## 4 Conclusions  
The FTIR and XPS analyses confirm the different steps in the chemical modification of algal/alginate/PEI beads (i.e., nitrilation followed by amidoximation). These techniques also bring information on (a) the modes of interaction involved in Sr(II) sorption (contribution of carboxylic, amine and amidoxime groups), and (b) the stability of the sorbent (AO-APEI) after desorption and recycling. The porous structure (characterized by specific surface area close to 40 m\({}^{2}\) g\({}^{-1}\) and pore volume close to 0.21 cm\({}^{3}\) g\({}^{-1}\)) may explain the relatively fast sorption of Sr(II): equilibrium is reached within 60-90 min. The pseudo-first-order rate equation fits the kinetic profile well; though the resistance to intraparticle diffusion also slightly affects mass transfer, the diffusion coefficient is about one order of magnitude lower than the self-diffusivity of Sr(II) in water. The amidoximation strongly increases sorption capacity (up to 188 mg Sr g\({}^{-1}\), 2.15 mmol Sr g\({}^{-1}\)). The sorption isotherm is well fitted by the Langmuir equation. Metal desorption is highly efficient using 0.2 HCl/0.5 M CaCl\({}_{2}\) solutions, and the sorbent can be reused for five cycles of sorption and desorption with a limited decrease in performances. The beads were successfully applied in a dynamic system (fixed-bed columns) though the wall effects (driven by the inadequacy of bead size compared to column diameter) affected binding performance. The Thomas model gives a relatively correct fit of experimental profiles. In multi-metal solutions (including equimolar concentrations of Ca(II), Sr(II), Na(I) Mg(II), and Al(III)), AO-APEI has a marked preference for Sr(II), especially at pH close to 7: the effect of pH (slope of the distribution coefficient) can be correlated to the softness parameter of the metal ions. This preferential affinity for Sr(II) is confirmed by the application to the recovery of Sr(II) from seawater. Despite the presence of a large excess of alkali (such as Na(I) and K(I)) and alkali-earth (such as Ca(II) and Mg(II)) metal ions, the selectivity coefficients for Sr(II) over these metals vary between 200 and 800. This is opening the way for future applications on \({}^{90}\)Sr recovery from contaminated seawater. The promising results obtained with amidoxime-functionalized APEI material opens the application for the recovery of strontium from complex solutions. The comparison of sorption properties with more conventional sorbents clearly demonstrates the interest for preparing new resins (APEI) based on renewables resources (algal biomass, alginate) and environmentally friendly polymers (PEI) as precursors. This new generation of APEI beads can serve as supports for manufacturing different functionalized resins; we recently developed a quaternarized functionalized material (Q-APEI) that develops very attractive performances for Sc(III) sorption [93]. The sorbent could be used for Sr removal from radioactive effluents, providing complementary tests show the stability of these materials under irradiation. The issue of metal desorption would be secondary in this case, the objective being concentrating and confining dispersed highly hazardous compounds (at micro-level concentrations). The following are available online, Scheme S1. Schematic presentation of the different steps involved in APEI functionalization. Table S1. SEM and EDX analysis of the raw material (APEI), nitrilated material (CN-APEI), and amidoxime-functionalized material (AO-APEI), before and after Sr(II) sorption, after Sr(II) desorption and after the fifth cycle of sorption and desorption, and after treatment with sodium chloride for either raw materials and AO sorbent. Table S2. Assignments and characteristic wavenumbers (cm\({}^{-1}\)) of the different peaks of the spectra of sorbent (at the different steps of the synthesis and after metal sorption and after 5 recycling cycles). Table S3. XPS characterization of APEI and CN-APEI materials. Table S4.