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 degrees 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 degrees 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 degrees 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 degrees C or 700 degrees 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 in-dicates that biochar generated by adding 10 % CaCO3 has the highest AP concentration (41.38 mg/g) at 700 degrees 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.