Investigation of the surface properties and microstructure of TiO2 sorbents prepared in supercritical CO2 for the treatment of Sr2+ contaminated effluents

Abstract

Nuclear facilities generate contaminated effluents containing radionuclides (such as Cs, Sr, Co…) that need to be removed for human health and environment protection reasons. Inorganic sorbents are attractive candidate materials because of their high thermochemical and radiation stability. Furthermore, their microstructural and surface properties can be adjusted to increase the radionuclide extraction efficiency. In this study, nanostructured sorbents consisting of aggregated TiO 2 nanocrystals with different surface properties and microstructures were prepared in supercritical CO 2 by varying the synthesis temperature. The Sr 2+ sorption process was characterized by measuring the surface properties and extraction capacity of the samples as a function of pH. In basic effluents, the Sr sorption capacity of these materials is directly linked to their specific surface area and sorption site density through a classic physisorption mechanism. Sr 2+ diffusion into the mesopores leads to rapid initial sorption, which is followed by a slower process driven by a proposed multistep mechanism. This mechanism involves the initial adsorption of partially hydrated Sr 2+ ions up to complete TiO 2 surface coverage, which implies slower Sr ion diffusion due to steric hindrance in small mesopores thus limiting access to additional secondary sites with lower adsorption energies.kinetics. Mesoporous powders with a high surface to volume ratio are beneficial because the active sites are easily accessible. Adsorption performance also seems to depend on the organization of the active sites and therefore on the crystalline structure of the sorbent [11]. Synthesizing nanostructured anatase TiO 2 with a high surface area and a well-defined mesoporous network would thus be a key advance for sorption-based decontamination processes.The TiO 2 sorbents investigated here were synthesized using supercritical carbon dioxide (SC-CO 2 ) as the reaction solvent, which has been shown to offer several advantages for the preparation and processing of inorganic nanostructured materials (nanopowders, thin films, impregnation of catalysts…) [12,13]. The unique diffusion properties and solvent power of SC-CO 2 allow mesoporous anatase powders with a high specific area to be prepared at low temperatures without any surfactant [14]. The density of the CO 2 is known to be a crucial parameter in controlling the particle size and morphology of metal oxides synthesized in SC-CO 2 [15]. In this study, TiO 2 particles were synthesized at a fixed pressure (~ 300 bar) and the effect of the SC-CO 2 temperature was investigated on their size, morphology, microstructure, surface properties, and the nature and density of the sorption sites. The adsorption mechanism and Sr 2+ extraction efficiency of these materials were then investigated at different pHs and correlated with their microstructure and surface properties. Finally, the Sr sorption properties of the particles (kinetics, isotherms, maximum capacity, distribution coefficient, and thermodynamic sorption constan...