Stepwise pH-Regulated Homogeneous Crystallization Enables Selective Phosphorus Recovery from Hydrothermally-Leached Sludge toward Value-Added Material Synthesis

Abstract Sludge phosphorus recovery is constrained by incomplete mobilization, impurity-rich leachates, and the limited value of conventional recovered materials. Here, a cascade process that couples selective phosphorus recovery with material-oriented conversion was developed, integrating thermal hydrolysis, citric acid leaching, membrane-enabled purification, stepwise homogeneous crystallization, and solid-state synthesis. Phosphorus release reached 66.1% via 170 °C thermal hydrolysis (TH) followed by citric acid (CA) at 0.2 g CA·g–1 TS leaching. Subsequent membrane processing decoupled solute composition by removing organic ligands and concentrating multivalent ions, thereby creating controlled conditions for crystallization. Guided by thermodynamics and kinetics, sequential homogeneous crystallization at pH 1.5 and 8.5 enabled the selective recovery of FePO4·2H2O and Ca10(PO4)6(OH)2, respectively. The recovered crystalline phases were further converted into precursors materials (LiFePO4 and Ca9Fe(PO4)7) via solid-state synthesis following mechanical activation, demonstrating feasibility of coupling resource recovery with downstream precursors production as key industrial raw materials. Rather than introducing new units, this work establishes a process-chemistry-material integration framework that leverages the intrinsic complexity of sludge to enable selective transformation and valorization. This strategy offers a potentially more sustainable and economically viable pathway for phosphorus recovery, advancing the transition from waste treatment to resource-oriented materials engineering and supporting circular innovation.