Acid rock drainage (ARD) or acid mine drainage refers to the acidic water that is created when sulphide minerals are exposed to air and water and, through a natural chemical reaction, produce sulphuric acid.

2. What is Acid Mine Drainage
(AMD)?
 Acid mine drainage, acid and metalliferous drainage (AMD), or acid rock
drainage (ARD) is the outflow of acidic water from metal mines or coal
mines.
 Acid rock drainage occurs naturally within some environments as part of the
rock weathering process but is exacerbated by large-scale earth
disturbances (construction sites, subdivisions, and transportation corridors)
usually within rocks containing an abundance of sulfide minerals.
 In many localities, the liquid that drains from coal stocks, coal handling
facilities, coal washeries, and coal waste tips can be highly acidic.
 This liquid often contains toxic metals, such as copper or iron. These,
combined with reduced pH, have a detrimental impact on the streams
aquatic environments.
 An important environmental issue in many areas where mining has taken
place.

3. Sources of Acid Mine
Drainage
 Mining of gold, silver, copper, iron, zinc, lead (or combined
metals), and coal
 During exploration, operation, and closure of mine, from the
mine’s:
• dewatering system
• tailings disposal
• waste heap
NOMENCLATURE
– Historically, the acidic discharges from active or abandoned mines
were called acid mine drainage, or AMD
– The term acid rock drainage, or ARD, was introduced in the
1980s and 1990s to indicate that acidic drainage can originate
from sources other than mines

4. Process of Acid Mine Drainage
 Geochemical and microbial reactions during weathering of sulfide minerals
(pyrite) in coal, refuse, or mine overburden
– Oxidation of sulfide minerals in the presence of air, water, and bacteria
– Formation of sulfuric acid and increase in acidity
– Solubilization of metals due to low Ph.
 At some mines, acidic drainage is detected within 2–5 years after mining
begins, whereas at other mines, it is not detected for several decades.

5. – When a mine is abandoned, the pumping ceases, and water floods the mine. This
introduction of water is the initial step in most acid rock drainage situations.
Tailings piles or ponds, mine waste rock dumps,and coal spoils are also an
important source of acid mine drainage.
– After being exposed to air and water, oxidation of metal sulfides within the
surrounding rock and overburden generates acidity. Colonies of bacteria and
archaea greatly accelerate the decomposition of metal ions, although the
reactions also occur in an abiotic environment.
– Metal mines may generate highly acidic discharges where the ore is a sulfide
mineral or is associated with pyrite. The most commonly mined ore of copper,
chalcopyrite, is itself a copper-iron-sulfide .Thus, copper mines are often major
culprits of acid mine drainage.

6. Chemistry of Acid Mine
Drainage
Reaction 1
2FeS2 + 7O2 + 2H2O → 4Fe 2+ + 4SO4 +
4H+
• weathering of pyrite in the presence of
oxygen and water to produce iron(II), sulfate,
and hydrogen ions
Reaction 2
4Fe2+ + 7O2 + 2H2O → 4Fe3+ + 2H2O
• oxidation of Fe(II) to Fe(III)
• rate determining step
Reaction 3
2Fe3+ + 12H2O → 4Fe(OH)3 + 12H+
• hydrolysis of Fe(III)
• precipitation of iron(III) hydroxide if pH >
3.5
Reaction 4
FeS2 + 14Fe3+ + 8H2O → 15Fe2+ + 2SO42- +
16H+
• oxidation of additional pyrite (from steps 1
and 2) by Fe(III) -- here iron is the oxidizing
agent, not oxygen
• cyclic and self-propagating step

7. Yellow boy:When the pH of acid mine drainage is raised past 3, either
through contact with fresh water or neutralizing minerals, previously
soluble iron(III) ions precipitate as iron(III) hydroxide, a yellow-orange
solid colloquially known as yellow boy

8. Effects of Acid Mine Drainage
– Water resources
– Increased acidity
–Depleted oxygen
– Increased weathering of minerals ⇒
release of heavy metals/toxic elements into
stream
– Precipitation of Fe(OH)3 ⇒ bright orange
color of water and rocks
– Biological resources
– Low pH and oxygen content ⇒ water
unsuitable for aquatic life – Precipitation of
Fe(OH)3
• Increased turbidity and decreased
photosynthesis
• Clogging of interstitial pore space in coars
aquatic substrate habitat
– Elimination of aquatic plants ⇒ change in
channel hydraulics – Stress on other biota
associated with aquatic habitats
– Human resources – Corrosion of pipes,
pumps, bridges, etc. – Degradation of
drinking water supplies – Harm to
fisheries

9. Identification and prediction
Geochemical assessment of mine materials
– 1. Sampling;
– 2. Static geochemical testwork (e.g. acid-base
accounting, sulfur speciation);
– 3. Kinetic geochemical testwork - Conducting
oxygen consumption tests,
– 4. Modelling of oxidation, pollutant generation
and release; and
– 5. Modelling of material composition.

10. • Chemical Treatment
– 1. Calcium Oxide
– 2. Anhydrous Ammonia
• Passive Treatment
– 1. Wetlands
– 2. Open Limestone channel

11. Calcium Oxide
– Its Alkaline in nature
• Addition of calcium oxide increase the PH level of AMD.
• Cause many of the metals present in solution to precipitate
as hydroxides and carbonates.
• It has the lowest material cost and is the safest and easiest
to handle

12. Anhydrous Ammonia
– In the gaseous state, ammonia is extremely soluble and
reacts rapidly.
• It behaves as a strong base and can easily raise the pH of
receiving water.
• Injection of ammonia into AMD is one of the quickest ways
to raise water pH.
• It should be injected into flowing water at the entrance of
the pond to ensure good mixing because ammonia is lighter
than water.

13. Wetlands
– Wetlands have several functions that aid in the removal of metals in drainage
• It acts on filtering mechanism of the dense plant root system which catches any
of the suspended solid and flocculated particles as they pass through the wetland.
1. Aerobic wetlands 2. Anaerobic wetlands
– Aerobic wetlands are shallow (1- to 3-foot deep) ponds; they filled with soil or
organic matter. They facilitate natural oxidation of the metals and precipitate
iron, manganese, and other metals.
– Anaerobic wetlands are shallow ponds filled with organic matter, such as
compost, and underlain by limestone gravel. It used to neutralize acidity and
reduce metals to the sulfide form

14. Open Limestone Channel
• It’s an open, free-flowing channel lined with coarse
limestone.
• These systems oxidize and precipitate metals and add
alkalinity to the water.
SOURCE:
WIKIPEDIA
SLIDESHARE.COM
USGS