Sequestration and speciation of Eu(iii) on gamma alumina: role of temperature and contact order.

The speciation, migration and transport of radionuclides in the environment are significantly influenced by their interactions with the natural minerals and humic substances therein. In view of this, the effect of temperature and contact order on the sorption behaviors of trivalent Eu(iii) in the γ-Al2O3/Eu(iii) and γ-Al2O3/HA/Eu(iii) systems was studied using batch experiments and the extended X-ray absorption fine structure spectroscopy (EXAFS) technique. The endothermic sorption behavior of Eu(iii) in the γ-Al2O3/Eu(iii) systems was induced by the hydrolysis reaction of Eu(iii) in solution and the complexation of Eu(iii) with the γ-Al2O3 surface sites. The endothermic sorption of Eu(iii) in the γ-Al2O3/HA/Eu(iii) systems was attributed to the endothermic binding of HA on γ-Al2O3 and the endothermic complexation between Eu(iii) and HA. EXAFS analysis suggested the formation of type B ternary complexes and their thermodynamic stability improves with rising temperature. The different sorption percentages under various contact orders were closely related to the binding mode of Eu(iii) on the exposed γ-Al2O3 surfaces or the γ-Al2O3/HA colloids. The findings obtained herein are important to evaluate the security of the radioactive waste repository and predict the fate of trivalent actinides (e.g., Am(iii), Cm(iii), Pu(iii), etc.) near the geological repository.