## 3 Results and Discussion
### Dependence of Extraction on pH  
SC4A contains OH groups, and therefore its extraction abilities can be influenced by hydrogen ions or hydrogen-containing compounds through hydrogen bonding. To evaluate the impact of the pH, the extraction of the metals with SC4A at various pH values was investigated and the results are  
Figure 2: Dependence of the distribution ratio (\(D\)) of the tested elements onto SC4A with the change in contact time. Conditions: \(T\) = 298 K, \([\rm{SC4A}]_{(\alpha)}\) = 1 \(\times\) 10\({}^{-3}\) M, \([\rm{Metal}]_{(\alpha)}\) = 1 \(\times\) 10\({}^{-4}\) M, pH = 6 \(\pm\) 0.05, shaking speed: 150 rpm. displayed in Figure 3. Separation factors \(SF_{\text{Cs}}\) and \(SF_{\text{Sr}}\) to other metals at different pH values were calculated and are listed in Table 1.  
As can be seen in Figure 3a, \(D_{\text{Cs}}\) and \(D_{\text{Sr}}\) increased along with the increase of the solution pH. \(D_{\text{Cs}}\) reached 27 at pH 6 and \(D_{\text{Sr}}\) reached 20 at pH 5, respectively. These implied the extraction abilities of SC4A in neutral or weak acidic solution to Cs(I) and Sr(II) are better than that in strong acidic solution. However, the distribution ratios of Na(I), K(I), Pb(II), Zn(II), Co(II) and Ni(II) were less than 0.4, indicating poor or almost no extraction with SC4A. Although the maximum distribution ratios for Cd(II) and Ce(III) were 2.55 and 4.90 respectively at pH 6, the \(SF_{\text{Cs/Cd}}\), \(SF_{\text{Cs/Ce}}\), \(SF_{\text{Sr/Cd}}\), and \(SF_{\text{Sr/Ce}}\) values under identical experimental conditions were 6.28, 10.81, 8.10, and 4.70, respectively, as can be seen in Table 1. This result indicated that SC4A had efficiently selective separation ability for Cs(I) or Sr(II), and the above mentioned coexistent metals may have no adverse impacts on the extraction and separation of the target ions. Experiments were not carried out at pH > 7 because \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Separation Factors**} & \multicolumn{5}{c}{**pH (\(\pm\)0.05)**} \\ \cline{2-6}  & **2** & **3** & **4** & **5** & **6** \\ \hline SF\({}_{\text{Cs/Na}}\) & - & 1.94 & 28.36 & 64.85 & 58.94 \\ SF\({}_{\text{Cs/K}}\) & 9.14 & 25.89 & 24.71 & 77.90 & 115.4 \\ SF\({}_{\text{Cs/Pb}}\) & 96.37 & 0.60 & 10.24 & 42.75 & 61.05 \\ SF\({}_{\text{Cs/Ce}}\) & 21.51 & 0.10 & 1.37 & 3.94 & 6.28 \\ SF\({}_{\text{Cs/Cd}}\) & 1.12 & 0.88 & 2.56 & 8.42 & 10.81 \\ SF\({}_{\text{Cs/Zn}}\) & - & 160 & 162 & \textgreater{}162 & \textgreater{}162 \\ SF\({}_{\text{Cs/Co}}\) & - & - & 121 & \textgreater{}121 & \textgreater{}121 \\ SF\({}_{\text{Cs/Ni}}\) & - & - & - & \textgreater{}103 & \textgreater{}103 \\ SF\({}_{\text{Sr/Na}}\) & - & 1.35 & 8.36 & 42.25 & 44.17 \\ SF\({}_{\text{Sr/K}}\) & 7.64 & 17.95 & 7.28 & 14.72 & 14.72 \\ SF\({}_{\text{Sr/Pt}}\) & 80.60 & 0.42 & 3.02 & 38.07 & 45.74 \\ SF\({}_{\text{Sr/Ce}}\) & 17.99 & 0.07 & 0.40 & 3.74 & 4.70 \\ SF\({}_{\text{Sr/Cd}}\) & 0.94 & 0.61 & 0.76 & 7.59 & 8.10 \\ SF\({}_{\text{Sr/Zn}}\) & - & 39.4 & 183 & \textgreater{}183 & \textgreater{}183 \\ SF\({}_{\text{Sr/Co}}\) & - & - & 137 & \textgreater{}137 & \textgreater{}137 \\ SF\({}_{\text{Sr/Ni}}\) & - & - & - & \textgreater{}103 & \textgreater{}103 \\ \hline \hline \end{tabular} Conditions: \(T=298\) K, [SC4A]\({}_{\text{(o)}}=1\times 10^{-3}\) M, [Metal]\({}_{\text{(o)}}=1\times 10^{-4}\) M, contact time = 60 min, shaking speed: 150 rpm.  
\end{table} 
Table 1: Separation Factor of Cs(I) and Sr(II) to other metals at different initial pH. Figure 3: (**a**) Dependence of the distribution ratio (\(D\)) of the tested elements onto SC4A with a change in initial pH; and (**b**) relationship between log \(D\) and pH\({}_{\text{e}}\). Conditions: \(T=298\) K, [SC4A]\({}_{\text{(o)}}=1\times 10^{-3}\) M, [Metal]\({}_{\text{(a)}}=1\times 10^{-4}\) M, contact time = 120 min, shaking speed: 150 rpm. metal precipitation occurs at higher pH. Therefore, pH 6 was chosen as the optimum pH value for the following extraction experiments.