Chemical speciation determines Se solubility and therefore its bioavailability and potential for transport in the environment. In this study we investigated the speciation of Se in soil developed on reclaimed mine sites in the U.S. Western Phosphate Resource Area (WPRA) using micro-X-ray absorption near-edge structure (micro-XANES) spectroscopy and micro-X-ray fluorescence (micro-XRF) mapping. Selenium was nonuniformly distributed in the soils and positively correlated with Fe, Mn, Cu, Zn, and Ni. Sixteen points of interest (POI) from three soil samples were analyzed with micro-XANES spectroscopy. The XANES data indicated that Se is present in the soils in at least three oxidation states, Se(-II, 0), Se(IV), and Se(VI). Selenides or elemental Se dominated 7 of the 16 POI. Selenate was the dominant species at only one of the POI. The remaining eight POI were composed of both Se(IV) and Se(VI), with minor Se(-II, 0) contributions. The results of this research suggest that the reduced Se species in the soil parent material are oxidizing to Se(VI), one of the more mobile species of Se in the environment. This information can be used to better predict and manage Se availability in soils.

Mining activities in the US Western Phosphate Resource Area ͑WPRA͒ have released Se into the environment. Selenium has several different oxidation states and species, each having varying degrees of solubility, reactivity, and bioavailability. In this study we are investigating the speciation of Se in mine-waste rocks. Selenium speciation was determined using bulk and micro-x-ray absorption spectroscopy ͑XAS͒, as well as micro-x-ray fluorescence mapping. Rocks used for bulk-XAS were ground into fine powders. Shale used for micro-XAS was broken along depositional planes to expose unweathered surfaces. The near edge region of the XAS spectra ͑XANES͒ for the bulk rock samples revealed multiple oxidation states, with peaks indicative of Se͑ϪII͒, Se͑IV͒, and Se͑ϩVI͒ species. Micro-XANES analysis of the shale indicated that three unique Se-bearing species were present. Using the XANES data together with ab initio fitting of the extended x-ray absorption fine structure region of the micro-XAS data ͑micro-EXAFS͒ the three Se-bearing species were identified as dzharkenite, a di-selenide carbon compound, and Se-substituted pyrite. Results from this research will allow for a better understanding of the biogeochemical cycling of Se in the WPRA.

Mining activities in the US Western Phosphate Resource Area ͑WPRA͒ have released Se into the environment. Selenium has several different oxidation states and species, each having varying degrees of solubility, reactivity, and bioavailability. In this study we are investigating the speciation of Se in mine-waste rocks. Selenium speciation was determined using bulk and micro-x-ray absorption spectroscopy ͑XAS͒, as well as micro-x-ray fluorescence mapping. Rocks used for bulk-XAS were ground into fine powders. Shale used for micro-XAS was broken along depositional planes to expose unweathered surfaces. The near edge region of the XAS spectra ͑XANES͒ for the bulk rock samples revealed multiple oxidation states, with peaks indicative of Se͑ϪII͒, Se͑IV͒, and Se͑ϩVI͒ species. Micro-XANES analysis of the shale indicated that three unique Se-bearing species were present. Using the XANES data together with ab initio fitting of the extended x-ray absorption fine structure region of the micro-XAS data ͑micro-EXAFS͒ the three Se-bearing species were identified as dzharkenite, a di-selenide carbon compound, and Se-substituted pyrite. Results from this research will allow for a better understanding of the biogeochemical cycling of Se in the WPRA.

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.