Mesoporous spinel CoFe2O4 as an efficient adsorbent for arsenite removal from water: high efficiency via control of the particle assemblage configuration

Abstract

A 3D mesoporous network of ∼6 nm cobalt ferrite (CoFe₂O₄) nanoparticles (NPs), synthesized through a polymer-assisted aggregation self-assembly method, is presented. The single-phase crystallinity and high porosity of the obtained material were confirmed with X-ray diffraction, transition electron microscopy, Raman and N₂ porosimetry studies. Porous CoFe₂O₄ assemblies, obtained after heat treatment of the hybrid networks, possess an open-pore structure with a BET surface area of 160 m² g⁻¹ and pores with an average size of ∼5.7 nm. Owing to its 3D network assemblage, this mesoporous CoFe₂O₄ exhibits an exceptional As^III uptake capacity of 252.8 mg g⁻¹, which is much higher than those of random CoFe₂O₄ NP aggregates (47.3 mg g⁻¹) and bulk-like CoFe₂O₄ microparticles (43.6 mg g⁻¹). A comprehensive surface complexation model is presented, allowing a quantitative description of the As^III adsorption on Fe- and Co-sites. Our results indicate that the As^III uptake can be attributed to specific FeOH and CoOH sites located on the outer surface and interior pore voids of the material. Confinement inside the pores is found to be responsible for strong lateral interactions among the adsorbed [H₃AsO₃] species. The As^III uptake of the present CoFe₂O₄ material is 3 to 10-fold higher than those of other high-performance adsorbents, such as graphite oxide, ZVI/activated carbon and CoFe₂O₄ and NiFe₂O₄ nanostructures. This exemplifies that, apart from surface chemistry, fine tuning of the spatial arrangement of NPs can offer advantageous tools towards highly-efficient As^III adsorbents.