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Paper No. 8
Presentation Time: 10:10 AM


ANDRE, Benjamin J.1, RAJARAM, Harihar2 and SILVERSTEIN, Joann2, (1)Earth Sciences Division, Lawrence Berkeley National Laboratory, 1 Cyclotron Rd, MS 90-1116, Berkeley, CA 94720, (2)Department of Civil, Environmental, and Architectural Engineering, University of Colorado at Boulder, Engineering Center ECOT 441, UCB 428, Boulder, CO 80309,

Acid rock drainage, ARD, results from the oxidation of metal sulfide minerals (e.g. pyrite), producing ferrous iron and sulfuric acid. Acidophilic autotrophic bacteria such as Acidithiobacillus ferrooxidans and Leptospirillum ferrooxidans obtain energy by oxidizing ferrous iron back to ferric iron, using oxygen as the electron acceptor. Despite being extensively studied for the last thirty years, there is still not a consensus in the literature about the basic mechanisms, limiting factors or rate expressions for microbially enhanced oxidation of metal sulfides. Most existing models of ARD do not account for microbial kinetics or iron geochemistry rigorously. Instead they assume that oxygen limitation controls pyrite oxidation and thus focus on oxygen transport. These models have been successfully used for simulating conditions where oxygen availability is a limiting factor (e.g. source prevention by capping), but have not been shown to effectively model acid generation and effluent chemistry under a wider range of conditions.

An indirect leaching mechanism (chemical oxidation of pyrite by ferric iron to produce ferrous iron, with regeneration of ferric iron by microbial oxidation of ferrous iron) is used as the foundation of a conceptual model for microbially enhanced oxidation of pyrite. Using literature data, a rate expression for microbial consumption of ferrous iron is developed that accounts for oxygen, ferrous iron and pH limitation. Reaction rate expressions for oxidation of pyrite and chemical oxidation of ferrous iron are selected from the literature. A completely mixed stirred tank reactor (CSTR) model is implemented coupling the kinetic rate expressions, speciation calculations and flow. The model simulates generation of ARD and effluent chemistry that qualitatively agrees with column reactor and single rock experiments. A one dimensional reaction diffusion model at the scale of a single rock is developed incorporating the proposed kinetic rate expressions. Simulations of initiation, washout and ARD flows are discussed to gain a better understanding of the role of porosity, effective diffusivity and reactive surface area in generating ARD. Simulations indicate that flow boundary conditions control generation of acid rock drainage as porosity increases.

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