Adsorption of dyes onto modified titanium dioxide

Titanium dioxide (TiO₂) nanoparticles with different sizes and crystalloid structures produced by the thermal method and doped with silver iodide (AgI), nitrogen (N), sulfur (S), and carbon (C) were applied as adsorbents. The adsorption of methyl violet (MV), methylene blue (MB), methyl orange (MO), and Orange II on the surface of these particles was studied. The photocatalytic activity of some particles for the destruction of MV and Orange II was evaluated under sunlight and visible light. The equilibrium adsorption data were fitted to the Langmuir, Freundlich, Langmuir-Freundlich, and Temkin isotherms. The equilibrium data show that TiO₂ particles with larger sizes and doped with AgI, N, S, and C have the highest adsorption capacity for the dyes. The kinetic data followed the pseudo-first-order and pseudo-second-order models, while desorption data fitted the zero-order, first-order, and second-order models. The highest adsorption rate constant was observed for the TiO₂ with the highest anatase phase percentage. Factors such as anatase crystalloid structure, particle size, and doping with AgI affect the photocatalytic activity significantly. Increasing the rutile phase percentage also decreases the tendency to desorption for NTiO₂ and STiO₂. Adsorption was not found to be important in the photocatalytic decomposition of MV in an investigation with differently sized AgITiO₂ nanoparticles. Nevertheless, CTiO₂ was found to have higher adsorption activity onto Orange II, as the adsorption role of carbon approached synchronicity with the oxidation role.