Gram-grade Cr (VI) adsorption on porous Fe@SiO2 hierarchical microcapsules

• Porous Fe@SiO2 hierarchical capsules were prepared by a two-step solution method.
• The maximum Cr (VI) adsorption capacity of Fe@SiO2 capsules was ca. 1400 mg g−1.
• The initial Cr (VI) adsorption rate of Fe@SiO2 capsules was ca. 19 mg g−1 min−1.
• The Fe@SiO2 capsules can rapidly respond to an external magnetic field.

In this work, porous Fe@SiO2 hierarchical microcapsules were fabricated by reducing α-Fe2O3@SiO2 micro-spindles, which were prepared by using a two-step solution method. These Fe@SiO2 microcapsules were then employed to remove Cr (VI) ion from water. On the basis of Langmuir isotherms model, the maximum Cr (VI) adsorption capacities were estimated to be ca. 1400 and 1300 mg g−1 at 305 and 310 K, respectively. These values, to our knowledge, were much higher than those in previous reports for many other Fe-based adsorbents. Besides, the initial Cr (VI) adsorption rate was calculated to be ca. 19 mg g−1 min−1 that was significantly higher than those reported in previous literatures, and hence improving the adsorptive efficiency of Fe@SiO2 microcapsules in practical water treatment. Thermodynamic analyses of enthalpy, entropy and Gibbs free energy further evidenced that these outstanding kinetics and equilibrium results were mainly due to a physical adsorption process, accompanied by a chemisorption associated with the reduction of Cr (VI) to Cr (III). The gram-grade Cr (VI) adsorption capacities and easy magnetic separation are highly desirable for their widespread potential applications in water treatment.