Magnesium Oxide-Modified Alumina for Enhanced Adsorption of Multi-Sulfonated Azo Dyes: Performance and Mechanistic Insights.

Multi-sulfonated anionic azo dyes are difficult to remove from water because their high solubility and ionized sulfonate groups reduce their affinity toward many oxide adsorbents. In this study, magnesium oxide-modified alumina composites were prepared by loading magnesium oxide onto mesoporous alumina to increase the density of surface hydroxyl groups. Sunset Yellow and Amaranth were selected as model dyes containing two and three sulfonate groups, respectively. Compared with pristine alumina, the modified adsorbents exhibited higher isoelectric points, stronger hydroxyl-related infrared signals, and significantly enhanced adsorption capacities. At pH 4.0, the maximum adsorption capacities reached 787 mg/g for Sunset Yellow and 709 mg/g for Amaranth. Notably, MgA-2 exhibited the highest utilization efficiency of MgO active sites. Adsorption kinetics were well described by the pseudo-second-order model, while equilibrium data followed the Langmuir model. Mechanistic analysis indicated that adsorption process mainly proceeded through ion exchange between surface hydroxyl groups and dye sulfonate groups. The higher adsorption capacity of Sunset Yellow was attributed to its lower number of sulfonate groups and lower demand for hydroxyl binding sites. These results demonstrate that magnesium oxide modification is an effective strategy for enhancing alumina-based adsorbents for the removal of multi-sulfonated anionic dyes from water.