Fabrication of a magnetite/diazonium functionalized-reduced graphene oxide hybrid as an easily regenerated adsorbent for efficient removal of chlorophenols from aqueous solution

Abstract

A magnetic hybrid nanomaterial, which contains magnetite (Fe₃O₄) particles and diazonium functionalized-reduced graphene oxide (DF-RGO), was fabricated via a three-pot reaction. First, the reduced graphene oxide (RGO) was synthesized via a redox reaction. Second, diazonium functionalized-RGO was prepared via a feasible chemical reaction. Third, Fe₃O₄ particles were loaded onto the surface of DF-RGO by covalent bonding, fabricating the M-DF-RGO hybrid. The fabricated hybrid was characterized by SEM, TEM, AFM, XRD, XPS, FT-IR, TGA, Raman spectroscopy, and magnetometry. The resulting M-DF-RGO hybrid possessed unique magnetic properties and was applied to remove 4-chlorophenol (4-CP) and 2,4-dichlorophenol (2,4-DCP) from aqueous solution. The adsorption of 4-CP and 2,4-DCP on the M-DF-RGO hybrid was performed under various conditions, with respect to initial chlorophenol concentration, pH, and contact time. The results suggest that the adsorption of 4-CP and 2,4-DCP onto the M-DF-RGO hybrid is strongly dependent on pH and weakly dependent on contact time. In addition, the adsorption isotherm of 4-CP and 2,4-DCP on the M-DF-RGO hybrid fits the Freundlich model well and the adsorption capacities of 4-CP and 2,4-DCP on M-DF-RGO reached 55.09 and 127.33 mg g^-1, respectively, at pH 6 and 25 °C. In this situation, intermolecular interactions including π-π interactions and hydrogen bonding are operative. The calculated results of density functional theory further demonstrate that 2,4-DCP molecules could be more easily absorbed than 4-CP molecules by the M-DF-RGO hybrid. Moreover, the M-DF-RGO hybrid could be easily separated by a magnetic separation process, and showed good recyclability of more than five cycles.

Fig. 1. TEM images of (a) GO, (b) RGO, (c) DF-RGO, and (d) M-DF-RGO; and SEM images of (e) GO, (f) RGO, (g) DF-RGO, and (h) M-DF-RGO.

Fig. 2. Characterization of samples. (a) XRD spectra; (b) FTIR spectra; (c) Raman spectra; (d) TGA curves.

Fig. 3. The magnetization curves of the samples, with the inset showing the separation efficiencies of M-DF-RGO.

Fig. 4. High resolution C 1s core level XPS spectra of (a) GO, (b) RGO, (c) DF-RGO, and (d) M-DF-RGO.

Fig. 5. The effect of pH on the removal efficiencies of 4-CP and 2,4-DCP on M-DF-RGO (experimental conditions: initial chlorophenol concentration = 100 mg L⁻¹, adsorbent dosage = 1.0 g L⁻¹, contact time = 12 h at 298 K).

Fig. 6. Adsorption kinetics of chlorophenols on M-DF-RGO (experimental conditions: initial chlorophenol concentration = 100 mg L⁻¹, pH = 6 and adsorbent dosage = 1.0 g L⁻¹ at 298 K).

Fig. 7. Adsorption isotherms of 4-CP and 2,4-DCP on M-DF-RGO (experimental conditions: initial chlorophenol concentration = 10–300 mg L⁻¹, pH = 6, adsorbent dosage = 1.0 g L⁻¹ and contact time = 30 min at 298 K).

Fig. 8. Optimized structures of the adsorption of (a) 4-CP and (b) 2,4-DCP on DF-RGO (carbon atom: yellow; chlorine atom: green; oxygen atom: red; hydrogen atom: blue).

Fig. 9. Recyclability of M-DF-RGO in the removal of chlorophenols (experimental conditions: initial chlorophenol concentration = 100 mg L⁻¹, pH = 6, adsorbent dosage = 1.0 g L⁻¹ and contact time = 30 min at 298 K).