Solubility and dissociation of lignitic humic acids in water suspension

Behavior of humic acids (HAs) extracted from lignite in water suspension was studied by means of potentiometry. Although humic acids are operationally defined as that fraction of humic substances, which is not soluble under acidic conditions, it does not mean that solid preparation easily dissolves in water. Presented results suggest that dissolving of solid humic acids in water environment is more complex than conventional solubility behaviour of sparingly soluble solids. Multi-step mechanism of solid humic acids interaction with water, including partial equilibrium dissolution as well as direct equilibrium dissociation from the solid state, was proposed and proved experimentally. Only very small part of solid humic acids can be dissolved in water and not giving necessarily saturation at the same time. In contrast to common sparingly soluble solids, solid humic phase thus possesses non-constant chemical potential even under constant temperature and pressure and its activity does not equal to unity, in general.