Interpretation of lanthanoid oxide and hydroxide solubilities based on their bulk and surface properties

バルク特性と表面特性に基づくランタノイド酸化物および水酸化物の溶解度に関する解釈

小林 大志*; Moniruzzaman, M.*; 元川 竜平   ; 佐々木 隆之*

Kobayashi, Taishi*; Moniruzzaman, M.*; Motokawa, Ryuhei; Sasaki, Takayuki*

The geochemical environment for lanthanide (Ln) oxides and hydroxides involves aquatic system processes where Lns are mobilized or immobilized by dissolution and precipitation. In the current study, solubility experiments were performed using lanthanum (La), europium (Eu), and thulium (Tm) oxides as starting materials in the undersaturation method and La, Eu, and Tm nitrate solutions as starting materials in the oversaturation method. After aging at 25 or 90C for given periods, the crystal structure, particle size, and solid-phase surface-charge states were investigated to interpret the La, Eu, and Tm solubilities. In aqueous systems, La is stable as La(OH)(cr), which dominates the bulk structure, surface-charge state, and solubility. In the oversaturation method at 25C, the presence of fine primary La(OH)(cr,np) particles with higher solubility was observed, suggesting the possible particle-size effect. However, in the undersaturation method at 90C, the surface-charge state was found to be more negative than that of other solid phases, resulting in lower solubility. The EuO and Eu(OH) stabilities are competitive in aqueous systems. The bulk solid phase was Eu(OH)(cr) in the oversaturation method at 25 and 90C, and in the undersaturation method at 90C, whereas the surface-charge state appeared to be controlled by EuO when aged at 90C. The stability of TmO increased further, so that TmO(cr) dominated the bulk, surface-charge state, and solubility, aged at both 25 and 90C in the undersaturation method. In contrast, Tm(OH)(am) was formed using the oversaturation method and dominated the bulk, surface-charge state, and solubility at both temperatures. These findings are expected to provide important clues for understanding the evolution of lighter-to-heavier Ln oxides and hydroxides in aqueous systems, which is important for radioactive waste disposal.