Rational Synthesis of Sulfonic Acid-Functionalized Covalent Organic Frameworks for Selective Capture of Trace Strontium in Natural Mineral Water

As a typical long-lived radionuclide, strontium-90 (90Sr) possesses persistent radiological hazards and exhibits high migratory and diffusive behaviors in groundwater. However, the development of adsorbents for the efficient and highly selective capture of trace Sr(II) remains a great challenge. In this study, covalent organic frameworks (COFs, TpPa-2SO3H and TpPa-SO3H) with different numbers of sulfonic acid groups were rationally synthesized and applied for capturing Sr(II). The introduction of sulfonic acid groups onto TpPa-2SO3H and TpPa-SO3H endowed them with high adsorption capacities of 136.4 and 68.3 mg g–1 toward Sr(II), which were 12-fold and 6-fold higher than that of TpPa without sulfonic acid groups, respectively. Meanwhile, TpPa-2SO3H and TpPa-SO3H exhibited fast adsorption kinetics, capturing 99% of Sr(II) within 3 and 10 min, respectively. In addition, TpPa-2SO3H also exhibited high selectivity, excellent anti-interference performance, and regeneration ability without high calcium backgrounds. Characterizations and theoretical calculations demonstrated that sulfonic acid groups in TpPa-2SO3H promoted Sr(II) capture through strong ion-exchange interaction. TpPa-2SO3H was further applied to extract trace Sr(II) in natural Sr-containing mineral water samples. The results showed that the extraction efficiency for Sr(II) exceeded 95% within 5 min. Furthermore, the selectivity for Sr(II) was investigated by the distribution coefficients (Kd), and the Kd was 5.6 × 104 mL g–1 for TpPa-2SO3H toward Sr(II), approximately 400 times higher than those of Na(I) and K(I), respectively. This work not only developed a promising strategy to extract trace Sr(II) from real samples but also broadened the application of COFs in separation science.