Passive Treatment of Coal-mine Drainage by a Sulfate-reducing Bioreactor: A Case Study from the Illinois Coal Basin, USA

摘要

The purpose of the study was to identify and monitor the geochemical and biogeochemical mechanisms that control the attenuation of coal-mine drainage during the first four years of treatment by an anaerobic sulfate-reducing bioreactor and associated oxidation ponds. This information is needed to investigate bioreactor design parameters, performance and longevity. The acid-mine drainage (AMD) at the case example site, the Tab-Simco Mine, had been highly acidic with an average pH of 2.9, a net acidity of 1,665 mg/L calcium carbonate equivalent-CCE, and high levels of dissolved SO42-, Al, Fe, and Mn. The severity of the AMD at this site may be considered as beyond the treatment capabilities of conventional passive treatment techniques. Nevertheless, the treatment system has increased the pH of the acid mine drainage (AMD) to ~6.0 and decreased the median acidity from 1,647 to 64.6 mg/L CCE, SO_4~(2-) from 2,995 to 1,691 mg/L, Fe from 443.3 to 4.6 mg/1, Al from 116.5 to 0.69 mg/L, and Mn from 36.5 to 23.5 mg/L. Geochemical modeling indicates that the bioreactor discharge is saturated with respect to the minerals alunite, gibbsite, siderite, rhodochrosite, jarosite, and iron hydroxide precipitates. The δ~(34)S value of SO_4~(2-) increased in the bioreactor from an average of +6.9%o to +9.2%o, suggesting the presence of bacterial sulfate reduction. Preliminary results of a bacterial community analysis show that DNA sequences corresponding to bacteria capable of sulfate reduction were present in the bioreactor outflow discharge, but were outnumbered by sequences similar to bacteria capable of re-oxidizing reduced sulfur species. The temporal trends include seasonal variation in SO_4~(2-) and metal concentration and a general decline in the amount of alkalinity. Recently, these declines have reduced the ability of the receiving stream to buffer the system discharge during dry seasons. Overall, this study illustrates the dynamic biogeochemical nature of the passive treatment systems.