K6P2W18O62 encapsulated into magnetic Fe3O4/MIL-101 (Cr) metal-organic framework: a novel magnetically recoverable nanoporous adsorbent for ultrafast treatment of aqueous organic pollutants solutions

Abstract

In this study, a Wells-Dawson type K₆P₂W₁₈O₆₂ polyoxometalate was encapsulated into the magnetic Fe₃O₄/MIL-101 (Cr) metal-organic framework and applied as a new magnetically recoverable ternary adsorbent to remove organic dyes from aqueous solutions. The as-prepared ternary magnetically recyclable hybrid (denoted as P₂W₁₈O₆₂@Fe₃O₄/MIL-101 (Cr)) was characterized by FT-IR spectroscopy, powder X-ray diffraction (XRD), Raman spectroscopy, EDX, SEM, BET surface area, and magnetic measurements. The results showed the successful encapsulation of K₆P₂W₁₈O₆₂ (∼26.5 wt%) into the magnetic Fe₃O₄/MIL-101 (Cr) framework. The magnetic hybrid had a high specific surface area of 934.89 m² g^-1. The adsorption efficiency of this nanohybrid for the removal of methylene blue (MB), rhodamine B (RhB), and methyl orange (MO) from aqueous solutions was evaluated. The magnetic nanohybrid demonstrated the fast and selective adsorption of cationic dyes from mixed dye solutions. The adsorption rate and capacity of P₂W₁₈O₆₂@Fe₃O₄/MIL-101 (Cr) were increased as compared with MIL-101 (Cr), P₂W₁₈O₆₂, and Fe₃O₄/MIL-101 samples due to the increased electrostatic attraction. The effects of parameters such as the adsorbent dosage, temperature, dye concentration, and pH were investigated on the adsorption process. The adsorption kinetics was analyzed by the Freundlich, Langmuir, and Temkin isotherm models and pseudo-second-order and pseudo-first-order kinetics models, with the Langmuir isotherm and pseudo-second-order kinetic model found to be suitable to describe the equilibrium data. Also, the thermodynamic results of the nanohybrid indicated that the adsorption was an endothermic and spontaneous process. After the adsorption reaction, the magnetic nanohybrid could be easily separated and reused without any change in structure. Based on the results of this study, the nanohybrid was an efficient adsorbent for eliminating cationic dyes.

Fig. 1. FT-IR spectra of (a) MIL-101 (Cr), (b) P₂W₁₈O_62, (c) Fe₃O₄ (d) P₂W₁₈O₆₂@MIL-101 (Cr), (e) Fe₃O₄/MIL-101 (Cr), and (f) P₂W₁₈O₆₂@Fe₃O₄/MIL-101 (Cr).

Fig. 2. XRD patterns of (a) MIL-101 (Cr), (b) P₂W₁₈O₆₂, (c) Fe₃O₄, (d) P₂W₁₈O₆₂@MIL-101 (Cr), (e) Fe₃O₄/MIL-101 (Cr), and (f) P₂W₁₈O₆₂@Fe₃O₄/MIL-101 (Cr).

Fig. 3. Raman spectra of (a) MIL-101 (Cr), (b) P₂W₁₈O_62, (c) P₂W₁₈O₆₂@MIL-101 (Cr), (d) P₂W₁₈O₆₂@Fe₃O₄/MIL-101 (Cr).

Fig. 4. SEM images of (a and b) MIL-101 (Cr), (c and d) P₂W₁₈O₆₂@MIL-101 (Cr), (e and f) Fe₃O₄/MIL-101 (Cr), and (g and h) P₂W₁₈O₆₂@Fe₃O₄/MIL-101 (Cr).

Fig. 5. (a) EDX spectrum, and (b)–(i) a representative SEM image of the P₂W₁₈O₆₂@Fe₃O₄/MIL-101 (Cr) magnetic nanohybrid with corresponding EDX elemental mappings.

Fig. 6. (a) Nitrogen adsorption–desorption isotherm and (b) pore-size distributions of MIL-101 (Cr), P₂W₁₈O₆₂@MIL-101 (Cr), and P₂W₁₈O₆₂@ Fe₃O₄/MIL-101 (Cr) nanohybrid.

Fig. 7. Magnetic hysteresis loop of (a) Fe₃O₄, (b) Fe₃O₄/MIL-101 (Cr), and (c) P₂W₁₈O₆₂@Fe₃O₄/MIL-101 (Cr) at room temperature. The inset shows the behavior of the nanohybrid under an external magnetic field.

Fig. 8. UV-Vis spectral changes of dyes aqueous solutions with P₂W₁₈O₆₂@Fe₃O₄/MIL-101 (Cr) at different time intervals: (a) MB dye, (b) RhB dye. [Dye] = 25 mg L⁻¹, adsorbent: 30 mg at 25 °C.

Fig. 9. Effect of (a) pH, (b) adsorbent dosage, (c) dye concentration, and (d) different temperatures on the removal of MB and RhB dyes.

Fig. 10. Effects of different loading amounts of P₂W₁₈O₆₂ in MIL-101 (Cr) on the removal of MB and RhB dyes.

Fig. 11. Adsorption efficiency of MB and RhB dyes in the presence of different adsorbent samples.

Fig. 12. Selective adsorption ability of P₂W₁₈O₆₂@Fe₃O₄/MIL-101 (Cr) toward mixed dyes solution of (a) MB&MO, (b) MB&RhB, (c) RhB&MO, (d) MB&RhB&MO. Conditions: C₀ (MB) = C₀ (RhB) = C₀ (MO) = 25 mg L⁻¹, and adsorbent dose = 30 mg/30 mL.

Fig. 13. Effect of contact time and initial dye concentration on the adsorption process onto the P₂W₁₈O₆₂@Fe₃O₄/MIL-101 (Cr) adsorbent.

Fig. 14. Pseudo-second order kinetics for dyes adsorption: (a) MB, (b) RhB.

Fig. 15. (a) Recyclability of the P₂W₁₈O₆₂@Fe₃O₄/MIL-101 (Cr) hybrid nanomaterial in the removal of MB dye, (b) SEM images, (c) FT-IR spectrum, and (d) XRD pattern of the fresh and recovered nanohybrid after three runs.