Influence of microwave-assisted pyrolysis parameters and additives on phosphorus speciation and transformation in phosphorus-enriched biochar derived from municipal sewage sludge

Municipal sewage sludge is rich in phosphorus, the conversion of which into phosphorus-enriched biochar via microwave-assisted pyrolysis (MWAP) can realize the efficient treatment of municipal sewage sludge and the recycling of phosphorus. This follows the concepts of the sustainable development of waste management and “cleaner production”. At present, fundamental knowledge is lacking on the use of MWAP to recover phosphorus from municipal sewage sludge. In this study, the effect of MWAP parameters (temperature, power, and residence time) and additives on the properties of phosphorus-enriched biochar and the migration and transformation of phosphorus were investigated. The pH, surface area, and total phosphorous increased with increasing temperature and power, whereas a greater residence time promoted the conversion of non-apatite inorganic phosphorus (NAIP) into apatite inorganic phosphorous (AP). CaO addition promoted the conversion of NAIP into AP, whereas MgCl2 promoted the conversion of AP into NAIP. The simultaneous addition of CaO and MgCl2 led to antagonism, with CaO being dominant, demonstrating that the original phosphorus components prefer to combine with CaO. Phosphorus recovery was optimal at temperatures 500–600 °C and a residence time of 150 min. The addition of 9% CaO or 6% CaO +6% MgCl2 was sufficient to convert sewage sludge into phosphorus into AP. Under such conditions, AP contents in phosphorus-enriched biochar increased 1.4 times compared to that of additive-free biochar. Our study highlights the potential of MWAP for closing the phosphorus cycle and for producing fertilizers from complex materials, such as municipal sewage sludge.