Sugar-capped magnetite nanoparticles for selective and efficient removal of Alizarin Red S dye from aqueous...

Water pollution has become a big concern worldwide, due to the enormous amounts of recalcitrant chemicals and organic dyes that are released from factories on a daily basis. This study evaluates removal of Alizarin Red S (ARS) dye from water using sugar-capped (i.e. chitosan, starch, and glucose) magnetic iron oxide nanoparticles (SMNPs). The physiochemical, structural, morphological, compositional, and magnetic properties of SMNPs were fully characterized using various techniques including TEM, XRD, FTIR, DLS, and VSM. Experimental dye removal studies showed that chitosan-coated MNPs attained the highest adsorption efficiency, rapidly removing the anionic ARS dye quantitatively within only few seconds, achieving very high adsorption capacities (q _e = ∼90 mg g^-1). Starch- and glucose-MNPs were also able to remove the dye, albeit at lower adsorption abilities (q _e = ∼46 and 19.5 mg g^-1, respectively). Adsorption isotherm and kinetic studies indicate that the process primarily follows the Langmuir (R ² = 0.967) or Freundlich (R ² = 0.978) isotherm models, with adsorption behavior best described by pseudo-second-order kinetics. Thermodynamic study showed a positive ΔH° value (+21.01 kJ mol^-1), confirming the endothermic nature of the adsorption process which is mainly governed by physisorption. Furthermore, the negative ΔG° values (-3.187 to -5.767 kJ mol^-1) confirm that the adsorption process is spontaneous and thermodynamically favorable. Theoretical density functional theory (DFT) calculations were employed to understand the mechanism of the dye-nano-adsorbent interactions, confirming that the observed behavior is primarily due to electrostatic and hydrogen-bonding interactions between the chitosan-MNPs and ARS anionic dye. By comparing these three different sugars, our results clearly verified how functionalization can influence dye removal efficiencies. Interestingly, regeneration studies demonstrated that chitosan-MNPs could be reused over multiple cycles while maintaining excellent recyclability (≥80%). This combination of reusability and strong adsorption efficiency highlights the potential use of chitosan-MNPs for large-scale, cost-effective, and wastewater treatment applications.