Diploma & Master Thesis
Sorption of Selen onto Maghemite
Production of electricity by nuclear power plants inevitably generates radioactive waste. Concerning the high-level and long-lived radioactive waste, a solution considered by several nuclear waste management agencies is to store them into deep underground repositories. The principle of such a concept is to provide a multi-barrier system to avoid the release of the radioactive waste through the biosphere for very long time scales (up to hundred thousand of years).
Long term safety assessments have shown that selenium-79, a fission product being very radiotoxic and with a very long half-life (1.1 x 106 years) could be one of the major isotopes contributing to the overall radioactivity potentially reaching the biosphere. Selenium has a quite complex speciation, with four main oxidation states, depending on both the pH and the redox potential of the surrounding environment. It is thus of great importance to be able to characterize both at a macroscopic and a microscopic level the different processes (retention, reduction, surface precipitation, ...) that can take place onto mineral surfaces and thus affect the availability and the mobility of the selenium species. This information can be inserted in surface complexation models for the description and prediction of selenium interaction at the solid/liquid interface with several sorbent surfaces.
This study focuses on selenium(VI) and selenium(IV) sorption onto maghemite, which was not thoroughly characterized yet. Maghemite was chosen because it’s a ubiquitous iron oxide mineral present in the environment. In addition, maghemite has been identified as a corrosion product of steel waste canisters and iron archaeological analogues, which are used to understand and validate the long term corrosion processes/mechanisms of iron in soils.
The maghemite will be characterized using several analytical methods: X-ray diffraction, Mössbauer and FT-IR. The specific surface area will be determined using a Multi-point Beckman Coulter surface analyzer SA 3100.
The selenium(VI) and selenium(IV) stocks solutions and the background electrolyte for the sorption experiments will be prepared in a glove-box under N2 atmosphere.
The Batch sorption experiments of selenium(VI) and selenium(IV) includes time-dependence experiments, pH and ionic strength effects.
To elucidate the binding behavior and the local coordination of both selenium(VI) and selenium(IV) oxyanions onto maghemite, at a molecular scale zeta potential measurements, Attenuated Total Reflection Fourier Transform-Infrared, as well as EXAFS will be used.
The concentration of Se left in the supernatants will be measured by ICP-MS, while the oxidation state of Se will be determined using a continuous flow hydride generator atomic absorption spectrometer.