Unraveling the adsorption mechanisms of cationic dyes on defective UiO-66: Insights from kinetics, isotherms, and spectroscopic analyses

Abstract

The efficient removal of cationic dyes from wastewater is crucial because of their environmental persistence and toxicity. This study investigates the adsorption behavior of methylene blue (MB) and crystal violet (CV) onto defective UiO-66 (dUiO-66), which is a metal-organic framework (MOF) with enhanced porosity and surface chemistry. The adsorption kinetics followed a pseudo-second-order model, indicating that the rate-limiting step is affected by the concentrations of both adsorbent and adsorbate. MB exhibited faster uptake owing to its smaller molecular size, whereas CV adsorption was influenced by steric hindrance and external surface interactions. Equilibrium adsorption was best described by the Sips model, which suggests heterogeneous surface interactions. The dUiO-66 framework exhibited significantly higher adsorption capacities (q(max )= 366 mg/g for MB and 561 mg/g for CV) than powdered activated carbon, confirming its superior performance. Spectroscopic analyses, including X-ray photoelectron spectroscopy and surface-enhanced Raman spectroscopy, provided molecularlevel insights concerning the adsorption mechanisms and revealed it-it interactions, electrostatic attraction, and hydrogen bonding as dominant forces. Structural integrity analysis confirmed that the adsorption process preserved the crystalline framework of dUiO-66, while inducing changes at the defect sites. These findings highlight the potential of dUiO-66 for selective dye removal and contribute to the rational design of MOF-based adsorbents for wastewater treatment.