## 3 Results and Discussion  
### Soil Leaching by Inorganic Acids  
#### 3.1.1 Effects of Inorganic Acids on the Lead Removal from Protogenetic Soil  
Four kinds of inorganic acids (HCl, HNO\({}_{3}\), H\({}_{2}\)SO\({}_{4}\), and H\({}_{3}\)PO\({}_{4}\)) and deionized water were used for leaching remediation of lead-contaminated soil, and the results are shown in Figure 1. It is indicated that the remediation efficiency of deionized water was 0.195%, which was similar to that reported by [PERSON] [23]. The soil pH value had a great influence on the form of lead in the soil. Generally, the content of soluble lead is higher in the acidic soil, in which the pH was 7.89 (Table 1), resulting in lead ions that were very tightly bound to the soil particles. The inorganic compounds of lead in the alkaline contaminated soil are rarely tetravalent and mainly exist in the insoluble divalent state, such as Pb(OH)\({}_{2}\), PbS, PbCO\({}_{3}\), and Pb\({}_{3}\)(CO\({}_{3}\))\({}_{2}\)(OH)\({}_{2}\); thus, it was difficult to obtain a higher remediation efficiency using only deionized water. In the case of Pb removal, soil leaching using HCl and HNO\({}_{3}\) showed a significant effect. After leaching with HCl and HNO\({}_{3}\), the lead concentration in the contaminated soil was reduced from 8043 mg\(\cdot\)kg\({}^{-1}\) to 2091.2 and 2429 mg\(\cdot\)kg\({}^{-1}\), respectively, which satisfied the control value standard of national construction land (the second type of land). When HCl and HNO\({}_{3}\) were used as eluents, a large amount of H\({}^{+}\) entered the solution and reacted with the inorganic compounds of lead in the soil, as shown in Equations (3)-(5), making the insoluble lead ions in the soil transform into soluble lead ions. Additionally, the other reason was that the amphoteric properties of functional groups such as hydroxyl groups in soil particles tend to absorb protons under highly acidic conditions [34], resulting in the decrease in the available anion concentration in the surface layer of soil and weakening the soil adsorption capacity for Pb\({}^{2+}\). Compared with HNO\({}_{3}\), Cl\({}^{-}\) can form soluble lead ion complexes ([PbCl\({}_{4}\)]\({}^{2-}\)) with Pb\({}^{2+}\), as shown in Equations (6)-(8), and the strong oxidation of HNO\({}_{3}\) may lead to the formation of insoluble metal compounds [23]; therefore, the remediation efficiency of HCl was slightly higher than that of HNO\({}_{3}\). The remediation efficiencies of H\({}_{2}\)SO\({}_{4}\) and H\({}_{3}\)PO\({}_{4}\) were only 1.81% and 28.0%, respectively. Though they could also produce a lot of H\({}^{+}\), promoting the dissolution of Pb\({}^{2+}\) from soil, their anions could react with lead ions to form precipitates, as shown in Equations (9) and (10). Both PbSO\({}_{4}\) and Pb\({}_{3}\)(PO\({}_{4}\))\({}_{2}\) could be precipitated as insoluble salts, which have a very small solubility product constant, making them remain in the soil. [PERSON] [35] studied the removal efficiencies of Zn by various inorganic acids, and they exhibited efficiencies in the order of HCl \(>\) HNO\({}_{3}\) \(>\) H\({}_{2}\)SO\({}_{4}\) \(>\) H\({}_{3}\)PO\({}_{4}\), which is similar to our result. Hence, HCl is the most suitable eluent for the remediation of lead-contaminated soil.