A field study of bioavailable polycyclic aromatic hydrocarbons (PAHs) in sewage sludge and biochar amended soils

The bioavailable PAHs (freely dissolved concentration, Cfree) were determined in sewage sludge (SL) or sewage sludge and biochar (BC) amended soil. SL or SL with a 2.5, 5 or 10% of BC was applied to the soil. The study was conducted as a long-term field experiment. Addition of BC to SL at a dose of 2.5 and 5% did not affect the content of Cfree PAHs in soils. However a significant difference (by 13%) in Cfree PAHs content was noted in experiment with 10% addition of BC. During the experiment, the concentration of Cfree PAHs in SL- and SL/BC-amended soil decreased. In particular sampling terms the content of Cfree PAHs in SL/BC-amended soil was significantly lower comparing to the Cfree PAHs content in SL-amended soil. After 18 months, Cfree PAH content was significantly lower in SL/BC-amended soil than in the experiment with SL alone, and did not differ significantly from the Σ16 Cfree content in the control soil. The largest decrease relative to the soil with sewage sludge alone was observed for 3-, 5- and 6-ring PAHs. This is the first field-based evidence that biochar soil amendment was effective to reduce of the Cfree of PAHs in sewage sludge-amended soils.