Enhanced phosphorus bioavailability of biochar derived from sewage sludge co-pyrolyzed with K, Ca-rich biomass ash

Sewage sludge has great potential for phosphorus (P) recovery. However, sewage sludge-derived biochar suffers from low P bioavailability in land application. K, Ca-rich biomass ash was used to co-pyrolyze with sewage sludge to enhance P bioavailability of synthesized biochar. Phase transformation mechanism of P during the co-pyrolysis process was studied. When sunflower straw ash (SSA) was used as an additive (50 wt%) for co-pyrolysis with sludge at 600 °C, the ratio of bioavailable P (Bio-P, determined by 2 wt% citric acid) to total P (TP) of the co-pyrolyzed biochar could achieve 92.1 wt%, which was remarkably higher than that of biochar pyrolyzed by sludge alone (9.5 wt%). The K and Ca elements in SSA significantly contributed to the conversion of the Fe-phosphate phase (FePO₄) in sludge into K, Ca-phosphates (KCaPO₄, K₂CaP₂O₇ and K₂CaP₂O₇·4H₂O) and Ca-phosphate (Ca₅(PO₄)₃OH) phases, therefore enhancing the Bio-P content in the co-pyrolyzed biochar. Model compound pyrolysis results indicate that KCl/K₂CO₃ and CaCO₃ phases in SSA play a synergistic role in enhancing the P bioavailability of co-pyrolyzed biochar. Based on the DFT calculations, the absolute value of the adsorption energy (|ΔE_ads|) of CH₃COO^- presented an order: K₂CaP₂O₇ (2.43 eV) > KCaPO₄ (1.70 eV) > Ca₅(PO₄)₃OH (1.64 eV)> FePO₄ (1.08 eV), indicating that K₂CaP₂O₇, KCaPO₄, and Ca₅(PO₄)₃OH are more likely to interact with organic acid and have higher bioavailability than FePO₄. Furthermore, the co-pyrolyzed biochar reaches the release rate standard for P, K-slow-release fertilizer. This study proposes a promising and sustainable solution to simultaneously realize sludge utilization and P resource recycling.