Preparation of Biochar from Co-pyrolysis of Napier Grass and Food Waste Digestate for Pb²⁺ Removal from Wastewater
Lead (Pb) is a highly toxic heavy metal that poses severe risks to environmental and human health, particularly affecting children's neurological development. This study investigates the adsorption performance and mechanisms of biochars derived from pyrolysis of Napier grass (Pennisetum purpureum), food waste digestate, and their mixtures for Pb²⁺ removal from aqueous solutions. Biochars were prepared at different mass ratios, and the optimal material (HP3SD1-B, Napier grass:digestate = 3:1) exhibited a maximum equilibrium adsorption capacity of 306.45 mg/g and a theoretical Langmuir maximum capacity of 447.62 mg/g, significantly outperforming pure digestate biochar and lower-ratio blends. Adsorption kinetics followed a three-stage profile: rapid liquid-film diffusion (0–180 min), intraparticle diffusion (180–360 min), and equilibrium at 360 min. The adsorption process was well described by the pseudo-second-order kinetic model (R² > 0.99) and the Langmuir isotherm, indicating monolayer chemisorption. Characterization via FTIR, XPS, SEM-EDS, and Zeta potential revealed that Pb²⁺ immobilization occurs primarily through surface precipitation (Pb₃(CO₃)₂(OH)₂ and PbO), complexation with hydroxyl, ether, and aromatic C=C groups, and auxiliary mechanisms including electrostatic attraction and K⁺/Mg²⁺ ion exchange. Optimal adsorption occurred at pH 6, correlating with the point of zero charge (PZC ≈ 2). This study demonstrates that co-pyrolysis of agricultural and organic solid wastes offers a cost-effective, high-performance biochar for heavy metal remediation, aligning with circular economy principles.