Manganese dioxide-loaded mesoporous SBA-15 silica composites for effective removal of strontium from aqueous solution

Manganese dioxide-loaded mesoporous SBA-15 silica (MnO₂/SBA-15) composites with short pore length were aprepared for the first time by simply immersing SBA-15 into a KMnO₄ and MnCl₂ mixed solution. Adsorption of Sr²⁺ from aqueous solution by using the MnO₂/SBA-15 was investigated by varying the pH, contact time, initial Sr²⁺ concentration, MnO₂ content and temperature. The adsorption process was rapid during the first 40 min and reached equilibrium in 120 min. The Sr²⁺ adsorption capacity increased with increasing pH, MnO₂ content and temperature, and the adsorption capacity of SBA-15 was significantly improved by the loading of MnO₂. Moreover, the experimental adsorption data were analyzed using different equilibrium isotherm, kinetic and thermodynamic models. The results showed that the isotherm data were well-described by the Langmuir model. The maximum Sr²⁺ adsorption capacity was determined to be 75.1 mg g^-1 at 283 K based on the Langmuir model. The analyzed kinetic data indicated that the Sr²⁺ adsorption process was well fitted by the pseudo-second order model. Furthermore, the thermodynamic parameters of adsorption were also determined from the equilibrium constant values obtained at different temperatures. The results suggested that the adsorption process was spontaneous and endothermic, and the overall mechanism of Sr²⁺ adsorption was a combination of physical and chemical processes.