Retained yet redistributed: Crystallization and reduction of iron phases reshape phosphorus availability during iron-enhanced primary sludge pyrolysis.

Phosphorus (P) recovery from iron-rich sludge is critical for sustainable resource management, yet the Fe-mediated mechanisms governing P transformation during pyrolysis remain poorly understood. Here, iron-enhanced primary sludge was pyrolyzed (200-900°C) to elucidate the role of Fe speciation evolution in controlling P redistribution and bioavailability. P and Fe were quantitatively retained during pyrolysis (∼100% recovery), while P speciation underwent pronounced redistribution from Fe-associated P (NaOH-P and BD-P) to Ca-associated P (HCl-P) and ultimately to residual P (Res-P), which reached 57.8% at 900°C. Integrated spectroscopic analyses (XRD, XPS, and 2D-COS) demonstrate that Fe evolves from poorly crystalline Fe (hydr)oxides to crystalline Fe oxides and further to zero-valent iron and iron phosphide species. Thermodynamic and density functional theory calculations further indicate that Fe mineral transformation facilitates the release of Fe-bound P, while Ca phases stabilize released phosphate as Ca-P at 400°C-600°C. Seed germination and correlation analyses linked this intermediate-temperature Ca-associated P window to improved root and leaf growth, whereas high-temperature biochar showed reduced P bioavailability despite higher P stability. These findings demonstrate that pyrolysis temperature can be used to tune Fe-mediated P transformation, balancing P bioavailability and long-term stabilization in Fe-rich sludge-derived biochar.