In situ one-step synthesis of FeO@MIL-100(Fe) core-shells for adsorption of methylene blue from water.

A new route for FeO@MIL-100(Fe) core-shells is proposed via in situ one-step hydrothermal strategy, in which FeO microspheres not only serve as magnetic cores but also provide Fe(III) for MIL-100(Fe) synthesis. The MIL-100(Fe) is uniformly grown as a shell on the surface of FeO, and the shell thickness can be fine-tuned from 73.5 to 148nm by simply controlling the reaction time. Compared with FeO, the surface area and pore volume of the FeO@MIL-100(Fe) are significantly increased while the magnetism is barely affected. The application of FeO@MIL-100(Fe) in adsorption was tested using several dyes as model analytes, and showed high adsorption capacity (221mgg) towards methylene blue (MB), which is based on electrostatic interactions and size filter effect. The MB adsorption isotherm follows Langmuir model and pseudo second-order kinetic model. Intra-particle diffusion model reveals that both film and pore diffusions are involved in the rate limiting steps. The adsorption is controlled by enthalpy change rather than entropy effect. ΔH, ΔS and ΔG values indicated that the adsorption process was spontaneous and exothermic. Simple synthesis procedure, immense magnetism, high adsorption capacity and excellent reusability of FeO@MIL-100(Fe) make it an attractive candidate for application of MB removal from polluted environmental samples.