Reassessing the role of carbon shells in magnetite nanoparticles: a comparative adsorption study of ionic dyes

Carbon-coated magnetite nanoparticles have been widely explored for water treatment applications; however, the fundamental role, necessity, and true performance advantages of the carbon shell relative to bare magnetite remain insufficiently understood. We present a systematic comparative study of ionic dye adsorption on bare Fe₃O₄ (b-Fe₃O₄) and carbon-coated Fe₃O₄ (c-Fe₃O₄) nanoparticles. Kraft Lignin was the carbon precursor for the carbon shell developed herein. Transmission electron microscopy (TEM) revealed that b-Fe₃O₄ NPs possessed an average diameter of 74 ± 16 nm, while the carbon shell thickness in c-Fe₃O₄ NPs measured 7.2 ± 3.0 nm. BET analysis showed a substantial decrease in specific surface area upon coating application, from 71.30 ± 0.21 m² g^-1 for b-Fe₃O₄ to 12.01 ± 0.06 m² g^-1 for c-Fe₃O₄. FTIR spectra confirmed the incorporation of aromatic carbon structures, and XRD patterns indicated identical crystallographic peaks for both samples, demonstrating preservation of the magnetite core. Adsorption experiments with anionic Congo red (CRed) and cationic methylene blue (MBlue) were conducted across a pH range of 2-10. Interestingly, b-Fe₃O₄ NPs exhibited high adsorption of cationic CRed (more than 90%) throughout the entire tested pH range, while displaying negligible adsorption of anionic MBlue. In contrast, c-Fe₃O₄ showed minimal interaction with CRed except at pH 2, but achieved strong MBlue removal, reaching 85% at pH 10. Kinetic analyses revealed rapid CRed uptake by b-Fe₃O₄, reaching equilibrium within ∼10 minutes at pH 4, whereas c-Fe₃O₄ required more than 24 hours to reach equilibrium for both dyes. Isotherm studies further confirmed distinct adsorption behaviors associated with each material system. These findings highlight that bare magnetite offers superior performance for anionic CRed, whereas carbon coating selectively improves uptake of cationic MBlue, highlighting that the benefits of carbon shells are system-dependent rather than universal.