## 3 Results and Discussion
### Comparison of Heavy Metal Removal Efficiency According to Washing Time  
Except for the acid solution, which had very high and fast removal efficiency, and CaCl\({}_{2}\), which had the lowest removal efficiency, the heavy metal removal efficiency according to the washing time was evaluated for the remaining three types of washing solution (Figure 6). [PERSON] and [PERSON] [32] reported that HCl solution could easily remove Cd, Pb, and Ni within 1 h. In particular, the equilibrium was reached within 1 h in most cases regardless of the concentration of HCl. [PERSON] et al. [33] tested four types of extraction solution and revealed that the heavy metal removal efficiency is the highest in HCl solution rather than citric acid, EDTA, and CaCl\({}_{2}\). In addition, the removal equilibrium was reached within  
Figure 4: The effect of the washing solution and its concentration on Pb removal efficiency. Red dotted line indicates the minimum removal efficiency of heavy metals to meet the legal standards (200 mg kg\({}^{-1}\) for Pb).  
30 min. In the 1 M citric acid, the equilibrium time of Cu was 120 min, indicating that desorption reached equilibrium the fastest, and it was evaluated that the washing reaction reached equilibrium in all other heavy metals at 240 min. In the 1 M oxalic acid, it was investigated that Cd and Pb reached equilibrium at 30 min, and Cu and Zn reached equilibrium at 60 min. In the EDTA (50 mM), Cd and Pb were reached at the same equilibration time of 120 min, and the remaining two metals reached equilibrium at 240 min.  
Based on the equilibration time, it is judged that the washing reaction reaches equilibrium at the fastest time in the 1 M oxalic acid treatment. However, when actually comparing the removal efficiency, the removal efficiency of the other heavy metals except for Cu (81%) was significantly lower than that of citric acid and EDTA. It was predicted that Cd could only be removed at 70% and 79% levels, respectively, compared to the two treatment groups, and Zn was also evaluated to be only 87% higher than citric acid. In addition, in the case of Pb, the removal efficiency was less than 50% (47% compared to citric acid, 49% compared to EDTA), so it is judged that it is necessary to consider the removal efficiency as well as the reaction equilibrium time. Meeting the remediation goal was observed only for Cu and Pb. It was predicted that Cu took less than 1 min in the citric acid and oxalic acid, respectively, and it is expected that it will take about 10.7 min longer in the EDTA than citric acid and oxalic acid. It was evaluated that Pb required 68.4 min for citric acid and 50.0 min for EDTA, respectively.  
Figure 5: The effect of the washing solution and its concentration on Zn removal efficiency. Red dotted line indicates the minimum removal efficiency of heavy metals to meet the legal standards (300 mg kg\({}^{-1}\) for Zn).  
Figure 6: The effect of the washing time on heavy metals removal efficiency by 1 M citric acid (**a**), 1 M oxalic acid (**b**), and 50 mM EDTA (**c**). Red dotted line indicates the minimum removal efficiency of heavy metals to meet the legal standards.  
The rapid extraction at the beginning of agitation confirmed in this experiment can be viewed as a result of the presence of metals in the soil. In the case of general soil, it takes a long time to remove heavy metals because the fraction of residual form is high, resulting in the fact that the dissolution and movement of metals into the extractants are limited [33]. On the other hand, it can be seen that heavy metals were extracted and removed in a relatively short period of time in the soil of the abandoned mine area, as there are many carbonate bonds and Fe/Mn-oxide bonds, which have weak bonding with soil components. In this study, it can be seen that the residual fraction is all less than 20% for four types of heavy metals except Ni, which is not highly contaminated (Figure 1).