Phosphonation of Alginate-Polyethyleneimine Beads for the Enhanced Removal of Cs(I) and Sr(II) from Aqueous Solutions

Abstract

Although Cs(I) and Sr(II) are not strategic and hazardous metal ions, their recovery from aqueous solutions is of great concern for the nuclear industry. The objective of this work consists of designing a new sorbent for the simultaneous recovery of these metals with selectivity against other metals. The strategy is based on the functionalization of algal/polyethyleneimine hydrogel beads by phosphonation. The materials are characterized by textural, thermo-degradation, FTIR, elemental, titration, and SEM-EDX analyses to confirm the chemical modification. To evaluate the validity of this modification, the sorption of Cs(I) and Sr(II) is compared with pristine support under different operating conditions: the pH effect, kinetics, and isotherms are investigated in mono-component and binary solutions, before investigating the selectivity (against competitor metals) and the possibility to reuse the sorbent. The functionalized sorbent shows a preference for Sr(II), enhanced sorption capacities, a higher stability at recycling, and greater selectivity against alkali, alkaline-earth, and heavy metal ions. Finally, the sorption properties are compared for Cs(I) and Sr(II) removal in a complex solution (seawater sample). The combination of these results confirms the superiority of phosphonated sorbent over pristine support with promising performances to be further evaluated with effluents containing radionuclides.

Keywords: cesium; composite hydrogel; functionalization; metal sorption; modeling; reuse cycles; selectivity; sorption isotherm; strontium; uptake kinetics.

Figure A1

SEM photos for shape and size evaluation of sorbent particles.

Figure A2

Textural analysis of APO-PEI sorbents: (a) N₂ sorption and desorption isotherms (BET method) and (b) pore size distribution (BJH method).

Figure A3

Characterization of thermal degradation of ALG-PEI (a,c) and APO-PEI sorbents (b,d): TGA curves (weight loss, (a,b)) and DTG curves (c,d).

Figure A3

Characterization of thermal degradation of ALG-PEI (a,c) and APO-PEI sorbents (b,d): TGA curves (weight loss, (a,b)) and DTG curves (c,d).

Figure A4

SEM observation (left panels) and semi-quantitative EDX analysis (right panels) of ALG-PEI (a) and APO-PEI (b).

Figure A5

SEM observation (left panels) and semi-quantitative EDX analysis (right panels) of ALG-PEI after Sr(II) sorption (a) and Cs(I) sorption (b), and of APO-PEI after Sr(II) sorption (c) and Cs(I) sorption (d).

Figure A6

SEM-EDX analysis of cross-sections of ALG-PEI sorbent before and after Cs(I) and Sr(II) sorption.

Figure A7

SEM-EDX analysis of cross-sections of APO-PEI sorbent before and after Cs(I) and Sr(II) sorption.

Figure A8

Determination of pH_PZC values of ALG-PEI and APO-PEI sorbents (pH-drift method; background salt solution: 0.1 M NaCl; sorbent dose, SD: 2 g L⁻¹; time: 48 h; agitation, v: 210 rpm; T: 21 ± 1 °C).

Figure A9

Speciation diagrams of Cs(I) (a) and Sr(II) (b) (under the experimental conditions selected for the study of pH effect, calculations using Visual MINTEQ software [76]).

Figure A10

Effect of pH on Cs(I) and Sr(II) sorption: (a) log₁₀ D vs. pH_eq plot and (b) pH variation during metal sorption for ALG-PEI and APO-PEI sorbents (C₀: 0.802 mmol Cs L⁻¹ or 2.128 mmol Sr L⁻¹; sorbent dose, SD: 0.67 g L⁻¹; v: 210 rpm; time: 48 h; T: 21 ± 1 °C).

Figure A11

Effect of the pH on the selectivity coefficient (SC_Cs/Sr) using ALG-PEI and APO-PEI sorbents (binary solutions: 0.754 mmol Cs L⁻¹ and 1.227 mmol Sr L⁻¹; SD: 0.67 g L⁻¹; v: 210 rpm; time: 48 h; T: 21 ± 1 °C).

Figure A12

Uptake kinetics for Cs(I) and Sr(II) sorption using APO-PEI sorbents from binary solution (C₀: 0.751 mmol Cs L⁻¹ and 1.148 mmol Sr L⁻¹; sorbent dose, SD: 0.67 g L⁻¹; v: 210 rpm; pH₀: 7; T: 21 ± 1 °C).

Figure A13

Sorption isotherms for Cs(I) (a) and Sr(II) (b) using ALG-PEI and APO-PEI sorbents: modeling with the Langmuir equation (pH₀: 7; C₀: 0.08–6.11 mmol Cs L⁻¹ or 0.12–9.32 mmol Sr L⁻¹; SD: 0.67 g L⁻¹; v: 210 rpm; time: 48 h; t: 21 ± 1 °C).

Figure A14

Modeling of Cs(I) and Sr(II) sorption isotherms (onto APO-PEI beads) using the Langmuir dual site model (LDS) (for experimental conditions, see Figure A12).

Figure A15

Cs(I) and Sr(II) sorption isotherms from binary solutions using ALG-PEI (a) and APO-PEI (b) sorbents (compared with isotherms from mono-component solutions—C₀: 0.07–6.11 mmol Cs L⁻¹ and 0.11–9.23 mmol Sr L⁻¹, with Sr/Cs molar ratio ≈1.5; pH₀: 7; SD: 0.67 g L⁻¹; v: 210 rpm; T: 21 ± 1 °C).

Figure A16

Metal sorption from multicomponent solutions using ALG-PEI (a) and APO-PEI (b) sorbents: effect of pH_eq on log₁₀D vs. pH_eq (C₀, mmol L⁻¹: 0.883 Na(I), 0.530 Ca(II), 1.020 Mg(II), 0.881 Fe(III), 0.875 Al(III), 1.088 Cs(I), and 0.959 Sr(II); time: 24 h; v: 210 rpm; T: 21 ± 1 °C).

Figure A17

Sorption capacity (proportionality given by the size of the bubble) vs. the positioning of individual metals in the covalent index/ionic index space (data collected from [1,63,100]).

Figure A18

Cs(I) and Sr(II) desorption kinetics for ALG-PEI and APO-PEI sorbents: case of mono-component systems (metal-loaded samples collected at equilibrium from the relevant kinetics; SD: 2.67 g L⁻¹; eluent: 0.2 M HNO₃; v: 210 rpm; T: 21 ± 1 °C).

Figure A19

Cs(I) and Sr(II) desorption kinetics for APO-PEI sorbent: case of binary systems (metal-loaded samples collected at equilibrium from the relevant kinetics; SD: 2.67 g L⁻¹; eluent: 0.2 M HNO₃; v: 210 rpm; T: 21 ± 1 °C).

Figure A20

Location of sample collection (Beihai, China).

Figure A21

Time evolution of sorption efficiency for major elements (a) and trace elements (b) using ALG-PEI (empty symbols) and APO-PEI (filled symbols) sorbents (initial concentrations see Table A8; SD: 0.2 g L⁻¹; pH₀: 7.59; pH_eq: 7.51; v: 210 rpm; T: 21 ± 1 °C).

Figure A22

SEM observation (left panel) and semi-quantitative EDX analysis (right panels) of ALG-PEI (a) and APO-PEI (b) after the treatment of seawater.

Scheme 1

Prospective binding mechanisms for Cs(I) and Sr(II) sorption onto ALG-PEI and APO-PEI sorbents.

Figure 1

FTIR spectra of ALG-PEI (a) and APO-PEI (b) sorbents at different stages of use: pristine material, after conditioning at pH 7 (the pH of metal sorption, referenced as “sorbent + Soln.”), after Cs(I) or Sr(II) sorption, after five cycles of sorption/desorption; wavenumber range: 1800–400 cm⁻¹.

Figure 2

Effect of pH on Cs(I) and Sr(II) sorption using ALG-PEI and APO-PEI sorbents (C₀: 0.802 mmol Cs L⁻¹ or 2.128 mmol Sr L⁻¹; sorbent dose, SD: 0.67 g L⁻¹; v: 210 rpm; time: 48 h; T: 21 ± 1 °C).

Figure 3

Uptake kinetics for Cs(I) and Sr(II) sorption using ALG-PEI and APO-PEI sorbents (C₀: 0.760 mmol Cs L⁻¹ or 1.188 mmol Sr L⁻¹; sorbent dose, SD: 0.67 g L⁻¹; v: 210 rpm; pH₀: 7; T: 21 ± 1 °C).

Figure 4

Sorption isotherms for Cs(I) (a) and Sr(II) (b) using ALG-PEI and APO-PEI sorbents: modeling with Sips equation (pH₀: 7; C₀: 0.08–6.11 mmol Cs L⁻¹ or 0.12–9.32 mmol Sr L⁻¹; SD: 0.67 g L⁻¹; v: 210 rpm; time: 48 h; t: 21 ±1 °C).

Figure 5

Metal sorption from multicomponent solutions using ALG-PEI (a,c) and APO-PEI (b,d) sorbents: effect of pH_eq on SC_Cs/metal (a,c) and SC_Sr/metal (b,d) (C₀, mmol L⁻¹: 0.883 Na(I), 0.530 Ca(II), 1.020 Mg(II), 0.881 Fe(III), 0.875 Al(III), 1.088 Cs(I), and 0.959 Sr(II); time: 24 h; v: 210 rpm; T: 21 ± 1 °C; SC_Cs/Cs and SC_Sr/Sr are mentioned as internal standard = 1).

Figure 6

Concentration factor (C = q_eq/C₀, L g⁻¹) (a) and distribution ratio (D = q_eq/C_eq, L g⁻¹) (b) for selected elements for the treatment of the seawater sample using ALG-PEI and APO-PEI sorbents (for initial concentrations, see Table A8; SD: 0.2 g L⁻¹; pH₀: 7.59; pH_eq: 7.51; time: 24 h; v: 210 rpm; T: 21 ± 1 °C; individual numbers represent the enhancement factor associated with the functionalization of ALG-PEI sorbent).

Scheme 2

Synthesis procedures for the preparation of ALG-PEI and APO-PEI sorbents.