Objective  In order to provide a theoretical basis for the in-situ remediation on lead (Pb)- cadmium (Cd) contaminated alkaline soil, biochar and its mixture of zeolite were used as passivating agents to study their effects on the physical and chemical properties and the availability of Pb and Cd in soil.

  Method  Using Cd-Pb contaminated soils (Pb 227.75 mg kg⁻¹, Cd 3.38 mg kg⁻¹) collected from vegetable fields near the metal smelter in Henan Province, pot culture experiments were conducted to evaluate the effects of different amounts of biochar (0, 1%, 2.5%, and 5%) and its mixture of zeolite (biochar∶zeolite = 1∶1) on the soil physical and chemical properties, the concentration of available Pb and Cd in the tested soil, and accumulation and translocation of Cd and Pb in the green vegetable.

  Result  Compared with the control treatment, the application of biochar and its mixture of zeolite increased soil pH by 0.03 - 0.17 and increased significantly the contents of soil organic matter, alkali-hydrolyzed nitrogen, available phosphorus, and potassium (P < 0.05). The concentrations of available Pb and Cd in the soil decreased by 10.69% - 32.75% and 7.63% - 26.72% with the increase of amendment agents. The effect of the mixture agents was greater than that of the biochar under the same application amount. The passivating agents promoted green vegetable growth and reduced the accumulation of Pb in the green vegetables. The contents of Pb in the root, stem and leaf decreased by 2.79% - 64.90% and 4.76% - 76.19%, respectively. And there was a significant decrease when the amount of passivating agent was 5% (P < 0.05). The agents also reduced the contents of Cd in the root, but they failed to reduce those in the stem and leaf. The accumulation and translocation capacity of green vegetable was greater for Cd than that for Pb. A related analysis showed that the availabilities of Pb and Cd in green vegetables were less affected by pH changes, but they were significantly and negatively associated with the soil organic matter content (P < 0.05).

  Conclusion  In Pb-Cd contaminated alkaline soil, the biochar and its compound of zeolite both changed soil's physical and chemical properties. Compared with the application of biochar, the compound treatment played the more role in reducing the effectiveness of soil Pb and Cd in the soil. The availabilities of Pb and Cd were less affected by soil pH change, but greatly affected by organic matter content. There existed a higher accumulation and translocation capacity of Cd than that of Pb in the green vegetables.