Bio-inspired magnetic iron oxide nanoparticles: performance evaluation in cationic and anionic dye remediation.
Abstract
The increasing discharge of synthetic azo dyes into aquatic ecosystems necessitates the development of efficient, low-cost, and sustainable remediation strategies. This study investigates the adsorption capacity of green hydrothermally synthesized magnetic iron oxide (Fe3O4) nanoparticles (IONPs) for cationic and anionic dyes. Comprehensive characterization confirmed the formation of crystalline Fe3O4. Microscopic analysis revealed polydispersed spherical particles (≈33 nm) capped with phytochemicals, while Vibrating Sample Magnetometer (VSM) analysis demonstrated superparamagnetic behavior with a saturation magnetization of ≈47 emu g-1. The adsorption mechanism of the oppositely charged model dyes, anionic Congo Red (CR) and cationic Methylene Blue (MB), is primarily governed by the surface charge density and pH sensitivity of the IONPs relative to their point of zero charge (pHPZC = 7.5). Surface protonation at low pH facilitates the electrostatic removal of anionic CR. In contrast, deprotonation at high pH enhances the sequestration of cationic MB. Kinetic data for both dyes followed a pseudo-second-order model, while isotherm studies showed that CR follows the Langmuir model (q m = 190.11 mg g-1) and MB aligns with both Langmuir (q m = 181.49 mg g-1) and Freundlich models. At the optimized pH, the IONPs' surface promotes a synergy of electrostatic attraction and π-π stacking. Furthermore, the IONPs showed excellent reusability, enabling easy magnetic recovery and structural stability, maintaining high removal efficiency and a consistent Fe-O FTIR signature over multiple adsorption-desorption cycles. Therefore, green-synthesized IONPs provide efficient, magnetically recoverable, pH-responsive dye adsorption, offering a sustainable and high-performance solution for future large-scale industrial wastewater remediation.
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