## 3 Results and Discussion
### Phytoremediation of Cd and Zn by Taracum officinal
#### 3.1.3 Effect of Other Competitive Metals on the Adsorption of Cd and Zn  
The concentrations of Cd and Zn in the shoots or roots of _Taraxacum officinal_ under the presence of other competitive metals (i.e., Al, Pb, Bi, and Ni) by adding various concentrations of EDDS are shown in Figure 4a for Cd and in Figure 4b for Zn, and each of the TF values are shown in Figure S2.  
In Figure 4, it can be seen that the concentrations of Cd or Zn did not necessarily decrease under the presence of other competitive metals (i.e., Al, Pb, Bi, and Ni). Comparing these results with that of Figure S1 (i.e., the case under the absence of other competitive  
Figure 4: Effect of the EDDS concentration on the absorption of (**a**) Cd and (**b**) Zn by _Taraxacum officinal_ under the presence of Al, Bi, Ni, and Pb. BLK (Blank) represents the pot for no addition of heavy metals (i.e., Al, Pb, Bi, and Ni) as well as no addition of EDDS (i.e., Pot No. 1 in Table 2).  
metals), the effect of competition with other metals on the concentrations of Cd or Zn was not constant and may also depend on the concentration of EDDS. In addition, from Figure S3, it can be concluded that the value of TF is generally higher than unity, although no large increase was observed compared to the case of the absence of other competitive metals (i.e., Figure S3). It can be said that the effect of phytoremediation of Cd and Zn by _Taraxacum officinal_ was detected under the presence of the same concentration of other competitive metals (i.e., Al, Pb, Bi, and Ni) to some extent. Comparing Pot Nos. 1 and 2 in Table S2 under the condition of Table 2, the biomass of the shoots (or roots) increased by adding other competitive metals (i.e., Al, Bi, Ni, and Pb).  
In [PERSON] et al.'s work [34], it was reported that multi-contaminated conditions of soil promote the uptake of heavy metals by the plant. A similar tendency was detected in this work.  
As mentioned in Section 3.1.1, the stability constants of EDDS with Cd\({}^{2+}\) and Zn\({}^{2+}\) are \(10^{11.5}\) and \(10^{13.5}\), respectively, which are relatively high. Looking at the results from the experiments, we considered that there is not so large selectivity, although EDDS has a higher stability constant with Zn\({}^{2+}\) compared to Cd\({}^{2+}\) (unlike the case of stability constant for EDTA). In the case of other metals, the stability constants of EDTA with Pb\({}^{2+}\) and Ni\({}^{2+}\) are known to \(10^{18.0}\) and \(10^{18.4}\), respectively [41]. This shows that its stability with Cd\({}^{2+}\) and Zn\({}^{2+}\) is either equal or slightly larger. In this way, it is considered that that chelating agents generally have high stability constant and lower selectivity with any metals.  
From our experiments, it may be certainly suggested that the amount of Cd and Zn taken up and transported by plants is not affected by the presence of other investigated metal ions in the soil. As mentioned above, it can be said that the chelating ligands bind these metal ions as well. Estimating from the setting condition shown in Table 1 or Table 2 and the molecular weight of chelating agent (for example, that of EDDS is 292.24), the concentration ratio of metal ions to ligand is quite small (that is from 1/300 to 1/50), and it can be said that the ligand concentration is much higher than the metal ion concentration; hence, the results obtained from our experiments are considered to be reasonable.