Feasibility of phosphorus saturated freshwater sludge derived carbonaceous materials as soil amendments: Molecular insights into speciation evolution of nutrients

Freshwater sludge (FS) is daily produced from drinking water treatment plants in large quantities and mostly disposed of in landfills worldwide. Here, we evaluated the feasibility of reutilization of P-saturated FS and derived biochar (FBC) as soil amendments for resource recycling. The results indicate that FS pyrolyzed at 300 °C (FBC300) had the highest adsorption capacity (40.54 mgP/g), and the rate-limiting step of adsorption was film diffusion but gradually altered to intraparticle diffusion with increasing pyrolysis temperature. Adsorbed P primarily existed as stable P fractions (NaOH-P, HCl-P and residue-P) or speciations (AlPO 4 and hydroxyapatite based on X-ray absorption fine structure spectra), but NaOH-P was found to gradually transform into available P (AP) in amended soils with time, achieving continuous release of AP and also reducing P loss through surface run-off. This study offers a novel and feasible reutilization strategy of freshwater sludge as phosphorus adsorbents and subsequent soil amendments to achieve the “zero waste” goal.