Combining biochar and lime amendments for remediation of Cd-contaminated paddy soil around e-waste dismantling site: a study from laboratory to field.

Cadmium (Cd) contamination from irregular e-waste dismantling poses a significant threat to agricultural safety and human health in surrounding paddy fields, necessitating the development of effective remediation strategies. Biochar and lime-based materials are recognized as representative organic and inorganic adsorbents, respectively, for remediating heavy metal-contaminated soils. However, the applicability and mechanisms of biochar, alone or in combination with lime, for remediating Cd-contaminated paddy soils remain inadequately understood. Herein, we investigated the effectiveness and mechanisms of biochars (peanut straw biochar, PB; corn straw biochar, CB), limes (limestone, LS; alkaline slag, AS), and their combinations in ameliorating soil acidity and remediating Cd contamination in a typical paddy soil adjacent to an e-waste dismantling area in China through a multi-scale approach combining laboratory incubation, greenhouse pot experiments, and field trials. PB was more effective than CB in reducing soil acidity among the biochars, while LS outperformed AS among inorganic amendments. The sole application of PB could immobilize Cd and reduce acidity, but it required impractically high application rates; doses ≤3.84 t ha-1 showed no significant effect. At current cost-effectiveness levels, using biochar alone for Cd remediation is economically unfeasible for large-scale field application. Under field conditions, combining biochar with limestone (2.64 t ha-1 LS + 1.92 t ha-1 PB) or applying LS alone (5.26 t ha-1) reduced soil bioavailable Cd by ∼90 % and decreased Cd concentration in rice grains by ∼50 %, bringing grain Cd content below the Chinese limit (0.2 mg kg-1). The primary mechanism for Cd reduction was the effective alleviation of soil acidity by these amendments. Additionally, the remediation efficacy was influenced by spatial heterogeneity within the field, a factor that should be considered in both technology development and precision remediation. This multi-scale investigation provides crucial technical guidance for remediating heavy metal-contaminated soils around e-waste dismantling zones.