Acid Mine Drainage (AMD) is the outflow of acidic water from mine waste usually containing toxic heavy metals and metalloids. This occurs through the process of sulfidic ores being exposed to the atmosphere or water, causing the sulfides to oxidize into sulfuric acid.
I am Jillian, SULTAN Early Stage Researcher 14, and my research is focused on the environmental and health impacts of sulfidic mine wastes from the tailings to the potential downstream products (i.e. ceramics, geopolymers, and green cement from valorized tailings). Given my background in environmental engineering, it is only natural that I would like to talk even more about the environmental aspects of mining.
When it comes to the environmental impacts of mine waste, especially sulfidic mine waste, an ongoing issue is acid mine drainage (AMD). AMD is the outflow of acidic water from mine waste usually containing toxic heavy metals and metalloids. This occurs through the process of sulfidic ores being exposed to the atmosphere or water, causing the sulfides to oxidize into sulfuric acid. As a result, metals and other toxins can then be easily dissolved into the acidic water, posing a threat to the environment, including humans and wildlife. I was astonished to hear that approximately 20,000 km of streams and rivers in the United States are degraded by AMD.
While the colors may be appealing if you encounter a stream containing AMD on a walk through nature, rest assured, you do not want to go swimming there.
Figure 1. Examples of AMD near mine waste sites2
There are three main steps to limit AMD: prevention, control, and treatment. There are many different prevention and treatment methods available, but can often be impractical or costly to implement 3 and the precise method differs per situation. In the ideal case, the waste would be reprocessed and valorized, such as in the SULTAN project. However, if not all of the waste can be valorized and some waste still remains exposed to environmental conditions, a preventative method should be applied instead. In the case of some underground mines, it may be difficult to remove the mine waste and in this case, a possible preventative method is inundation (also referred to as flooding).
In order to prevent AMD from forming, the oxygen and/or water in contact with the minerals needs to be minimized or completely depleted. This is where the flooding method plays a role. The abandoned underground mine is flooded and sometimes sealed in order to deplete the oxygen in contact with the sulfide ores. Any oxygen that is then left will be consumed by the microorganisms that are present and no excess oxygen should be able to enter the system.
In the case where AMD is already forming in an underground mine, flooding and sealing can be coupled with an active or passive method such as an aerobic wetland system or adjusting the pH by adding an alkaline substance (i.e. lime) to treat the already contaminated outflow.
The flooding method has been applied in multiple cases, for example in the Lusatia region of East Germany, a former coal mining industry was completely transformed into a habitable lake district (See article 4). The former mines were flooded to create artificial lakes and the polluted water was treated. As a result, the aquatic life has returned and the area has become a popular destination for people too 4. The project involved creating 140 sq km of lakes throughout the region as well as keeping the history alive by also providing tours about the industrial heritage4.
Figure 2. Aerial view of the Lusatian Lakeland, a former industrial mining area5
While flooding is a commonly used method and there are some seemingly successful implementation stories like in the Lusatia region, there are still some drawbacks. One major limitation is the availability of surface water, such as a river, to be diverted or dammed in order to flood the closed mine site 6. It is also crucial for this to be a non-oxygen-containing water source. Therefore, the flooding method is not ideal or even possible in every situation.
In addition to the flooding method, there are also numerous other remediation methods for AMD, both abiotic or biotic. Adding an alkalizing agent, such as lime, as I mentioned previously, is a commonly used standalone active abiotic method. Most treatment methods can be categorized as either active or passive. Active treatment methods typically require equipment, regular operation, and maintenance, continuous dosing of chemicals and/or power but are more reliable than passive systems7; whereas passive treatment methods do not require continuous chemical inputs and take advantage of naturally-occurring chemical and biological processes to treat contaminated mine waters1. In general, active technologies require more maintenance and have more recurring costs than passive technologies. Other examples of passive technologies consist of constructed wetland ecosystems, in which plants and microbes play a big role too. I encourage you to refer to Tamara (ESR 8)’s blog on biomining to learn more about the microbial aspect that can occur naturally to remove metals.
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References and sources:
1. Skousen, J. G., Sextone, A. & Ziemkiewicz, P. F. Acid Mine Drainage Control and Treatment. Am. Soc. Agron. Am. Soc. Surf. Min. Reclamation. (2000). doi:10.2134/agronmonogr41.c6
2. Acid Mine Drainage Being Managed, Says IMC. Environment News South Africa (2013). Available at: www.environment.co.za/acid-mine-drainage-amd/acid-mine-drainage-managed-says-imc.html.
3. Moodley, I., Sheridan, C. M., Kappelmeyer, U. & Akcil, A. Environmentally sustainable acid mine drainage remediation: Research developments with a focus on waste/by-products. Miner. Eng. 126, 207–220 (2018).
4. Mellgard, P. Life after Lignite: How Lusatia Has Returned to Nature. The Guardian News and Media (2014). Available at: www.theguardian.com/environment/2014/sep/10/lusatia-lignite-mining-germany-lake-district.
5. Sullivan, P. East Germany’s Old Mines Transformed into New Lake District. The Guardian News and Media (2016). Available at: www.theguardian.com/travel/2016/sep/17/lusatian-lake-district-project-east-germany.
6. Johnson, D. B. & Hallberg, K. B. Acid mine drainage remediation options: A review. Sci. Total Environ. 338, 3–14 (2005).
7. Trumm, D. Selection of active and passive treatment systems for AMD—flow charts for New Zealand conditions. New Zeal. J. Geol. Geophys. 53, 195–210 (2010).