Fenton pretreatment promotes selective heavy metal removal while retaining phosphorus during sludge...

Sewage sludge represents a promising secondary phosphorus resource; however, its thermochemical valorization is severely hindered by the presence of hazardous heavy metals (HMs). This study integrates Fenton pretreatment with a dual-stage pyrolysis-calcination process to achieve selective HM removal while maximizing P retention in the solid phase. Municipal sludge was pretreated with varying concentrations of Fenton reagents, followed by pyrolysis at 500 °C and calcination at 900 °C. At the optimal Fenton concentration of 90 mM, total HM removal efficiency improved by 70.91%, with particularly high volatilization of Zn and Mn, while P retention remained stable at approximately 81%. Sequential extraction revealed that pretreatment effectively mobilized HMs from stable fractions into more labile forms, facilitating their thermal migration. Concurrently, P speciation analysis showed a transition from organic phosphorus toward inorganic forms, specifically non-apatite inorganic phosphorus (NAIP), enhancing P stability in the final biochar. Spectroscopic analyses (FTIR, XRD, and XPS) indicated that Fenton-driven oxidation destabilized organic-ligand and carbonate-associated host environments. Density Functional Theory (DFT) calculations further demonstrated that hydroxyl-radical attack induces electron density redistribution within Zn-O coordination structures, significantly weakening Zn-organic and Zn-mineral interactions. These findings demonstrate that Fenton-assisted thermal conversion can simultaneously mitigate HM risks and preserve the nutrient value of sludge-derived biochar, offering a robust pathway for sustainable sludge valorization.