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.