Phytostabilization of cadmium and lead in maize using biochar and nanobiochar: effects on soil properties and metal dynamics

Although biochar is widely used for the remediation of potentially toxic elements (PTEs), comparative evaluations of biochar (BC) and its nanoform (nanobiochar, NB) for soil-based phytostabilization remain limited. This study assessed the efficiency of BC and NB (applied at 2.5%, 5%, and 10% w/w) in immobilizing cadmium (Cd) and lead (Pb) in contaminated soils using maize (<i>Zea mays</i> L.) as a test crop in a completely randomized design. Both amendments significantly improved soil physicochemical properties, with NB 10% producing the greatest increase in organic matter (from 0.85% to 1.80%) and cation exchange capacity (from 9.8 to 18.4 cmol/kg). Compared with the unamended control, NB 10% reduced bioavailable Cd and Pb fractions by 76.1% and 83.7%, respectively, while increasing pseudo-residual metal forms. Maize germination increased from 64% in the control to 88% under NB 10%, and root growth inhibition decreased by 62%, indicating reduced metal phytotoxicity. Shoot and root Cd and Pb concentrations declined by more than 50%, resulting in markedly lower bioaccumulation and translocation factors. The highest phytostabilization efficiency (90% for Cd, 94% for Pb) and pseudo-residual fractions (90.7% Cd, 94.1% Pb) were recorded in NB 10%-amended soil. Overall, NB outperformed conventional BC due to its greater surface reactivity, porosity, and functional properties, demonstrating strong potential for sustainable stabilization of PTE-contaminated soils. These results demonstrate that NB can serve as an efficient and sustainable soil amendment for reducing metal bioavailability and ensuring safer crop production in contaminated agricultural soils.