Pyrolysis of sewage sludge to biochar: Transformation mechanism of phosphorus

Biochar produced by pyrolysis of sewage sludge (SS) is considered as a great resource for P recycling. However, the unclear transformation mechanism of phosphorus species during pyrolysis limits the further application of biochar. The study systematically investigated the effects of pyrolysis temperature (300 − 500 ℃) and additives (CaO and MgO) on P speciation in SS and its derived biochar, proposed the transformation pathways of P and clarified its transformation mechanism. The results indicated that the ester bonds in orthophosphate diesters were broken before 400 ℃ to form orthophosphate monoesters and pyrophosphate. Various P forms in SS were gradually converted to Ca10(PO4)6(OH)2, Ca9Al(PO4)7, and Ca3(PO4)2. The addition of CaO further promoted the formation of Ca−P, and the maximum of Ca−P was 2.5 times than that of pyrolysis alone. Phosphorus mainly existed in the forms of Ca3(PO4)2. The promoting effect of MgO on the formation of apatite phosphorus (AP) was not as obvious as CaO. The study provides more theoretical support for the feasibility of SS-pyrolysis biochar for agricultural applications.