Role of lipids in the structural organization of humic acid
Abstract
The role of lipid components in the structural organization of humic acid in the solution- and the solid-state has been investigated using surface tensiometry and differential scanning calorimetry. A combination of aqueous alkaline and organic solvent extractions was used to isolate two humic-like fractions (HA1 and HA2) and one lipid-like fraction from humic acid. Fraction HA1 represents approximately two-thirds of the total organic carbon of the original humic acid and under alkaline conditions is a very weak surfactant. Fraction HA2 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, HA2 does not show micelle-like aggregation over the concentration range studied. The solid-state structural organization was examined by directly comparing the organization of the humic acid as isolated with a physical mixture of the same chemical composition composed of its three fractions using differential scanning calorimetry. Comparative measurements of the specific heat capacity as a function of temperature of the original humic acid reveal differences when compared to a mixture of the components with the same chemical composition. These differences provide direct evidence that humic acid's solid state structure is more than just a mixture of components and is determined by specific interactions between its components. This study indicates that humic acid has a hierarchical or "structure-within a structure" architecture. The lower-level structure is determined by the self-assembly of amphiphilic components of humic acid with lipids into a nanostructured composite material. A higher-level structure is formed by the association of this composite material with the remainder of humic acid's components.
- Publication:
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AGU Spring Meeting Abstracts
- Pub Date:
- May 2009
- Bibcode:
- 2009AGUSM.B32B..03R
- Keywords:
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- 0418 Bioremediation;
- 0428 Carbon cycling (4806);
- 1055 Organic and biogenic geochemistry