## 3 Results and Discussion
### Comparison of Heavy Metal Removal Efficiency According to Solid-Solution Ratio  
The soil-solution ratio has been known as a key factor controlling the sorption and desorption of metal on soil [34]. In this study, except for CaCl\({}_{2}\), which had the lowest removal efficiency, the heavy metal removal efficiency according to the solid-solution ratio was evaluated for the remaining four types of washing solution (Figures 7 and 8). In 1 M citric acid, the removal efficiency of Cd, Cu, and Zn reached equilibrium at all solid-solution ratios, respectively, and only in the case of Pb, the reaction reached equilibrium at a 1:20 ratio. The maximum removal efficiency at each equilibrium solid-solution ratio was 84%, 80%, 89%, and 93% for Cd, Cu, Pb, and Zn, respectively. In the oxalic acid (1 M), all heavy metals were found to be reached at the maximum removal efficiency at 1:100 ratio, and the amount of heavy metal removal at 1:100 ratio was Cd 88%, Cu 92%, Pb 74%, and Zn 96%. That is, except for Pb, all heavy metals exhibited higher removal efficiency than citric acid. In the EDTA, the reaction of the remaining three heavy metals except for Zn reached equilibrium at 1:10 and Zn reached equilibrium at 1:20. However, the amount of heavy metal removal was relatively low compared to the rest of the washing solutions, showing 64% Cd, 72% Cu, 81% Pb, and 58% Zn. In the acid solution (pH 0.5), it was analyzed that all heavy metals reached equilibrium at the 1:5 treatment ratio and that the reaction reached equilibrium at the lowest solid-solution ratio among all solutions applied to the soil. In particular, the acid solution showed the highest treatment efficiency as all of the maximum removal efficiency was over 90% despite the low solid-solution ratio. Achieving the remediation goal was observed in all treatments except for the EDTA. Citric acid satisfied all criteria at a treatment ratio of 1:2 or higher in all treatment groups except for Cd (1:10), and oxalic acid satisfied the criteria at 1:100 or higher in all treatment groups except for Cu (1:20). In the acidic solution with the highest treatment efficiency, all heavy metals satisfied the criteria at 1:5 or higher, but EDTA was found to satisfy the criteria at 1:5 and 1:10 solid-solution ratio only for Cu and Pb, respectively. [PERSON] and [PERSON] [35] examined citric acid, EDTA, and oxalic acid as a washing solution and reported that the removal efficiency showed a tendency to increase as the solid-solution ratio increased.  
Figure 7: The effect of the solid-solution ratio on the heavy metal removal efficiency by 1 M citric acid (**a**), 1 M oxalic acid (**b**). An asterisk indicates significant difference from maximum removal efficiency.  
Although the washing condition is (types of solution, time, and solid-solution ratio) the same, the removal efficiency of each heavy metal is significantly different. This is because the behavior characteristics in soil are different for each type of heavy metal. [PERSON] et al. [36] reported that the uptake of heavy metal from soil to lawn grasses was conducted more easily as follows: Cd > Cu > Zn > Ni > Pb. This result was found to be affected by the type of heavy metals bound to the soil assessed by sequential extraction. For example, the behavior of Cu has been influenced by metal oxides and interactions with other elements, resulting in changes in Cu-binding fractions [37].  
It goes without saying that the greater the amount of washing solution compared to the soil, the greater the amount of removal efficiency [33]. However, the amount of heavy metals removed by the volume of washing solution decreases [38]. In addition, due to the problem of cost, it is important to find the optimal ratio.