Trace Elements

All coals contain small concentrations of trace elements. Trace elements enter the atmosphere through natural processes, and sources of trace elements include soil, seawater, and volcanic eruptions. Human activities, such as power generation and industrial and commercial sectors, also lead to emissions of some elements. Although these elements are present in small concentrations in the coal (i.e., parts per million, ppm, by weight), the large amount of coal burned annually mobilizes tons of these pollutants as particles or gases.

Title III of the U.S. Clean Air Act Amendments (CAAA) of 1990 designates 188 hazardous air pollutants (HAPs). Included in the list are eleven trace elements: antimony (Sb), arsenic (As), beryllium (Be), cadmium (Cd), chromium (Cr), cobalt (Co), lead (Pb), manganese (Mn), mercury (Hg), nickel (Ni), and selenium (Se). In addition, barium (Ba) is regulated by the Resources Conservation and Recovery Act, and boron (B) and molybdenum (Mo) are regulated by Irrigation Water Standards [37]. Vanadium (V) is regulated based on its oxidation state, and vanadium pentoxide (V2O5) is a highly toxic regulated compound. Other elements, such as fluorine (F) and chlorine (Cl), which produce acid gases (i.e., HF and HCl) upon combustion, and radionuclides such as radon (Rn), thorium (Th), and uranium (U) are also of interest.

The distribution of trace elements in the bottom ash, ash collected in the air pollution control device, and fly ash and gaseous constituents emitted into the atmosphere depends on many factors, including the volatility of the elements, temperature profiles across the system, pollution control devices, and operating conditions [27,40]. Numerous studies have shown that trace elements can be classified into three broad categories based on

Hg Cl F

B Se

Al Fe Mn Si Th

Class III Volatilized and Emitted Rn ) Fully in the Vapor Phase—

Not Enriched in the Fly Ash

Class II Enriched in the Fly Ash and Depleted in the Bottom Ash

Class I Equally Distributed Between the Bottom Ash and the Fly Ash

FIGURE 3-1. Classification scheme for selected trace elements relative to their volatility and partitioning in power plants. (Adapted from Miller et al. [37] and Clarke and Sloss [40].)

their partitioning during coal combustion. A summary of these studies is presented by Clarke and Sloss [40], and Figure 3-1 illustrates the classification scheme for selected elements.

Class I elements are the least volatile and are concentrated in the coarse residues (i.e., bottom ash) or are equally divided between coarse residues and finer particles (i.e., fly ash). Class II elements will volatilize in the boiler but condense downstream and are concentrated in the finer-sized particles. Class III elements are the most volatile and exist entirely in the vapor phase. Overlap between classifications exists and is a function of fuel, combustion system design, and operating conditions, especially temperature [40].

Environmental Effects

The environmental effects of trace elements are a function of the chemical and physical form in which they are found [40]. Environmental effects may occur due to the element itself or as a result of a combination of the element and other compounds. Linking a specific environmental effect to an individual element is difficult as is determining the contribution from human activities because the trace elements also occur naturally.

Some trace elements may have an immediate effect in the atmosphere. Trace element metals such as Mn(II) and Fe(III) may contribute to acid rain by promoting oxidation of sulfur dioxide to sulfate in water droplets [40]. Trace elements may also be involved in the complex atmospheric chemistry that forms photochemical smog and may affect cloud formation.

Soils may contain high concentrations of certain trace elements due to natural minerals and ores. In addition, deposition of trace elements downwind from power plants can lead to high concentrations in the soils and uptake by plants. Some elements found in coal are major plant nutrients— specifically, calcium, magnesium, and potassium [40]. These elements are not considered trace elements because they occur in quantities greater than 1000 ppm (>0.1% by weight). Other elements (both major and trace) are considered minor plant nutrients, such as iron, manganese, copper, zinc, molybdenum, cobalt, and selenium. Elements such as aluminum, sodium, and vanadium are considered essential for some species while others are potentially toxic including chromium, nickel, lead, arsenic, and cadmium.

Cadmium Cadmium (Cd) is a silvery metal and is both toxic and carcinogenic [41]. The correlation between soil concentration of cadmium and plant uptake is not clear; consequently, there is concern about cadmium concentrations in the environment and ingestion of plant-based foods by the general population.

Mercury Mercury (Hg) is a liquid, silvery metal that is considered toxic; in the form of methyl mercury, it is extremely toxic [35,41]. The mercury directly emitted from power plants is measured as three forms: elemental (Hg°), oxidized (Hg+2), and condensed on ash particles (Hgp). In the natural environment, mercury can go through a series of chemical transformations to convert it to a highly toxic form, methylmercury (CH3Hg), which is concentrated in fish and birds [42]. Methylation rates in the ecosystems are a function of mercury availability, bacterial population, nutrient load, acidity and oxidizing conditions, sediment load, and sedimentation rates. Methylmercury enters the food chain, particularly in aquatic organisms, and bioaccumulates. Volatile elements that are emitted from power plants, such as mercury, are mostly found either in gaseous form or enriched on the surface of fine particles and physically should be available for uptake by plants [40]. There is evidence, however, that almost no mercury from the soil is taken into the shoots of the plants; hence, plants appear to be an important barrier against entry of mercury into the above-ground ecosystem, even if accumulation in the soil has occurred.

Lead Lead (Pb) is the only metal currently listed as a criteria pollutant. Lead is a gray metal with a low melting point. It is soft, malleable, ductile, resistant to corrosion, and a relatively poor electrical conductor [28]. For these reasons, lead has been used for over 4000 years for plates and cups, food storage vessels, paints, piping, roofing, storage containers for corrosive materials, radiation shields, lead-acid batteries, and as an organolead additive in gasoline. As a result, lead can be found throughout the world, including trace amounts in Antarctica and the Arctic [28]. In the past, automotive sources were the major contributor of lead emissions to the atmosphere [29]. Due to the EPA's regulatory efforts to remove lead from gasoline, along with banning lead from paint pigments and solder, a decline in lead emissions has been observed. The highest concentrations of lead are found in the vicinity of nonferrous and ferrous smelters and battery manufacturers. Lead can be deposited on the leaves of plants, presenting a hazard to grazing animals.

Selenium Crops, including animal forages are sensitive to the addition of small amounts of selenium (Se) in the soil [40]. Selenium is a silvery metallic allotrope or red amorphous powder [41] and is toxic to plants at low concentrations. It has been shown to cause stunting and brown spots in some varieties of beans and to reduce germination and cause stunting in cereals and cotton.

Other Trace Elements Clarke and Sloss [40] listed examples where trace elements from coal combustion may have beneficial effects in some areas. Boron is a micronutrient that is required in trace amounts by many plants and animals, and the amount of boron released from coal-fired power plants is likely to be beneficial to local agriculture. Copper, iron, manganese, and zinc are necessary for normal growth of plants. It is recognized, however, that excessive concentrations of copper and zinc lead to damage to root formation and growth, but the quantities being deposited around coal-fired power stations are likely to be beneficial to local soils. Also, manganese can be harmful in large doses, especially in acidic soils.

Health Effects

Trace element emissions have the potential to cause a number of harmful effects on human health. While there is no evidence that most trace elements from coal-fired power plants are causing health effects at their low ambient air concentration, there is concern that pollutants may accumulate throughout the food chain. This is especially true of mercury, which is discussed in more detail later in this section.

Arsenic The combustion of most coals is unlikely to contribute toxic amounts of arsenic (As) to air [43]. There is some concern, however, that arsenic in fly ash disposal and coal cleaning wastes may be leached into the groundwater. Arsenic, which is gray, metallic, soft, and brittle, may be considered essential; however, it is toxic in small doses [41]. Arsenic can cause anemia, gastric disturbance, renal symptoms, ulceration, and skin and lung cancer [40]. In addition, arsenic can damage peripheral nerves and bloodvessels and is a suspected teratogen (i.e., causes damage to embryos and fetuses). The chemical form of arsenic can affect its toxicity, with organic forms of arsenic being more toxic than elemental arsenic. An extreme example of chronic arsenic poisoning is occurring in the Guizhou Province of China [39,44]. In this province, the villagers bring their chili pepper harvest indoors in the autumn to dry. They hang their peppers over open-burning stoves, where arsenic-rich coal (up to 35,000 ppm) is used to heat and cook. These chili peppers, which normally contain <1 ppm arsenic, can contain as much as 500 ppm arsenic after drying. About 3000 people are exhibiting typical symptoms of arsenic poisoning, including hyperpigmentation (flushed appearance, freckles), hyperkeratosis (scaly lesions on the skin, generally concentrated on the hands and feet), Bowen's disease (dark, horny, precancerous lesions of the skin), and squamous cell carcinoma.

Boron Boron (B) is similar to arsenic in that the concentration emitted to the atmosphere is small and is unlikely to cause problems as an airborne pollutant [43]; however, boron in the fly ash can become soluble in ash disposal sites. Boron, a dark powder, is essential for plants but can be toxic in excess [41].

Beryllium Beryllium (Be), a silvery, lustrous, relatively soft metal, is toxic and carcinogenic [41]. It can cause respiratory disease and lymphatic, liver, spleen, and kidney effects [40].

Cadmium Cadmium (Cd) has no known biological function and is therefore not a nutritional requirement [40]. Cadmium is toxic, carcinogenic, and teratogenic [41] and can cause emphysema, fibrosis of the lung, renal injury, and possibly cardiovascular disease.

Chromium Chromium (Cr), a hard, blue-white metal, is an essential trace element; however, its chromates are toxic and carcinogenic [41]. Chromium, which is ingested by humans through food and drink, can be toxic when it accumulates in the liver and spleen [40]. The oxidation state of chromium affects its mobility and toxicity. Chromium (III) is nontoxic and has a tendency to absorb to clays, sediments, and organic matter and therefore is not very mobile. Chromium (IV), however, is more mobile and toxic and may account for ~5% of the total chromium particles emitted from power plants.

Fluorine Fluorine (F) is a pale yellow gas and is the most reactive of all elements [41]. It is an essential element and is commonly used for protecting the enamel of teeth, but excess fluoride is toxic. Fluorosis includes mottling of tooth enamel (dental fluorosis) and various forms of skeletal damage, including osteosclerosis, limited movement of the joints, and outward manifestations such as knock-knees, bow legs, and spinal curvature [39]. Fluorosis, combined with nutritional deficiencies in children, can result in severe bone deformation. An extreme example of this is exhibited in China, where the health problems caused by fluorine volatilized during domestic coal use are far more extensive than those caused by arsenic [39,44].

More than 10 million people in the Guizhou Province and surrounding areas suffer from various forms of fluorosis caused by corn being dried over unvented ovens burning high-fluorine (>200 ppm) coal.

Mercury Mercury (Hg) exists in trace amounts in fossil fuels, including coal, vegetation, crustal material, and waste products. Through combustion or natural processes, mercury vapor can be released to the atmosphere, where it can drift for a year or more, spreading with air currents over vast regions of the world [45]. Research indicates that mercury poses adverse human health effects, and fish consumption is the primary pathway for human and wildlife exposure. Mercury bioaccumulates in fish as methylmercury (CH3Hg) and poses a serious health hazard for humans. Other research suggests that other forms of mercury may be harmful as well [46,47]. Ingested mercury in elemental, organic, and inorganic form is converted to mercuric mercury, which is slowly eliminated from the kidneys but remains fixed in the brain indefinitely [47]. Exposure to high levels of metallic, inorganic, or organic mercury can permanently damage the brain, kidneys, and developing fetus [46]. Documented associations have been reported between low-dose, prenatal exposure to methylmercury and neurodevelopmental effects on attention, motor function, language, visual-spatial, and verbal abilities [48]. Loss of sight has been associated with cases of extreme mercury ingestion [39]. Chronic thallium poisoning has been reported in the Guizhou Province in China, where vegetables are grown on mercury-/thallium-rich mining slag. Most symptoms that have been reported, such as hair loss, are typical of thallium poisoning; however, many patients from this region have lost their vision, which is being attributed to mercury poisoning as the mercury concentration of this coal is 55 ppm, or about 200 times the average mercury concentration in U.S. coals.

Manganese Manganese (Mn) is unlikely to cause health problems as an airborne pollutant from the combustion of most coals, but leaching of ash may be a concern [43]. Manganese, a hard, brittle, silvery metal, is considered an essential nutrient, is nontoxic, and is a suspected carcinogen [41]. It is also reported to cause respiratory problems [40].

Molybdenum Molybdenum (Mo), a lustrous, silvery, and fairly soft metal, is an essential nutrient, is moderately toxic, and is a teratogen [41]. Under some conditions, molybdenosis may occur in animals, notably ruminants, due to consumption of vegetation with relatively high concentrations of molybdenum [43].

Nickel Nickel (Ni), a silvery, lustrous, malleable, and ductile metal, has no known biological role, and nickel and nickel oxide are carcinogenic [41]. Nickel can cause dermatitis and intestinal disorders [40].

Lead Exposure to lead (Pb) occurs mainly through inhalation of air or ingestion in food, water, soil, or dust. It accumulates in the blood, bones, and soft tissues [29]. Lead can adversely affect the kidneys, liver, nervous system, and other organs. Excessive exposure to lead may cause neurological impairments, such as seizures, mental retardation, and behavioral disorders. Even at low doses, lead exposure is associated with damage to the nervous system of fetuses and young children. Lead may be a factor in high blood pressure and heart disease.

Selenium Selenium (Se) is considered an essential element but is toxic in excess of dietary requirements and is also a carcinogen [41]. Livestock consuming plants with excessive amounts of selenium can suffer two diseases, alkali disease or blind staggers, and can experience infertility and cirrhosis of the liver, as well as death in extreme cases [40]. In humans, selenium can cause gastrointestinal disturbance, liver and spleen damage, and anemia and is a suspected teratogen. Symptoms of selenium poisoning include hair and nail loss. Selenosis has been reported in southwest China, where selenium-rich carbonaceous shales, known locally as "stone coals,'' are used for home heating and cooking [39]. The ash from this selenium-rich coal (as much as 8400 ppm) is then used as a soil amendment, thereby introducing high concentrations of selenium into the soil that is subsequently taken up by crops.

Vanadium Vanadium (V), a shiny, silvery, soft metal, is an essential trace element, although some compounds, specifically vanadium pentoxide (V2O5), are quite toxic [41]. Health effects associated with vanadium include acute and chronic respiratory dysfunction [40].

Radionuclides Radionuclides are listed generically as 1990 CAAA HAPs. Radioactivity arises mainly from isotopes of lead, radium, radon, thorium, and uranium [37,40,43,49]. Health effects from radiation are well documented and include various forms of cancer; however, radionuclide emissions from power plants are quite low [37,40]. During coal combustion, most of the uranium and thorium and their decay products are released from the original coal matrix and are distributed between the gas phase and solid combustion products [49]. Virtually 100% of the radon gas present in the coal feed is transferred to the gas phase and is lost in stack emissions. In contrast, less volatile elements, such as thorium and uranium, and the majority of their decay products are retained in the solid combustion wastes [37,49]. Fly ash is commonly used as an additive to concrete building products, but the radioactivity of typical fly ash is not significantly different from that of more conventional concrete additives or other building materials such as granite or red brick [49].

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