Lead remediation in contaminated soils using magnetic biochar hydrogel beads via swelling-driven hydraulic gradients and Fe3+-mediated acidification.

Lead (Pb) contamination in soils poses a persistent threat to food safety and human health. Here, we report a novel remediation strategy employing porous Fe2O3/BC@SA hydrogel beads-composed of Fe2O3-modified biochar (BC) embedded within a sodium alginate (SA) matrix. This composite synergizes the high Pb adsorption capacity of BC with the hydrophilicity and mechanical resilience of hydrogels, yielding a robust and environmentally compatible adsorbent. The beads exhibited a maximum Pb2+ adsorption capacity of 238.5 mg g-1. In soil incubation assays, 5.0 wt% bead amendment removed 75.8% of bioavailable Pb within 60 days and retained high removal efficiency after five wet-dry cycles, underscoring its durability under practical conditions. Mechanistically, hydrogel swelling upon hydration establishes a hydraulic gradient that drives Pb2+ migration toward the beads. Simultaneously, Fe3+ hydrolysis releases protons, inducing localized acidification that remobilizes sequestered Pb species, which are subsequently captured via coordination, ion exchange, and electrostatic interactions. Magnetic recovery further enables bead retrieval and reuse, minimizing secondary contamination risk. This work presents a sustainable, low-cost, and scalable approach for in-situ Pb immobilization in contaminated soils, with significant implications for environmental remediation and agricultural safety.