Mining operations worldwide serve as the foundation for accessing essential resources vital to global development. Critical minerals play an imperative role in advancing the ongoing energy transition and achieving global climate goals. However, it is essential to recognise that mining activities can come with inherent and significant environmental impacts, prompting the need for sustainable improvements.Recently highlighted by the United Nations as the second biggest global environmental threat, second only to climate change, is that of acid mine drainage.

What is acid mine drainage waste?

Acid Mine Drainage (AMD) occurs when the tailings and overburden waste (the soil, rock and plant life that needs to be removed to reach the targeted mining material), become exposed to air and water. This typically occurs where sulphide minerals are present and the exposure triggers a chemical reaction known as oxidation, ultimately leading to the production of acid. This acid causes the dissolution of heavy metals contained in mined materials, resulting in the formation of an acidic solution with elevated levels of contaminants such as iron, manganese, aluminium, lead, copper, zinc, arsenic, cobalt and cadmium, amongst others.

What makes acid mine drainage so harmful is that its generation can occur indefinitely, years after the mining activity has ceased. The severity of acid mine drainage can be determined by natural factors such as the size of the extracted material volume, the exposed surface area, and the concentration of sulphide minerals contained in mine waste. This acidic solution infiltrates the ground, making it one of the largest pollutants of surface and groundwater with significant environmental impacts.

The challenge of acid mine drainage waste

Acid mine drainage poses a significant challenge to water bodies worldwide. In the United States, for instance, more than 22,000 kilometres of streams and 180,000 acres of freshwater reservoirs have been affected by acidic contamination. The Rio Tinto in Spain, for example, has waters maintaining a pH of around 2.5, rendering the environment extinct of both plants and aquatic organisms. In South Africa, the legacy of coal and gold mining spanning over a century has had profound effects not only on plants, aquatic life but human health. Farmers in particular have suffered losses, with reports of livestock deaths and illnesses among communities who inadvertently consumed contaminated acidic water.

Can acid mine drainage waste be treated?

Thankfully, there are various technologies available to clean up contaminated areas. These technologies are generally divided into two categories ‘passive’ and ‘active’, both of which try to lower the concentrations of harmful metals and raise pH levels above acidic to ultimately improve the ecological viability of the area.

In passive remediation approaches, physical, chemical, and biological mechanisms can all be used to lessen the impacts of acid mine drainage. These treatments are often considered to provide a dependable, economical, and ecologically friendly solution by efficiently eliminating pollutants. Alternatively, chemical compounds like limestone, calcium oxide or slurry of calcium hydroxide are usually used in active remediation processes in order to neutralise acidity and eliminate harmful heavy metals. Active remediation treatments are more costly than passive treatments, but they are quicker and more reliable.

Both passive and active acid waste treatment options have their own associated environmental costs, however. Life cycle analysis (LCA) studies of acid mine drainage treatment technologies reveal the broader environmental footprint of each approach. For instance, while active treatments may offer immediate remediation of acid mine drainage, their high energy requirements and chemical use contribute significantly to their life cycle environmental impacts. Conversely, passive systems, although slower and potentially less effective in extreme conditions, often have a lower overall environmental impact due to their reliance on natural processes and lower energy consumption.

The importance of LCA and preventative waste measures

While remediation techniques can address already contaminated water bodies, preemptive measures should be focused on to not only prevent severe environmental impact but to avoid the need for later, expensive, and additionally environmentally costly remediation treatments. Again, this is where LCA emerges as a critical tool, offering multi-impact, holistic insights into the environmental cost of mining practices at every life cycle stage, including that of potential remediation activities.

LCA serves as a deeply informative approach in identifying environmental impact hotspots where acid mine drainage-related environmental impacts including ecotoxicity, acidification and eutrophication are most pronounced. By pinpointing these hot spots, LCA enables science-driven sustainability choices for comparing and choosing less environmentally impactful critical mineral mining operations and value chains as early in the mine design stage as possible. A notable example of such advancements is the implementation of waste management technologies, such as reprocessing tailings.

The issue of acid mine drainage presents significant environmental challenges and requires well-thought out solutions. By using tools like life cycle assessment and implementing proactive measures, we can clearly compare, assess and address the life cycle environmental impacts of critical mineral mining value chains and acid mine drainage on water bodies and ecosystems – ultimately to ensure that the renewable energy transition does not cost more than it should.