Selenide [Se(–II)] retention and mobilization in anoxic, iron-rich environments: Impacts of iron phase precipitation and transformation

Abstract:

The radionuclide Selenium-79 (Se-79) is predicted to be a key contributor to the long-term radiologic hazards associated with geological high-level waste (HLW) repositories. Se exists predominantly as selenide [Se(–II)] under anoxic conditions in the deep subsurface, and is likely immobilized via precipitation with, or adsorption on, Fe(II) minerals. However, the initial immobilization mechanisms with metastable Fe(II) minerals, and Se(–II)’s subsequent response to iron phase transformation in the long term remain poorly understood. In this work, we investigated the retention and mobilization behavior of Se(–II) as a function of pH and the mode of its initial interaction with aqueous or solid Fe(II) phases.

We carried out batch precipitation and transformation experiments under N₂ atmosphere and reducing conditions. We examined two cases: in the first, Se(–II) was reacted with aqueous Fe(II), while in the second, Fe(OH)₂ was first precipitated before reaction with Se(–II); both experiments were carried out at pH 8 and 12. In both cases, Se(–II) introduction resulted in the immediate precipitation of black particles. EXAFS and TEM characterization showed that Se(–II) precipitated as iron selenide nanoparticles in both cases regardless of pH. Aging of these initial precipitates at 90°C resulted in magnetite crystallization. Regardless of whether Se(–II) was reacted with aqueous Fe(II) or solid phases, it was completely retained as discrete, crystalline FeSe₂ at pH 8. At pH 12, Se(–II) was mostly remobilized; however, we observed evidence of partial retention via the precipitation of nanocrystalline iron selenide (Fe₃Se₄ or Fe₇Se₈) on the surface of magnetite, as well as by incorporation in defects on the edges of magnetite crystals. These results show that pH controls long-term Se(–II) behavior and that magnetite crystallization may play a role in the retention of Se(–II), particularly at high pH.

*This work was part of “The project for validating near-field assessment methodology in geological disposal (FY2019)” supported by the Ministry of Economy, Trade and Industry of Japan.