Аdsorption of Co2+ and radioactive 60Со by mesoporous TiO2

Introduction shows the important of this scientific direction. 60Co with half-life of 5.3 years is one of the few anthropogenic, gamma-emitting radionuclides, that can be detected in aquatic environments affected by liquid effluent discharged from nuclear facilities. The need for control of the content of 60Co in the environment, determines the search for new adsorption materials with high adsorption capacity and chemical, thermal, and radiation resistance. The aim of present work is to investigate the adsorption of Co2+ and 60Co by mesoporous TiO2 from aqueous solutions. Experimental techniques describes the adsorption studies in detail. The mesoporous TiO2 with the initial pore size ratio (Smeso/S = 58 %; Vmeso/V = 64 %) was selected as adsorbent. Synthesis of adsorbents was carried out by the method of liquid phase hydrolysis of aqua complex of TiCl4. The dependence of adsorption value on agitation time, solutions acidity, and equilibrium concentration of Co2+ was investigated in butch mode. The presence of cobalt on the surface of mesoporous TiO2 was confirmed using XRF-analysis. The initial and residual concentration of cobalt was controlled by complexonometric titration with xylenol orange as indicator. Four simplified kinetic models: pseudo-first order and pseudo-second order equations, firstly applied by Lagergren, intraparticle diffusion and Elovich (Roginsky-Zeldovich) kinetic models were applied to experimental data. Langmuir and Dubinin-Radushkevich adsorption theory applied for experimental equilibrium data of adsorption of cobalt cations by mesoporous TiO2. The adsorption energy was measured using Dubinin-Radushkevich equation. The results obtained have shown that the experimental data on the adsorption kinetics of Co2+ by mesoporous TiO2 fit well by Lagergren pseudo-second kinetic model. Applying of Elovich kinetic model gives also high correlation’s coefficients, close to unit (R2 > 0.9). The equilibrium adsorption data are well approximated by Langmuir adsorption theory. Maximal adsorption value obtained experimentally (49±4 mg/g) is in good agreement with calculated by Langmuir adsorption theory (63.81 mg/g). The adsorption energy calculated using Dubinin-Radushkevich equation is 8.104±0.361 kJ/mol, which correspond to physical adsorption mechanism. However, for each values of Polanyi’s potential (ε) (which correspond to certain equilibrium concentration Ce, mg/L) adsorption energy is different. It smooth decreases with increasing concentration of adsorbate in the solution. Although the experimental results are well describing by the Langmuir model, the adsorption energy of Co2+ ions by mesoporous TiO2 depends on the degree of surface filling, which means that the adsorption centers of this sample are not independent. At the low equilibrium concentration of Co2+ (38 mg/L), the adsorption energy is much higher than the corresponding value for adsorption by the physical mechanism. To our opinion, that is why applying of Elovich kinetic model to experimental dada gives high R2. The adsorption of Co2+ by mesoporous TiO2 strongly depends on solutions acidity. To simulate conditions close to real, the adsorption of 60Co by mesoporous TiO2 was investigated. The percentage of 60Co, adsorbed onto TiO2 is more than 90 %. The main conclusion is that mesoporous TiO2 could be useful as an adsorbent for water purification from Co2+ and in decontaminating of radioactive waste containing 60Co.