Effect of carrier gas during pyrolysis on the persistence and bioavailability of polycyclic aromatic hydrocarbons in biochar-amended soil

In this study the persistence (based on extractable, C-tot) and bioavailability (based on freely dissolved content, C-free) of polycyclic aromatic hydrocarbons (PAHs) in biochar-amended soil was investigated. Biochar produced at 500 or 700 degrees C from sewage sludge (BC) or sewage sludge and willow (W) mixture (BCW) in an atmosphere of nitrogen (N-2) or carbon dioxide (CO2) was evaluated. The biochars were applied to the real soil (podzolic loamy sand) at a dose of 2% (w/w). The content of C-tot and C-free PAHs was monitored for 180 days. The biochar production conditions determined the C-tot and C-free PAHs in the soil. A change of carrier gas from N-2 to CO2 caused an increase in C-tot PAH losses in the soil from 19 to 75% for the biochar produced from SL and from 49 to 206% for the co-pyrolyzed biochar. As regards C-free PAHs, the change from N-2 to CO2 increased the losses of C-free PAHs only for the biochar derived from SL at a temperature of 500 degrees C (by 21%). In the soil with the other biochars (produced at 700 degrees C from SL as well as at 500 and 700 degrees C from SL/W), the C-free increased from 17 to 26% compared to the same biochars produced in an atmosphere of N-2.