Redox reactivity of Selenium in environmental geomedia

Due to its long half-life of 2.95 × 105 a [1] and high activity, 79Se is an important radionuclide to monitor for a correct assessment of the safety of nuclear waste repositories. Selenium is present in five oxidation states in nature ranging from −II, −I, 0, +IV and +VI. Selenium solubility is controlled by its oxidation state, hence depends on the redox conditions present in soils, sediments, and aquifers. The −II, −I and 0 oxidation states are commonly predominant in “reducing” anoxic environments, while the +IV and +VI states predominate in “oxidizing” environments [2]. Selenate [Se(VI)], the most oxidized form of Se, is an anion with high mobility in the pore waters of the clayey formations that will host the French repository for high-activity and long-life nuclear wastes. It is therefore important to evaluate the possible redox reactions that could lead to a decrease of its mobility in the environmental conditions of the repository. It is known that pyrite and siderite, two Fe(I