Biogeochemistry of Selenium on Remediated Phosphate-Mine Tailings in Southeastern Idaho

Abstract

Abstract. Selenium (Se) occurs in a wide variety of oxidation states and is a common element present in sedimentary geologic materials such as shale. Phosphate mining activities in southeastern Idaho have left shale materials near the surface, thus releasing the natural Se into a new weathering environment. In this study we are investigating the biogeochemistry of Se at remediated phosphate mining sites. Soil samples were collected from the rhizosphere of remediated sites and analyzed using microscopically focused X-ray absorption fine structure (XAFS) spectroscopy. A greenhouse study to measure the impacts of sulfate and manure amendments on plant bioavailability is also discussed in this paper. XAFS spectroscopy results provide knowledge of mineralogy and Se-oxidation state in the soils. XAFS spectroscopy revealed that Se exists in several different reduced forms in the parent shale materials, including elemental Se and ferroselite-type minerals. In weathered soils, Se was present as Se(IV) and reduced Se minerals. All of the Se(IV) species were associated with iron, likely goethite. Soil pore water analysis revealed that the soil solution contained both Se(IV) and Se(VI), the most soluble and plant-available forms. We hypothesize that the reduced Se in the shale-parent materials is weathering to Se(IV)-goethite type minerals, which further oxidize to Se(VI) in the rhizosphere and is taken up into the plant foliage. The mechanism that rhizosphere oxidation occurs is unknown, and may be biotic or abiotic. Results from this study provide insight into the weathering mechanism by which Se is made plant available, and will help in developing improved management strategies that will reduce Se exposure to animals.