## 2 Results and Discussion
### Sorption Properties
#### 2.2.4 Multicomponent Sorption  
The sorption selectivity is an important criterion for designing a separation process. Composite solutions (constituted of equimolar concentrations of different metal ions; i.e., 1.8 mmol metal L\({}^{-1}\)) were treated with AO-APEI, and the residual metal concentrations were analyzed for determining the sorption capacities of the different metals. The selectivity coefficient SC\({}_{\text{Sr/metal}}\) is defined as the ratio of the distribution ratios K\({}_{\text{d}}\)(Sr)/K\({}_{\text{d}}\)(metal) (with K\({}_{\text{d}}\) = qeq/Ceq\({}_{\text{eq}}\) L g\({}^{-1}\)). Figure 5 shows the effect of equilibrium pH on the selectivity coefficients of AO-APEI for Sr(II) over Al(III), Ca(II), Mg(II), and Na(I). The sorbent has a marked preference for Sr(II) over the competitor ions; however, this preference is strongly impacted by the pH and the type of metal. For Al(III), Mg(II), and Na(I), the selectivity profiles follow the same trends:  
(1) At pH 2.3, the SC\({}_{\text{Sr/metal}}\) is higher than 20,  
(2) The SC strongly decreases (down to 8.7-26) when the pH increases (up to 5),  
(3) The SC strongly increases above pH 5 and reaches optimum values as high as SC: 51-129, the maximum selectivity is reported for Sr(II) over Al(III) (i.e., 129 against 76). (4) Above pH 7, the selectivity tends to decrease again (though maintaining a good SC; i.e., higher than 40). In the case of Ca(II), the profiles are roughly the same, the main changes are observed with a maximum SC\({}_{\text{Sr/Ca}}\) close to 111 at pH 4.8 and a relatively high SC (i.e., close to 92) at pH 2.3. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Sorbent** & **pH0** & **Time** & **qm** & **b\textgreater{}102** & **Reference** \\  & **(min)** & **(mg Sr g\({}^{-1}\))** & **(L mg\({}^{-1}\))** & **Reference** \\ \hline Clay & 3.4 * & 300 & 2.98 & 0.336 & [26] \\ Bentonite & 6 & 120 & 38.6 & – & [27] \\ Zeolite microsphere & 6 & 20 & 109.89 & 15.72 & [73] \\ Activated porous calcium silicate & 7 & 120 & 142.3 & - & [74] \\ Oxidized Multiwall CNTs & 7 & 100 & 6.62 & 0.608 & [22] \\ Moss & 6 & 240 & 13.1 & 5.71 & [7] \\ Zr-Sb oxide/PAN & 4.74 & 270 & 43.67 & 84.0 & [75] \\ Resorcinol-formaldehyde-imimodiacetic acid resin & 9 & 1200 & 188.4 & - & [76] \\ Impregnated resin & 1 M & 120 & 4.83 & 5.8 & [77] \\ Extractant impregnated & 2 M & & & & \\ silica/polymer resin & HNO\({}_{3}\) & 360 & 454.5 & 0.013 & [78] \\ Ni-K Ferrocyanide/TiO\({}_{2}\) & 7.8 & 180 & 143 & 0.61 & [31] \\ Prussian blue analog on chitosan/CNT & 6 & 240 & 205.1 & 0.46 & [79] \\ Amidoxime chitosan beads & 8.5 & 180 & 153 & - & [68] \\ Aminophosphate chitosan/Co(II) & 6 & 60 & 3.41 & 89.0 & [80] \\ _Saccharomyces cerevisiae_ & 3–4 ** & 1800 & 37.4 & 3.51 & [16] \\ Bacteria-derived carbonaceous & 4.5 & 1440 & 67.11 & 44.7 & [53] \\ nanofibers & & & & & \\ EDTA-mercerized bacterial cellulose membrane & 6.0 & 600 & 44.86 & 8.5 & [81] \\ Magnetic chitosan beads & 8.2 & 360 & 11.58 & - & [82] \\ Alginate beads & 6 & 1440 & 111.1 & 9.4 & [83] \\ AO-APEI & 6.0 & 90 & 189 & 2.29 & This work \\ \hline \hline \end{tabular}  
*: equilibrium pH, **: complex solution (simulate of high-level liquid waste).  
\end{table} 
Table 3: Comparison of Sr(II) sorption properties for various sorbents. Table S7 reports the sorption capacities reached at equilibrium and the distribution ratios for the different metal ions at different equilibrium pH values. Figure S8 shows the log\({}_{10}\)-plots of the distribution ratios (K\({}_{\rm d}\), L g\({}^{-1}\)) for the different metals (in multi-component solutions) as a function of equilibrium pH. Experimental points are linearly distributed with slopes ranging between 0.124 for Al(III) and 0.324 for Ca(II).