Bioavailability of Polycyclic Aromatic Hydrocarbons in the Soils Contaminated with Airborne Dust Depositions

A model experiment on the biodegradation of polycyclic aromatic hydrocarbons (PAHs) in the upper horizons of urban soils with different organic matter contents was carried out. The airborne dust depositions containing PAHs were added to the soil samples; the samples were at constant humidity and temperature. The total PAH content and the contents of potentially bioavailable fractions of phenanthrene, pyrene, and benzo(a)pyrene were determined in the samples after 1, 51, 102, 190, and 365 days of incubation. The total PAH content determined by the exhaustive extraction and the content of potentially bioavailable fractions extracted with n-butanol exponentially decreased over 365 days of the experiment in both the control samples and the samples artificially contaminated with atmospheric dust. The rate of PAH biodegradation was proportional to the absolute content of the bioavailable fractions in soils and negatively correlated with the hydrophobicity of the three examined PAHs in the soils with a high organic matter content. The relative content of the bioavailable fractions of phenanthrene and pyrene decreased during the experiment, whereas the relative content of bioavailable benzo(a)pyrene remained almost constant. Based on the obtained results, a conceptual scheme for the transformation of the PAHs from airborne dust depositions in soils is proposed. It takes into account not only the molecular parameters of polyarenes but also the phase composition of the polluting source material containing PAH when assessing PAH bioavailability. The procedure for determining the potentially bioavailable fraction of polyarenes in soil by direct measuring their concentration in n-butanol together with determination of the total PAH content can be used as a method for environmental assessment of the PAH state in urban soils when predicting the rates of accumulation and transformation of hydrophobic pollutants.