Facile synthesis of hierarchical MCM-41 spheres with an ultrahigh surface area and their application for removal of methylene blue from aqueous solutions

Abstract

Hierarchical mesoporous silica spheres (HS) were prepared by a simple sol–gel method and used for the removal of methylene blue from aqueous solutions. The macrostructure of HS can be tuned simply by adding different amounts of ethanol as a reactant. The prepared HS were characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, nitrogen adsorption–desorption isotherms, and thermogravimetric analysis. The results showed that HS possessed both macropores and mesopores, with an average diameter of 5.6 μm, a surface area of 2280 m² g⁻¹, and a pore volume of 1.15 cm³ g⁻¹. The influence of pH, temperature, dosage of adsorbent, and initial methylene blue (MB) concentration on the adsorption behavior were investigated. The experimental results showed that HS exhibited a high adsorption capacity (654.5 mg g⁻¹) and extremely rapid adsorption rate (<2 min) due to their unique hierarchical structure and very high surface area. The kinetic studies showed that the experimental data fitted well to the pseudo-first-order kinetic model. The Langmuir adsorption isotherm model was found to present an accurate description of these adsorption data. The experimental results suggest that HS are potentially useful materials for effective adsorption and removal of methylene blue in aqueous solution.