Analysis of molecular aggregation in humic substances in solution

The aim of our work is firstly to investigate the presence of stable molecular aggregates in different selected humic substances (HS), and secondly, to identify the possible presence of different molecular behaviors (macromolecular and supramolecular) in these HS. Molecular aggregation was studied through the analysis of the molecular size distribution changes induced by the acidification with different acids (hydrochloric and acetic acids) and further realkalinization with sodium hydroxide of HS samples. Size distribution changes were analyzed by the complementary use of ultrafiltration, size exclusion chromatography and dynamic light scattering. Our results indicated that the HS studied appeared to be composed of two main fractions. A fraction, which presented a clear macromolecular behavior in solution, with macromolecules and/or very stable molecular aggregates present; and another fraction that was principally formed by molecular aggregates (supramolecular associations), which also included molecules with low molecular weight and unclear macromolecular nature. Therefore, macromolecules, small molecules and supramolecular associations all seem to coexist in the HS studied. (C) 2007 Elsevier B.V. All rights reserved.