Methylene blue adsorption by TiO2-based nano-adsorbents: performance evaluation and kinetic study

This study deals with the synthesis of TiO2-based nano-adsorbents and their performance in methylene blue adsorption. Nano-adsorbents were made up of micronized powder of anatase TiO2 as a relatively low-cost precursor and a compound alkaline solution containing 5 mol of NaOH and 5 mol of KOH. To synthesize nano-adsorbents, the hydrothermal method was used at three different temperatures of 110, 155, and 200 °C and two calcination temperatures of 450 and 700 °C. The results of methylene blue adsorption tests indicated that the nano-structure synthesized at 155 °C was of maximum adsorption capacity and calcination process resulted in a significant decrease in adsorption capacity. This meso-porous nano-adsorbent was characterized by such techniques as Scanning Electrom Microscopy (SEM), Transmission Electron Microscopy (TEM), Field-Emission Scanning Electron Microscopy (FESEM), X-ray Diffractometry (XRD), Brunauer-Emmett-Teller (BET) analysis, Barrett-Joyner-Halenda (BJH) analysis, and Fourier-Transform Infrared (FTIR) spectroscopy. Pseudo-first-order and pseudo-second-order adsorption kinetics, as well as the Weber and Morris model, were then used to conduct a kinetic study. The adsorption kinetics was also analyzed with three adsorption isotherm models—i.e., Langmuir, Temkin, and Freundlich. The obtained results suggested that the nano-adsorbent exhibiting the maximum adsorption capacity of 96 mg/g was well fitted to the pseudo-second-order model with a correlation coefficient of 0.9938. The findings also showed that the Freundlich model with a correlation coefficient of 0.9592 was the most compatible adsorption isotherm model for this nano-adsorbent.