Contribution of phenanthrene in different binding sites to its biodegradation in biochar-amended soils

Highlights: • PHE biodegradability in different binding sites was studied in soils with biochar. • Biodegradation of PHE was more strongly inhibited by BC400 than by BC700. • Both PHE and microbial biomass highly concentrated on the biochar. • Only PHE in rapidly desorbing sites was degraded in soils with BC400. • PHE degradation in slowly desorbing sites played the key role in soils with BC700. Biochars can strongly sorb hydrophobic organic contaminants in soils. However, contribution of contaminants in different binding sites to their biodegradation in biochar-amended soils is not clear. In this work, wheat straw biochars were prepared at pyrolysis temperatures of 400 °C (BC400) and 700 °C (BC700). During a 42-day experiment, degradation rate constant of phenanthrene in soils was in the order of treatment without biochar (1.64 × 10⁻² d⁻¹) > treatment with BC700 (0.96 × 10⁻² d⁻¹) > treatment with BC400 (0.30 × 10⁻² d⁻¹). At the beginning, amendment of BC400 and BC700 reduced the rapidly desorbing fraction of phenanthrene in soils by 44.8% and 92.5%, respectively. At the end, both phenanthrene and microbial biomass highly concentrated on the biochar separated from soils. The results of a coupled model of desorption and biodegradation revealed that only phenanthrene in rapidly desorbing sites was degraded in BC400-amended soils, whereas degradation of phenanthrene in both rapidly and slowly desorbing sites occurred in BC700-amended soils, contributing 24.4% and 75.6% of the degradation, respectively. High fraction (>95%) of biodegradable phenanthrene in slowly desorbing sites was the key reason for higher biodegradation rate of phenanthrene in soils with BC700 than in soils with BC400.