• • HP3SD1-B (Napier grass:digestate = 3:1) achieved a maximum equilibrium adsorption capacity of 306.45 mg/g and a theoretical Langmuir capacity of 447.62 mg/g, outperforming single-feedstock biochars by >40%, enabling cost-effective Pb²⁺ removal from industrial wastewater.
• • Adsorption kinetics followed a three-stage profile: rapid uptake (0–180 min), intraparticle diffusion (180–360 min), and equilibrium at 360 min, with pseudo-second-order fitting (R² > 0.99) confirming chemisorption as the rate-limiting step, essential for designing contact time in continuous flow systems.
• • Optimal pH for adsorption was 6.0, consistent with the material's point of zero charge (PZC ≈ 2) and Pb²⁺ speciation; this narrow operational window requires pH adjustment for acidic effluents, impacting process integration.
• • Mechanistic analysis (XPS, FTIR, SEM-EDS) revealed that Pb²⁺ immobilization is dominated by surface precipitation (Pb₃(CO₃)₂(OH)₂, PbO) and complexation with oxygen-containing functional groups, with auxiliary ion exchange (K⁺/Mg²⁺) and electrostatic attraction, ensuring strong binding and low leaching risk.