In the smelting process, sulfide ores of copper (Cu), Pb, and zinc (Zn) are
oxidized (roasted), forming metallic oxides. For example, zinc sulfide (ZnS) is
converted in a smelter to zinc oxide (ZnO), releasing SO 2 :
2ZnS þ 3O 2 ! 2ZnO þ 2SO 2
ð8:3Þ
8.2.2 CHARACTERISTICS OF SO 2
SO 2 is highly soluble in water (solubility: 11.3 g per 100 ml). When SO 2 is
emitted into the atmosphere, it can dissolve in fog or cloud droplets, forming
sulfurous acid (H 2 SO 3 ), which is readily oxidized by molecular oxygen (O 2 ) to
sulphuric acid (H 2 SO 4 ). The formation of H 2 SO 4 by this process is greatly
facilitated by some metal salts, which are also dissolved in the droplets. Any
ammonia (NH 3 ) present in the atmosphere will rapidly react with the H 2 SO 3 or
H 2 SO 4 droplets to form ammonium sulfate or ammonium bisulfate.
1
Atmospheric SO 2 may be removed by several competing processes: direct
removal by deposition as bisulfate in precipitation, incorporation into fog and
cloud droplets (where it is oxidized catalytically and photochemically to
sulfate), or diffusion to plant surfaces where it is adsorbed and reacts
chemically. According to Fox,
2 both dry and wet forms of H 2 SO 4 produced
in the atmosphere may be removed by deposition to the earth’s surface.
Studies show that the photochemistry of the free hydroxyl radical (OH
Á )
controls the rate at which many trace gases, including SO 2 , are oxidized and
removed from the atmosphere.
3 The photochemistry involving the OH
Á radical
is shown in Figure 8.1.
8.2.3 EFFECTS ON PLANTS
SO 2 enters plant leaves predominantly by gaseous diffusion through stomatal
pores, as do other atmospheric pollutants. The number of stomata and the size
of aperture are important factors affecting SO 2 uptake. Other factors, such as
light, humidity, temperature, and wind velocity, are also important because
they influence the turgidity of stomatal guard cells. Low concentrations of SO 2
can injure epidermal and guard cells, resulting in elevated stomatal conductance and greater entry of SO 2 into plants.
Following uptake by plant leaves, SO 2 is rapidly translocated through the
plant. It can then affect photosynthesis, transpiration, and respiration, the
three major functions of plant leaves. A slight increase in both net
photosynthesis and transpiration may occur at low SO 2 concentrations for
short periods, followed by a decrease in both processes. Higher SO 2
concentrations induce immediate decreases in these processes. Plant injuries
may be manifested by leaf chlorosis and spotty necrotic lesions (Figure 8.2). As
noted previously (Table 5.1), a synergistic effect on leaf damage occurs when
plants are exposed to SO 2 and O 3 simultaneously. Damage to mesophyll cells
commonly occurs, which is the main cause of observed changes in photo112
Environmental Toxicology
[16:53 26/8/04 P:/CRC PRESS/4365 MING-HO.751 (1670)/4365-008.3d]
Ref: 4365 MING-HO YU Chap-008 Page: 112 111-134
oxidized (roasted), forming metallic oxides. For example, zinc sulfide (ZnS) is
converted in a smelter to zinc oxide (ZnO), releasing SO 2 :
2ZnS þ 3O 2 ! 2ZnO þ 2SO 2
ð8:3Þ
8.2.2 CHARACTERISTICS OF SO 2
SO 2 is highly soluble in water (solubility: 11.3 g per 100 ml). When SO 2 is
emitted into the atmosphere, it can dissolve in fog or cloud droplets, forming
sulfurous acid (H 2 SO 3 ), which is readily oxidized by molecular oxygen (O 2 ) to
sulphuric acid (H 2 SO 4 ). The formation of H 2 SO 4 by this process is greatly
facilitated by some metal salts, which are also dissolved in the droplets. Any
ammonia (NH 3 ) present in the atmosphere will rapidly react with the H 2 SO 3 or
H 2 SO 4 droplets to form ammonium sulfate or ammonium bisulfate.
1
Atmospheric SO 2 may be removed by several competing processes: direct
removal by deposition as bisulfate in precipitation, incorporation into fog and
cloud droplets (where it is oxidized catalytically and photochemically to
sulfate), or diffusion to plant surfaces where it is adsorbed and reacts
chemically. According to Fox,
2 both dry and wet forms of H 2 SO 4 produced
in the atmosphere may be removed by deposition to the earth’s surface.
Studies show that the photochemistry of the free hydroxyl radical (OH
Á )
controls the rate at which many trace gases, including SO 2 , are oxidized and
removed from the atmosphere.
3 The photochemistry involving the OH
Á radical
is shown in Figure 8.1.
8.2.3 EFFECTS ON PLANTS
SO 2 enters plant leaves predominantly by gaseous diffusion through stomatal
pores, as do other atmospheric pollutants. The number of stomata and the size
of aperture are important factors affecting SO 2 uptake. Other factors, such as
light, humidity, temperature, and wind velocity, are also important because
they influence the turgidity of stomatal guard cells. Low concentrations of SO 2
can injure epidermal and guard cells, resulting in elevated stomatal conductance and greater entry of SO 2 into plants.
Following uptake by plant leaves, SO 2 is rapidly translocated through the
plant. It can then affect photosynthesis, transpiration, and respiration, the
three major functions of plant leaves. A slight increase in both net
photosynthesis and transpiration may occur at low SO 2 concentrations for
short periods, followed by a decrease in both processes. Higher SO 2
concentrations induce immediate decreases in these processes. Plant injuries
may be manifested by leaf chlorosis and spotty necrotic lesions (Figure 8.2). As
noted previously (Table 5.1), a synergistic effect on leaf damage occurs when
plants are exposed to SO 2 and O 3 simultaneously. Damage to mesophyll cells
commonly occurs, which is the main cause of observed changes in photo112
Environmental Toxicology
[16:53 26/8/04 P:/CRC PRESS/4365 MING-HO.751 (1670)/4365-008.3d]
Ref: 4365 MING-HO YU Chap-008 Page: 112 111-134