Niobium pentoxide as an adsorbent for methylene blue removal: Synthesis, characterization and thermal stability

Abstract

This study investigated the synthesis of niobium pentoxide (Nb2O5) by peroxide oxidation with hydrothermal treatment and evaluated its application for adsorption of the organic pollutant methylene blue (MB) in aqueous medium. Hydrothermal treatment was performed at 150 °C for different times (4, 12, and 24 h). The resulting materials were characterized by X-ray diffractometry, Raman spectroscopy, thermogravimetry, differential scanning calorimetry, Fourier-transform infrared spectroscopy (FTIR), N2 adsorption isotherm analysis, zeta-potential analysis, and scanning electron microscopy (SEM). Nb2O5 samples had high surface area (174 m2 g−1) and orthorhombic crystal structure. FTIR revealed that an increase in synthesis time produced an increase in surface hydroxylation. Particles dispersed in water had zeta-potential values lower than −31.3 mV, suggesting colloidal stability in aqueous suspension. SEM analyses showed no significant variation in nanoparticle morphology as a function of synthesis time. MB adsorption studies using the sample treated at 150 °C for 24 h showed that the adsorption was higher at pH 6.0, and the process follows pseudo-second order kinetics, suggesting chemical diffusion as the dominant process. The Freundlich isotherm model provided the best fit to experimental data. Thermal analyses indicated a phase transition at about 600 °C, corroborated by analysis of samples after calcination at 500 and 700 °C. Thus, the obtained materials have high specific surface area, adsorption capacity, suspension stability, and thermal stability.