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Microbial Contributions to Soluble and Volatile Arsenic Dynamics in Retorted Oil Shale¹

ABSTRACT

Arsenic volatilization and solubilization from samples of Paraho and Lurgi retorted shale were evaluated with and without added soybean meal [Glycine max (L.) Merr.], dimethylarsinic acid (DMAA), methanearsonic acid (MAA), and sodium arsenate (SA) in 12-week experiments. Using a Paraho process shale from an experimental reclamation site in western Colorado, As was only volatilized when nutrients were present, using 23°C incubation. Organic As forms added at 267 x 10^–6 mol As kg^–1 shale tended to remain more in solution, and were volatilized to a greater extent (45.7 and 6.4 x 10^–6 mol kg^–1 shale as DMAA and MAA, respectively) than observed with less soluble SA or innate As. Soluble As dynamics were also influenced by nutrient availability. With added DMAA, soluble As levels decreased over the course of the experiment, and decreases followed by rerelease were observed with MAA and SA. The low initial levels of soluble innate As increased slightly over the course of the experiment. In contrast, without nutrients present, other than with added DMAA, the soluble As levels decreased. Incubation at 45°C resulted in a marked reduction of As mobilization, and after heat sterilization, no As mobilization could be observed. Addition of an unsterilized shale inoculum led to a reestablishment of these As mobilization processes in the sterilized samples. In addition, As mobilization was inhibited under anaerobic conditions. With a sample of Lurgi process shale, mobilization in the presence of nutrients was observed, but As volatilization could not be detected, suggesting that variations in oil shale sources and retorting may influence microbial As mobilization. These results suggest that measurable amounts of As can be mobilized from some retorted shale samples, if nutrients are available and environmental conditions allow these processes to occur.

Key Words: dimethylarsinic acid • methanearsonic acid • sodium arsenate • reclamation • mobilization • trace metals

¹ Contribution of the Dep. of Microbiology, Colorado State Univ., Fort Collins, CO 80523, and the Univ. of Colorado at Denver, Denver, CO 80202.

² Formerly Research Assistant, Dep. of Microbiology, Colorado State Univ., Fort Collins, now Research Scientist, Synergen, Boulder, CO 80301; Professor, Dep. of Microbiology, Colorado State Univ., Fort Collins; and Analytical Laboratory Director, Univ. of Colorado, Denver, respectively.