I’ve always wondered about acid mine drainage, but I never really new what it was or what caused it. So, I decided to do some research. Without getting into too much detail, here are some of the basics of acid mine drainage.
What causes acid mine drainage? Well, mining. Sorry, I couldn’t help stating the obvious. But really, acid mine drainage usually occurs due to tailings from a mine- the big piles of dirt they excavate and dump outside the mine. The chemical/mineral responsible for the acid drainage is pyrite (FeS2). That’s right, fool’s gold is the root cause of acid mine drainage!
How can pyrite cause acid mine drainage? It’s true, you wouldn’t really expect fool’s gold to cause acidic conditions. It all starts with pyrite that was excavated and placed above ground. Next comes the rain, which soaks the pyrite. From here on out, the best way to explain how pyrite causes acid drainage is to present the reactions:
2FeS2 (pyrite) + 7O2 + 2H2O –> 2Fe+2 + 4SO4-2 + 4H+
The H+ is the acidic part, and the resulting pH can be as low as 1. For comparison, lemon juice has a pH of about 2.
Just when you think it’s done, even more acid is produced! In the following reaction, Fe+2 reacts with some of the acid produced from the first reaction.
2Fe+2 + 1/2 O2 + 2H+ –> 2Fe+3 + H2O
The resulting Fe+3 can go two routes from here. It can either precipitate in the form of various oxides (the red scum visible on plants in areas of acid mine drainage) or it can go on to form even more acid! Here’s the reaction for further acid production:
FeS2 + 14Fe+3 + 8H2O –> 15Fe+2 + 2SO-2 + 16H+
What if you want to include all the acid production and precipitation in one reaction? Here it is:
4FeS2 + 15O2 + 14H2O <--> 4Fe(OH)3 (precipitate) + 8H2SO4
That’s right, sulfuric acid is produced. It’s very acidic, harmful to the environment, and can even cause other toxic heavy metals to come into solution.
Is there any way to prevent acid mine drainage? Other than not mining, there is no all encompassing solution. There’s the option of reburying the mine tailings, however some acid will still be created and put into the water table. Here are the methods current mining corporations are dealing with acid mine drainage:
- Spray mine tailings with a polymer. Essentially the tailings are sprayed with a plastic coating so that no O2 or water can being the reaction with pyrite.
- Dump the mine tailings in acid consuming rock. For example, dumping the tailings in a bed on limestone. Acid production will occur, but it will be neutralized by the carbonate in the limestone. Sure, CO2 will be emitted, but it’s better than having acid everywhere!
- Collect the leachate and treat it. Needless to say, this takes a lot of energy and money.
Want to see a bad case of acid mine drainage? Look up Rio Tinto in Spain.