One-step synthesis of Fe/Mg-modified sludge biochar for phosphate recovery from sludge dewatering supernatant: Performance, mechanism and cost analysis

Sludge dewatering supernatant is phosphorus-rich yet underutilized because its high ionic strength and dissolved organic matter limit conventional adsorbents. Here, an Fe/Mg bimetallic sludge biochar (FeMg@SBC) was synthesized via one-step impregnation-pyrolysis for phosphate capture. FeMg@SBC exhibited a maximum adsorption capacity of 116.8 mg/g and maintained efficient phosphate removal over pH 3–11, consistent with the synergistic roles of Fe and Mg sites. Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) revealed Fe-O-P and Mg-O-P linkages formed through ligand exchange with surface hydroxyl/oxide groups, while X-ray diffraction (XRD) confirmed adsorption-induced surface precipitation via metal-phosphate formation. Together with density functional theory (DFT) calculations identifying Fe/Mg-associated oxygen-containing groups as key active sites, these results indicate that phosphate immobilization was governed by coupled ligand exchange and surface precipitation. In real sludge dewatering supernatant, 1.0 g/L FeMg@SBC reduced phosphate from 131.9 to 40.2 mg/L, retained 65% of its initial capacity after five regeneration cycles, and showed low Fe and Mg leaching of 0.25 and 0.13 mg/L, respectively. The estimated production cost was 1.60 USD/kg. A conceptual process integration was further proposed, providing a cost-effective pathway for sludge valorization and closed-loop phosphorus recovery from sidestreams.