The Solubility of Metallic Selenium under Anoxic Conditions
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The Solubility of Metallic Selenium under Anoxic Conditions
Yoshihisa Iida1, Tetsuji Yamaguchi1, Shinichi Nakayama1, Tomoko Nakajima2 and Yoshiaki Sakamoto1 1 Japan Atomic Energy Research Institute, Tokai, Naka, Ibaraki 319-1195, Japan Email: [email protected] 2 Hokkaido University, Kitaku, Sapporo, Hokkaido, 060-0810, Japan
ABSTRACT The solubility of metallic selenium was measured in a mixture of 0.1M-NaCl and 0.05M-N2H4 under anoxic conditions (O2 < 1 ppm) by both oversaturation and undersaturation methods. Equilibrium was attained in 40 days. The aqueous selenium species identified were HSe- at pH between 5 and 8, and Se42- at pH between 10 and 13, by UV-Vis absorption spectrometry. The solid phase was identified as Se (cr) by X-ray diffraction. The equilibrium constants of Se(cr) + H+ + 2e- = HSe- logK0 = -6.5±0.5 and 4Se(cr) + 2e- = Se42-
logK0 = -16.8±0.5
were determined. The standard molar free energy of formation of HSe- and Se42- was determined to be (37.1±2.9) and (95.9±2.9) kJ/mol, respectively. INTRODUCTION Performance assessment calculations [1] for a high-level radioactive waste repository shows that selenium-79 (half-life 106 years) is one of the radionuclides that dominates long-term radiological hazard. Release of 79Se from a repository is expected to be limited by solubility of the compounds. Therefore, thermodynamic data of selenium are necessary for performance assessment calculations. Since the chemical conditions under deep underground environments are likely to be anoxic and reducing, HSe- is considered to be a dominant selenium species in groundwater [2]. Selenium can be more soluble as polyselenide anions, Sen2- in alkaline solutions [3,4]. The value for the standard molar free energy of formation (∆Gf0) of HSe- has been reported to be 44.0 kJ/mol [5-7] with a large uncertainty of ±8.0 kJ/mol [5]. This uncertainty in ∆Gf0 introduces uncertainty of ±1.4 orders of magnitude in solubility estimations. This large uncertainty results from indirect determination of ∆Gf0 of HSe-. Since the equilibrium constant for the reaction Se(cr) + H+ + 2e- = HSe-
(1)
is critical in estimating the solubility of selenium compounds in aqueous solutions, it should be determined directly through solubility experiments of metallic selenium. The equilibrium constant of the reaction
nSe(cr) + 2e- = Sen2-
(2)
has not been determined directly. The equilibrium constant for the reaction (2) had been calculated [3] from the standard molar free energy of formation of Se2- [6] and the equilibrium constants determined for the stepwise formation of polyselenide anions, Sen2- (n = 1-4) [3]. Since the equilibrium constant for reaction (2) is critical in estimating the solubility of selenium compounds in alkaline water, e.g., water in contact with leachable concrete or cement, it should be directly determined through solubility experiments. We have performed solubility experiments of metallic selenium in aqueous solutions under anoxic conditions and pH between 5 and 13, to obtain equilibrium dissolution con
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