Surface functionalization of mesoporous silica MCM-41 with 3-aminopropyltrimethoxysilane for dye removal: kinetic, equilibrium, and thermodynamic studies

Mesoporous silica MCM-41 was synthesized and functionalized with a silane coupling agent (3-aminopropyltrimethoxysilane, APTMS) for the removal of Acid Blue 25 (AB25, anionic dye) and Methylene Blue (MB, cationic dye) from aqueous solutions as adsorbents. The synthesized (MCM-41) and functionalized (f-MCM-41) materials were characterized using field emission scanning electron microscopy, energy-dispersive X-ray spectrometry, transmission electron microscopy, particle size analysis, nitrogen gas adsorption-desorption analysis, X-ray diffraction spectrometry, and Fourier transform infrared spectrometry. The results indicate that the surface modification of MCM-41 with APTMS was successfully carried out. Batch experiments were performed to examine AB25 and MB removal by MCM-41 and f-MCM-41 under various experimental conditions. The results show that f-MCM-41 was a good adsorbent for anionic AB25, but not for cationic MB. This could be attributed to the presence of amine groups on the surfaces of f-MCM-41 from the APTMS modification. Conversely, MCM-41 was good for MB removal, but not for AB25 removal. AB25 removal in f-MCM-41 was sensitive to solution pH, with decreasing adsorption capacity as pH increased from 4.1 to 9.4. MB removal in MCM-41 was sensitive to solution pH in the opposite manner, with increasing adsorption capacity as pH increased from 4.1 to 10.3. Thermodynamic analysis showed that the adsorption of AB25 to f-MCM-41 and of MB to MCM-41 decreased with increasing temperature from 15 to 45 degrees C, indicating the exothermic nature of the sorption process. This study demonstrated that surface functionalization with silane coupling agents can help to make MCM-41 useful for the removal of various types of dyes.