A simple, environmentally friendly synthesis of recyclable magnetic γ-Fe2O3/Cd2+-Ni2+-Fe3+-CO32− layered double hydroxides for the removal of fluoride and cadmium ions. Adsorption capacity and the underlying mechanisms

Abstract

The homogeneous precipitation assisted by fluoride ions as a complexing agent was successfully applied for the synthesis of novel, raster-shaped γ-Fe2O3/Cd2+-Ni2+-Fe3+- layered double hydroxides (LDHs) starting from NiCl2·6H2O, FeCl3·6H2O, and wastewater enriched with F− and Cd2+ ions. The material comprises large, platelet-shaped particles with uniformly dispersed γ-Fe2O3 on the LDHs surface that induce strong magnetic properties and allow easy separation during the recycling process. Moreover, large specific surface area (156 m2/g) and small band gap (2.40 eV) of γ-Fe2O3/Cd2+-Ni2+-Fe3+-CO32−LDHs indicate great potential applicability as adsorbents and photocatalysts. Based on the experimental data, the formation mechanism of γ-Fe2O3/Cd2+-Ni2+-Fe3+-CO32−LDHs was proposed. Fluoride ions form [FeF6]3− complexes with a portion of iron in the solution and reduce the amount of iron(III)-hydroxide, which increases the pH range for Fe3+ deposition. This promotes co-precipitation of Ni2+ and Cd2+ and the formation of Cd2+-Ni2+-Fe3+-CO32−LDHs at higher pH values. The presence of Cd2+ in the solution induces the transformation of Fe(OH)3 into γ-Fe2O3 particles located at the surface of the Cd2+-Ni2+-Fe3+-CO32−LDHs platelet. The main mechanism for Cd2+ removal from the aqueous solution was coprecipitation via the formation of Cd2+-Ni2+-Fe3+-CO32− LDHs. Owing to the small ionic radius of F− and the high adsorption capacity of LDHs, the fluoride ions were adsorbed between the layers of LDHs, and their concentration was effectively reduced at the desired level. This novel method for the preparation of magnetic LDHs simultaneously removes harmful ions from wastewater and improves the magnetic properties of recyclable materials, which provides a new pathway for the material preparation and application.