Solvothermal synthesis of different phase N-TiO₂ and their kinetics, isotherm and thermodynamic studies on the adsorption of methyl orange

The different crystal forms of nitrogen doped-titanium oxide (N-TiO₂) with different particle sizes were produced by precipitation-solvothermal method and their adsorption mechanism were also investigated. The adsorption kinetics showed that rutile N-TiO₂ displayed higher adsorption capacity than anatase for methyl orange (MO) and its adsorption behavior followed the pseudo-second-order kinetics. The equilibrium adsorption rate of N-TiO₂ for MO was well fitted by the Langmuir isotherm model and the adsorption process was monolayer adsorption. The adsorption capacity decreased with increasing temperature. The average correlation coefficient was beyond 97%. The thermodynamic parameters (δ_aG^θ_m, δ_aH^θ_m, and δ_aS^θ_m) were calculated. It was found that anatase and rutile N-TiO₂ had different adsorption enthalpy and entropy. The smaller the particle size, the greater the surface area and surface energy was, then δ_aG^θ_m decreased and the standard equilibrium constant increased at the same time. The adsorption process onto different crystalline phase N-TiO₂ was exothermic and non-spontaneous.