## 3 Results and Discussion
### Soil Leaching by Low-Molecular-Weight Organic Acids
#### 3.2.3 Effect of CA Concentration  
Consequently, a concentration experiment was carried out, and the concentration gradient was designed to be 0.01, 0.03, 0.05, 0.07, 0.1, and 0.3 mol\(\cdot\)L\({}^{-1}\). The results are shown in Figure 5. With the increase in CA concentration from 0.01 to 0.3 mol\(\cdot\)L\({}^{-1}\), the remediation efficiency of Pb increased from 0.61% to 61.6% (Figure 5a), and the content of Pb decreased from 7994 to 3087 mg\(\cdot\)kg\({}^{-1}\) (Figure 5b), which is slightly lower than that of HCl. When the concentration of CA was too low, such as 0.01 mol\(\cdot\)L\({}^{-1}\), the remediation efficiency was very poor. This may be because the CA content was too low to dissolve lead ions. The remediation efficiency was significantly increased and reached 54.9%, while the concentration of CA increased from 0.01 to 0.05 mol\(\cdot\)L\({}^{-1}\). Herein, CA had strong acid dissolving effect and complexing ability. Generally speaking, the higher the concentration of CA is, the more lead ions are complexed. However, when the concentration exceeded 0.05 mol\(\cdot\)L\({}^{-1}\), there was little increase in the remediation efficiency, which may be that the complexing capacity of CA had reached its limit. Lead in contaminated soil, such as residual lead, is usually difficult to leach using CA. Hence, 0.05 mol\(\cdot\)L\({}^{-1}\) was chosen as the optimum reaction concentration for the leaching experiments. Figure 5: Lead-contaminated soil is remediated by CA: (**a**) remediation efficiency, (**b**) Pb content in soil after leaching. Not only is the removal effect of CA on lead related to acid dissolution effect, but it also depends on the complexation ability of the anion acid group to a certain extent. In order to further understand the removal mechanism of metal ions by CA, it is necessary to consider the interaction between lead ions and anionic ligands, and the complex characteristic of CA and lead ions forming a variety of 1:1 stable soluble complexes. As reported in [40], CA can be dissociated into four forms (H\({}_{3}\)A, H\({}_{2}\)A\({}^{-}\), HA\({}^{2-}\), A\({}^{3-}\)) in solution if it is expressed as H\({}_{3}\)A, so the reaction between the lead ion and the anion of CA under different pH conditions can be interpreted as follows:  
\[\mathrm{Pb^{2+}+H_{2}A^{-}}\rightleftharpoons(\mathrm{PbH_{2}A})^{+}\qquad \qquad(\mathrm{pH}\;<\;4) \tag{13}\]  
\[\mathrm{Pb^{2+}+HA^{2-}}\rightleftharpoons(\mathrm{PbHA})\qquad\qquad(4\;<\; \mathrm{pH}\;<\;6) \tag{14}\]  
\[\mathrm{Pb^{2+}+A^{3-}}\rightleftharpoons(\mathrm{PbA})^{-}\qquad\qquad( \mathrm{pH}\;>\;6) \tag{15}\]  
The order of the complexation equilibrium constant of CA and lead ions is as follows: (PbA)- \(>\) (PbH\({}_{2}\)A)+ \(>\) PbHA [40]. When the temperature is fixed, the existing forms of CA are only related to pH. When pH < 4, strong acidic conditions are conducive to the dissociation of Pb\({}^{2+}\) from soil colloids, and the complexation is strong at the same time. Therefore, CA has a higher remediation efficiency of lead at low pH (<4), and the existing form of lead complexes is mainly (PbH\({}_{2}\)A)+. H\({}_{2}\)A\({}^{-}\) gradually transforms into HA\({}^{2-}\) and a small amount of A\({}^{3-}\) when pH is between 4 and 6. At present, the acid dissolution effect of CA is weak, and the complexing ability plays a leading role. The main existing form of lead complexes is (PbHA). A further increase in pH will lead to the complete failure of acid dissolution effect, and only complex Pb is produced. Although the complexation ability of CA is strong at this time, the remediation efficiency is low due to the hydrolysis of Pb\({}^{2+}\). As a result, the acid dissolution's effect and complexation ability play different roles at different pH values, and the pH value is affected by CA concentration, which is the essence of changing the CA concentration and thus affecting the remediation efficiency.