Bioavailability of polycyclic aromatic hydrocarbons in soils contaminated with airborn dust depositions

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