Development of phosphorus composite biochar for simultaneous enhanced carbon sink and heavy metal immobilization in...

As a porous and carbon material, biochar is focused on respectively in sequestrating carbon and stabilizing metals in soil, while few studies attempted to design biochar for simultaneously achieving these two targets. This study proposed to produce phosphorus-composite biochar for synchronously enhancing carbon sequestration and heavy metals immobilization. Two phosphorus materials from tailings, Ca(H₂PO₄)₂ and Ca₅(PO₄)₃(OH), were selected as modifier to load into biomass prior to pyrolysis. Results showed that incorporating P not only increased pyrolytic C retention in biochar by 36.1-50.1%, but also obtained biochar with higher stability by chemically formation of COP, C-PO₃ and C₂-PO₂. After 90-day incubation with soil, more C was sequestrated in the P-biochar amended soil (59.6-67.0%) than those pristine biochar (43.2-46.6%). Highly soluble Ca(H₂PO₄)₂ was more efficient than Ca₅(PO₄)₃(OH) in this regard. Meanwhile, these P-composite biochar exhibited more Pb/Cd immobilization (31.3-92.3%) compared with the pristine biochar (9.5-47.2%), which was mainly due to the formation of stable precipitates Pb₅(PO₄)₃Cl and Cd₃(PO₄)₂, especially for Ca₅(PO₄)₃(OH) modification. Additionally, P-composite biochar "intelligently" altered soil microbial community, i.e., they suppressed Actinobacteria proliferation, which is correlated to carbon degradation, while promoted Proteobacteria growth, facilitating phosphate dissolution for ready reaction with heavy metals to form precipitate, benefiting the Pb and Cd immobilization. A dual functions biochar was engineered via simply loading phosphorous prior to pyrolysis and simultaneously enhanced carbon sequestration and heavy metal immobilization.