Methylene blue adsorption from aqueous solutions using undoped and silver-doped nanotubes of anatase-titania synthesized via modified hydrothermal method

The nanotubes of undoped and silver (Ag)-doped anatase-titania (TiO2) have been synthesized via the modified hydrothermal method involving the use of acetic acid-modified sol–gel processed nanocrystalline undoped and Ag-doped anatase-TiO2 as precursors. Their dye adsorption characteristics have been investigated using a cationic dye such as the methylene blue (MB). For the undoped nanotubes of anatase-TiO2, the MB dye adsorption measurements have been obtained in the aqueous solutions by varying both the initial dye concentration (~7.5–250 μM) and initial solution pH (~2.5–11). The dye adsorption capacity of undoped nanotubes of anatase-TiO2 has been determined to be 32 mg g−1 at the initial solution pH of ~10. Similar measurements have been obtained for the varying amount of Ag doping (Ag/Ti ratio of 0.01 and 0.05) at the initial solution pH of ~10. The Ag-doped nanotubes of anatase-TiO2 having the Ag/Ti ratio of 0.01 exhibit the highest dye adsorption capacity (39 mg g−1) within the investigated range of Ag/Ti ratio. Hence, controlling the initial solution pH and the amount of Ag doping appear to be two effective approaches for enhancing the amount of dye adsorbed on the surface of hydrothermally synthesized nanotubes of anatase-TiO2. Further analyses reveal that the kinetics of MB adsorption on the surface of nanotubes of undoped anatase-TiO2 follows the pseudo-second-order model and the Langmuir isotherm model shows the best fit to the equilibrium adsorption data typically for higher initial solution pH values (≥10), both of which are not affected by the amount of Ag doping.