Comprehensive adsorption study of methyl red onto MFe2O4 using DFT calculations and Monte Carlo simulations

Abstract

This work aims to elucidate the chemical properties of methyl red (MR) and the mechanisms underlying its interactions with MFe₂O₄ (311) (M = Co, Cu, Mg, and Zn) in aqueous environment. Quantum computational descriptors were calculated based on the lowest unoccupied molecular orbital energy and the highest occupied molecular orbital energy. Additionally, Molecular descriptors were obtained through the Adsorption Locator module. The minimal ΔE_gap indicates a high reactivity of MR, which correlates with adsorption configurations predominantly adopting a parallel orientation relative to the surfaces. The dynamic descriptors reveal that the adsorption mechanism is spontaneous and exothermic. Accordingly, the MFe₂O₄/MR/500H₂O systems exhibited higher adsorption energies, ranging from −776.08 to −703.98 kcal/mol, compared to MFe₂O₄/MR/H₂O systems with fewer water molecules (400, 300, and 100H₂O). Among the MFe₂O₄/MR/500H₂O systems, the MR/CuFe₂O₄ complex demonstrated the highest E_ads at −776.08 kcal/mol, followed by MR/CoFe₂O₄ (−723.74 kcal/mol), MR/MgFe₂O₄ (−713.35 kcal/mol), and MR/ZnFe₂O₄ (−703.98 kcal/mol). Across all studied systems, dE_ads/dNi_MR values significantly exceed the dE_ads/dNi_H2O values, implying that H₂O molecules on the MFe₂O₄ surface could be partially substituted. The most stable configurations imply chemisorption between the adsorbate and the surfaces, a conclusion further supported by radial distribution function (RDF) analysis.