Bilayer-Coated Magnetite Nanoparticles as Magnetic Adsorbents for Efficient Paraquat Removal

The environmental persistence and acute toxicity of paraquat (PQ²⁺) highlight the urgent need for efficient and reusable adsorbents for water remediation. In this study, linoleic acid (LA)- and palmitic acid (PA)-coated magnetite nanoparticles (Fe₃O₄@LA and Fe₃O₄@PA) were synthesized via a continuous-flow coprecipitation process to form bilayer structures. Structural analyses (TEM, XRD, FTIR, TGA, BET, and VSM) confirmed successful bilayer coating and preserved superparamagnetic properties, ensuring easy magnetic recovery. Batch adsorption experiments revealed that Fe₃O₄@LA exhibited a maximum Langmuir capacity (<i>Q</i> _max) of 14.8 ± 0.4 mg g^-1, outperforming Fe₃O₄@PA (11.2 ± 0.3 mg g^-1), owing to enhanced electrostatic and hydrophobic interactions provided by the unsaturated linoleic acid bilayer. Adsorption kinetics followed a pseudo-second-order model, indicating chemisorption, while thermodynamic parameters confirmed that the process is exothermic and spontaneous. Both nanoadsorbents demonstrated good recyclability over six adsorption-desorption cycles with negligible capacity loss. These results demonstrate that fatty acid bilayer-coated Fe₃O₄ nanoparticles are cost-effective, magnetically recoverable, and environmentally benign materials for the remediation of herbicide-contaminated water.