The textile industry is a major user of dyes and hence an important source of water pollution, which poses a threat to aquatic as well as human life. Adsorption is a widely used technique for separating and removing dye from wastewater. Recently the use of mesoporous silica (MPSs) as an adsorbent in wastewater treatment has received considerable attention. MPSs are emerging as potential adsorbents because of their surface area, tunable and uniform pore structure, high pore volume, ordered pore structure, thermal and mechanical stability, and extraordinarily wide possibilities for functionalization to increase their adsorption capacity. In this paper the ability of MPSs to absorb dyes from aqueous solutions is reviewed. This article provides information on adsorption studies carried out under different operational conditions such as contact time, solution pH, temperature, agitation speed, etc. This review also summarizes and attempts to compare the equilibrium isotherm and kinetic models, and the thermodynamic studies that report the adsorption of dyes onto MPSs. The literature reviewed, indicate that SBA-MPSs have a higher adsorption capacity than MCM-MPSs and functionalized MPSs a better adsorption capacity than MPSs. It is evident from the literature that the potential of MPS based nano-sorbents being used for removing dyes from aqueous solution is very high. However, still more research work is needed on developing cost effective and more efficient MPS based nano-sorbents for use commercially.

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A B S T R A C TSBA-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 (0.5-3.5 g L −1 ), temperature (25-40˚C), and initial concentrations (10-30 mg L −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 3 g L −1 , MB concentration of 10 mg L −1 , and temperature of 25˚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.

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