Adsorption of Azo Dye Congo Red on the Sn-Doped TiO2 Surface

In this paper, the effect of SnO2 impurity on the surface charge and adsorption properties of TiO2 samples is investigated. The experimental value of the zero charge point for TiO2 with 3 %, 6 % and 12 % of SnO2 equals 3.53, 3.97 and 3.2, respectively. The adsorption activity of the samples was studied on model solutions of the anionic dye - Congo red. The maximum adsorption capacity (q(exp)) equals 24.6 mg/g for 3Sn/TiO2, 25.0 mg/g for 6Sn/TiO2 and 39.1 mg/g for 12Sn/TiO2. Langmuir, Freundlich, and Dubinin-Radushkevich models were used to describe the adsorption mechanism of Congo red dye on the surface of Sn/TiO2 samples. Based on the results of the studies of Congo red adsorption by the surface of titanium dioxide doped with Sn, all samples agree best with the Langmuir model. The correlation coefficients for the Langmuir isotherms are in the range of 0.9927 - 0.9996, while the values of R-2 for the Freundlich and Dubinin-Radushkevich isotherms are in the range of 0.721 - 0.8329 and 0.8283 - 0.9433, respectively. The low correlation of the experimental data with the Freundlich model indicates that these samples are not characterized by multilayer adsorption and inhomogeneous adsorbent surface. The results of approximation (Dubinin-Radushkevich model) indicate that the process of binding Congo red molecules to the Sn/TiO2 surface is ion exchange, since the adsorption energy (E) is in the range of 8 - 16 kJ.mol(-1.) Experimental data obtained from adsorption isotherms show that the TiO2 sample containing 12 % SnO2 is the most active. The introduction of additional Sn ions into the TiO2 structure appears to lead to an increase in the adsorption capacity and the efficiency of dye removal. The best result of Congo red removal occurred at a concentration of C-o = 5 mg/l (% of removed dye approximate to 83 % for 12Sn/TiO2; 81 % for 6Sn/TiO2 and 71 % for 3Sn/TiO2). Therefore, the studied samples of TiO2 doped with SnO2 can be used as effective adsorbents of Congo red from aqueous solutions.