The Generation and Passive Treatment of Acid Mine Drainage
Today we are faced with numerous environmental problems due to the lack of industry
regulation during previous decades. One such problem is acidic water in streams and lakes
caused by the United State’s long history of coal mining. Acid mine drainage (AMD) is the
production of acidic water from areas where the earth has been disturbed such as mining sites.
When rocks that contain the mineral iron sulfide are exposed to air, a series of chemical
reactions occur that contribute to the formation of AMD. Measures must be taken to remediate
bodies of water affected by AMD so that aquatic life and vegetation can thrive in their
environment. Passive treatments systems are one option for treating streams affected by AMD.
This chapter will explore the chemistry of acid mine drainage as well as the four components of
a passive treatment system.
Chemistry of Acid Mine Drainage
Exposing iron sulfide to oxidizing conditions leads to a number of chemical reactions that
results in the generation of acid mine drainage. Land excavation exposes sulfide minerals to
water and oxygen. Pyrite, an iron sulfide, is the most common acid producer in coal mining
areas. The mineral is found in coal seams and surrounding strata where mining practices
expose sizeable amounts of pyrite to the atmosphere. The uncovered pyrite is then oxidized in
the presence of oxygen and water to form sulfuric acid and ferrous iron. This sequence initiates
a series of chemical reactions that further degrades the health of receiving streams:
1. Rainwater produces a solution of sulfuric acid that trickles into ground and surface
water sources
2. This solution dissolves metals such as iron, aluminum, manganese, calcium, and sodium
from neighboring rocks and minerals
3. Ferrous iron is oxidized to ferric iron
4. This ion in turn hydrolyzes and forms iron hydroxide and more acidity
The outcome of acid mine drainage
is water with low pH, elevated
concentrations of sulfate, and
dissolved metals. Passive treatment
systems use chemical and biological
processes to treat AMD. This
treatment system utilizes a vertical
flow pond, aeration system, settling
pond, aerobic wetland, and
manganese removal bed in order to
treat AMD.
Figure 1. Overview of a Passive Treatment System (West Virginia
University 2012)
Vertical Flow Pond
The first component of a passive treatment system is a vertical flow pond. Vertical flow
ponds consist of a layer of organic compost with limestone fines, a layer of limestone,
and an under drain system. The AMD first flows downwards through overlying water
into the organic compost with limestone fines. The reason for the organic layer is to
establish anoxic (without oxygen) conditions so that iron entering the limestone layer is
in the reduced ferrous form. Ferrous iron precipitates at a pH higher than that of the
vertical flow pond and therefore remains soluble in the system. If oxidation were to
occur, ferric iron would precipitate, causing a need for the solids to be periodically
removed from the vertical flow pond to prevent clogging. The organic layer contains
limestone fines in order to remove aluminum from the system before it clogs the
limestone layer. This layer also reduces sulfate to sulfide. Iron sulfide may precipitate in
the organic layer. The water then flows through a layer of limestone. This is where the
majority of alkalinity is generated as a result of calcium carbonate dissolution. Once the
water flows through the limestone it exits through an under drain system.
(2.0 ft)
1
2
(3.0
ft)
(2.0
ft)
Aeration System and
Figure 2. Vertical Flow Pond Cross Section (Hedin Environmental 2012)
Settling Pond
The water coming from the vertical flow pond must be aerated before it passes into the
settling pond in order to oxidize ferrous iron to ferric iron, which will then precipitate
out of the system. A series of concrete steps and a notched trough directly before the
settling pond can be used to aerate the water and dissipate the energy as it comes from
the vertical flow pond. The steps are used primarily for aeration while the perforated
trough aids in aeration and distributes the water evenly into the settling pond. A settling
pond is required to settle out precipitates that form after aeration.
Aerobic Wetland
Some iron may remain in the solution after the settling pond. The water next passes
through an aerobic wetland in order to physically remove this excess iron. Aerobic
wetlands consist of wetland vegetation planted in shallow, relatively impermeable
sediments comprised of soil or clay.
The disadvantages to using an aerobic
wetland are the limited capacity of
solid storage available, which means
removal and replanting will be
required, and short-circuiting of
treatment water flowing through the
wetland is possible.
Figure 2(Canadian Institute of Mining, Metallurgy, and
Petroleum 2011)
Manganese Removal Bed
The last stage in this treatment system is a manganese removal bed. In passive
treatment, manganese (II) can be oxidized to manganese (III or IV) and precipitated as a
manganese oxide in a manganese removal bed. This bed is a large plot of limestone at
least one foot in depth with ditches that allow for oxidation conditions.
Conclusion
The purpose of a passive treatment system is to take advantage of naturally occurring
processes to treat AMD and therefore function without any power or continuous chemical
input. Once in place a passive treatment system can treat AMD for an extensive amount of
time with little to no upkeep. As a result passive treatment systems are generally more cost
effective than other available treatment options.