The influence of lipids on the energetics of uptake of polycyclic aromatic hydrocarbons by natural organic matter

Although most of the organic carbon in soils and sediments may be composed of humic substances, their interaction with other compounds, especially their sorption interactions, may be significantly affected by the presence of small amounts of the other components of natural organic matter (NOM). In this investigation, the influence of the lipid fraction of NOM on the sorption thermodynamics of fluorene, phenanthrene, and pyrene to several geosorbent samples was examined before and after extraction of lipids. Batch experiments were performed at the same concentration for all polycyclic aromatic hydrocarbons (PAHs) (0.025 x their solubility in water) at different temperatures (10, 20, 30, and 40 degrees C), and the thermodynamic parameters were calculated. Removal of the lipids increases the sorption capacity of the samples as well as the exothermicity of the process. The free energy change was negative for all the samples and no significant differences were noticed on lipid removal. The entropy changes were small and positive for the whole geosorbent samples, but even smaller or more negative when the lipids were removed. This indicates that the interaction of PAHs with soils and sediments in the absence of extractable lipids is stronger and the mechanisms involved may be different, changing from a partitioning-like mechanism to specific adsorption. Because of the competition between lipids and PAHs for the same sorption sites, the lipids can be viewed as an "implicit sorbate."