Statistical physics modeling and interpretation of methyl orange adsorption on high–order mesoporous composite of MCM–48 silica with treated rice husk

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In this study, a high–order composite of MCM–48 silica with rice husk ash (RHA) was synthesized. The composition and morphological properties of this composite (MCM–48/RHA) were assessed though XRD, SEM and FTIR techniques. The adsorption of methyl orange (MO) on the as–synthesized MCM–48/RHA was very fast (i.e., after 5 min of interaction time) and the maximum removal efficiency (99%) was achieved at pH 3.0. Langmuir, Freundlich, and Dubinin–Radushkevich models were used to fit the experimental data without clarifying the adsorption mechanisms. Therefore, the modeling of MO experimental adsorption via statistical physics theory was used to understand the dye removal mechanism. Results indicated that a monolayer model with two energies was the best to fit experimental adsorption isotherms. Statistical physics parameters controlling the MO adsorption mechanism were determined and interpreted. The adsorption energies (E1 = 29.103 and E2 = 19.076 kJ/mol) suggested that dipole bond forces and electrostatic interactions were involved in MO adsorption. The parameters of thermodynamic functions indicated that the MO adsorption onto the MCM–48/RHA composite was endothermic and spontaneous. Based on Kelvin equation in the liquid phase, the pore size distribution (PSD) and the adsorption energy distribution (AED) of the prepared composite were determined. The PSD results exhibited that MO molecules can penetrate the high–order composite pores, while the AED confirmed the physical nature of MO adsorption process.