An Introduction to Acid Mine
Drainage
by
George Mitchell
and Tim Craddock
AMD affects over 2000 miles of
streams in West Virginia
Extent of AMD in the mid-Appalachian region
What is
Acid Mine Drainage?
Basically, AMD formation
is an oxidation process
Oxidation is defined as the interaction between oxygen
molecules and all the different substances they may contact,
from metal to living tissue. Technically, however, with the
discovery of electrons, oxidation came to be more precisely
defined as the loss of at least one electron when two or more
substances interact. Those substances may or may not include
oxygen. (Incidentally, the opposite of oxidation is reduction —
the addition of at least one electron when substances come
into contact with each other.)
The culprit
Pyrite
or other sulfide minerals
Pyrite Oxygen Water
Acid Mine Drainage
The chemistry of oxidation of pyrites, the production of ferrous ions and
subsequently ferric ions, is very complex, and this complexity has
considerably inhibited the design of effective treatment options. Although
a host of chemical processes contribute to AMD, pyrite oxidation is by far
the greatest contributor. General equations for this process are:
2FeS2 + 7O2+ 2H2O → 2Fe2+ + 4SO42- + 4H+
The oxidation of the sulfide to sulfate solubilizes the ferrous iron (iron(II)),
which is subsequently oxidized to ferric iron (iron(III)):
4Fe2+ + O2 + 4H+ → 4Fe3+ + 2H2O
Either of these reactions can occur spontaneously or can be catalyzed by
microorganisms that derive energy from the oxidation reaction. The ferric
irons produced can also oxidize additional pyrite:
FeS2 + 14Fe3+ + 8H2O → 15Fe2++ 2SO42- + 16H+
The net effect of these reactions is to release H+, which lowers the pH and
maintains the solubility of the ferric ion.
When pyrite is enclosed
within rock, only minimal
amounts of pyrite are oxidized
through natural weathering,
thereby generating only small
amounts of acid. When fully
exposed to air and water, the
chemical reactions forming
AMD occur at a faster rate.
This acid is sometimes naturally diluted or
neutralized by surrounding alkaline rocks.
Remember, pyrite is exposed only when large
volume of rocks are disturbed, as in mining or
other major land disturbances.
The acid produced
eventually finds its
way to the ground
and surface waters.
Why is AMD Harmful?
The resulting acid and dissolved metals
stress or are toxic to aquatic life.
Common metals found in AMD
Iron (Fe)
Aluminum (Al)
Manganese (Mn)
Clear streams do not necessarily
mean clean streams
When acid levels are high, the pH is
low. In low pH conditions, the
metals are dissolved and leave no
physical indication of their presence.
However, acid mine drainage
eventually leaves its mark.
Water samples are collected to determine the
extent of acid mine drainage.
Discharge is the most
important measurement.
Pollutant load calculations
L = F x C x D, where
F = conversion factor/units
C = concentration
D = discharge
Discharge determines the loading, or volume,
of metals and acid found in the water.
Common field parameters Laboratory parameters
• pH • pH
• Specific conductivity • Specific conductivity
• Water temperature • Acidity
• Dissolved oxygen • Alkalinity
• Discharge • Sulfates
• Total Iron
• Aluminum
• Manganese
Once the volume of metals
and acid are known, a
treatment system can be
designed.
Chemical Treatment Examples
Anhydrous ammonia Hydrogen peroxide
Calcium oxide
Passive Treatment Examples
Wetlands Alkaline beds
Open limestone channels
The best method to treat
AMD is prevention
This can be done by
using proper
reclamation methods,
which prevents air
and/or water from
reaching the pyritic
materials.
West Virginia’s waters can and will be protected
from the degradation of AMD.
