Pyrolysis temperature governs the immobilization mechanisms and selectivity of medulla stachyuri-derived biochar for...

The development of efficient and sustainable adsorbents is crucial for remediation heavy metal contamination in both aqueous and terrestrial environments. Biomass-derived biochar shows great promise, yet its adsorption performance is highly dependent on both the feedstock properties and the pyrolysis temperature. However, a systematic understanding of how temperature dictates the adsorption mechanisms, especially for multi-metal systems, remains limited for novel biomass precursors. Therefore, the stem pith of Medulla stachyuri (MS) was utilized to prepare biochars at different pyrolysis temperatures (400, 600, and 800 °C, denoted as MBCs) and to investigate their adsorption behavior for Pb²⁺ and Cu²⁺. The results indicated that higher pyrolysis temperatures significantly enhanced the specific surface area (reaching 322.94 m²/g for MBC-800) and ash content but decreased the oxygen-containing functional groups. MBC-800 exhibited superior adsorption capacities for Pb²⁺ (2139.25 mg/g) and Cu²⁺ (970.68 mg/g), with remarkable selectivity for Pb²⁺ in binary systems. Mechanistic studies revealed that the dominant adsorption mechanism shifted from surface complexation at lower temperatures to precipitation induced by the biochar's inherent inorganic components (e.g., SO₄²⁻ forming PbSO₄) at higher temperatures. Component contribution analysis quantitatively confirmed that the water-soluble fraction in MBC-800 was responsible for over 78% of Pb²⁺ immobilization. Furthermore, MBC-800 demonstrated excellent stability with the lowest desorption rate, indicating a low risk of secondary pollution. This work highlights the superiority of high-temperature biochar from MS for efficient and stable heavy metal removal, providing new insights into the precipitation-dominated mechanism and the high-value utilization of medicinal plant residues.