Nitrogen-Doped Calcite Derived from Ca-MOFs as an Efficient Adsorbent for Sr(II) Removal from Water: The Role of Nitrogen and Calcium

Strontium (90Sr) is a typical radionuclide, which can act as a contaminant and poses a big problem for the eco-environment if left untreated. In this study, an original nitrogen-doped calcite (N-CaCO3) was synthesized using a solvothermal and calcination method and used to remove Sr(II) from simulated water. XRD, SEM, and XPS analyses proved that N was successfully doped into CaCO3, resulting in porous CaCO3 with a regular morphology. The specific surface area of N-CaCO3 (136.53 m2/g) can reach 2.19-fold greater than that of CaCO3. The results based on the batch adsorption data indicated that the pseudo-second-order kinetic model (R2 = 0.9964) and the ion exchange model (R2 = 0.9859) fitted the adsorption data well. The as-synthesized N-CaCO3 exhibited better adsorption performance in regard to low concentrations of Sr(II) (below 64.5 mg/L) compared with commercial CaCO3. The structural analysis suggested that Ca and N played pivotal roles in the adsorption process and that the adsorption mechanism was dominated by ion exchange and surface complexation. This study successfully fabricated a nitrogen-doped calcite for Sr(II) cleanup, presenting an efficient strategy to modulate the microstructure of CaCO3, or other materials, to enhance its adsorption performance.