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

Abstract

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 degrees 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.