Adsorption of methylene blue onto spinel magnesium aluminate nanoparticles: Adsorption isotherms, kinetic and thermodynamic studies

Magnesium aluminate spinel has been synthesized at nanoscale by coprecipitation method. The synthesized material was characterized for the phase composition, quantitative description, surface morphology, surface area, pore volume and pore diameter. X-ray diffraction analysis confirmed the formation of the cubic spinel phase with no peaks corresponding to the impurity phases. The Scherrer crystallite size calculated from line broadening is 20 nm. The pore surface area was found to be 343 m2/g by N2 vapor adsorption experiments and material was characterized as mesoporous on the basis of pore diameter analysis. The scanning electron micrograph shows the agglomeration of particles while the nanosize is confirmed by the transmission electron micrograph. The porous nature of the material was then explored for carrying out the adsorption of methylene blue solutions onto the surface. Adsorption studies were carried out with 0.1 g of the material and the effect of pH and shaking times were studied and both were found to influence the adsorption. Adsorption data was also fitted to Temkin, Freundlich and the Langmuir adsorption models in order to study the mechanism of adsorption by interpreting the calculated parameters like, heat of adsorption (BT), binding energy (KT), degree of adsorption (KF), heterogeneous factor (n), and energy of adsorption (KL), binding forces (aL), and the separation factor (RL). The pseudo-second order rate constant (Kp−2), initial sorption rate (Srate) and the half adsorption time (t1/2) were also calculated and explained to clarify the mechanism of adsorption onto MgAl2O4 surface. Gibbs free energy was also calculated from the adsorption data at room temperature.

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► Nanosized MgAl2O4 synthesized by coprecipitation method.
► Mesoporous material having large surface area was obtained.
► Adsorption favored at neutral pH.
► Chemisorption process is involved.
► Multilayer adsorption occurs with interactions between the adsorbed molecules.