Enhanced recovery of phosphorus in sewage sludge-derived biochar with CaCO3: Phosphorus speciation and slow-release phosphorus behavior.

[Display omitted] • The addition of CaCO 3 enhance the conversion of NAIP to AP in biochar. • The AP content in CaCO 3 -amended biochar at 700°C was higher than that at 500 °C. • The main form of P in CaCO 3 -amended biochar was Ca 5 (PO 4) 3 OH. • CS700-Ca10 has the potential to be used as a slow-release P fertilizer for soil. The recovery of phosphorus (P) from sewage sludge through pyrolysis is one of the most promising alternatives to alleviate the shortage of P supply in the future. Adding calcium additives is beneficial to the conversion of available P and helps to improve the recovery and utilization efficiency of P in biochar. This study thoroughly examined the influence of CaCO 3 on the speciation and slow-release behavior of P in sludge during pyrolysis at 500 and 700 °C. The addition of CaCO 3 could enhance the conversion of non-apatite inorganic phosphorus (NAIP) into more accessible apatite phosphorus (AP). We observed that adding CaCO 3 (2.5–20 %) to raw sludge at 500 °C increased the proportion of AP in inorganic phosphorus (IP) from 65.2 to 97.3 % but did not increase the concentration of AP, which remained between 31.03 and 31.64 mg/g. At 700 °C, introducing CaCO 3 (2.5–20 %) boosted the AP content from 35.08 to 41.38 mg/g and increased the proportion of AP in IP from 67.7 to 98.6 %. Moreover, irrespective of the pyrolysis temperature being 500 °C or 700 °C, the final main form of P in biochar with CaCO 3 was Ca 5 (PO 4) 3 OH, according to experimental characterization results (X-ray photoelectron spectroscopy and X-ray diffraction) and thermochemical calculations. Additionally, the efficacy of citric acid (CA) in releasing biochar-bearing P was tested in a laboratory extraction experiment for 21 days. CaCO 3 -amended biochar showed a significantly slower total phosphorus (TP) release than the biochar without CaCO 3 daily, and the amount of TP decreased with increasing amounts of added CaCO 3. The finding of this study indicates that biochar generated by adding 10 % CaCO 3 has the highest AP concentration (41.38 mg/g) at 700 °C, and its TP-release efficiency in CA is 0.29–1.98 mg/g in 21 days, which shows remarkable potential as a soil fertilizer with slow P release. [ABSTRACT FROM AUTHOR]