Synthesis and characterization of citric acid grafted MCM-41 and its adsorption of cationic dyes

In this study, citric acid (CA) grafted MCM-41 (SCA-MCM-41) was synthesized for improvement of its adsorptive capacity for methylene blue (MB), malachite green (MG) and brilliant green (BG). Characterization of the adsorbent was carried out using scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM), elemental analysis (EA), X-ray diffraction analysis (XRD), textural analysis, thermo-gravimetric analysis (TGA), solid state nuclear magnetic resonance analysis (SSNMR) and fourier transform infrared analysis (FTIR). MB, MG and BG adsorption studies were carried out on citric acid modified MCM-41, varying parameters such as initial dye concentration, pH, adsorbent dose, contact time and temperature. The maximum adsorption capacity of the modified material improved considerably for all the dyes with a basic pH and temperature of 25 °C for all the adsorbates. The qm values obtained were 204.08 mg/g, 334 mg/g and 227 mg/g for MB, MG and BG, respectively. Langmuir, Freundlich, Temkin and Dubinin-Radushkevich models were used in analyzing equilibrium data. The adsorption kinetics favoured the pseudo-second-order model. Adsorption of the dyes into SCA-MCM-41 was spontaneous and exothermic.