Enhanced Recovery of Nonextractable Benzo[a]pyrene Residues in Contrasting Soils Using Exhaustive Methanolic and Nonmethanolic Alkaline Treatments

The fate, impacts, and significance of polycyclic aromatic hydrocarbon (PAH) nonextractable residues (NERs) in soils remain largely unexplored in risk-based contaminated land management. In this study, seven different methanolic and nonmethanolic alkaline treatments, and the conventional methanolic saponification, were used to extract benzo[a]pyrene (B[a]P) NERs that had been aged for 180 d from four contrasting soils. Up to 16% and 55% of the amount of B[a]P spiked (50 mg/kg) into soils was nonextractable after 2 d and 180 of aging, respectively, indicating rapid and progressive B[a]P sequestration in soils over time. The recovery of B[a]P from soils after 180 d of aging was increased by up to 48% by the seven different alkaline extractions, although the extraction efficiencies of the different alkaline treatments did not differ significantly (p > 0.05). Approximately 40% of B[a]P NERs in the sandy-clay-loam organic matter-rich soil was recovered by the exhaustive alkaline extractions after 180 d of aging, compared to only 10% using conventional methanolic saponification. However, the amounts of B[a]P NERs recovered depend on soil properties and the amounts of NERs in soils. A significant correlation (R-2 = 0.69, p < 0.001) was also observed between the amounts of B[a]P recovered by each of the seven alkaline extractions in the contrasting soils and corresponding NERs at 180 d of aging, indicating a potential association warranting further investigations. Extraction techniques that estimate the amounts of PAH NERs recoverable in soil can help give a better understanding of the fate of NERs in soil.