Tobacco (Nicotiana tabacum L.) is a crop that is able to accumulate metals. In this study, humic acid was selected as a Cd, Cu, Pb, and Zn passivator, and added to calcareous field soil in amounts of 6.4, 10.3, and 14.8 kg·ha−1. Its impact on the soil fractions of the metals in the soil was extracted by the Community Bureau of Reference (BCR) sequential extraction method, and their accumulation of the metals in tobacco leaves was investigated. Application of 14.8 kg·ha−1 humic acid decreased the DTPA-extracted concentrations of Pb, Cd, Zn, and Cu by 39%, 37%, 29%, and 18%, respectively, as compared with untreated soil. The fractions of Cd, Pb, Cu, and Zn in soil were extracted by the BCR sequential extraction method, and the relationship between the difference metal fractions in the soil and the metal contents in the plant materials were analyzed. The exchangeable fractions of Cd, Pb, Cu, and Zn and the reducible fractions of Pb and Cu are the main bioavailable fractions. Additionally, the reducible fractions of Cd and Zn, the oxidizable fractions of Pb, Cu, and Zn, and all residual fractions of metals were nonbioavailable fractions in the soil. The soils were treated with humic acid (HA) to shift bioavailable metals to stable phases that were less bioavailable. The available Cd, Pb, Cu, and Zn were strongly retained in the soil after the application of humic acid, which decreased the uptake in tobacco in the upper, middle, and lower leaves.

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A 56d incubation experiment was conducted to explore the effects of the silk-worm excrement biochar (500 °C, BC) combined with different iron-based materials (FeCl3, FeSO4, and reduced iron powder) on the speciation of cadmium (Cd) and lead (Pb) in a contaminated soil. Application rate of BC and iron-based materials is 1% (W/W) and 0.2% (W/W) of the soil, respectively. At the same time, the soil physicochemical properties, such as pH, cation exchange capacity (CEC), and the structure of soil, were determined in order to explore the influence mechanism of amendments to forms of Cd and Pb in soil. The results show that the stabilization effects on Cd is (BC + FeSO4) > (BC + FeCl3) > (BC + Fe) > (BC) and Pb is (BC + Fe) > (BC + FeSO4) > (BC + FeCl3) > (BC) at the end of incubation, compared with the effect of the control group. The treatment of (BC + FeSO4) is the most effective in terms of the stabilization of Cd and Pb, which makes the percentages of organic-bound and residual Cd and Pb increase by 40.90% and 23.51% respectively. In addition, with different ways of treatment, the pH value and CEC of soil see a remarkable increase by 1.65–2.01 units and 2.01–2.58 cmol·kg−1 respectively. X-ray diffraction (XRD) patterns show that the soil imprisons Cd and Pb in different mineral phases. As such the treatment of (BC + FeSO4) can significantly improve soil environment, increase soil pH value & CEC and exert a relatively good stabilization effect on both Cd and Pb.