Selective, effective, and rapid removal of cationic and anionic organic dyes using various tailored polymer-coated (PEG, PVP, PAA/starch) magnetite nanoparticles

The development of novel, selective, and effective nanosorbents that can efficiently eliminate both anionic and cationic organic dyes in wastewater treatment remains a challenge. Herein, we prepared different panels of magnetic nanoparticles (MNPs) coated with various polymers (PVP, PEG, and PAA/starch) and studied their adsorption capacity towards two common dyes: Anionic Congo red (CR) and cationic Methylene blue (MB). The physiochemical, structural, morphological, compositional, and magnetic properties of the polymer-coated MNPs were fully characterized using various electronic and spectroscopic techniques including TEM, XRD, FTIR, and VSM. The obtained polymer-coated MNPs exhibited spherical nanosized particles (∼5–15 nm), pure well-crystallized cubic spinel Fe₃O₄ phases, and high saturation magnetization with superparamagnetic behavior, rendering easy-magnetic separation. The dye removal studies showed that the tailor-made MNPs efficiently adsorb the dyes rapidly (> 99% in less than 2 minutes) with very high adsorption capacities (q_e = ∼85–110 mg g⁻¹). Interestingly, it was found that the adsorption is selective where PAA/starch-MNPs were the best at adsorbing the cationic MB, while PEG-MNPs completely removed the anionic CR. Adsorption isotherms and kinetics studies suggest that the adsorption occurs by the Langmuir or Freundlich isothermal adsorption models (R² = 0.995) following pseudo-second-order adsorption kinetics (R² = 0.999). Thermodynamic data showed that adsorption processes of both CR and MB onto MNPs were endothermic and spontaneous in nature. Moreover, an integrated theoretical density functional theory (DFT) method was employed to fully understand the mechanism behind the dye-adsorbent interactions. DFT studies confirmed that these observations are mainly attributed to strong electrostatic and H-bonding interactions between the grafted functionalized polymer on MNP surface and the specific dye. Delightfully, a regeneration study showed that the magnetic nanoadsorbents could be reused for six consecutive cycles with high removal efficiencies (> 65%), without the use of any desorption agents. These unique findings highlight the potential of the designed polymer-coated MNPs as effective magnetic nanosorbents for quick and efficient industrial wastewater treatment.