FIGHTING ACID MINE DRAINAGE
Environmental article by Harold Hough
Louis Pasteur and Alexander Fleming aren’t considered mining pioneers, but in reality, their medical contributions are helping us better understand and control acid rock drainage (ARD). That’s because more and more miners are looking at the problem of ARD not solely as a mining problem, but as an infection. By addressing the problem this way, mines may be able to drastically reduce acid drainage instead of treating water decades after the mine has closed.
Although many blame mining for ARD, the real fault belongs to a lowly bacteria called Thiobacillus ferrooxidans, which oxidizes pyriates and sulfide ores while producing sulfuric acid. In reality, all the mine does is bring more “food” to the surface. In that regard, the mine is suffering from an infection from the Thiobacillus bacteria.
Traditional treatment of ARD has been as backwards at many medical treatments of the Dark Ages. Instead of combating the problem directly, mines have treated the symptoms, which often included treating the drainage before it entered the water supply. These included lime neutralization, carbonate neutralization, and ion exchange. In that regard, it was like treating someone with a high temperature by just bathing them in ice water.
Then there was the all natural remedy, wetlands creation. Like natural remedies found in your medicine chest, they work, but not as effectively or as quickly. They usually require pre-treated water that has gone through something like lime neutralization. The wetlands then precipitate out the metals in the water.
The advantage of wetlands is that they fit into a long term mine reclamation plan that encourages more wildlife and even offers some recreational value. The downside is that they are more expensive in terms of cost per ton of metal removed from the water.
Natural methods also include re-grading tailings and re-vegetation so plant life and natural occurring bacteria take root and keep the Thiobacillus from establishing itself. The problem occurs when the Thiobacillus has gained a foothold and has made the ecosystem too acidic for other life to thrive
Situations like that require the “penicillin” method – killing the bacteria. By killing the bacteria, a mine can stop the acid production and give nature enough time to establish a flourishing colony of bacterial that promotes plant growth and crowds out the Thiobacillus. In fact, there are chemicals that specifically target the dangerous bacteria, while providing a food source for helpful heterotrophic germs. These are surfactants that wash away the greasy membrane that protects the Thiobacillus from the sulfuric acid. In the final irony, the germ is killed by the same acid it produces.
The chemical, sodium dodecylbenzene sulfonate, can be used at several stages in the mining process. It can be sprayed on piles of water rock or stockpiles. And as long as the piles aren’t disturbed, the chemical continues to work. There are even time release pellets that can stop the bacteria for years. In the process, humus and organic acids are produced, which encourage good bacterial growth.
Another method for fighting ARD is the “germ versus germ” method where a mine can introduce a new bacterium that will counteract the Thiobacillus. These are what are known as “sulfate reducing bacteria” and we know them best because they are the type of lifeforms that live deep in the ocean near hot water vents. They use sulfur much as other organisms use oxygen. The bacteria produce hydrogen sulfide and bicarbonate when supplied with sources of carbon and sulfate. Hydrogen sulfide reacts with metal ions in the drainage, and causes them to drop out of the water as metal sulfides. The bicarbonate serves to help neutralize the drainage.
Although ARD continues to be a problem, a greater understanding of the bacteria that causes the problem has made it easier to come up with innovative solutions that counteract them. With luck, the current methods of treating ARD will resemble the out-of-date cures our great grand parents once took.