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 SO42- 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 similar to 0.4 mg TP g(-1) SSB, e.g., without CaO 0.5 similar to 5 mg TP g(-1) SSB) and slow release tests (21 days, with CaO: 1.2 similar to 4.1 mg TP g(-1) SSB, e.g., without CaO 1.8 similar to 5.7 mg TP g(-1) SSB). Ortho-P release was more remarkable for the SSB amended with CaO (similar to 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.

Automated summary

The pyrolyzation of sewage sludge can convert inherent phosphorus into bioavailable phosphate forms, making sewage sludge-derived biochar a potential soil fertilizer. This study investigated the release behavior of phosphorus from biochar and CaO-amended biochar under different pyrolysis temperatures, showing that higher temperature and CaO addition enhanced the conversion of non-apatite inorganic phosphorus into more bioavailable forms. Acidic and alkaline conditions favored the fast release of phosphorus from biochar, while biochar with CaO addition exhibited a slower release of phosphorus compared to biochar without CaO.