## Materials and Methods
### Sorption Procedures  
Most of the experiments were performed in batch tests. A fixed volume (L) of solution (at concentration C\({}_{0}\), mmol Sr L\({}^{-1}\)) and fixed pH value was mixed with a given amount of sorbent (m, g) for variable contact times (uptake kinetics) or at equilibrium time (set to 48 h). Collected samples were filtrated on filter membranes (1.2 \(\mu\)m) and analyzed for residual metal concentration (C\({}_{\mathrm{eq}}\), mmol Sr g\({}^{-1}\)) using an inductively coupled plasma atomic emission spectrometer (ICPS-7510 Shimadzu, Tokyo, Japan). The sorption capacity at equilibrium (q\({}_{\mathrm{eq}}\), mmol Sr g\({}^{-1}\)) was calculated using the mass balance equation: q\({}_{\mathrm{eq}}\) = (C\({}_{0}-\)C\({}_{\mathrm{eq}}\))V/m. For fixed-bed column experiments, the column (5 mm diameter) was filled (depth: 4.4 cm) with a small amount of sorbent (70 mg, d.w.); it is noteworthy that the size of the beads was too large to avoid wall effects, but these preliminary tests give the first indication on the feasibility of the application of these beads for dynamic treatment. The preferential channeling and theloss of packing density close to the wall of the column induce the heterogeneous distribution of the metal ions in the cross-section of the reactor and, consequently, a less effective mass transfer, which leads to non-homogenous transfer of the solution through the bed. To prevent this effect, it is usually necessary to maintain the size ratio bead/column diameters (d/D) less than 1/10 or 1/12. The amount of sorbent available for these dynamic tests (performed at the end of the study) was not sufficient for performing experiments consistent with the appropriate d/D ratio. All experiments were performed at room temperature (i.e., \(25\pm 1\)\({}^{\circ}\)C). Sorption tests in multi-component solutions were carried out to evaluate the selectivity of metal sorption for Sr(II) at different pH values. The solutions containing 1.8 mmol L\({}^{-1}\) of Mg(II), Al(III), Ca(II), Na(I), and Sr(II) (as chloride salts) at initial pH ranging between 2 and 7 were mixed with the sorbent (sorbent dosage, SD: 0.125 g L\({}^{-1}\)) for 48 h. The mass balance equation allowed calculating the sorption capacity for each metal, to deduce the distribution ratio (K\({}_{d}\), L g\({}^{-1}\)), and then the selectivity coefficient (SC, dimensionless) of Sr(II) over competitor metal ions: SC = K\({}_{d}\)(Sr)/K\({}_{d}\)(metal).  
Desorption tests were carried out in batch system by contact of Sr(II)-loaded beads with eluent solution (0.5 M CaCl\({}_{2}\)/0.2 M HCl) for 2 h, under agitation (150 rpm). The sorbent dosage (SD, g L\({}^{-1}\)) was fixed to 2 g L\({}^{-1}\). Recycling tests (5 cycles) were carried out using the same procedures for both sorption (S) and desorption (D); a washing step was operated between each S and D operations using 10 mL of demineralized water (twice).  
Experiments were tested on complex matrices (seawater) for testing the recovery of Sr(II). The samples were collected on two sites: Beihai (China) and Da Nang (Vietnam); initial pH values were: 7.63 and 7.98, respectively. The sorption tests were carried out in a batch with a sorbent dosage of 0.2 g L\({}^{-1}\) (at natural pH). Samples were collected at different contact times (up to 96 h) and analyzed by ICP-AES or AAS (AA-7000, Shimadzu, Tokyo, Japan).  
Detailed experimental conditions are systematically reported in the caption of figures. The modeling of uptake kinetics and sorption isotherms are summarized in Tables S16 and S17.