneous molecules which are operationally defined on the ba­ sis of solubility: fulvic acid is completely soluble in aqueous solution, while humic acid is soluble in al­ kali but precipitates in acid solutions. A knowledge of the solubility characteristics of HA is important for understanding the behaviour of and relationship between humic substances in soil and natural waters [1]. In solution, HA behaves as micellar colloids. The colloid particles have a negative charge and this iso­ electric point is in the acid region. It has been found that chemical properties of HA are similar to the other acidic polyelectrolytes. The solubility of HA depends on pH, ionic strength, and many other factors [2]. In view of the fact that HA is a relatively high-molecularmass species containing polar and nonpolar moieties [3], we speculate that there occurs a partition-like in­ teraction of the surfactants with the soil HA. The pur­ pose of the present work is to study the solubility of HA at different pH values and influence of an anionic surfactant addition on its solubility curve.