Composition, particle size, charge, and colloidal stability of pH-fractionated humic acids

Abstract

The purpose of this study was to divide humic acids into several fractions by means of their dissolving in buffers with different pH values and to characterize obtained humic fractions with respect to their composition, structure, particle size, and charge. Relationships between determined characteristics of fractionated humic acids and pH values of used buffer solutions and the method of fractionation was investigated. Humic acids were fractionated by means of two different methods: the subsequent dissolution in buffers adjusted to different pH and the sequential dissolution in buffers with increasing pH values. Composition, structure, and properties of the obtained humic fractions were studied using elemental analysis, FT-IR spectroscopy, UV/VIS spectroscopy, and light scattering methods. As expected, fractions obtained by subsequent dissolution were more heterogeneous than those prepared by sequential extraction. Fractions obtained at lower pH values contained higher amounts of aromatic and carboxylic groups, while those extracted at higher pH values were rich in aliphatic and/or peptide groups. Fractions extracted close to neutral pH region had some specific properties. Functional groups dissociated in a range of pH values depending on the chemical structure of the molecules. Weaker carboxylic groups could dissociate in less acidic solutions, and more aromatic fractions could be dissolved. Conformational changes and deaggregation process differed with the fractionation procedure and concentration of studied solutions. A bimodal distribution of particle sizes and higher values of polydispersity were obtained for some for less concentrated solutions of humic fractions. Obtained humic fractions behave as anionic heterogeneous ligands with many carboxylic and phenolic groups of different strength, and present polyfunctional and polyelectrostatic effects due to different functional sites and net charged groups. Two main processes can affect their properties and behavior in aqueous solutions: dissociation of acidic functional groups and breaking of humic aggregates into smaller molecular associations or humic molecules. An important parameter affecting the spatial arrangement of obtained humic fractions is their concentration.