Thiol-rich and ion-imprinted alginate hydrogel as a highly adsorptive and recyclable filtration membrane for rapid and selective Sr(II) removal

Radioactive metal ion such as strontium ion, ⁹⁰Sr²⁺, has posed severe threats to environments and humans since the wide application of nuclear power plants around the world, while a rapid remediation of Sr²⁺ contaminated water still remains challenging. The current study developed an economical biomaterial-based hydrogel adsorbent with excellent Sr²⁺ adsorption performance achieved by ion-imprinting and abundant thiol groups, which was adaptable as an adsorptive filtration membrane for efficient and rapid purification of Sr²⁺ polluted water.   

The hydrogel was synthesized via a three-step route based on sodium alginate (SA). First, SA was emulsified and converted via Sr²⁺ complexation to hydrogel (SA-Sr); secondly, a thiol-rich carboxyethyl grafted pentaerythritol tetrakis (thioglycolic acid) ester (PA) synthesized by click chemistry was used to covalently crosslink the hydrogel (SA-PA-Sr) with abundant thiol groups simultaneously introduced. Lastly, a Sr²⁺-imprinted adsorbent (SA-PA-H) was obtained via acid elution of the SA-PA-Sr gel.

The SA-PA-H was demonstrated to exhibit a superior Sr²⁺adsorption capacity (~151.7 mg/g), a rapid adsorption kinetics following pseudo-second order with a rate constant of 0.669 g mg^-1 min^-1, decent selectivity for Sr²⁺, a value ~ 1.14×10³ mL g^-1 when adsorbing 10 ppm Sr²⁺ from concentrated (100 ppm) solutions of competitive ions (Na⁺ or Mg²⁺). The good performance was maintained over a wide range of pH (4-10) and temperature (25-40 °C), and the adsorption mechanism was attributed to the prevalent Sr²⁺ bindings to thiol groups and Sr²⁺-imprinted cavities.

Moreover, high elasticity with a storage shear modulus ~ 10 MPa at low strains whilst rapid and full self-recovery after being repeatedly damaged by large strains of the SA-PA-H were demonstrated by rheology. This allowed the SA-PA-H to be adapted as a membrane for vacuum filtration, giving a high removal efficiency (> 99.2%) of Sr²⁺ under a high liquid flux (~ 40 L m^-2 h^-1). In addition, the adsorbent can be regenerated by acid washing and after four consecutive adsorption-desorption cycles, the drop in removal efficiency was minor (53.51% to 36.88% for 100 ppm Sr²⁺). This investigation demonstrated a novel hydrogel adsorbent advantageous in cost, performance, processability, and sustainability, being applicable for rapid and complete decontamination of nuclear wastewater via adsorptive membrane filtration.