Solubility and solid phase of trivalent lanthanide hydroxides and oxides

Abstract

For the safety assessment of radioactive waste disposal, it is necessary to predict the migration behavior of actinide elements under relevant geochemical conditions, as they are included in the waste as alpha-emitting radionuclides wit h long half-lives. Actinide elements of thorium, uranium, neptunium, and plutonium can exist in a tetravalent oxidation state under reducing geochemical conditions, deep underground and easily precipitate as a sparingly soluble amorphous hydroxide solid phase (An(IV)(OH)4(am)) under neutral to alkaline pH conditions of the waste repository systems [1-4]. The solubilities of An(IV)(OH)4(am), hence, play an important role in understanding their migration behavior. It is known that a crystalline oxide solid phase as An(IV)O2(cr) is thermodynamically more stable and it has been reported that the crystallization of An(IV)(OH)4(am) towards An(IV)O2(cr) proceeded under certain solution conditions such as strong alkaline pH or elevated temperatures [5-7]. The solubilities of An(IV)O2(cr) have been reported to several orders of magnitude lower than those of An(IV)(OH)4(am) [1-7]. Trivalent actinide elements of americium and curium also exhibit a strong hydrolysis reactions under neutral to alkaline pH conditions to precipitate the sparingly soluble amorphous hydroxide solid phase (An(III)(OH)3(am)) [8-11]. In contrast to the tetravalent actinide elements, no crystalline oxide solid phase (An(III)2O3(cr)) was observed in the solubility experiments [2,12]. A few literatures have observed crystalline hydroxide solid phase (An(III)(OH)3(cr)) from X-ray diffraction patterns instead of An(III)2O3(cr) and showed an order of magnitude lower solubility values than those of An(III) (OH)3(am) [13,14]. This can be explained by thermodynamic data of An(III)2O3(cr). For example, the standard enthalpy (∆fHm°) and entropy (Sm°) of Am2O3(cr) have been reported to be ∆fHm° = −1690.4±8.0 kJ/mol and Sm° = 133.6±6.0 J/K/mol resulted in the standard formation Gibbs energy of ∆fGm° = −1605.449±8.284 [2]. Combined with the thermodynamic data for Am and H2O [2], the standard reaction Gibbs energy (∆rGm°) for 1/2 Am2O3(cr) + 3H Am + 3/2 H2O was calculated to be ∆rGm° = −151.59 kJ/mol, leading to the solubility product (Ks°) of log Ks° = 26.56. This value is approximately 10 orders of magnitude h igher than those repor ted for Am(OH)3(am) and Am(OH)3(cr) [2,4], hinting the oxide solid phase is less stable in aqueous systems. However, due to experimental limitations for handling macro amounts of trivalent actinide elements, only few studies have investigated the An(III) solubility with a definite solid phase characterization [8,11,13] and the stability of An(III)2O3(cr) in aqueous systems has not been well experimentally clarified. Trivalent lanthanide elements are often used as analogues of trivalent actinide elements. A number of literatures have investigated the hydrolysis behavior, solubilities and solid phases of lighter to heavier lanthanide elements and reported their thermodynamic data [15-24]. Several works occasionally summarized the state of knowledge on the solubilities of trivalent lanthanide elements [25-28]. Most recently, Brown and Ekberg [4] have compiled the literature data on the hydrolysis of lanthanide elements and selected the recommended values for the solubility product of hydroxide solid phase (Ln(III) (OH)3(s)) and related thermodynamic data. It is noted that the solubility product of amorphous hydroxide solid phase (Ln(III) (OH)3(am)) obtained after short aging periods was excluded in the review due to poor identification of the solid phase [4]. Konings et al. [29] conducted a comprehensive review on the thermodynamic properties of lanthanide and actinide oxides, where recommended values of ∆fHm° and Sm° for the lanthanide oxide solid phase (Ln(III)2O3(cr)) were selected. For example, the values of ∆fHm° and Sm° for La2O3(cr) were selected to be ∆fHm° = −1791.6±2.0 kJ/mol and Sm° = 127.32±0.84 J/K/mol, based on the reported results by solution calorimetry and heat capacity measurements [29]. The calculated ∆fGm° and subsequently ∆rGm° for 1/2 La2O3(cr) + 3H La + 3/2 H2O was −192.33 kJ/mol, leading to log Ks° = 33.70, which is much larger than the selected Ks° value for La(OH)3(s) (log Ks° = 19.72) [4]. The transformation of Ln(III)2O3(cr) to Ln(III) (OH)3(cr) in aqueous systems was observed in a few literatures [30-32]. The formation of Ln(III)(OH)3(cr) was observed by precipitating Ln(III) solutions with NaOH, after complete dissolution of initial Ln(III)2O3(cr) with a hot nitrate solution [31]. Neck et al. [32] observed that the initial material of Nd2O3(cr) was converted to Nd(OH)3(cr) in a purified water at 25 °C after a few months, prior to the their solubility experiment of Solubility and solid phase of trivalent lanthanide hydroxides and oxides