MgO modified ordered porous materials for efficient methylene blue adsorption: Effects of support, synthesis temperature and method on adsorption performance

In this study, we prepared a series of adsorbents for the removal of methylene blue (MB) using SBA-15 and MCM-41 as supports. We employed different synthetic methods (solid-state grinding and impregnation) and temperatures (400 and 600 °C) to clarify the key parameters governing the adsorption of MB. Characterization results indicated that solid-state grinding was more favorable for the dispersion of MgO nanoparticles within the support pores, whereas impregnation led to MgO agglomeration on the surface. The adsorbents prepared via the impregnation method faster achieved adsorption equilibrium at both low and high MB concentrations because of easily accessible MgO sites on the surface. However, adsorption isotherms showed superior MB adsorption capacity of the adsorbents prepared via solid-state grinding, which was attributed to the highly dispersed MgO within the mesopores. The adsorbents prepared at 600 °C exhibited higher MB adsorption capacity than those prepared at 400 °C. Particularly, the maximum adsorption capacity of SBA-15@Mg-600 for MB was 827.54 mg/g. The MB adsorption capacity increased with pH because of the strengthening of electrostatic attraction. The presence of ions in water, especially Ca^2 + , inhibited MB adsorption via competitive adsorption. The adsorption mechanism primarily involved the formation of hydrogen bonds, electrostatic interaction, and pore filling. After five regeneration cycles, SBA-15@Mg-600 maintained stable adsorption performance, whereas the adsorption capacity of MCM-41-based adsorbents notably decreased. These results indicate that SBA-15@Mg-600 can serve as a highly effective and stable adsorbent for MB removal. This study highlights the crucial role of MgO dispersion and support stability in the design of efficient adsorbents for MB removal.