Behavior of fast and slow phosphorus release from sewage sludge-derived biochar amended with CaO

The pyrolyzation of sewage sludge (SS) could efficiently transform inherent phosphorus (P) into bioavailable phosphate forms, which endows SS-derived biochar (SSB) the potential as a soil fertilizer. However, the details about the release behavior of P in SSB have not been systematically investigated. This study evaluated the fast and slow P releasing behaviors from SSB and CaO-amended SSB prepared under different pyrolysis temperature. The higher pyrolysis temperature and CaO addition could enhance the conversion of non-apatite inorganic phosphorus (NAIP) into more bioavailable apatite inorganic phosphorous (AP). Acidic and alkaline conditions were favorable for the fast release of P from SSB. Higher ionic strength condition gave greater releasing amounts of TP and the SO₄^2- facilitating a rapid release of TP than those for Cl^- and NO₃^-. SSBs with CaO addition showed a much slower TP release than those without CaO both in fast release (24 h, with CaO: 0.05~0.4 mg TP g^-1 SSB, e.g., without CaO 0.5~5 mg TP g^-1 SSB) and slow release tests (21 days, with CaO: 1.2~4.1 mg TP g^-1 SSB, e.g., without CaO 1.8~5.7 mg TP g^-1 SSB). Ortho-P release was more remarkable for the SSB amended with CaO (~54% of TP), which was likely due to the formation of orthophosphate. The results of this study suggested that SSB prepared by high pyrolysis temperature and CaO addition had high potential as a slow P-releasing fertilizer for the soil.