Remediation of coal-mine drainage by a sulfate-reducing bioreactor: A case study from the Illinois coal basin,USA

This study reports changes in coal-mine drainage constituent concentrations through an anaerobic SO₄-reducing bioreactor monitored over a 3-a period. The purpose of the study was to identify and monitor over time the biogeochemical mechanisms that control the attenuation of toxic compounds in the mine drainage. This information is needed to investigate bioreactor performance and longevity. The water treated at the case example site, the Tab-Simco Mine, was highly acidic with an average pH of 2.9, a net acidity of 1674 mg/L CaCO₃ equivalent-CCE, and high levels of dissolved ${\text{SO}}_4^{2-}$, Al, Fe and Mn. The results of this study indicated that the treatment system increased the pH of the acid mine drainage (AMD) to 6.2 and decreased the median acidity to 22.7 mg/L CCE, ${\text{SO}}_4^{2-}$ from 2981 to 1750 mg/L, Fe from 450.6 to 1.76 mg/L, Al from 113 to 0.42 mg/L, and Mn from 36.4 to 23.3 mg/L. Geochemical modeling indicates that the bioreactor discharge is saturated with respect to the minerals alunite, gibbsite, siderite, rhodochrosite, jarosite, and Fe hydroxide precipitates. The observed trends also include seasonal variations in ${\text{SO}}_4^{2-}$ reduction and a general decline in the amount of alkalinity produced. The average δ³⁴S value of the ${\text{SO}}_4^{2-}$ in the untreated AMD was +7.3‰. In the bioreactor, δ³⁴S value of ${\text{SO}}_4^{2-}$ increased from an average of +6.9‰ to +9.2‰, suggesting the presence of bacterial SO₄ reduction processes. Preliminary results of a bacterial community analysis show that DNA sequences corresponding to bacteria capable of SO₄ reduction were present in the bioreactor outflow sample. However, these sequences were outnumbered by sequences similar to bacteria capable of reoxdizing reduced sulfur species. This study illustrates the dynamic nature of metal removal in SO₄-reducing bioreactor-based treatment systems.