Distribution Behavior of Pyrene to Adsorbed Humic Acids on Kaolin

Abstract

The distribution behavior of pyrene on humic acid (HA)–kaolin complexes, prepared by adsorbing HA on kaolin, was investigated by batch experiments. The distribution coefficient (Kd) of pyrene on the HA–kaolin complex increased with the fraction (foc) of organic carbon adsorbed to the surface of the kaolin. This can be attributed to hydrophobic interactions between pyrene and the adsorbed HAs. The effects of adsorbed HAs were quantitatively evaluated by calculating the distribution coefficient (Koc) and affinity constant (Kadsoc) for pyrene to the adsorbed HAs. A fluorescence quenching method was employed to determine the affinity constant (Kaqoc) of pyrene to HAs dissolved in an aqueous solution. When the Koc values were compared with the Kaqoc values, the Koc values were found to be 4 to 11 times larger than the Kaqoc values. On the other hand, the Kadsoc values were 4 to 9 times larger than the Kaqoc values. These indicate that the affinity for pyrene is enhanced by the adsorption of HAs to kaolin. In addition, the Koc values increased with increasing average molecular weights of the HAs. These results demonstrate that HAs, when they are adsorbed to clay minerals, play an important role in the deposition of polycyclic aromatic hydrocarbons (PAHs) in a soil environment.