Adsorption behavior of aqueous europium on kaolinite under various disposal conditions

This work investigated the adsorption behavior of europium on kaolinite under various disposal conditions. Batch-wise adsorption and precipitation experiments and equilibrium model calculations were performed over a pH range of 4–10 and CO₂ concentration range of 0%, 0.03%, and 10%. Experimental precipitation behaviors are in agreement with the results of equilibrium model calculations using the geochemical code MINTEQA2. Aqueous species of Eu³⁺ exists mainly at pH 5 or below and solid phases of Eu(OH)₃(s), Eu(OH)CO₃(s), and Eu₂(CO₃)₃·3H₂O(s) are formed at higher pH ranges. Adsorption behavior of Eu on kaolinite in the low pH range can be explained by interlayer ion-exchange reaction. The significant increase in adsorbed amount at pH 5–6 is due to the surface complexation at the edge site of kaolinite. In the high pH range, precipitation of Eu contributes mainly to the adsorption quantity. The rapid decrease in adsorbed amount above pH 7 under 10% CO₂ condition occurs by the formation of anionic europium species of Eu(CO₃) ₂ ^- .The adsorption of Eu on kaolinite could be well interpreted by the Freundlich adsorption isotherm. The data except for the highest equilibrium concentration ranges were also explained by Langmuir isotherm and the maximum adsorbed quantity of Eu on kaolinite,b, is 1.2 mg/g.