Remediation Effects and Mechanisms of Biochar Derived from Agricultural Waste on Soils Contaminated with Cadmium (Cd) and Lead (Pb)

Cadmium (Cd) and lead (Pb) are ubiquitous toxic heavy metals in farmland soils, posing a threat to agricultural product safety and human health through food chain transmission. Biochar is widely used for in situ immobilization of heavy metals; however, systematic comparisons of the immobilization performance of rice straw biochar (RSB) and sugarcane bagasse biochar (SCB) under single and combined Cd–Pb contamination remain limited. This study systematically evaluated their immobilization performance and mechanisms through pot and batch adsorption experiments. Without altering total soil Cd and Pb contents, both biochars significantly regulated heavy metal bioavailability in the soil–plant system. In batch adsorption, RSB exhibited maximum Cd and Pb adsorption capacities 2.1 and 3.0 times those of SCB, respectively, with chemisorption as the dominant mechanism. In pot experiments, RSB reduced Pb uptake in pakchoi by 60.0% and 81.0%, but increased Cd uptake. SCB increased Cd uptake under single Cd contamination, had no significant effect on Pb under single Pb contamination, yet reduced Cd and Pb uptake under co-contamination by 44.4% and 31.6%, respectively. These differential effects are attributed to distinct mechanisms: Pb was primarily immobilized via stable mineral precipitation, whereas Cd was bound through weakly reversible ion exchange. Both biochars improved soil fertility and maintained core bacterial ecological functions without posing additional ecological risks. This study clarifies the feedstock-dependency and metal-specificity of biochar in remediating Cd- and Pb-contaminated farmlands, guiding precise biochar selection under varying contamination scenarios.