After a mine is abandoned, a web of tunnels is left behind. Water often runs through these tunnels, picking up mineral deposits from inside the mine. In West Virginia, these mines are almost solely coal mines. A mineral called pyrite, or fools’ gold, naturally occurs near coal, so pyrite is usually all over the tunnel walls. When pyrite (FeS2, or iron and sulfur) is exposed to air, a chemical change occurs. Two hydrogen molecules and four oxygen molecules from the air bind with each sulfur molecule from the pyrite, creating sulfuric acid. This chemical reaction leaves a few molecules left over: iron and extra hydrogen. The iron stays in the water as particulates, and it binds with oxygen, turning contaminated water orange from rust. The hydrogen ions also stay in the water; since pH is a measure of the ratio of hydroxide ions (OH–) to hydrogen ions (H2+), the increased hydrogen causes the pH to drop.
Other metals are also commonly found in AMD-affected streams, but an AMD trademark is the orange coloring and lowered pH. AMD can be treated by removing the metal particulates and adding basic substances (such as limestone) to raise the pH. It is an expensive process, but we are making progress towards cleaning it all up.
Made using: Khan Academy Computer Science.
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