10.3.5 COMBUSTION OF COAL
As mentioned previously, coals mined in U.S. contain about 0.001 to 0.048%
F, usually as fluorapatite or fluorspar. Combustion of coal in power plants,
therefore, emits considerable quantities of F into the atmosphere. During
combustion, about half of the F in coal is emitted as gaseous HF and SiF 4 and
particulate matter.
The dramatic increase in the use of coal as an energy source in many cities
and areas in the world has caused atmospheric F-pollution to increase steadily.
This trend is especially conspicuous in a number of less-developed countries.
For example, studies show that several cities in China, including Chongqing
and Beijing, are experiencing severe fluoride air pollution problems arising
mostly from coal combustion.
2,6
In Beijing, coal is the dominant energy source, accounting for more than
75% of the total energy consumption. Combustion of coal for heating in winter
accounts for 23% of the annual coal combustion. Furthermore, the F content
of coal consumed in the city is reported to be 163 mg/g, more than double the
mean value of 80 mg/g for coals of other parts of the world.
2 Another
important source of F in Beijing is dust from fresh concrete used for
construction. Factors such as these have contributed to the elevated F
concentrations of wet depositions in the city. For example, the annual
volume-weighted average concentration of soluble F of ambient aerosol is
reportedly 60 mg/m
3
, which is 75 times higher than the concentration observed
in the air sample taken in the city of Morioka, a city without fluoride pollution,
in northern part of Japan.
6
Fluoride has also been traced to runoff from application of insecticides and
herbicides. In addition to direct runoff into surface waters, airborne F may be
deposited into surface water and onto the ground, and eventually taken up by
soils, plants, and animals (Figure 10.2).
10.4 EFFECTS ON PLANTS
HF is the most phytotoxic air pollutant. The high toxicity of F and its
compounds is due to their rapid absorption and the inherent toxicity of the
element. F can cause injury to susceptible plants at concentrations below 1 ppb
(0.8 mg/m
3 ) for exposure periods of 7 days or less.
7,8,9 Exposure to F can result
in marked increases in foliage F levels. The extent of increases depends upon
factors such as duration of exposure, atmospheric F levels, and species or
variety of plants. F-induced effects in plants may be viewed based on four levels
of biologic organization: cellular, tissue or organ, organism, and ecosystem
(Table 10.2).
1
F accumulates in plant leaves mainly as a result of diffusion from the
atmosphere through the stomata or through absorption from soil by root. In
contrast to other major air pollutants, such as sulfur dioxide (SO 2 ), nitrogen
dioxide (NO 2 ), and ozone (O 3 ) , discussed in Chapter 8, F accumulates in the
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Environmental Toxicology
[16:52 26/8/04 P:/CRC PRESS/4365 MING-HO.751 (1670)/4365-010.3d]
Ref: 4365 MING-HO YU Chap-010 Page: 154 149-170
As mentioned previously, coals mined in U.S. contain about 0.001 to 0.048%
F, usually as fluorapatite or fluorspar. Combustion of coal in power plants,
therefore, emits considerable quantities of F into the atmosphere. During
combustion, about half of the F in coal is emitted as gaseous HF and SiF 4 and
particulate matter.
The dramatic increase in the use of coal as an energy source in many cities
and areas in the world has caused atmospheric F-pollution to increase steadily.
This trend is especially conspicuous in a number of less-developed countries.
For example, studies show that several cities in China, including Chongqing
and Beijing, are experiencing severe fluoride air pollution problems arising
mostly from coal combustion.
2,6
In Beijing, coal is the dominant energy source, accounting for more than
75% of the total energy consumption. Combustion of coal for heating in winter
accounts for 23% of the annual coal combustion. Furthermore, the F content
of coal consumed in the city is reported to be 163 mg/g, more than double the
mean value of 80 mg/g for coals of other parts of the world.
2 Another
important source of F in Beijing is dust from fresh concrete used for
construction. Factors such as these have contributed to the elevated F
concentrations of wet depositions in the city. For example, the annual
volume-weighted average concentration of soluble F of ambient aerosol is
reportedly 60 mg/m
3
, which is 75 times higher than the concentration observed
in the air sample taken in the city of Morioka, a city without fluoride pollution,
in northern part of Japan.
6
Fluoride has also been traced to runoff from application of insecticides and
herbicides. In addition to direct runoff into surface waters, airborne F may be
deposited into surface water and onto the ground, and eventually taken up by
soils, plants, and animals (Figure 10.2).
10.4 EFFECTS ON PLANTS
HF is the most phytotoxic air pollutant. The high toxicity of F and its
compounds is due to their rapid absorption and the inherent toxicity of the
element. F can cause injury to susceptible plants at concentrations below 1 ppb
(0.8 mg/m
3 ) for exposure periods of 7 days or less.
7,8,9 Exposure to F can result
in marked increases in foliage F levels. The extent of increases depends upon
factors such as duration of exposure, atmospheric F levels, and species or
variety of plants. F-induced effects in plants may be viewed based on four levels
of biologic organization: cellular, tissue or organ, organism, and ecosystem
(Table 10.2).
1
F accumulates in plant leaves mainly as a result of diffusion from the
atmosphere through the stomata or through absorption from soil by root. In
contrast to other major air pollutants, such as sulfur dioxide (SO 2 ), nitrogen
dioxide (NO 2 ), and ozone (O 3 ) , discussed in Chapter 8, F accumulates in the
154
Environmental Toxicology
[16:52 26/8/04 P:/CRC PRESS/4365 MING-HO.751 (1670)/4365-010.3d]
Ref: 4365 MING-HO YU Chap-010 Page: 154 149-170