Introduction to ENVIRONMENTAL TOXICOLOGY Impacts of Chemicals Upon Ecological Systems - CHAPTER 7

CHAPTER 7 Inorganic Gaseous Pollutants In this Chapter, five of the major gaseous air pollutants are considered, i.e., sulfur oxides (SOx), nitrogen oxides (NOx), ozone (O3), carbon monoxide (CO), and fluoride (F). Although much of the fluoride emitted into the atmosphere from various sources is in the particulate form, fluoride is included in our discussion here with other inorganic gaseous pollutants mainly because gaseous fluoride causes the most damage to living organisms, especially plants. SULFUR OXIDES Sulfur oxides include both sulfur dioxide (SO2) and sulfur trioxide (SO3), of which SO2 is more important as an air pollutant. Sulfur trioxide may be formed in the furnace by reaction between sulfur and O2 or SO2 and O2. Sulfur dioxide is probably the most dangerous of all gaseous pollutants on the basis of amounts emitted. Sources of SO2 Sulfur oxide emission results from the combustion of sulfur-containing fossil fuels, such as coal and oil. The sulfur content of coal ranges from 0.3 to 7% and the sulfur is in both organic and inorganic forms, while in oil, sulfur content ranges from 0.2 to 1.7% and its sulfur is in organic form. The most important sulfur compound in coal is iron disulfide (FeS2) or pyrite. When heated at high tempera-tures, pyrite undergoes the following reactions: FeS2 + 3 O2 ® FeSO4 + SO2 (7.1) 4 FeS2 + 11 O2 ® 2 Fe2O3 + 8 SO2 (7.2) In the smelting process, sulfide ores of copper, lead, and zinc are oxidized (roasted) to convert a sulfide compound into an oxide. For example, zinc sulfide undergoes the oxidation process in a smelter forming ZnO and SO2 as shown below: 2 ZnS + 3 O2 ® 2 ZnO + 2 SO2 (7.3) In the U.S., SO2 emission from stationary sources and industry accounts for about 95% of all SO2 emission. Characteristics of SO2 SO2 is highly soluble in water, with a solubility of 11.3g/100 ml. Once emitted into the atmosphere, SO2 may undergo oxidation in the gaseous phase, forming H2SO4 aerosol. Gaseous SO2 also may become dissolved in water droplets and, following oxidation, form H2SO4 aerosol droplets. Both forms of H2SO4 thus pro-duced may be removed by deposition to the Earth’s surface (Figure 7.1). Figure 7.1 SO transport, transformation, and deposition processes.Initially SO is mixed into the atmosphere (I). Gaseous SO may undergo oxidation in the gaseous phase with subsequent formation of H SO aerosol (II). Both gaseous SO and H SO aerosol may be deposited at the Earth’s surface (III). Gaseous SO may become dissolved in a water droplet (IV). The dissolved SO can be oxidized in solution to form H SO aerosol droplets (V). The H SO aerosol and the H SO droplet may be removed to the Earth’s surface by wet deposition (VI). (From Fox, D.L. 1986.The transformation of pollutants. In Air Pollution, 3rd ed., Vol.VI, A.C. Stern, Ed., Academic Press, New York, pp. 86-87. With permission.) Recent studies have shown that the photochemistry of the free hydroxyl radical controls the rate at which many trace gases, including SO2, are oxidized and removed from the atmosphere. The photochemistry involving the OH radical is illustrated in Figure 7.2. Figure 7.2 Photochemistry of the OH radical controls the trace gas concentration.The photo-chemistry of the free hydroxyl radical controls the rate at which many trace gases are oxidized and removed from the atmosphere. Processes that are of primary importance in controlling the concentration of OH in the troposphere are indicated by solid lines in the schematic diagram; those that have a negligible effect on OH levels but are important because they control the concentrations of associated reaction and products are indicated by broken lines. Circles indicate reservoirs of species in the atmosphere; arrows indicate reactions that convert one species to another, with the reactant or photon needed for each reaction indicated along each arrow. Multistep reactions actually consist of two or more sequential elementary reactions. HX = HCl, HBr, HI or HF. C H denotes hydrocarbons. (From W.L. Chameides and D.D. Davis, Chem. Eng. News, 60 (40): 38-52, 1982. American Chemical Society. With permission.) Effects on Plants For SO2, the stomatal pores are the main entry ports to the internal air spaces of plant leaves. Absorption takes place mainly by gaseous diffusion through these pores. The number of stomata and size of aperture are important factors affecting the uptake of SO2. Other factors such as light, humidity, wind velocity, and temper-ature also are important, as these influence the turgidity of guard cells. Low con-centrations of SO2 can injure epidermal and guard cells, leading to increased stomatal conductance and greater entry of SO2 into the plant. Following the uptake by plant leaves, SO2 is rapidly translocated through the plant and affects photosynthesis, transpiration, and respiration, the three major functions of plant leaves. A slight increase in both net photosynthesis and transpiration may occur at low SO2 concen-trations for short time periods, followed by a decrease in both processes. Higher SO2 concentrations induce immediate decreases in these processes. Plant injuries may be manifested by leaf chlorosis and spotty necrotic lesions. Damage to meso-phyll cells is commonly observed in microscopic studies. Once within the substomatal air spaces of the leaf, SO2 comes into contact with cell walls of the mesophyll cells. SO2 readily dissolves in the intercellular water to form sulfite (SO32–), bisulfite (HSO3–), and other ionic species. Both SO32– and HSO3– have been shown to be phytotoxic, as they affect many biochemical and physiological processes (Malhotra and Hocking 1976). Both SO32– and HSO3– have a lone pair of electrons on the sulfur atom that strongly favor reactions with electron-deficient sites in other molecules. The phytotoxicity of SO32– and HSO3– can be overcome by conversion of these species to less toxic forms such as SO42–. Oxidation of HSO3– to the less toxic sulfate can occur by both enzymatic and nonenzyme mechanisms. Several factors, including cellular enzymes such as peroxidase and cytochrome oxidase, metals, ultraviolet light, and O2 ·stimulate the oxidation of SO2. In the presence of SO32– and HSO3–, more O–·is formed by free-radical chain oxidation. Other free radicals can be formed as well. These oxidizing radicals can have detri-mental effects on the cell. Plant metabolism is affected by SO2 in a variety of ways. For instance, stimu-lation of phosphorus metabolism (Plesnicar 1983) and reduction in foliar chlorophyll concentration (Lauenroth and Dodd 1981). Carbohydrate concentrations were increased by low levels of SO2 and decreased by higher levels (Koziol and Jordon 1978). Effects of SO2 on enzyme systems have been investigated in many studies. The enzymes studied include alanine and aspartate aminotransferases, glutamate dehydrogenase, malate dehydrogenase, glycolate oxidase, glyceraldehyde-3-phos-phate dehydrogenase, glucose-6-phosphate dehydrogenase, fructose-1, 6- bisphos-phatase, and ribulose-5-phosphate kinase. Enzyme activity may be increased or decreased by exposure to SO2 at different concentrations. As mentioned previously, there are differences in sensitivity of plant species to SO2 under similar biophysical conditions. This suggests that delicate biochemical and physiological differences operating in different plant species could affect the sensitivity of a particular plant to SO2. Effect on Animals Although SO2 is an irritating gas for the eyes and upper respiratory tract, no major injury from exposure to any reasonable concentrations of this gas has been demonstrated in experimental animals. Even exposure to pure gaseous SO2 at con-centrations 50 or more times the ambient values produced little distress (Alarie et al. 1970; Alarie et al. 1973). Concentrations of 100 or more times are required to kill small animals. Mortality is associated with lung congestion and hemorrhage, pul-monary edema, thickening of the interalveolar septa, and other relatively nonspecific changes of the lungs. For example, mice exposed to 10 ppm SO2 for 72 h showed necrosis and sloughing of the nasal epithelium (Giddens and Fairchild 1972). The lesions were more severe in animals with preexisting infection. Other symptoms include decreased weight gains, loss of hair, nephrosis in kidneys, myocardial degen-eration, and accelerated aging. Many studies have demonstrated increase in the response of animals to SO2 in the presence of particulate matter and elevations of relative humidity. Thus, H2SO4 mist and some particulate sulfates enhance the reactions of animals to SO2, suggest-ing that alteration of SO2 to a higher oxidation state may increase its irritability in animals. These interactions have important implications in air pollution control, as the rate of conversion of SO2 to acid sulfates may have greater health significance than the concentration of SO2 in the air. Effect on Humans Sulfur dioxide is rapidly absorbed in the nasopharynx of humans. Humans exposed to 5 ppm of the gas showed increased respiratory frequency and decreased tidal volume. Similar to observations made with animals, human exposure to SO2 alters the mode of respiration, as exhibited by increased frequency, decreased tidal volume, and lowered respiratory and expiration flow rates. Synergism and elevated airway resistance with SO2 and aerosols of water and saline have been demon-strated. It was previously thought that SO2 and black suspended particulate matter inter-acted and that both had to be elevated in order to exhibit health effects. New findings and analyses have changed such perception concerning the health effects of this group of pollutants. Emitted SO2 is generally thought to be oxidized slowly by atmospheric oxygen to SO3, which readily combines with water to form H2SO4. Ultimately the aerosol reacts with atmospheric particles or surfaces to form sulfates. The World Health Organization recommended that the air quality standards reflect the joint presence of SO2 and the resulting acid sulfates. Recent experimental and epidemiological data do not provide evidence for a specific effect of sulfate aerosol. However, airway reactivity is variable among subjects. Individuals with airway hyperactivity, e.g., asthmatics, have been shown to exhibit increased pulmonary flow resistance when exposed to SO2 by mouthpiece, while the increase was less with nasal breathing (Frank et al. 1962). Exercise augments responses to the pollutants. ... - tailieumienphi.vn