## 3 Results and Discussion
### Effects of Leading on Lead Speciation and Soil Quality  
The biological toxicity of heavy metals in soil is not only determined by its total amount; it is more dependent on the existing forms. Hence, the speciation analysis of heavy metals in soil will help to further explore the effectiveness of the repair technology, degrading the bioavailability of heavy metals. For this reason, this study further compared the speciation evolution of lead in contaminated soil before and after leaching with HCl and CA, respectively. Under optimal conditions, lead was separately leached by HCl and CA from the contaminated soil. The lead speciation in soil after leaching was divided into four fractions and measured by a modified three-step sequential extraction BCR procedure. The speciation concentration distribution of lead in soil before and after leaching with 0.02 mol\(\cdot\)L\({}^{-1}\) HCl and 0.05 mol\(\cdot\)L\({}^{-1}\) CA are shown in Figure 6a, respectively. In the protogenetic soil, the existing forms of lead are mainly acid-soluble and residual fractions. The content of acid-soluble lead is as high as 2765 mg\(\cdot\)kg\({}^{-1}\), accounting for 34.4% of the total lead, indicating that much of the lead in the contaminated soil has a high mobility and bioavailability. The content of residual lead is 2625 mg\(\cdot\)kg\({}^{-1}\), accounting for 32.6% of the total lead, and it mainly exists in soil crystal lattices such as silicate and primary and secondary minerals, which is the result of natural geological weathering processes. Compared with other forms lead, it is not easily released under normal conditions and can be stable in the sediment for a long time. The \(M_{\rm F}\) value of protogenetic soil, as shown in Figure 6c, is 34.4%; that is, such higher mobility of Pb will lead to the great soil toxicity and harmfulness. It can be seen from Figure 6a,b that, after leaching, the form distribution of lead in the soil changed greatly, and the contents of the acid-soluble fraction, reducible fraction, oxidizable fraction, and residual fraction decreased, and the acid-soluble state decreased the most. Ke [40] indicated that CA can mainly remove exchangeable, carbonate-bound, oxide-bound forms of the four heavy metals after leaching, which is similar to our results. The acid-soluble lead is mainly composed of two parts: one is the clay minerals adsorbed on the sediment and other components; the other part is the lead combined with the carbonate precipitation. The acid-soluble fraction is a very unstable form because it is sensitive to environmental changes and easily migrates and transforms. Especially at a low pH, it can easily be washed out. The results showed that, after leaching by HCl and CA, the content of acid-soluble lead in the soil decreased from 2765 mg\(\cdot\)kg\({}^{-1}\) to 185 and 460 mg\(\cdot\)kg\({}^{-1}\), a decrease of 93% and 83%, respectively, indicating that the removal capacity of CA to the active lead fraction is comparable to that of HCl. Furthermore, the \(M_{\rm F}\) values of the soil were 7.74% and 12.3%, and the pH values of the soil were 3.88 and 6.97, respectively. Both of them can obviously reduce the mobility of lead ions in the soil and its biological toxicity, but the high-concentration HCl leaching will make the soil severely acidified, destroying its structure. Figure 6: Speciation distribution of Pb in soils before and after leaching: (**a**) concentration of different forms of Pb, (**b**) removal rates of different forms of Pb, (**c**) the \(M_{\rm F}\) value and pH of soil before and after leaching. \(F_{1}\): Acid-soluble fraction. \(F_{2}\): Reducible fraction. \(F_{3}\): Oxidizable fraction. \(F_{4}\): Residual fraction. The removal ability of HCl for other less active lead fractions is stronger than CA, which is why HCl acid has a higher remediation efficiency for lead-contaminated soil. The reducible fraction mainly refers to the part combined with oxides such as Fe and Mn in the soil.