POLLUTION FROM MINE DRAINAGE 1017
FeS 3O FeSO SO
2FeS 7O 2H O 2FeSO 2H SO4FeSO 2H S22 4 2
222 424
42
→
→OO O 2Fe (SO ) 2H O
4FeSO O 2H O 4Fe(OH)SOFe (SO )42 243 242 2 424
→↓
→↓332 4 24
22 4 4 322H O 2Fe(OH)SO H SO
Ca(HCO ) FeSO CaSO Fe(HCO )4
→↓
→FFe(HCO ) O 2H O 4Fe(OH) 8CO
Ca(HCO ) Fe (SO ) 4H22 2 2 3 2
22 2 43
→↓↑224322422 2 4 4OCaSO 2Fe(OH) 2CO 2H SO
Ca(HCO ) H SO CaSO 2CO→↓↑
→
222 ↑ 2H O.There has been a continuing debate among the scientific
community over the role which bacteria may play in the for-
mation of acid mine damage. Bacteria of the Ferrobacillus
ferroxidans family are almost always found in large pools of
acid mine drainage. The bacteria have been shown to have
the capability of oxidizing ferrous iron to the ferric state.
However, they thrive in a very limited pH range (approxi-
mately pH 3.5) and there is no evidence to indicate that they
contribute directly to the primary oxidation of the pyretic
substance. Ferric ion can oxidize sulfide sulfur and this could
possibly provide a mechanism for the bacteria to increase
the rate of oxidation of pyrite in some circumstances. Other
recent studies have shown that the transfer of oxygen from
the atmosphere to the pyrite surface is the rate limiting
factor of the reaction, making moot arguments put forth as
to whether bacterial or chemical oxidation is the principal
mechanism of acid drainage formation.
Bacteria may play a significant role in the oxidation of
the ferrous to ferric ion in mine drainage. This fact can be of
considerable importance in the treatment of mine drainage
as ferrous iron tends to retard the rate of neutralization reac-
tions. It almost appears that this is Nature’s first step in the
neutralization of acid drainage. The bacterial oxidization of
iron allows the alkalinity of the associated earth strata and
of diluting waters to be more readily reacted with the acid-
ity of the mine drainage waters. This concept is rather radi-
cally at variance with former concepts that considered the
sterilization of a mine as a possible method of reducing or
eliminating the formation of acid drainage. Bacteria has been
used deliberately as a step in the treatment of mine drain-
age. This process allows the ferrous iron to be oxidized and
accelerate the neutralization of the acid salts.
The formation of acid drainages from surface and under-
ground mines is essentially the same process, and the two
drainages are indistinguishable on the basis of the chemi-
cal qualities of the waters. In general the iron contained in
drainage from surface mines and also from coal refuse piles
is in the ferric state. The drainage from surface mining may
contain very substantial amounts of suspended solids or sedi-
ment. Because many of the drainage problems of strip and
deep mines are directly interrelated there is almost no rationalway of separating the treatment or preventive measures which
may be applied to the two types of mining.EXTENT OF ENVIRONMENTAL IMPACTThe problem of environmental degradation caused by mine
drainage is widespread and serious. Some form of mining
occurs in each of the fifty states and many states are exten-
sively mined. The aquatic degradation caused by coal mining
in the eastern and Appalachian region is best known and has
been best documented. For this reason, most statements of
the damages caused by mine drainage cite only the degra-
dation in the Appalachian area. The Appalachian Regional
Commission reported that some 10,500 miles of streams in
that region are affected by mine drainage and acid drainage
continually pollutes nearly 5700 stream miles. Since data
are not available in many mining areas, particularly in the
Rocky Mountain and western areas, firm total statements of
the extent of the problem cannot be made. However, enough
information is available to indicate that it is very substantial.
The extent and amount of degradation which may be
caused by the presence of mine drainage, and the environ-
mental and economic impacts which may be felt, vary, of
course, with the type of mine, the strata surrounding it, and
other localized conditions.
For example, coal mine drainage, which is usually acidic,
kills fish and other forms of aquatic biota by lowering the pH
of the water and also may have an adverse economic effect
upon the human population. The acidity accelerates the cor-
rosion of bridges, culverts, boats and navigational facilities,
making replacement at shorter intervals necessary. High cost
water treatment may be required to make the local water
supply potable or suitable for industrial use. Water contact
sports may not be possible in the area, causing a loss of
potential tourist industry revenues.
Although metal mining may cause the same adverse
effects, it usually occurs in less densely populated regions.
Additionally, metal mine drainage may render the waters
toxic to humans as well as aquatic life by the presence of
heavy metals.REMEDIES FOR MINE DRAINAGE POLLUTIONIn the past a substantial number of investigators have simply
discussed the problem and observed its extent so that more is
known about the nature of the problem than about the mech-
anisms which may be applicable to correcting or mitigating
it. Mine drainage may be characterized on the basis of its
source and possible remedies considered by this categoriza-
tion even though the chemical nature and biological impact
of the drainage from the several sources is identical. For the
purposes of this discussion let us consider three types of
mine drainage by the source: drainage from active or operat-
ing mines, drainage from non-operating (sometimes called
abandoned) mines and drainage which will be generated in
the future from mines which have not yet begun operation.C016_010_r03.indd 1017C016_010_r03.indd 1017 11/18/2005 11:03:04 AM11/18/2005 11:03:04 AM