Enhanced reactivity of hydroxylated polycyclic aromatic hydrocarbons to birnessite in soil: reaction kinetics and nonextractable residue formation.
Phenanthrene and pyrene were not transformed by birnessite (delta-MnO2) in the presence of phenol. The phenoxy radicals generated from phenol by birnessite did not act as a mediator for polycyclic aromatic hydrocarbon radical reaction under the studied conditions. In contrast, 9-hydroxyphenanthrene and 1-hydroxypyrene were remarkably sensitive to birnessite. The disappearance patterns of the test compounds both in the aqueous phase and soil followed first-order kinetics, with a linear relationship found between the rate constants and the surface area of birnessite. Moreover, the data indicated that the reaction was faster in the presence of soil than in the aqueous phase probably because of the presence of hydroxyl groups in soil organic matter. Sequential solvent extraction was not successful in the recovery of 9-hydroxyphenanthrene from the birnessite-treated soil samples, and capillary electrophoresis data suggest the formation of nonextractable residues of the compound in soil. In addition, the acute toxicity determined by Microtox declined approximately 8.3 times in the soil samples treated with birnessite compared to untreated samples, demonstrating that the toxic compound was no longer present as its parent form.