Dynamic light scattering measurements of particle size development in aqueous humic materials

Dynamic light scattering (DLS) has been used to monitor changes in aggregate sizes of aqueous humic materials as a function of solution properties. Humic and fulvic acids were dissolved at relatively low concentrations (15-30 mg L(-1)) in solutions of different temperature, cation and ethanol content, and pH. The results could be explained in terms of intramolecular contraction and intermolecular aggregation of humic polymers. The former were prevalent in soil humic acids, and less so in aquatic humic acids and fulvic acids. Increasing the temperature of humic solutions generally led to an increase in particle sizes, which was ascribed to an effect akin to surfactant clouding. The addition of cations led to either contraction or expansion, depending on the charge and concentration of the ion, and the nature of the humic material. Reducing the pH initially caused contraction, followed by growth and precipitation in more highly acidic media.