## 3 Results and Discussion
### Comparison of Heavy Metal Removal Efficiency According to the Type of Washing Solution  
The total concentration of Cd was 17.34 mg kg\({}^{-1}\). To meet the remediation goal, 4 mg kg\({}^{-1}\), about 77% of the total concentration should be decreased (Figure 2). For Cd, the maximum removal efficiency of washing solutions were 86.8%, 67.0%, 64.5%, 47.5%, and  
Figure 1: Binding fraction (%) of heavy metals in soil assessed by BCR method. 15.6% for acid solution (HCl 3 M), citric acid (100 mM), EDTA (100 mM), oxalic acid (1 M), and CaCl\({}_{2}\) (100 mM), respectively (Figure 2). NOEC showed the lowest concentration of EDTA at 50 mM and citric acid and oxalic acid at 100 mM and 1 M, respectively. The remediation goal was found to be satisfied only in the acid solution at pH\({}_{\text{eq}}\) 5.7. The total concentration of Cu was 307.76 mg kg\({}^{-1}\). To meet the remediation goal, 150 mg kg\({}^{-1}\), about 51% of the total concentration should be decreased (Figure 3). For Cu, the maximum removal efficiency of washing solutions were 91.6%, 75.8%, 53.8%, 52.8%, and 0.5% for acid solution (3 M), oxalic acid (100 mM), citric acid (100 mM), EDTA (100 mM), and CaCl\({}_{2}\) (100 mM), respectively (Figure 3). NOEC showed the lowest concentration of oxalic acid at 10 mM and EDTA and citric acid at 50 mM and 100 mM, respectively. The remediation goal was found to be satisfied in the acid solution at pH\({}_{\text{eq}}\) 0.5, EDTA at 4.4 mM, citric acid at 11.1 mM, and oxalic acid at 5.6 mM, respectively. The total concentration of Pb was 714.78 mg kg\({}^{-1}\). To meet the remediation goal, 200 mg kg\({}^{-1}\), about 72% of the total concentration should be decreased (Figure 4). For Pb, the maximum removal efficiency of washing solutions were 100%, 81.0%, 62.1%, and 3.1% for acid solution (3 M), EDTA (100 mM), citric acid (1 M), and oxalic acid (1 M), respectively (Figure 4). NOEC showed the lowest concentration of EDTA at 50 mM and oxalic acid and citric acid at 500 mM and 1 M, respectively. The remediation goal was found to be satisfied only in the acid solution at pH\({}_{\text{eq}}\) 0.5 and EDTA at 3.4 mM, respectively. Figure 2: The effect of the washing solution and its concentration on Cd removal efficiency. Red dotted line indicates the minimum removal efficiency of heavy metals to meet the legal standards (4 mg kg\({}^{-1}\) for Cd). The total concentration of Zn was 1755.47 mg kg\({}^{-1}\). To meet the remediation goal, 300 mg kg\({}^{-1}\), about 83% of the total concentration should be decreased (Figure 5). For Zn, the maximum removal efficiency of washing solutions were 91.5, 64.1%, 56.7%, and 30.4% for acid solution (3 M), oxalic acid (1 M), citric acid (100 mM), and EDTA (100 mM), respectively (Figure 5). NOEC showed the lowest concentration of EDTA at 50 mM and oxalic acid and citric acid at 1 M and 100 mM, respectively. The remediation goal was found to be satisfied only in the acid solution at pH\({}_{\text{eq}}\) 0.6. The main mechanism of washing and removal of heavy metals in the soil can be explained by the increase in the mobility of the heavy metals due to the decrease in pH and the formation of ligand complexes by the heavy metals and chelating agents [6; 9; 27]. The acidic solution (pH adjustment by HCl) used in this study to wash and remove heavy metals, chelating agents such as organic acids (citric acid, oxalic acid, EDTA), and neutral salts (CaCl\({}_{2}\)) are also major heavy metal desorption agents based on the above two mechanisms. [PERSON] et al. [28] attempted to remove Cd from soil using various types of low-molecular-weight organic acids (LMOWAs). As a result, although there was a large difference in pH depending on the type of organic acid after washing, the actual Cd removal rate showed a greater difference according to the difference in the formation coefficient of the ligand complex of the organic acid rather than the change in pH.