Immobilisation of endogenous phosphorus and lead in sediments by composite water purification sludge hydrochar

Sediment is a critical component of aquatic ecosystems, that acts as a natural sink for diverse pollutants such as heavy metals and phosphorus (P). However, the current research on sediment remediation has predominantly focused on single contaminants. In this study, a novel composite material, calcium peroxide/lanthanum-loaded hydrochar (CaO₂–LaHyd), was synthesised through the hydrothermal carbonisation of water purification sludge, followed by the sequential loading of lanthanum ions and nano-calcium peroxide. The adsorption capacities of CaO₂–LaHyd for P and Pb were evaluated via adsorption experiments, and their passivation mechanisms were investigated through sediment capping simulations. Materials were characterised using scanning electron microscopy, X-ray photoelectron spectroscopy, X-ray diffraction, and energy-dispersive spectroscopy. Results indicated that CaO₂–LaHyd exhibited maximum adsorption capacities of 66.05 mg·g⁻¹ for P and 230.41 mg·g⁻¹ for Pb. In the simulated capping experiments, the addition of 5% CaO₂–LaHyd significantly reduced the phosphate concentrations in the overlying water. The slow release of oxygen from calcium peroxide improves sediment redox conditions, suppresses endogenous P release, and decreases interstitial P levels. Speciation analysis revealed that CaO₂–LaHyd promoted the endogenous P transformation into stable forms while reducing the bioavailable P fractions. Concurrently, it enhanced Pb passivation, thereby mitigating Pb leaching risks from the sediment.