Solubility Measurement of Iron-Selenium Compounds under Reducing Conditions
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6ROXELOLW\0HDVXUHPHQWRI,URQ6HOHQLXP&RPSRXQGVXQGHU5HGXFLQJ&RQGLWLRQV Akira Kitamura1, Masahiro Shibata1 and Hideo Kitao2 1 Japan Nuclear Cycle Development Institute, Tokai, Naka, Ibaraki 319-1194, Japan E-mail: [email protected] 2 Nuclear Development Corporation, Tokai, Naka, Ibaraki 319-1111, Japan $%675$&7 Chemical behavior of selenium (Se), which was one of the important elements for performance assessment of geological disposal of high-level radioactive waste, was investigated under reducing and iron-containing conditions. A washing method for an iron diselenide (FeSe2(cr)) reagent with acidic and basic solutions was carried out for the purification of FeSe2(cr) reagent, which was considered to be a solubility limiting solid for Se under the geological disposal conditions. Furthermore, solubility of FeSe2(cr) was measured in alkaline solutions under reducing conditions, and thermodynamic data on equilibrium reactions between Se in solution and Se precipitate were obtained. It was found that the dependencies of solubility values on pH and redox potential (Eh: vs. standard hydrogen electrode) were best interpreted assuming that the solubility limiting solid was not FeSe2(cr) but Se(cr) and the aqueous species was SeO32- in the present experimental conditions. The equilibrium constant between Se(cr) and SeO32- at zero ionic strength was determined and compared with literature values. The chemical behavior of Se under geological disposal conditions was discussed. ,1752'8&7,21 Selenium-79 (Se-79), which is one of the fission products with a half life of 1.13¯106 years [1], is one of the most important radionuclides for safety assessment of a geological disposal of high-level radioactive waste (HLW). Aqueous species of Se are usually anionic species such as selenide ion (Se2-), selenite ion (SeO32-) and selenate ion (SeO42-). In the engineered and natural barrier systems, sorption of these anionic species onto bentonite and rocks is expected to be very weak. Thus, solubility of Se is directly affecting to the safety assessment of the HLW disposal system, in which simulated groundwaters are a weak alkaline solution under reducing conditions, with a plenty of divalent iron (Fe(II)) from overpack. Thus, formation of iron-selenium compounds (FexSey) will be pointed out. Tachikawa et al. investigated the formation of iron-selenium compounds by an oversaturation method [2]. It was observed that the precipitate of Se(cr) was transformed to FeSe(cr) and FeSe2(cr) by X-ray diffractometry. Based on this result, it is expected that the solubility of Se is limited by a precipitate of iron diselenide (FeSe2(cr)), and that the aqueous species is hydrogenselenide ion (HSe-) in the HLW disposal system. However, the mechanism of the transformation from Se(cr) to FeSe(cr) and FeSe2(cr) has not been elucidated, and no thermodynamic data on Fe-Se compounds have been obtained by Tachikawa et al. Furthermore, the reliability of the thermodynamic data on Se, which usually have been obtained from pyrochemical methods [3], is
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