Bioavailability-Adjusted B[a]P Degradation In Soil And Bioaccumulation In Earthworm

As the most carcinogenic pollutant among polycyclic aromatic hydrocarbon compounds, Benzo(a)pyrene (B[a]P) can not only undergo metabolic degradation by microorganism in soil, but also can be accumulated and metabolized through food chains in vivo, with different metabolites still possessing carcinogenic and toxic effects as products. B[a]P is prone to adsorption and aging, which keep B[a]P from entering the organism and reduce the bioavailability of organic pollutants in soil. To date, environmental risk assessment for B[a]P is mainly based on the total amount of compound, lack of the consideration in bioavailability limit and risks of metabolites. In this study, the degradation and residual of B[a]P and its metabolites in soil was investigated. Bioavailability-adjusted degradation kinetics of B[a]P in soil was proposed by measuring the concentration of available B[a]P. Establishing the relationship model between earthworm bioaccumulation and bioavailability of B[a]P. Then new assessment indicators including B[a]P bioavailability and risks of metabolites were proposed.The method to extract and analyse available and total B[a]P and its metabolites (3-OHB[a]P, B[a]P-7,8-dihydrodiol, B[a]P-tetrol I and B[a]P-tetrol II) was established. The results show that the main degradation pattern of B[a]P in soil was microbial metabolism, with 3-OHB[a]P and B[a]P-7,8-dihydrodiol as products. The degradation of B[a]P in three soils followed the first-order kinetics (R2> 0.92), the degradation rate constants of available B[a]P ranged from 4.6 × 10-3 to 14.1 × 10-3 d-1, higher than that of total B[a]P with rate constants from 2.1 × 10-3 to 5.8 × 10-3 d-1, indicating a significant degradation difference between available and total B[a]P. The bioavailability of B[a]P evaluated by the concentration ratio between available B[a]P and total B[a]P decreased over time, which followed the exponential equation (R2> 0.83), indicating that the degradation of B[a]P in soil had an effect on B[a]P bioavailability. To further define the degradation of B[a]P in soil, a bioavailability-adjusted degradation kinetics model of B[a]P was established, with the degradation rate constants from 2.5 × 10-3 to 6.1 × 10'3 d-1 (R2> 0.96).The linear relationship (R2= 0.89) between available B[a]P and bioaccumulation in earthworms was established by measuring the concentrations of total B[a]P, available B[a]P and bioaccumulation in earthworms in 31 kinds of soils, indicating that the available B[a]P may be suitable to mimic the accumulation of earthworm. The correlation model on B[a]P bioavailability and earthworms bioaccumulation was established by incorporating B[a]P bioavailability into the accumulation process of earthworm. The B[a]P carcinogenic risk model (Crisk=CHPCDTp+Ctp/HPCDTtp) based on bioavailability of B[a]P and degradation metabolites was established by analyzing the content relation between B[a]P and its metabolites (3-OHB[a]P and B[a]P-7,8-dihydrodiol) as well as environmental risks in 31 soils. The results show that the carcinogenic risk concentrations of B[a]P were 1.1?6.8 fold higher than the total B[a]P concentration, indicating that the total B[a]P concentration in soils can underestimate the integrated environmental risks, especially far more underestimated in farmland soils.