Fate and stability of nonextractable residues of [14C]PAH in contaminated soils under environmental stress conditions

It has been recognized during recent years that polycyclic aromatic hydrocarbons (PAH) may form nonextractable residues in soil and that this process may be stimulated by microbial activities. To use that process intentionally for soil bioremediation, one must ensure that the formed nonextractable PAH residues will not be released from the soil on the long run. The long-term stability of four different nonextractable [(14)C]PAH residues ([(14)C]naphthalene, [(14)C]anthracene, [(14)C]pyrene, and [(14)C]benzo[a]pyrene) was therefore monitored under different ecological stress conditions, it was found that a considerable fraction of the total [(14)C]PAH residues could be released as (14)CO(2) from the soil being partly due to a biogenic reduction of the nonextractable (14)C residue fraction. The turnover of this fraction was comparable to the natural turnover rate for humic substances. Neither the addition of humus-degrading microorganisms nor a mechanical stress treatment of the soil structure by freezing and thawing led to a mobilization of the nonextractable [(14)C]PAH residues. However, a significant mobilization of the nonextractable (14)C activity occurred when EDTA was added to the soil. The metal-organic soil complexes were destabilized by this complexing agent and released (14)C activity that was attached to colloidal or dissolved organic matter.