Magnetic Carbon Nanocages: An Advanced Architecture with Surface- and Morphology-Enhanced Removal Capacity for Arsenites

Magnetic carbon nanocages (Mag@CNCs) were synthesized via a green one-step process using pine resin and iron nitrate salt as a carbon and iron source, respectively. To produce Mag@CNCs, pristine materials have been carbonized at high temperature under inert atmosphere. The structural, textural, and surface properties of as-synthesized Mag@CNCs were studied employing microscopic, spectroscopic, and surface physicochemical methods. The obtained results showed that the new Mag@CNCs have significant surface area (177 m² g–¹) with both microporosity and mesoporosity. Moreover, the material exhibits a homogeneous distribution of core–shell-type magnetic nanoparticles within the carbon matrix, formed by iron carbide (Fe₃C) and metallic iron (α-Fe), with sizes of 20–100 nm, surrounded by a few graphitic layers-walls. Most importantly, Mag@CNCs were tested as absorbents for As(III) removal from aqueous solutions, showing a total of 263.9 mg As(III)-uptake capacity per gram of material at pH = 7, a record sorption capacity value among all previously tested iron-based materials and one of highest values among all reported sorbents so far. The adsorbed As(III) species are anchored at the surface of Mag@CNCs, as demonstrated by high-resolution transmission electron microscopy and X-ray photoelectron spectroscopy measurements. The pH-edge As(III)-adsorption experiments combined with theoretical surface complexation modeling allowed a detailed understanding of the interfacial properties of Mag@CNCs, and hence the As(III) uptake mechanism. The analysis revealed that As(III) binds on two types of surface sites of Mag@CNCs, i.e., on carbon-surface species (≡CₓOH₂) and on Fe-oxide layer (≡FeOH₂) of nanoparticles. This exemplifies that the advanced morphology- and surface-driven synergistic properties of the Mag@CNCs material are crucial for its As(III)-uptake performance.