Study of Adsorption Equilibrium in the System Methylene Blue + Nanocomposit TiO2/Diatomite

The article presents the results on the adsorption of the dye methylene blue (MB) from an aqueous solution on a nanocomposite DDT (nanosized titanium dioxide in the anatase phase precipitated on diatomite) and its components – diatomite D and TiO₂. The effect on adsorption of the pH of the solution, the dose of the adsorbent, the   initial concentration, and temperature of the dye solution was studied. The equilibrium data were fitted to the Freundlich, Langmuir, and Dubinin-Radushkevich isotherms equations. The obtained parameters and correlation coefficients showed that the adsorption of the dye on D and TiO₂ was best described by the Langmuir isotherm model, while the adsorption isotherm of MB on DDT – by the Dubinin-Radushkevich model. The MB adsorption data on DDT and D, obtained at different solution temperatures (20, 30, and 40°C) revealed irreversible adsorption, which increased with an increment of T. Thermodynamic parameters of the process were calculated. The negative values of the Gibbs free standard energy ΔG^o show that the adsorption of MB on these adsorbents is spontaneous. Positive values of ΔH^o, characteristic of the endothermic process, were relatively high, which indicates that the interaction between the sorbent molecules and the adsorbate was not only physical, but also chemical. The dye adsorption on TiO₂ decreases with increasing the solution temperature; it is physical and exothermic in nature, ΔG^o being positive and ΔH^o negative. The values of  the activation  energy of adsorption E_a of MB on DDT, D, and TiO₂, calculated according to the Dubinin-Radushkevich equation, was equal to 10.74, 11.78, and -2.68 kJ/mol, respectively, thus indicating the ion-exchange mechanism of the adsorption of MB by DDT and initial diatomite and physical adsorption by TiO₂. A negative value of the activation energy for a sample of TiO₂ means the absence of a potential barrier during adsorption of MB on this adsorbent.