Selenide retention by mackinawite
N Finck1, K Dardenne, D Bosbach
1Institute for Nuclear Waste Disposal (INE), Karlsruhe Institute of Technology (KIT), Karlsruhe, Germany. nicolas.finck@kit.edu
Environmental Science & Technology
|August 21, 2012
Summary
Selenium-79 (79Se) mobility in nuclear waste disposal is a concern. This study shows selenide (Se(-II)) can be immobilized by coprecipitation and adsorption onto mackinawite (FeS), reducing its environmental risk.
Area of Science:
- Geochemistry
- Environmental Science
- Nuclear Waste Management
Background:
- Selenium-79 (79Se) poses risks in deep geological repositories due to its long half-life and mobility.
- Selenium mobility is oxidation-state dependent; reduced species (Se(0), Se(-II)) are less mobile than oxidized forms (Se(IV), Se(VI)).
- Limited research exists on selenide (Se(-II)) retention by repository barrier materials.
Purpose of the Study:
- To investigate the retention of selenide (Se(-II)) by mackinawite (FeS) through coprecipitation and adsorption.
- To understand the molecular-scale mechanisms of selenide interaction with mackinawite.
- To assess the potential of mackinawite as a barrier for immobilizing selenide in nuclear waste disposal.
Main Methods:
- Coprecipitation and adsorption experiments of selenide with mackinawite (FeS).
- X-ray Diffraction (XRD) and Scanning Electron Microscopy (SEM) for structural and morphological analysis.
- Multi-edge X-ray Absorption Spectroscopy (XAS), including Extended X-ray Absorption Fine Structure (EXAFS), for molecular-level characterization.
Main Results:
- XRD and SEM showed no significant structural or morphological changes in mackinawite due to selenium presence.
- XAS analysis confirmed selenium incorporation into a mackinawite-like sulfide environment during coprecipitation.
- Adsorption experiments indicated selenide interaction with FeS, likely via a dissolution-recrystallization mechanism of mackinawite.
Conclusions:
- Mackinawite (FeS) effectively immobilizes selenide (Se(-II)) through coprecipitation and adsorption.
- Selenium incorporation occurs within a sulfide matrix, suggesting mackinawite's potential as a barrier material.
- This study provides the first experimental evidence of selenide incorporation into iron monosulfide using multi-edge XAS.
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