Remediation of Acid Mine Drainage
by Constructed Wetlands
Coal Mine Drainage
• Prior to 1978, no one legally required to
treat mine water discharge at most sites;
• >12,000 miles of river and 180,000 acres
of lakes/reservoirs are adversely affected
• US companies now spend >$1million/day
to treat CMD prior to discharging legally;
• Industry would welcome alternatives!
Acid Mine Drainage
• Caused by a complex system of geochemical
and microbial reactions
• Water contacts pyrite in coal, culm, refuse, or
overburden
• Produces acidic discharge rich in metals; when
pH increases these precipitate as
– Ferric hydroxide-stains the bottom orange; may coat
rocks and substrate,choking out benthic organisms
– Manganous oxide-stains rocks and detritus black
– Aluminum oxide-stains white
Pyrite Weathering
Summary Reaction
4 FeS2 + 15 O2 + 14 H2O  4 Fe(OH)3¯ + 8 H2SO4
Pyrite + Oxygen + Water "Yellowboy" + Sulfuric Acid
2 FeS2 + 7 O2 + 2 H2O  2 Fe2+ + 4 SO42- + 4 H+
Pyrite + Oxygen + Water  Ferrous Iron + Sulfate + Acidity
4 Fe2+ + O2 + 4 H+  4 Fe3+ + 2 H2O
Ferrous Iron + Oxygen + Acidity  Ferric Iron + Water
4 Fe3+ + 12 H2O  4 Fe(OH)3 ¯ (ppt)+ 12 H+
Ferric Iron + Water  Ferric Hydroxide (yellowboy) + Acidity
FeS2 + 14 Fe3+ + 8 H2O  15 Fe2+ + 2 SO42- + 16 H+
Pyrite + Ferric Iron + Water  Ferrous Iron + Sulfate + Acidity
Treatment
• Early on, consisted of buffering acidity with
– Calcium carbonate
– Sodium hysdroxide
– Sodium bicarbonate
– Anydrous ammonia
• Precipitated the metals
• Expensive
– Chemicals, operation/maintenance, and
disposal of metal-rich sludges
Passive Treatment
• Allow biogeochemistry to occur in
treatment system rather than natural body
• Requires nearly no chemicals or energy
input compared to active treatment
Aerobic Wetlands
•Large surface area with 6-18” of horizontal flowWater must be alkaline
•Aeration by riffles/falls increases oxidation and therefore precipitation
How large of a wetland is needed?
Minimum wetland size (ac) = [Fe loading (lb/day) ¸ 180 (lb/ac/day)]
+ [Mn loading (lb/day) ¸ 9 (lb/ac/day)]
+[Acidity (lb/day) ¸ 60 (lb/day/acre)]
Anaerobic (Compost) Wetlands
•Act as reducing wetland; chemical and microbial processes
generate alkalinity increasing pH
•Goal is to reduce pollution levels that will restore the receiving
stream
Limestone Channels
•Simplest
•Either a limestone channel or blocks placed in stream
•Dissolution of limestone increases pH
•Coating of limestone (armoring) reduces its useful life
Diversion Wells
•Use more pulverized limestone than channels;
•agitation slows armoring process
Oscillatoria
Oedogonium
Microspora