Aggregation of humic acid in solution: Contributions of different fractions

The role of lipid components in the aggregation of several humic acids isolated from different sources was investigated using surface tensiometry. A combination of aqueous alkaline and organic solvent extractions was used to isolate two humic-like fractions (HA(1) and HA(2)) and one lipid-like fraction from each humic acid. Fraction HA(1) represents approximately two-thirds of the total organic carbon of the original humic acid and under alkaline conditions is a weak surfactant that lowers the surface tension of water by only a small amount. The HA(2) fraction represents up to one-third of the humic acid and significantly lowers the surface tension of water. It is also intimately associated with the lipid fraction. Unlike the original humic acid samples, HA(2) does not show micelle-like aggregation over the concentration range studied. Aggregate formation is discussed as an emergent characteristic that results from the interaction of the humic acid's components with the lipid components serving a facilitative role. (c) 2009 Elsevier Ltd. All rights reserved.