Radioactive selenium: origin and environmental dispersion scenarios

Radionuclides can be present in the environment from both natural and anthropogenic origins, showing characteristic biogeochemical behaviours according to the specific properties of the element. The environmental mobility and bioavailability of selenium (Se) strongly depends on the chemical species which, in turn, depends on aspects like redox state and microbiology. Among the most common oxidation states, species of Se(IV) and Se(VI) are considered relatively mobile and bioavailable. Once incorporated within an organism, Se shows a narrow band between dietary deficiency (e.g., used as a co-factor in functional proteins and RNA) and toxicity (e.g., selenosis, dependent on the concentrations and the chemical species involved; Jeffery et al., 2002). The recommended daily intake for adult humans is limited to 1 μg kg−1 of body weight, with a maximum allowable concentration in drinking water of 10 μg L−1 (WHO [World Health Organization], 2011). In addition, Se has no essential metabolism in plants but it is still readily taken up and accumulated due to its structural similarity with other oxyanion forms of bio-essential elements like sulfate and phosphate (Pilon-Smits et al., 2017).