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Microbial and Geochemical Responses in Acid-Producing Mine Tailings
Figure 3. Tailings columns at MSU’s Center for Biofilm Engineering.
Primary Issue Addressed: Source Control
Secondary Issues Addressed: Biological Treatment, Characterization, and Modeling
Project Site: Research performed at Montana State University using Tailings from Placer Dome’s Golden Sunlight Mine near Whitehall, Montana.
Collaborating Entities: MSE, Montana State University’s Center for Biofilm Engineering and Placer Dome’s Golden Sunlight Mine
Cost Share: In-kind services provided by Golden Sunlight Mine.
Project Description
During the past decade, there have been numerous projects attempting to use sulfate-reducing bacteria (SRB) to remediate mine waste-related problems. While some of these projects have been successful, others have not. This project attempted to shed much-needed light on poorly understood microbial and geochemical mechanisms. Therefore, the project goal was to understand and predict the physical, geochemical, and microbiological changes that result from stimulation of SRB in mine tailings through the addition of organic carbon. Major objectives for this work were to determine:
- optimum organic feedstock and feed rates to stimulate SRB;
- the mineral phases that precipitate as a result of SRB activity; and
- the long-term stability of these mineral phases.
This project built on extensive Mine Waste Technology Program experience using SRB and other microbial techniques to treat acid mine drainage but focused on the microbiological changes and mineral forms that result from microbial action.
The project was divided into three phases. Phase I consisted of a laboratory evaluation of the microbial, geochemical, and mineralogical changes occurring in mine tailings as a result of organic carbon addition. During Phase II, it was planned to evaluate the stability of changes in the tailings after cessation of organic carbon. Phase III was a pilot-scale test to validate the laboratory findings under field-relevant conditions. Phase I was only partially completed.
Status
During Phase I, column tests were used to evaluate the effect of organic
carbon addition on the geochemical, mineral, and microbial properties
of the tailings. These were evaluated with respect to the source of organic
carbon, application dosage, and application frequency.
Phase I laboratory evaluation work began as scheduled in March
2004 with the construction of twelve test columns at Montana State
University (MSU) in Bozeman (Figure 3). The columns were filled
with fresh tailings from the Golden Sunlight Mine (GSM) near Whitehall,
Montana. In accordance with the laboratory work plan, each column
received enough tap water each week to be equivalent to an annual
52 inches of precipitation. The columns were sampled for effluent
pH, effluent oxidation-reduction potential, and subsurface gas
composition on a weekly basis. Effluent microbial composition
and dissolved metals were sampled on a monthly basis, and gas
(O2, CO2) flux was measured weekly.
The experimental strategy outlined in the work plan and quality assurance project plan was to allow acidification of the columns to occur over the first several months of operation and then to institute organic carbon addition to assess microbial and geochemical changes per the project objectives. Unfortunately, acidification of the columns did not occur in 5 months of operation. After adding water to the columns for this entire time, the effluent was still near neutral.
Depending on their pyrite content, it is not uncommon for fresh mine tailings to take quite a while to become acidic. The tailings did not contain much pyrite (as measured by the iron content); but according to previous experience, they should have been producing acidic drainage after 5 months of water addition. Initial analysis of the GSM tailings indicated the presence of both iron and sulfur at concentrations of 5% to 7%. Past column work with highly weathered tailings from a proximal mine site (Mammoth Mine, Cardwell, Montana) containing similar iron (but less sulfur) acidified within 1 month of operation using similar water application rates.
Preliminary microbiological analyses showed large sulfate-reducing populations but no iron oxidizing bacteria (IOB). Previous experience indicated that IOB would be detectable as the tailings started to become acidic. Increasing populations of IOB would then accelerate acid production.
Because the columns were not producing acid by September 2004, other alternatives were evaluated, including initiating organic carbon treatment prior to acid generation. It was thought that the objectives of this experiment, i.e., establishing the response of various populations to organic carbon treatment and assessing mineralogical changes, could have been accomplished with non acidic columns. The long-term performance of control columns receiving no organic carbon would have been compared to those that received treatment. Treatment before acid generation would have more closely emulated field situations where organic carbon is applied near the time of tailings deposition (before acid generation commences).
However, since the initial project premise was treatment of acid-generating tailings, it was decided to continue with the original plan. Since the project was behind schedule, MSE Technology Applications, Inc., and MSU recommended to EPA that the columns be “jump started” by adding acidic effluent from existing Fox Lake tailings columns that had been producing acidic water with a healthy population of IOB. This would have added about 2 months to the schedule without sacrificing the scientific integrity of the project.
Unfortunately, EPA canceled the project in late 2004. Therefore, organic carbon amendments were never added, and the project was not finished. Hopefully, future researchers can find other ways to shed light on microbial and geochemical mechanisms of SRB-treated tailings that are currently poorly understood.