Phenanthrene metabolites bound to soil organic matter by birnessite following partial biodegradation

The hypothesis that phenanthrene, an aromatic compound without a hydroxyl group, can form nonextractable residues in soil with the aid of phenanthrene-biodegrading bacteria and birnessite was tested. The mutant strain Sphingobium yanoikuyae B8/36 successfully accumulated cis-phenanthrene dihydrodiol, and the intermediate was readily radicalized and coupled into soil organic matter in the presence of birnessite. Phenanthrene and the intermediate disappeared from the soil in 96 h in the presence of birnessite, but the intermediate accumulation occurred without birnessite. By determining the total organic carbon contents before and after birnessite treatment, it could be seen that birnessite did not mineralize cis-phenanthrene dihydrodiol. Fourier transform infrared and ultraviolet analyses suggest instead that the intermediate was incorporated into the soil organic matter, forming nonextractable, bound residues. Increases in the aromaticity and pH in birnessite-treated soil also present more evidence for bound residue formation. The soil in which bound residue formed did not exhibit an acute toxicity of phenanthrene, but evidence indicated that such toxicity existed in the freshly spiked soil. In addition, a long-term column test revealed that the bound residues could not be eluted by the combination of water, 80% methanol, and U.S. Environmental Protection Agency Toxicity Characteristic Leaching Procedure solution (pH 2.88) for four months, implying stability of the nonextractable residues in the soil.