Sorption study of MOF-5-imprinted manganese ferrite for the removal of Congo red and methylene blue dyes from aqueous solution.

Incessant pollution of water by azo dyes, especially Congo red (CR) and methylene blue (MB), is a global concern. Metal nanoparticles have been widely employed in adsorption due to their excellent physicochemical properties. However, iron oxide nanoparticle aggregates are formed when exposed to air. Hence, metal-organic framework (MOF-5) was imprinted on the nano ferrite to form a composite, addressing the limitation. MOF-5 is selected due to its excellent adsorbent properties. The metal nanoparticle synthesized is MnFe2O4, and its MOF-imprinted composite is MnFe2O4@MOF-5. In this study, MnFe2O4 and MnFe2O4@MOF-5 synthesized were investigated for their adsorptive capacities for removing CR and MB from water. From the scanning electron micrograph, it was observed that the surfaces of MnFe2O4 and MnFe2O4@MOF-5 are heterogeneous with particles that are not regularly shaped. Furthermore, the diffraction patterns from the XRD results confirmed the imprinting of MOF-5 on MnFe2O4 to form MnFe2O4@MOF-5. The adsorption capacities displayed by MnFe2O4@MOF-5 towards MB (88.9 mg g-1) and CR (100 mg g-1) are higher than those of MnFe2O4. Adsorption of CR and MB to MnFe2O4@MOF-5 was pH-dependent. A 100% adsorption of CR was also observed at neutral pH, and at an adsorbent (MnFe2O4@MOF-5) weight of 0.1 g (at 20 min). The pHpzc for MnFe₂O₄@MOF-5 is 6.2. An increase in adsorbent dosage has no significant effect on the sorption process. The sorption process fits better with the Langmuir isotherm. It can also be well described with the pseudo-second-order kinetic model. The surface of MnFe₂O₄@MOF-5 is best regenerated for MB with NaOH (0.1 M), whereas deionized water achieved a better result for CR with 99% regeneration. This study identified MnFe₂O₄@MOF-5 as an efficient adsorbent for the purification of water systems contaminated with MB or CR, highlighting its potential in environmental remediation.