Uranyl-sorption properties of amorphous and crystalline TiO 2/ZrO2 millimeter-sized hierarchically porous beads

Hierarchically porous TiO2/ZrO2 millimeter-sized beads were synthesized using a sol-gel templating technique, and investigated for suitability as radionuclide sorbents using uranyl as a radionuclide- representative probe. The bead properties were varied by altering either composition (22, 36, and 82 wt % Zr in the Ti/Zr composite) or calcination temperature (500 or 700 °C). Uranyl adsorption was higher for the crystalline beads (surface area: 52-59 m2 g-1) than the amorphous beads (surface area: 95-247 m2 g-1), reaching a maximum of 0.170 mmol g-1 for the 22 wt % Zr sample. This was attributed to the higher surface hydroxyl density (OH nm-2), presence of limited microporosity, and larger mesopores in the crystalline beads. Mass transport properties of the crystalline beads were not compromised by the large bead diameter: sorption rates comparable to those reported for powders were achieved and rates were higher than exclusively mesoporous reported systems, thereby highlighting the importance of pore hierarchy in designing materials with improved kinetics. Chemical stability of the sorbent, an important property for processes involving corrosive effluents (e.g., radioactive waste), was also assessed. Crystalline beads displayed superior resistance against matrix leaching in HNO3. Stability varied with composition: the 22 wt % Zr sample demonstrated the highest stability.