Chemistry of Acidification

When you see a stream with the characteristic yellow-orange coating on the rocks, like the one in the image, you can be certain that the stream either currently has a low pH, or did have a low pH in the very recent past. Chances are also high, especially if you live in an area where coal mining is common, that the acidity and yellow-boy are ultimately caused by mining.

The coal itself does not cause either the acidification or the yellow boy. Instead it is another mineral commonly found with coal, pyrite. When pyrite (FeS₂) is exposed to water and air, it decomposes into free iron(II) (Fe²⁺) and sulfuric acid (H₂SO₄) (eq. 1). Before mining, exposure happens only occasionally. But after removing large amounts of material from the mountain by mining, pyrite is in constant contact with water and oxygen inside the mine. Iron hydroxide and sulfuric acid then build up and seep into streams.

The pH in the streams then drops dramatically; how far depends on the amount of pyrite and other metal-sulfides in the mine, but typically ends up between pH 4 and 5. This causes a number of effects on the stream ecology.

1. Few organisms can live in a highly acidic environment. This is one of the reasons we add vinegar, which is acidic, when pickling vegetables - we want to kill off bacteria in the water. (We also add salt, heat, and pressure, which are other effective methods of killing bacteria.)

2. Heavy metals enter solution at low pHs. This means that an AMD streams will have relatively high concentrations of aluminum, manganese, iron, and others. This stresses organisms and contributes to their death.

3. Because most organisms are unable to survive these conditions, there is less competition for those who can, such as Thiobacillius thiooxidans and similar species. Thus they flourish.

As a bit of an aside, recall that there are two main ways for living things to get their energy. All creatures need sugar to make useable energy, so the methods only differ in how living things make their sugar. The first, photosynthesis, is what plants use to covert sunlight, water, and carbon dioxide, into sugar. The second, predation, is when animals eat plants (or other animals), and then break down their food into sugars. These bacteria, however use a third method, chemosynthesis. They do something similar to photosynthesis, but instead of using sunlight to make sugar, they use energy from chemical reactions. The chemical reaction Thiobacillius uses for their production energy converts ferrous iron (Fe²⁺) into ferric iron (Fe³⁺) (eq. 2).

Since these bacteria live in water, the ferric iron (Fe³⁺) immediately reacts with the water to make ferric hydroxide (Fe(OH)₃), also known as yellow-boy, the yellow-orange sludge on the bottom of the streams (eq 3).

This sludge suffocates algae and plants living on the rocks, killing them. It is also unsightly, preventing impoverished riverside communities from attracting tourists to the area.

Continue on to learn about the Chemistry of Treatment.