Atmospheric carbonation reduces bioaccessibility of PAHs in industrially contaminated soil

Sorptive Bioaccessibility Extraction (SBE) was used to monitor changes in accessibility of polycyclic aromatic hydrocarbons (PAHs) during storage of historically contaminated alkaline soil (Σ US EPA 16 + 2 further PAHs: 2452 ± 69 mg kg^-1, n = 3). While total concentrations of PAHs were rather stable during storage for 561 days at 4 °C, PAH accessibility declined by 95% due to atmospheric carbonation. The formation of carbonates was evidenced by an increase of inorganic soil carbon and by carbonate coatings on black soil particles (SEM-EDX) that could be dissolved by providing neutral to acidic soil conditions. Subjecting soil (252 days of storage) to biodegradation at pH 7 resulted in a degraded fraction of PAHs equivalent to the accessible PAH fraction of soil as received (PAHs with log K_ow <5). The present study addresses important interactions and relationships between carbonation of soil, aging of PAHs in soil and related changes in PAH accessibility. A main finding was the reversibility of this retention mechanism, a changing environment (e.g. reduction of pH below 8) can result in a rise of accessible PAHs and consequently in an increase of exposure and associated risk.