Methylene blue adsorption from aqueous solution by functionalized perlites: an experimental and computational chemistry study

In this study, 3-aminopropyltrimethoxy-silane (3-APTMS) and 3-mercaptopropyltrimethoxy-silane (3-MPTMS) were used to functionalize perlite in order to increase the adsorption capacity of methylene blue (MB). These materials were characterized by X-ray diffraction, X-ray fluorescence, Fourier transform infrared spectroscopy, scanning electron microscopy, and thermal (thermogravimetry/differential thermal analysis) analyzes. The performance of perlite and functionalized derivatives was tested by adsorption of MB in a batch system under a variable pH (2–11) and initial MB concentration (10–60 mg L–1). Nonlinear and linear Langmuir, Freundlich, Dubinin–Radushkevich (D–R), and Sips adsorption equations were applied to define equilibrium isotherms. The maximum dye sorption was found to be at a pH of 11.0 (96.44% for PA, 96.51% for PAPTMS, and 96.73% for PMPTMS). The adsorption capacity for the dye was found to be 4.9445, 5.2014, and 5.3112 mg g–1 for PA, PAPTMS, and PMPTMS, respectively, at 298 K. The adsorption kinetics was best described by the pseudo- second-order model. The obtained results confirm that amino and mercapto functional groups have positive effects on the adsorption of methylene blue by the adsorbents used in this work. Quantum chemical analysis supported better adsorption of methylene blue dye on amine and mercapto-functionalized perlite surfaces than the unmodified surface and the most suitable site for MB adsorption was suggested by Fukui function analysis.