## 3 Results and Discussion
### Pertinent Factors Affecting the Complex of Amino Acids and Cd  
The formation of Cd complexes might be influenced by various factors in water. As shown in Figure 5a, the uncomplexed Cd\({}^{2+}\) concentration increased with increasing ionic strength. In fact, the inhibition might be due to competitive binding onto the active functional groups of amino acids between the cationic ions and Cd\({}^{2+}\). A similar inhibition phenomenon has been observed in previously reported studies in which cationic ions in natural groundwater negatively affected the complexing of Cd\({}^{2+}\) with ligands [45]. It is apparent from Figure 5b that the uncomplexed Cd\({}^{2+}\) concentration decreased with increasing pH, indicating that the complex process was promoted at higher pH. [PERSON] et al. (1995) also reported that a higher pH was beneficial to ternary and binary chelation formation between metal and amino acids, resulting in more stability than complex formation [46]. This might be because the protonation of amino and carboxyl groups at low pH would reduce the available chelate sites [47]. As reported by [PERSON] et al. (2019), the proportion of coordinated metal increased with increasing pH, and nearly 100% copper ions were coordinated with amino acids at pH 7.4 [48].  
\begin{table}
\begin{tabular}{c c c c c c c} \hline
**Amino Acid** & **ThrCd2+** & **GluCd2+** & **GlyCd2+** & **AlaCd2+** & **LeuCd2+** & **PheCd2+** \\ \hline Complex energy & \(-24.33\) & \(-23.87\) & \(-22.26\) & \(-21.58\) & \(-20.31\) & \(-18.96\) \\ (kcal/mol) & \(-260.94\) & \(-248.42\) & \(-279.17\) & \(-272.23\) & \(-264.98\) & \(-254.29\) \\ \hline \end{tabular}
\end{table}
Table 4: Calculated energy of amino acids and cadmium complexes.  
Figure 5: The complexation of Cd\({}^{2+}\) with amino acids at different (**a**) amino acid concentrations, (**b**) pH, and (**c**) cation concentrations.  
The complex of amino acids and Cd can also be affected by the initial concentration of amino acids. As seen in Figure 5c, the uncomplexed Cd\({}^{2+}\) concentration decreased with increasing amino acid concentration from 0 to 0.1 mmol/L. An increase in ligands introduces more soluble complex agents for Cd and decreases Cd\({}^{2+}\) without precipitation [37]. The complexation of the ligands with Cd\({}^{2+}\) was fitted, and the fitting curve is shown in Figure 6. The minimum logCd\({}^{2+}\) was estimated to be \(-\)9.26 mol/L, \(-\)9.06 mol/L, \(-\)8.92 mol/L, \(-\)8.84 mol/L, \(-\)8.56 mol/L, and \(-\)8.54 mol/L for Thr, Glu, Gly, Ala, Leu, and Phe, respectively. The complexation constants were then calculated by Equation (3). As presented in Table 5, the resulting constant \"\(p\)\" value was less than 1, indicating that cadmium existed in the form of a monodentate complex. Moreover, the constant log\(\beta\) values of these complexes were 5.56, 5.05, 5.03, 4.96, and 4.77, for Thr, Glu, Gly, Leu, Ala, and Phe, respectively. The log\(\beta\) values of these complexes followed the order of Thr > Glu >> Gly > Ala > Leu >> Phe, in good agreement with their corresponding complex energy order in Section 3.1.5. All of these results implied that Thr and Glu had a greater ability to chelate Cd\({}^{2+}\) to form more stable complexes of ThrCd\({}^{2+}\) and GluCd\({}^{2+}\).  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Amino Acids** & **Thr** & **Glu** & **Gly** & **Ala** & **Leu** & **Phe** \\ \hline \(p\) & 0.67 & 0.5 & 0.57 & 0.5 & 0.5 & 0.5 \\ log\(\beta\) & 5.56 & 5.13 & 5.05 & 5.03 & 4.96 & 4.77 \\ R\({}^{2}\) & 0.99 & 0.99 & 0.99 & 0.99 & 0.99 & 0.99 \\ \hline \hline \end{tabular}
\end{table}
Table 5: Calculated complex parameters of the studied amino acids with Cd\({}^{2+}\).  
Figure 6: Fitted curves of (**a**) Thr, (**b**) Glu, (**c**) Gly, (**d**) Leu, (**e**) Ala, and (**f**) Phe with Cd\({}^{2+}\).