Microwave synthesis of silica nanoparticles and its application for methylene blue adsorption

Silica nanoparticles were obtained from rice husk through an alternative microwave process and applied to remove Methylene Blue dye (MB) from aqueous media by adsorption. The better characteristics of microwave modified nano-silica (MW-nSiO2), regarding to the standard nano-silica (nSiO2), were demonstrated by X-ray diffraction, infrared spectroscopy, N2 adsorption isotherms, gas picnometry, differential scanning calorimetry and scanning electron microscopy. nSiO2 and MW-nSiO2 were used as adsorbents to remove MB dye from aqueous media. Adsorption kinetics, isotherms and thermodynamics were evaluated. MW-nSiO2 presented higher values of surface area, pore volume, pore diameter and porosity than nSiO2. Pseudo-n order and HSDM models were adequate to represent the kinetic profile. To represent the equilibrium curves, Sips model was the most adequate. MW-nSiO2 attained adsorption capacity of 679.9 mg g−1 coupled with removal percentage of 80%, being and excellent adsorbent for MB dye. For both adsorbents, the thermodynamic results revealed a favorable, spontaneous and exothermic process. These findings show that microwave is a powerful technique to intensify the silica production process, generating a material with better characteristics for dyes adsorption.