Green Synthesis and Encapsulation of Superparamagnetic Magnetite for Mercury (II) Removal: Adsorption Isotherms, Adsorption Kinetics, and Thermodynamic Studies

Mercury (II) ions have significant ecological impacts. In the current study, magnetized iron oxide (Fe3O4) nanoparticles were green synthesized and encapsulated with alginate (ALG). The nanoparticles were converted into a composite (Fe3O4/ALG). The Fe3O4/ALG beads were used to remove Hg (II) from industrial wastewater. Characterization was performed by X-ray diffraction (XRD), scanning electron microscopy—energy dispersive spectroscopy (SEM-EDS), Fourier transform infrared spectroscopy (FT-IR), and thermogravimetric analysis-differential thermal analysis (TGA-DTA). A central composite design (CCD) approach was performed to define the experimental conditions providing the highest global adsorption efficiency and to optimize the pH, temperature and adsorbent mass. Hg (II) concentrations were determined by flame and electrothermal atomic absorption spectrometry (FAAS and ETAAS). The limits of detection (LOD) and quantification (LOQ) were 1.2 mg L−1 and 3.5 mg L−1 for FAAS and 0.2 µg L−1 and 0.5 µg L−1 for ETAAS, respectively. Additionally, Langmuir, Freundlich, and Temkin isotherm models were employed to determine the adsorption mechanism. The adsorption capacity of Fe3O4/ALG was determined to be 49.18 mg g−1 by the Langmuir model. Kinetic studies revealed that the adsorption of Hg (II) best fitted the pseudo-second order model. The Gibb’s free energy (ΔG°) values showed that the adsorption of Hg (II) on Fe3O4/ALG occurred spontaneously.