Numerous studies have reported the potential of silica as a biochar (BC) modifier. However, despite its high silica content, fly ash is rarely used for BC modification. Herein, modified BCs were produced by co-pyrolysis of corn stalks with alkali-fused fly ash (AFFA) at 200 and 600 °C (denoted as AFFA/BC). The Pb(II) adsorption mechanism and adsorption performance were investigated. The AFFA/BC had larger specific surface areas than the pure BC samples (2.54–137 vs. 0.50 m² g⁻¹) owing to their stable carbon structure. The Pb(II) adsorption capacity of AFFA/BC in water was approximately 6% higher than that of BC owing to the increased cation (Na⁺) exchange and new bonding sites, such as C–O and Si–O. AFFA/BC exhibited good Pb(II) adsorption performance in high-concentration simulated wastewater (pH 4–6), with a maximum adsorption capacity of 110.29 mg g⁻¹. The Pb(II) adsorption mechanism was in accordance with the pseudo-second-order kinetic and Langmuir isotherm models. At 25 °C and pH 5, the theoretical Pb(II) adsorption capacities of AFFA₂₀₀/BC and AFFA₆₀₀/BC were 201.66 and 186.81 mg g⁻¹, respectively, compared to 145.98 mg g⁻¹ of BC. Physical adsorption, precipitation, cation exchange, and complexation were identified as the main Pb(II) adsorption mechanisms through X-ray photoelectron spectrometry.

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