Modeling and optimization of cationic dye adsorption onto modified SBA-15 by application of response surface methodology

SBA-15 mesoporous silica was modified with melamine-based dendrimer amine (MDA) via grafting approach and used as adsorbent for the removal of methylene blue (MB) cationic dye from aqueous solution. The synthesized material, denoted MDA-SBA-15, was characterized by means of X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy, and N-2 adsorption-desorption, in order to prove the 2D hexagonal mesoporous structure and covalent grafting of MDA onto SBA-15. Central composite design combined with response surface methodology (RSM) was employed for statistical modeling, optimization, and analysis the effects of influence variables such as the initial pH (3-10), adsorbent doses (0.5-3.5gL(-1)), temperature (25-40 degrees C), and initial concentrations (10-30mgL(-1)) onto the MB removal. From the analysis of variance, pH and temperature were identified as the most influential factors onto each experimental design response. Maximum percentage removal (98%) under optimum conditions of variables (pH of 10, adsorbent dose of 3gL(-1), MB concentration of 10mgL(-1), and temperature of 25 degrees C), as predicted by RSM, was found to be very close to the experimentally determined value (95.5%). The Langmuir, Freundlich, and Temkin isotherm models were used to describe the equilibrium sorption of MB by MDA-SBA-15, and the Langmuir isotherm showed the best concordance as an equilibrium model. The obtained kinetic data manifested that adsorption kinetics was more accurately exposed by a pseudo-second-order model and film diffusion was the rate-determining step for the adsorption of MB onto MDA-SBA-15. The associated thermodynamic parameters reveal that the process of adsorption is spontaneous and exothermic nature within the studied temperature range.