Magnetically Double-Shelled Layered Double Oxide (LDO)/LDO/γ-Fe2O3 Composite for Highly Efficient Removal of Congo Red and Chromium(VI)

Abstract

Novel double-shelled core-shell-type magnetic composites MgFexAl-LDO/LDO/γ-Fe2O3 (x = 0, 0.1, and 0.5) were synthesized by a cost-effective two-step coprecipitation method followed by proper calcination. The composites are constructed by two layers of LDO nanosheets (∼80 × 10 nm) which shows vertically oriented relay growth on the surface of spherical γ-Fe2O3. The thickness of LDO shells is ca. 220 ∼ 260 nm, providing a large number of effective adsorption sites and numerous open channels composed of adjacent LDO nanosheets. All the composites show excellent adsorption capacities for Congo Red (CR) and Cr(VI). Especially, the MgAl-LDO/LDO/γ-Fe2O3 exhibits the maximum adsorption capacity (qmax = 123.4 mg g–1) for Cr(VI), which is due to the double-shelled morphology with a large Brunauer–Emmett–Teller area (219 m2 g–1), the electrostatic attraction between the positive LDO shells and Cr(VI) oxyanions, and the adsorption-coupled reduction, with the adsorbed Cr(VI) anion reduced to Cr(III) by hydroxyl groups of the hydrated metal ions and reconstructed into layer double hydroxide layers by the “memory effect”. The MgAl-LDO/LDO/γ-Fe2O3 and MgFe0.1Al-LDO/LDO/γ-Fe2O3 show extraordinary adsorption efficiency for CR with very close qmax values (3980 and 3832 mg g–1, respectively). The former can be attributed to its large SBET and strong LDO shells─CR anion electrostatic interaction, while the latter (99 m2 g–1) can still be ascribed to the complexing function of a small amount of Fe3+ species to CR. The MgAl-LDO/LDO/γ-Fe2O3 composite can be conveniently separated and recovered from the aqueous solution after adsorption by an external magnet. The fourth cycle efficiency of up to 84% implies a promising application prospect.