Study of the synthesis of TiO₂ layers on macroporous ceramic supports in supercritical (SC) CO₂ for processing radioactive aqueous effluents in dynamic mode

Public and military nuclear industry generates a significant amount of radioactive liquid waste which must be treated before being released into the environment. Decontamination methods alternative to the industrial techniques (evaporation, chemical treatment) are being developed, such as column treatments or coupled filtration/sorption processes. Current researches mainly focus on the development and shaping of specific sorbents. In this context, the objectives of this thesis were first to study the synthesis of TiO₂ layers on macroporous ceramic supports in supercritical (SC) CO₂ and then to evaluate their potential for radionuclide extraction in these alternative processes. A robust synthesis method has been developed, based on the thermal decomposition of titanium isopropoxide in SC CO₂ in the temperature range between 150 C and 350 C. Nano-structured TiO₂ films were formed on the macroporous supports (ceramic foams, tubular α-alumina supports) with good adhesion, already at 150 C. The effect of the synthesis temperature on sorbents physico-chemical characteristics and sorption properties has been studied with TiO₂ powders prepared under the same conditions as the supported films. The best sorption performance were observed for the powder prepared at 150 C, owing to its higher density of surface sites in comparison with powders prepared at either 250 C or 350 C. Consequently, this synthesis temperature (150 C) was selected for a detailed study of the composite sorbents (TiO₂/support), in order to assess their sorption performance in continuous treatment processes. The sorption experiments have shown that a column of alumina macroporous foam (Φpore = 400μm) coated with TiO₂ was suitable for processing effluents in dynamic mode with high throughputs. Both macro-pore sizes and column height were revealed as important parameters to be controlled. For the coupled filtration/sorption treatment, TiO₂ membranes exhibit good mechanical strength and are able to extract strontium. On the other hand in the case of strontium, composite sorbents (TiO₂/foam or TiO₂/tubular support) can be fully regenerated by using an acidic aqueous solution (pH = 3). (author)