Geology Reference
In-Depth Information
3.3.1.3 Acid Mine Drainage and Ochreous Deposits. One of the major
pollution problems affecting freshwaters is that of acid waters dis-
charged from coal and metal mines.
56
In an active underground coal
mine, pumping lowers the water table. The deeper strata thus become
exposed to air with the result that pyrite (FeS
2
) present is subject to
oxidation, generating acid conditions.
2FeS
2(s)
þ
2H
2
O
þ
7O
2(g)
"
2FeSO
4(s)
þ
2H
2
SO
4(1)
(3.81)
When mining and pumping cease, the water table returns to its natural
level. While the flooding of the mine stops the direct oxidation of pyrite,
it does bring the sulfuric acid and iron sulfates into solution. Some of the
ferrous ion (Fe
21
) may be oxidized to the ferric ion (Fe
31
), a slow
process at low pH, but one which can be catalysed by bacteria, and the
ferric ion may react further with pyrite.
57
4Fe
21
þ
O
2(g)
þ
4H
1
4Fe
31
þ
2H
2
O
"
(3.82)
FeS
2(s)
þ
14Fe
31
þ
8H
2
O
15Fe
21
þ
2SO
2
4
þ
16H
1
"
(3.83)
Underground in the Richmond Mine at Iron Mountain, California,
extremely acidic mine waters have been encountered, with pH ranging
from 1.5 to
3.6 and total dissolved metal (mainly iron) and sulfate
concentrations as high as 200 g L
1
and 760 g L
1
, respectively.
58
These
are the most acidic waters known.
On reaching the surface, the acidic mine drainage water is mixed with
air and oxygenated water, leading to rapid oxidation from the ferrous to
the ferric form and the precipitation of the characteristic unsightly orange
ochreous deposits of iron (hydr)oxides, e.g. ferrihydrite (Fe(OH)
3
),
goethite (FeOOH), observed along many stream and river beds.
4Fe
21
þ
O
2(g)
þ
4H
1
4Fe
31
þ
2H
2
O
"
(3.84)
Fe
3
þ
þ
3H
2
O
Ð
Fe
ð
OH
Þ
3
ð
s
Þ
ferrihydrite
þ
3H
þ
ð
3
:
85
Þ
Fe
3
þ
þ
2H
2
O
Ð
FeOOH
ð
s
Þ
goethite
þ
3H
þ
ð
3
:
86
Þ
The overall process may be represented as
2FeS
2(s)
þ
15/2O
2(g)
þ
7H
2
O
"
2Fe(OH)
3(s)
þ
4H
2
SO
4(1)
(3.87)
Microorganisms (e.g. Thiobacillus thiooxidans, Thiobacillus ferrooxi-
dans, Metallogenium) are involved as catalysts in many of the oxidizing
