Nanobiochar outperforms biochar in reducing cadmium and lead mobility through soil chemical and rhizosphere-mediated phytostabilization.

A soil-column experiment was conducted to compare biochar (BC) and nanobiochar (NB) at 2.5, 5, and 10% (w/w) to investigate their potential for stabilizing cadmium (Cd) and lead (Pb) under bare-soil and maize-planted conditions, enabling separation of amendment effects from rhizosphere-mediated influences. Soil properties, metal leaching behavior, microbial biomass nitrogen, plant growth, tissue metal accumulation, and phytostabilization indices were assessed. In contaminated unamended soil, soil organic matter, cation exchange capacity, and microbial biomass nitrogen declined, with substantial adverse effects in bare soil than in planted soil. Both amendments improved soil properties and shifted Cd and Pb from bioavailable to residual fractions, with a stronger effect by NB. Under 10% NB with plant cover, bioavailable Cd and Pb declined by 76% and 73%, respectively, compared with the unamended treatment, while residual fractions increased markedly. Nanobiochar strongly reduced mobility-related indices; the Cd leaching index and desorption ratio decreased by 93%, while those of Pb decreased by 90%, relative to the unamended bare-soil treatment. Plant metal concentrations followed the order root > shoot > leaf. Under 10% NB, shoot and leaf Cd declined by 95-96%, while shoot and leaf Pb declined by 94-95%. Overall, the NB-maize system showed the greatest stabilization efficiency.