Efficient and irreversible capture of strontium ions from aqueous solution using metal-organic frameworks with ion trapping groups

Efficient and irreversible capture of radioactive nuclides is an important environmental protection task when disposing of nuclear wastewater. This paper uses an "ion trapping" concept to design an efficient adsorbent based on a metal-organic framework (MOF), for removal of radioactive strontium from nuclear wastewater. Two functionalized MOFs were achieved by the introduction of sulfate or oxalate into the pore structure of MOF-808, giving MOF-808-SO4 and MOF-808-C2O4, respectively. These functionalized MOF-808 materials exhibit excellent ability to remove Sr2+ from acidic solution due to their particular trapping action, with the maximum Sr2+ adsorption capacities of 176.56 mg g(-1) and 206.34 mg g(-1) for MOF-808-SO4 and MOF-808-C2O4, respectively, surpassing most other inorganic materials. Additionally, 99% of Sr2+ is removed by MOF-808-SO4 and MOF-808-C2O4 after reaching the equilibrium. Remarkably, these two functionalized MOF-808 materials exhibited selectivity for the removal of Sr2+ from simulated mixed nuclear wastewater, even when there are 10 times as many co-existing ions as Sr2+ ions, demonstrating that anchoring of the trapping groups is a good way to improve Sr2+ adsorption capacity on MOFs. Moreover, both functionalized MOF-808 materials trap Sr2+ ions irreversibly, suggesting these trapping groups have strong ability to bind with Sr2+. A mechanism study indicated no proton ion exchange occurred during the adsorption process, so the functionalized groups anchored to MOFs play an important role in Sr2+ adsorption. Our present study provides an efficient way to design new adsorbents for the removal of radioactive strontium and other radionuclides from nuclear wastewater.