SULFUR ISOTOPIC EVALUATION OF A SULFATE-REDUCING BIOREACTOR TREATING ACID MINE DRAINAGE

Sulfate-reducing bioreactor cells (SRBCs) hold promise as a cost-effective option in the treatment of acid mine drainage (AMD). They are designed to promote bacterial sulfate reduction by providing the needed conditions for the bacteria to thrive. Laboratory and small-scale field studies have shown that these systems are capable of simultaneously removing sulfate and sequestering metal sulfides while generating excess alkalinity. However, there have been few studies of these systems in large-scale field settings that go beyond monitoring the inflow and outflow chemistry. Here we provide preliminary data on the internal workings of a large (2,100 m³) SRBC built to treat a low-flow AMD seep in Indiana.

Beginning in January, 2009 samples were collected from the inflow and outflow of the system as well as from an array of monitoring ports located at varying depths inside the cell. The internal sampling ports were placed in such a way as to observe 3D trends in activity occurring within the system. The AMD flowing into the SRBC has [SO₄] = 2840 mg/L with a δ³⁴S_VCDT of -6.55‰. By the time the water leaves the system, [SO₄] = 940 mg/L with a δ³⁴S_VCDT of -4.48‰, indicating that the dominate sulfate removal mechanism is unlikely to be bacterial sulfate reduction. At the same time, samples collected from the deepest portion of cell have δ³⁴S_VCDT of up to +35.71‰ for the residual sulfate ([SO₄] = 350 mg/L), signifying that bacterial sulfate reduction is occurring. Other sections, particularly near where the AMD enters the SRBC ([SO₄] = 1390 mg/L; δ³⁴S_VCDT -5.21‰), near the surface ([SO₄] = 1350 mg/L; δ³⁴S_VCDT -6.34‰), and along the perimeter ([SO₄] = 1600 mg/L; δ³⁴S_VCDT -2.87‰) have significantly lighter δ³⁴S_VCDT values. These findings suggest that while bacterial sulfate reduction is taking place (as evidenced by a fractionation of up to +42‰), it’s not occurring in a uniform manner throughout the system. The observed range in fractionation values may reflect an influence by gypsum precipitation or the development of preferential flow within the system. The information presented in this study highlights observations not typically attainable in smaller laboratory scale studies that need to be accounted for in future installations of this technology.