Geoscience Reference
In-Depth Information
Figure 10.7.
Liming of a lake in Sweden by helicopter. Photo by Tero Niemi, available from Johnner.
acidity. The net effect of adding slaked lime to acidified
water is
The same result occurs when soil dust particles that
contain CaCO
3
(s) collide with acidified raindrops. The
erosion of farmland and desert borders in many loca-
tions worldwide has enhanced the quantity of soil
dust in the air, inadvertently increasing the calcium
carbonate content of rainwater, decreasing rainwa-
ter acidity, and increasing rainwater pH in nearby
regions.
Unfortunately, the same process described by Reac-
tion 10.17 that decreases soil acidity is partly responsi-
ble for the erosion of great statues and buildings made
of or containing
marble
or
limestone
.These materi-
als, both of which contain calcite, erode when they
become coated with acidified water. Coating can occur
in at least two ways. The first is when acidified rain-
drops or aerosol particles deposit directly onto a marble
or limestone surface. The second is when a gas dis-
solves and forms an acid in dew or rainwater that has
recently coated a surface. For example, when SO
2
(g)
dissolves in water, it oxidizes to sulfuric acid (Sec-
tion 10.3.2).
When water containing sulfuric acid coats a calcite
surface, the hydrogen ion dissociated from the acid dis-
solves the calcite by Reaction 10.17, and the sulfate
ion reacts with the dissociated calcium ion to form the
mineral
gypsum
by
2H
+
Hydrogen
ion
Ca
2
+
Calcium
ion
2H
2
O(aq)
Liquid
water
(10.16)
Lime is commonly added to lakes in large amounts
(Figure 10.7) to reduce the effects of acid deposition.
Sweden, which had the largest liming program in the
world, added 200,000 tonnes of fine-ground limestone
to lakes and watercourses each year in the 1990s and
early 2000s. Since the 1970s, more than 7,000 of Swe-
den's 17,000 anthropogenically acidified lakes have
been limed. Because lime is consumed 2 to 3 years after
its application, acidified lakes need to be relimed reg-
ularly. Other countries that have had large liming pro-
grams include Norway, Finland, and Canada. Although
liming of forest soil has also been tried, it is not so cost
effective as lake liming (Bostedt et al., 2010).
Ca(OH)
2
Calcium
hydroxide
(aq)
+
+
10.6.3. Calcium Carbonate
Some soils that contain the minerals calcite or aragonite
[CaCO
3
(s),
calcium carbonate
]haveanatural ability
to neutralize acids. When acid rain falls onto calcite-
containing soils, the H
+
is removed by
Ca
2
+
Calcium
ion
2H
+
Hydrogen
ion
Ca
2
+
Calcium
ion
2
−
+
SO
+
2H
2
O(aq)
Liquid
water
CaSO
4
-2H
2
O(s)
Calcium sulfate
dihydrate
(gypsum)
CaCO
3
Calcium
carbonate
(s)
+
+
CO
2
(g)
Carbon
dioxide
gas
+
H
2
O(aq)
Liquid
water
(10.17)
4
Sulfate
ion
(10.18)
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