In-situ growth of iron oxides with MIL-100(Fe) enhances its adsorption for selenite

Composite materials of porous MIL-100(Fe) and bulk iron oxides, including MIL-100(Fe)@Aka and MIL-100(Fe)@Hem, were prepared through in-situ growth of iron oxides and MIL-100(Fe) using hydrothermal method. Powder X-ray diffraction (XRD), transmission electron microscopy (TEM), thermogravimetric (TG) and pore analysis revealed two phases of crystal MIL-100(Fe) and akaganeite/hematite in the composite materials. Iron oxides were not only cemented with the surface of MIL-100(Fe) but also integrated into the pores of MIL-100(Fe). Especially, the hematite was likely to get more favor to in-situ grow with MIL-100(Fe) and form the well-crystallized composite. Compared with MIL-100(Fe), MIL-100(Fe)@Aka exhibited more stable surface charge in the pH range of 3.0–8.0, and MIL-100(Fe)@Hem displayed a higher density of active sites. Both MIL-100(Fe)@Aka and MIL-100(Fe)@Hem showed spontaneous and exothermal adsorption for Se(IV), which could be well described by the linear driven forces model. MIL-100(Fe)@Aka and MIL-100(Fe)@Hem had an obvious synergistic effect on Se(IV) adsorption. The introduction of akaganeite and hematite into MIL-100(Fe) could enhance its adsorption capacity for Se(IV) by a maximum of 154% and 153% respectively, and the saturated adsorption capacity of MIL-100(Fe)@Hem0.1 was up to 301.93 mg/g as fitted by Sips model. The X-ray photoelectron spectrum (XPS) of external-electron revealed that the oxidation state of Fe atoms decreased after adsorbing Se(IV). The findings in this study are expected to reveal the synergistic effect of porous MIL-100(Fe) and bulk iron oxides for greatly improving the adsorption capacity towards pollutants, and provide some guidance for the development of effective adsorbents to be used in environmental remediation.