The convertion of sewage sludge to biochar as a sustainable tool of PAHs exposure reduction during agricultural utilization of sewage sludges

There is a discussion in the literature whether PAHs introduced with biochar are safe and whether they are persistent in the environment. The persistence of PAHs (C_tot – total and C_free – freely dissolved) in sewage sludge (SSL) or SSL-derived biochar-amended soils was investigated. Biochar were produced at 500, 600 and 700 °C. We also compared the persistence of PAHs in these experimental treatments depending on the plants cultivated (grass, clover and thale cress). We showed that the C_tot PAHs in the biochar-amended soils exhibited higher persistence than in the SSL-amended soil. The opposite trend was observed for C_free PAHs. A higher reduction of C_free PAHs was noted in the biochar-amended soils than in SSL-amended soil. The persistence of both C_free and C_tot PAHs clearly varied between the biochars produced at different temperatures. It should be stated that despite that for biochar the persistence of C_tot PAHs is higher compared to SSL-amended soils, an opposite trend is observed for the fraction of C_free (which is directly responsible for the toxic effect), and this entails a lower risk to the environment (lower mobility and bioavailability). The plants had a significant impact on C_tot PAHs content depending on the number of PAH rings.