Efficient removal of selenite in aqueous solution by MOF-801 and Fe3O4/MOF-801: Adsorptive behavior and mechanism study

Metal-organic framework materials (MOFs) and MOFs-based composite materials have been extensively applied to adsorb heavy metals due to their high specific surface area and high porosity. Herein, the MOF-801 and its complex Fe3O4/MOF-801 were respectively prepared and employed as adsorbents to capture the Se(IV) in an aqueous solution. And the Fe3O4/MOF-801 composite was post-synthesized from MOF-801 by a simple cryogenic hydrothermal process. The experimental results showed that the maximum adsorption capacity of MOF-801 and Fe3O4/MOF-801 was 101.2 and 206.6 mg/g under acidic conditions, respectively. The coexisting anions, except phosphate and sulfate ions, had negligible effect on the Se(IV) adsorption. The mechanism of Se(IV) removal by the two adsorbents was thoroughly analyzed by Zeta potential, Fourier transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS). And the results illustrates that the Se(IV) was captured on the surface hydroxyl sites of the MOF-801, while the Fe3O4/MOF-801 captured Se(IV) through electrostatic attraction, inner-sphere complexation, and the reduction effect. Furthermore, the existence of charge transfer in the Fe3O4/MOF-801, which facilitated the adsorption and reduction of Se(IV), was confirmed by the XPS analysis. This work provides a promising strategy to design high-performance magnetic MOFs composites for the removal of Se(IV) from wastewater.