## 4 Conclusions
XPS characterization of AO-APEI before and after Sr(II) sorption. Table S5. Assignments, Binding energies (BEs), and Atomic Fractions (AF, %) on the different stages of synthesis and after sorption. Table S6. Sorption capacities (q\({}_{\text{eq}}\), \(\mu\)mol g\({}^{-1}\)) and distribution ratio (K\({}_{\text{d}}\), L kg\({}^{-1}\)) for metal sorption from multi-metal equimolar concentrations at different equilibrium pH values. Table S7. SEM and EDX analysis of AO-APEI sorbent after loading with multi-metal solutions (Ca(II), Na(I), Mg(II), Al(III), and Sr(II)) at different pH values (i.e., 2-7). Table S8. Modeling of breakthrough curves for the sorption of Sr(II) using AO-PEI--Parameters of the Thomas model. Table S9. Modeling of kinetic profiles for Sr(II) desorption from metal-loaded AO-PEI--FORRE and PSORE equations. Table S10. Summary of the results obtained in the treatment of two seawater samples (after 18 h of contact). Table S11. Semi-quantitative EDX analysis of sorbent (surface and interior; average of 4 or 5 analyses (weight percentage) with standard deviation into parentheses) after treatment of two seawater samples (Beihai, China and Da Nan, Vietnam). Table S12. XPS characterization of APEI before and after exposure to 2 M NaCl solution for 48 h. Table S13. XPS characterization of AO-APEI before and after exposure to 2 M NaCl solution for 48 h. Table S14. Assignments, Binding energies (BEs), and Atomic Fractions (AF, %) of APEI and AO-APEI before and after exposure to 2 M NaCl solution for 48 h. Table S15. Macroscopic morphology of APEI and AO-APEI beads and size measurements. Table S16. Uptake kinetics models--PRORE (pseudo-first order rate equation), PSORE (pseudo-second order rate equation) and RIDE (resistance to intraoperative diffusion equation--Crank equation). Table S17. Sorption isotherm models Figure S1. Textural analysis of AO-APEI sorbent. Figure S2. Thermal analysis (TGA and DrDTG) of APEI and AO-APEI materials (Temperature ramp: 10 \({}^{\circ}\)C/min; N\({}_{2}\) atmosphere). Figure S3. FTIR spectra of APEI, CN-APEI, AO-APEI materials, sorbent after Sr(II) sorption, after metal desorption and after 5 cycles of sorption and desorption (test of sorbent stability at recycling). Figure S4. Determination of the pH\({}_{\text{PZC}}\) of AO-APEI--pH-drift method (Sorbent dosage, SD: 2 g L\({}^{-1}\); Time: 48 h; Background salt: 0.1 M NaCl). Figure S5. pH variation during Sr(II) sorption using APEI and AO-APEI exponents--(Duplicate experiments; Sorbent dosage, SD: 0.375 g L\({}^{-1}\); C\({}_{0}\): 29.7 mg Sr L\({}^{-1}\); room temperature: 25 \(\pm\) 1 \({}^{\circ}\)C; agitation speed: 140 rpm; contact time: 48 h). Figure S6. Effect of pH on the distribution coefficient (log\({}_{0}\) units) for Sr(II) onto APEI and AO-APEI sorbents--(Duplicate experiments; Sorbent dosage, SD: 0.375 g L\({}^{-1}\); C\({}_{0}\): 29.7 mg Sr L\({}^{-1}\); room temperature: 25 \(\pm\) 1 \({}^{\circ}\)C; agitation speed: 140 rpm; contact time: 48 h). Figure S7. Uptake kinetics for Sr(II) removal using AO-APEI sorbent--Modeling of kinetic profiles with the PSORE model (pH\({}_{0}\): 6.0; pH\({}_{\text{eq}}\): 7.1; Sorbent dosage, SD: 0.2 g L\({}^{-1}\); C\({}_{0}\): 55.5 and 50.0 mg Sr L\({}^{-1}\) for 1 st and 2 nd series, respectively; Temperature: 25 \(\pm\) 01 \({}^{\circ}\)C; agitation speed: 140 rpm). Figure S8. Effect of pH on distribution coefficients (log\({}_{10}\) units) for Sr(II), Al(III), Ca(II) Mg(II) and Na(I) recovery using AO-APEI from equimolar multi-component solutions (1.8 mmol L\({}^{-1}\); SD: 125 mg L\({}^{-1}\); Temperature: 25 \(\pm\) 1 \({}^{\circ}\)C; agitation speed: 140 rpm; contact time: 48 h). Figure S9. Correlation between the slope of the plots log\({}_{10}\) Kd vs. pH and the softness parameter of selected metal ions (data collected from Figure S8 and Marcus). Figure S10.