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

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 CaCO3 on the speciation and slow-release behavior of P in sludge during pyrolysis at 500 and 700 °C. The addition of CaCO3 could enhance the conversion of non-apatite inorganic phosphorus (NAIP) into more accessible apatite phosphorus (AP). We observed that adding CaCO3 (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 CaCO3 (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 CaCO3 was Ca5(PO4)3OH, 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. CaCO3-amended biochar showed a significantly slower total phosphorus (TP) release than the biochar without CaCO3 daily, and the amount of TP decreased with increasing amounts of added CaCO3. The finding of this study indicates that biochar generated by adding 10 % CaCO3 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.