Adsorption mechanisms of uranium aqueous species on highly porous materials

In France, former uranium mines and their draining effluents are kept under environmental monitoring due to the presence of mine tailings which can modify the chemistry of surface waters and increase their uranium concentration. In order to decrease the U(VI) concentration to the one set by the standards, water treatment facilities are used. However, an incoming regulation will drastically decrease the allowed U(VI) concentration in the released water, implying the changing of the current U(VI) removal methods. Aqueous adsorption is known to be an efficient and non expensive method. Ordered meso-porous materials have interesting adsorption properties due to their textural properties and their ability to be functionalized. Different types of meso-porous materials (functionalized or non functionalized silica, carbon, magnesium oxide) were synthesized, characterized, and experimented as U(VI) adsorbents. Studies made in batch and dynamic modes were achieved from reconstituted solutions and mine-water from the Bellezane's site, on the different materials, and adsorption mechanisms were studied in function of the aqueous uranium speciation and the presence of competitive ions. Results show that aminopropyl grafted SBA-15 displays the best adsorption capacities at pH 6. It is also shown that U(VI) adsorption proceeds through the formation of inner-sphere complex inside of the meso-porosity of the material, and that the presence of complexes involving U(VI) and dissolved carbonates significantly decrease the adsorption capacity of the material toward U(VI). (author)