Effect of Amino Functionality on the Uptake of Cationic Dye by Titanium-Based Metal Organic Frameworks

Titanium-based metal–organic frameworks (MOFs), named MIL-125 and NH2-MIL-125, have been successfully synthesized by the hydrothermal method. They were specially designed for the application of methylene blue (MB) removal from aqueous solution. The maximum adsorption capacity of MB was found to be 321.39 mg/g and 405.61 mg/g. The amount of dye that has been adsorbed 99.9% was 300 ppm within 20 and 120 intervals of time. The results show that NH2-MIL-125 is more effective in terms of both selectivity and capacity for the adsorption of MB cationic dye compared with MIL-125. The high adsorption selectivity was due to the unique electrostatic interaction between the amino groups of the dye molecules and NH2-MIL-125 but, on the other hand, owing to the more negative zeta potential (−32.4 mV), resulted from the charge balance for the protonation of −NH2. Moreover, reaction parameters including exposure time, adsorbent dose (0.02–0.05 mg), initial dye concentration (100–500 mg/L), and temperature were studied in detail. The adsorption processes in the two MOFs were determined to follow a pseudo-second-order pathway and obey a Langmuir isotherm model. Furthermore, the reaction was found to be spontaneous in nature yet thermodynamically an endothermic process. Characterization and structural analysis of the samples were evaluated by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared (FTIR) spectrometry, N2 adsorption/desorption (BET), X-ray photoelectron spectroscopy (XPS), and zeta potential.