Immobilization of Cd and Pb in soil using PEI (polyethyleneimine)-functionalization biochar derived from agricultural residues

The rapid development of industry and agriculture has led to a significant increase in the toxicity and pollution of cadmium (Cd) and lead (Pb) in soil. Consequently, soil remediation employing biochar or modified biochar has emerged as a cost-effective and environmentally sustainable approach to address the issue of heavy metal (HM) ion pollution. PEI-functionalization biochar (PBC) derived from corn straw (PBCC), wood straw (PBCW), and rice straw (PBCR) was synthesized to immobilize Cd and Pb in contaminated acidic yellow soil. Characterization revealed successful PEI grafting N-doped carbon XRD patterns at 26.7°. PBC increased average pore size by 25.23–30.60% (3.976–3.175 nm, 23.906–18.34 nm) but reduced specific surface area due to polymer occlusion, PBCW retaining the highest surface area (214.9 m²/g). Applied at 1–7% doses, PBC significantly elevated soil pH and electrical conductivity while enhancing soil organic carbon, total nitrogen, and phosphorus. The 5% dose of PBCR showed strong Cd immobilization (47.15% reduction in bioavailable Cd, 8.32–11.01 mg/kg, p < 0.001) and reduced bioavailable Pb by 24.43% (0.06–0.11 mg/kg, p < 0.01). Sequential extraction confirmed PBC converted weak acid-extractable Cd/Pb into stable residual fractions. This study demonstrates that 5% PEI-modified rice straw biochar (PBCR) effectively remediates Cd/Pb co-contaminated acidic soils while enhancing agricultural productivity, confirming simultaneous metal passivation and nutrient retention. This work establishes agricultural residue-derived PEI-biochar as a sustainable strategy for co-remediating heavy metals and enhancing productivity in contaminated farmland.