Oxidation and Mobilization of Selenium by Rhizosphere Processes and Chemicals.
Libbie Oram1, Daniel G. Strawn1, Greg Moller1, Jodi L. Johnson-Maynard1, Mathew Marcus2, and Sirine Fakra2. (1) Univ of Idaho, Soil and Land Resources Division, PO Box 442339 Ag Sci. 242, Moscow, ID 83844-2339, (2) Lawrence Berkeley National Laboratory, 1 Cyclotron Rd, Berkeley, CA 94720
Mining activities in the Western Phosphate Resource Area (WPRA) have moved selenium (Se) from a relatively stable setting to a much more weathered and dynamic environment. This has resulted in oxidation and subsequent mobilization of selenium, thereby causing elevated levels in soils, plants, and water in the region. However, Se bioavailability and toxicity are dependent on its speciation and complex biogeochemistry. Previous XAFS analysis indicated that Se in soils was primarily present as Se(0,II-) or Se(IV) with some Se(VI) present, while the plant Se is primarily organic selenides or Se(VI), suggesting that rhizosphere processes are oxidizing and mobilizing selenium from the mineral phase to Se(VI). Nitrates and organic acids are active soil and rhizosphere substances. Thermodynamic calculations for the oxidation of Se by nitrate and previous studies suggest that nitrate can oxidize selenium. Plant-exuded organic acids could increase available selenium in the rhizosphere via competition for sorption sites and by dissolution of oxide surfaces. The goal of this research is to investigate the mobilization of selenium from immobile soil selenium to oxidized, plant-available species that impact the terrestrial food chain. Experiments to achieve this goal include (1) greenhouse studies to further investigate the speciation of Se in rhizosphere soil and roots of plants grown in Se-contaminated soil and (2) batch and stirred-cell reactor studies to investigate the plant bioavailability of selenium as influenced by desorption and redox reactions. Se(IV) adsorbed onto goethite is an immobile phase of Se widespread in the WPRA and is the species used in these experiments. X-ray Absorption Spectroscopy (XAS) was used to determine the speciation of selenium in rhizosphere soil and in plant roots and to determine the influence of ligands, such as nitrates and organic acids, on the speciation and bioavailability of selenium. In the greenhouse studies, Se(VI) was the predominant species detected in rhizosphere soil, plant roots, and control soils after approximately eight weeks of watering, suggesting that selenite can be readily oxidized and mobilized in both rhizosphere soil and in bulk soil. The influences of nitrogen species on oxidation and organic acids on desorption of selenite adsorbed onto goethite and the interaction of these processes are under current investigation. Further, comparisons of plant-available selenium before and after the influence of nitrate and organic ligands will provide insight into the effects of important reactions that may occur in rhizosphere soil. Results from this research will allow for a better understanding of the biogeochemical cycling and environmental fate of selenium, and therefore, better environmental management techniques and decreased risks to habitat in the WPRA.