Unraveling the sorption mechanism of industrial dyes onto Zr-based MOFs: computational and experimental modelling for highly efficient removal†

The ability of NH₂-UiO-66 to remove the cationic dye rhodamine B (RhB) and the anionic dyes indigo carmine (IC) and orange 2 (O2) was evaluated. XRD, SEM/EDX, FTIR, N₂ sorption and TG/DTG analytical techniques were used to evaluate the physicochemical properties of NH₂-UiO-66 produced by the solvothermal method. For IC and O2 dyes, the NH₂-UiO-66 material showed an adsorption capacity of 265.8 mg g⁻¹ and 229.8 mg g⁻¹, respectively, while for RhB it was 91.6 mg g⁻¹. The most accurate model was Toth's isothermal model with R² > 0.90. The Elovich kinetic model provided the most accurate fit, with an R² > 0.95 for all dyes, suggesting a competition between physisorption and chemisorption. The HOMOs are significantly delocalised on the nitrogen atom, while the LUMOs are delocalised around the aromatic nucleus, according to DFT and Monte Carlo simulation studies. The chemical reactivity of the dyes IC, RhB and O2 interacting at the adsorbent surface was demonstrated by calculating quantum parameters such as E_HOMO, E_LUMO and gap energy (E_gap). The adsorption mechanism found was favorable, suggesting electrostatic attractions as well as pi–pi interactions between the benzene rings of the dye and the H₂N-H₂BDC linker. NH₂-UiO-66 showed high stability after 5 adsorption cycles.