Study on the existing forms and recovery efficiencies of phosphorus in biochar from excess sludge

With the background of carbon neutrality, the resource and energy utilization of excess sludge (ES) have become the focus of future research. The pyrolysis of ES can produce biochar and enrich phosphorus (P). In this paper, the existing forms and recovery efficiencies of P in biochar from ES (BCES) were investigated. The results showed that the total phosphorus (TP) content of BCES at 850 °C was 65.1 mg/g, and the inorganic phosphorus (IP) content was 64.2 mg/g. The TP content of BCES was two times heavier than that of ES. The main ingredient of ES was quartz (SiO₂), while the main phases of BCES were quartz (SiO₂) and aluminum phosphate (AlPO₄) at 650 -850 °C, and P mainly existed in the form of AlPO₄. When the pyrolysis temperature was 800 and 850 °C, two new minerals appeared: Ca₅(PO₄)₃OH and CaZn₂(PO₄)₂·2H₂O. Based on the conditions of a leaching time of 150 min, a H₂SO₄ concentration of 0.2 mol/L, a stirring rate of 220 rpm and a liquid-solid ratio of 50 mL/g, the leaching efficiency of P in BCES was found to be 100%. The pyrolysis temperature had no effect on leaching efficiencies of P; however, a higher pyrolysis temperature promoted metal leaching content.