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

Radionuclides such as strontium 90, 90Sr, pose severe threats to the environment and humans since the wide application of nuclear power plants around the world. In the environment, 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 and lastly, a Sr2+-imprinted adsorbent (SA-PA-H) was obtained via acid elution of the SA-PA-Sr hydrogel. The SA-PA-H demonstrated superior Sr2+adsorption capacity (∼151.7 mg/g), rapid adsorption kinetics following pseudo-second order, with a rate constant of 0.669 g mg−1 min−1, and 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 performance was maintained over a wide pH range (4–10) and temperatures (25–40 °C). The adsorption mechanism was attributed to the prevalent Sr2+ bindings to thiol groups and Sr2+imprinted cavities. Moreover, SA-PA-H showed high elasticity with a storage shear modulus ∼ 10 kPa at low strains, and rapid and full self-recovery after being repeatedly damaged by large strains . 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-2h−1). In addition, the adsorbent could 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 the rapid and complete decontamination of nuclear wastewater via adsorptive membrane filtration.