The concentration range between selenium essentiality and potential toxicity is narrow, ranging from 0.04 mg/kg to 4 mg/kg (Wang and Chen 2003). The essential or toxic nature of selenium depends not only on the amount of selenium found in soils but the species of occurrence. Four species of selenium, selenide (Se^2-), elemental Se (Se⁰), selenite (SeO₃^2-), and selenate (SeO₄^2-) exist in soil and geological materials. The actual species present directly affects absorption and bioavailability. The concentration of selenium depends on soil characteristics such as: pH, chemical and mineralogical composition, adsorbing surface, and oxidation-reduction status (Wang and Chen 2003). Equilibrium thermodynamics predict that in moderately reducing conditions the dominant species found in the system would be selenite. Under reducing conditions selenium would become less mobile, with the soluble phase abundance being controlled by an iron selenide phase (Masscheleyn et al 1990). The nature of selenium in the industrially impacted soils in the Nickel Belt of Northern Ontario is the focus of this study. These soils have an acidic pH and contain high concentrations of both natural and smelter emitted iron oxides in the surface horizons. Industrial activities have also led to the enrichment of airfall dusts containing significant amounts of trace elements such as Ni, Cu, As, Mn, Co, Pb, and Z. Although the regional soils are naturally selenium deficient, they have, within a radius of 150 km, become enriched with selenium from mining and smelting activities. An incubation experiment was conducted to determine the potential release of selenium and other associated metals to the matrix solution during the spring thaw period. Samples from seven contaminated sites were incubated for 50 days with redox potential and pH measurements taken at 20 intervals in this time period. Samples were then centrifuged and the supernatant was analyzed by ICP-MS for a series of environmentally and nutritionally important elements. The redox potential decreased for samples from across all sites as the incubation time increased. Although the pH change was not significant over this incubation period the release of selenium into soil solution also increased. Available thermodynamic data indicate that the dominant selenium species in the organic horizons would be selenite or selenide. However, in this study, the correlation (r² value of 0.90 or above) between the release of selenium and iron into solution in the organic horizons of these soils indicates that a major mechanism controlling the release of selenium into the soil solution is the association of the selenium oxyanions with iron oxyhydroxides. Ongoing research is focused on determining the species of selenium present, and on deciphering the mineral phase association.

References: Masscheleyn, P.H., Delaune, R.D. and Patrick, W.H. Jr. (1990). Transformations of selenium as affected by sediment oxidation-reduction potential and pH. Environmental Science and Technology, 24(1): 91-96. Wang, M.C. and H.M. Chen. 2003. Forms and distribution of selenium at different depths and among particle size fractions of three Taiwan soils. Chemosphere, 52:585-593.