Trace Elements in Coal

Transcription

1 by Humana Press Inc. All rights of any nature, whatsoever, reserved /99/ $12.00 Trace Elements in Coal Environmental and Health Significance ROBERT B. FINKELMAN U.S. Geological Survey, Mail Stop 956, Reston, VA ABSTRACT Received April 20, 1998; Accepted June 6, 1998 Trace elements can have profound adverse effects on the health of people burning coal in homes or living near coal deposits, coal mines, and coal-burning power plants. Trace elements such as arsenic emitted from coal-burning power plants in Europe and Asia have been shown to cause severe health problems. Perhaps the most widespread health problems are caused by domestic coal combustion in developing countries where millions of people suffer from fluorosis and thousands from arsenism. Better knowledge of coal quality characteristics may help to reduce some of these health problems. For example, information on concentrations and distributions of potentially toxic elements in coal may help delineate areas of a coal deposit to be avoided. Information on the modes of occurrence of these elements and the textural relations of the minerals in coal may help to predict the behavior of the potentially toxic trace metals during coal cleaning, combustion, weathering, and leaching. Index Entries: Coal combustion; health impacts; trace elements; trace metals; coal quality. INTRODUCTION Trace elements in the environment come from a variety of natural and anthropogenic sources (1). Perhaps none of the sources is as misunderstood as coal combustion. Under certain conditions, the emission of trace elements from coal combustion can be relatively benign. Under other conditions, these emissions present a potentially deadly threat that has caused severe health problems for tens of millions of people. Ironically, the concerns over the environmental and human health impacts of coal combustion appear to be inversely proportional to the damage done. Biological Trace Element Research 197 Vol. 67, 1999

2 198 Finkelman Table 1 Arithmetic and Geometric Means for Chemical Elements in US Coal Arithnaetic Geometric Component Mean S.D. Mean S.D. Max. Num. Ash % Aluminum (A1)% Antimony (Sb) ppm Arsenic (As) ppm Barium (Ba) ppm Beryllium (Be) ppm Bismuth (Bi) ppm (<1.0) n.d. n.d. n.d Boron (B) ppm I Bromine (Br) ppm Cadmium (Cd) ppm Calcium (Ca) % Carbon (C) % Cerium (Ce) ppm Cesium (Cs) ppm Chlorine (C1) ppm Chromium (Cr) ppm Cobalt (Co) ppm Copper (Cu) ppm Dysprosium (Dy) ppm Erbium (Er) ppm Europium (Eu) ppm Fluorine (F) ppm Gadolinium (Gd) ppm [1.8] n.d. n.d. n.d., Gallium (Ga) ppm Germanium (Ge) ppm Gold (Au) ppm (<0.05) n.d. n.d. n.d. n.d. n.d. Hafnium (Hf) ppm Holmium (I-Io) ppm [0.35] n.d. n.d. n.d Hydrogen (H) % Indium (In) ppm (<0.3) n.d. n.d. n.d. n.d. n.d. Iodine (I) ppm (<1.0) n.d. n.d. n.d. n.d. n.d. Iridium (I0 ppm (<0.001) n.d. n.d. n.d. n.d. n.d. Iron (Fe) ppm Lanthanum (La) ppm Lead (Pb) ppm Lithium (Li) ppm Lutetium (Lu) ppm Magnesium (Mg) %. I Coal plays a crucial role in the world's energy mix--about 26% of all the energy produced in the world in 1995 was derived from coal combustion (2). Nearly 60% of the electricity produced in the United States is generated from coal. Forecasts for the next 20 yr suggest that coal use in the United States will increase slightly; however, coal use in developing countries, especially China and India, should increase substantially because of population growth and industrialization (2). One consequence of the mining and combustion of coal is the mobilization of trace elements, especially trace metals, that may have environmental and human health significance.

3 Trace Elements in Coal 199 Table 1 (Continued) Arithmetic Geometric Component Mean S.D. Mean S.D. Max. Num. Manganese (Mn) ppm Mercury (Hg) ppm Molybdenum (Mo) ppm Neodymium (Nd) ppm [9.5] n.d. n.d. n.d Nickel (Ni) ppm Niobium (Nb) ppm Nitrogen (N) % Osmium (Os) ppm (<0.001) n.d. n.d. n.d. n.d. n.d. Oxygen (O) % Palladium (Pd) ppm (<0.001) n.d. n.d. n.d. n.d. n.d. Phosphorus (P) ppm Platinum (Pt) ppm (<0.001) n.d. n.d. n.d. n.d. n.d. Potassium (K) % Praseodymium (Pr) ppm [2.4] n.d. n.d. n.d Rhenium (Re) ppm (<0.001) n.d. n.d. n.d. n.d. n.d. Rhodium (Rh) ppm (<0.001) n.d. n.d. n.d. n.d. n.d. Rubidium (Rb) ppm Ruthenium (Ru) ppm (<0.001) n.d. n.d. n.d. n.d. n.d. Samarium (Sm) ppm Scandium (Sc) ppm Selenium (Se) ppm Silicon (Si) % Silver (Ag) ppm (<0.1) Sodium (Na) % Strontium (Sr) ppm Sulfur (S) % Tantalum (Ta) ppm Tellurium (Te) ppm (<0.1) n.d. n.d. n.d. n.d. n.d. Terbium (Tb) ppm Thallium (TI) ppm Thorium (Th) ppm Thulium (Tm) ppm [0.15] n.d. n.d. n.d Tin (Sn) ppm Titanium (Ti) % Tungsten (W) ppm Uranium (U) ppm Vanadium (V) ppm Ytterbium (Yb) ppm [0.95] n.d. n.d. n.d Yttrium 00 ppm Zinc (Zn) ppm Zirconium (Zr) ppm Note: All values are on a coal basis. Data are exclusively from the U.S. Geological Survey (USGS) except for estimated values in parenthesis, which are based on USGS and literature data. Values in brackets are calculated from cerium and lanthanum data and assuming a chondrite normalized rare earth element distribution pattern. (n.d. = no data; S.D. = standard deviation; Max. = maximum; Num. = number of samples). Coal contains detectable concentrations of virtually every element in the periodic table (Table 1). Many of these elements, including many potentially toxic metals, are enriched in coal (some by several orders of

4 200 Finkelman magnitude) relative to their concentrations in the Earth's surface. Elements in coal can be released into the environment by a variety of processes such as the following: 9 Leaching of in-ground coal by ground water 9 Ground or surface water leaching of coal exposed by mining 9 Leaching of coal in storage piles by precipitation 9 Atmospheric emissions from utility, industrial, and domestic coal combustion 9 Combustion of coal storage or waste piles 9 Natural combustion of coal beds at or near the Earth's surface 9 Leaching from buried coal combustion products or from construction materials made from coal combustion residues The environmental and human health impacts of most of these processes are not fully understood and deserve further attention. For most of these processes, the impacts will be local or, at most, regional. Coal combustion, however, can have international and even global impacts, as air masses transport emissions across the globe. This article describes several examples of how trace elements, mobilized by coal combustion, have affected the health of people in Asia and Eastern Europe. TRACE ELEMENTS IN COAL Concerns have been expressed in the United States (3) and elsewhere (4) over the release of As, Be, Cd, Co, Cr, F, Hg, Ni, Pb, Sb, Se, U, and other trace elements during the burning of coal. Trace elements such as arsenic, emitted from coal-burning power plants in Europe, have been shown to cause severe health problems. Bencko and Symon (5) studied 10-yr-old boys living in the vicinity of a power plant in Slovakia that burned high-arsenic lignite ( ppm As on a dry basis). They found significant hearing loss and attributed it to arsenic poisoning from the power plant emissions. Bencko (6) also found a significant increase of skin basalioma cancer in the region nearest the power plant. Bencko and others (7) also suggested that immunological changes detected in people living near a coal-burning power plant may be the result of beryllium mobilized by coal combustion. Recent risk assessment calculations from the U.S. Environmental Protection Agency (8) indicate that trace element emissions from power plants using coal having relatively low to moderate trace element concentrations or having efficient pollution control devices may not present a significant health risk. This report indicates that the risk of trace elements causing cancer from inhalation in a population near a power plant is less than 1 in a million. Nevertheless, there are other situations in which trace elements emitted from coal combustion can cause serious health problems. Perhaps the most widespread health problems are

5 Trace Elements in Coal 201 caused by domestic coal combustion in developing countries. Millions of people in China suffer from fluorosis (9) and thousands suffer from arsenism (10) caused by coal combustion in China. Selenium (11) and mercury poisoning have also been attributed to domestic coal combustion. Zheng and Huang (9) have demonstrated that adsorption of fluorine by corn dried over unvented ovens burning high (> 200 ppm in the coal) fluorine coal is the probable cause of extensive dental and skeletal fluorosis in southwest China. The problem is compounded by the use of clay as a binder for making briquettes. The clay used is a high-fluorine (about 900 ppm) residue from the intense leaching of the limestone substrate. The occurrence of arsenic poisoning in southwest China may have a similar etiology (10). The primary source of arsenic may be from chili peppers dried over unvented ovens burning high-arsenic coal. One coal sample analyzed at the U.S. Geological Survey laboratory had 35,000 ppm arsenic on an as-analyzed basis! The fresh chili peppers have less than 1 ppm arsenic. The chili peppers dried over the high-arsenic coal fires have more than 500 ppm arsenic (12). Ingestion of the chili peppers tainted with arsenic from the mineralized coal has caused thousands of cases of arsenosis, producing severe skin cancers (12). Zheng and others (11) report nearly 500 cases of human selenosis in southwest China that are attributed to the use of selenium-rich carbonaceous shales known locally as "stone coal." The stone coals have as much as 8390 ppm selenium (whole-rock basis). The selenosis is attributed to the practice of using combustion ash as a soil amendment. This process introduced large amounts of selenium into the soil and resulted in selenium uptake by crops. There is also considerable concern about the health effects of mercury and the proportion of anthropogenic mercury in the environment (13). So far, there is no direct evidence of health problems caused by mercury released from coal, but there are circumstances where poisoning from mercury released from coal combustion may be occurring. Zhou and Liu (14) reported on chronic thallium poisoning in Guizhou Province, China, where the source of the thallium poisoning appears to be from vegetables grown on a mercury/thallium-rich mining slag. Most symptoms, such as hair loss, are typical of thallium poisoning. However, loss of vision in several patients from this region was considered to be unique (15). Mineralogical analysis of the coal being used in the homes of patients having visual impairment revealed abundant mercury minerals. Chemical analysis of a coal sample being used in Guizhou Province, China indicates a mercury concentration of 55 ppm, which is more than 300 times the average mercury concentration in US coals. Iron concentrations in coal are generally in the percent range; thus, iron is not truly a trace constituent. Nevertheless, it is worthwhile noting that iron may act as an intermediary in causing common lung disorders of coal minors. Huang and co-workers (16) have suggested that acid-soluble ferrous iron may cause lung tissue damage leading to

6 202 Finkelman emphysema. The impact of the acid-soluble ferrous iron is tempered by the presence of carbonate minerals in the coal that neutralize the acids and allow the oxidation of the ferrous iron to benign ferric iron. COAL CHARACTERIZATION Determining the concentration of the elements in coal is complicated by the fact that coal contains virtually every element in the periodic table and by the wide range in element concentrations (from percent to parts per billion). The situation is further complicated by the wide range in properties, such as volatility, of the elements. To achieve a comprehensive chemical characterization of coal, several analytical techniques are generally employed. The most commonly used multielement techniques are inductively coupled plasma-mass spectroscopy and inductively coupled plasma-atomic emission spectroscopy. Instrumental neutron activation analysis has been successfully applied to coal characterization for many years because it can be used on whole coal, thus avoiding problems of volatilization. For the highly volatile elements, element-specific methods are employed: mercury and selenium are commonly determined by coal vapor and hydride generation atomic absorption spectroscopy, respectively, and fluorine and chlorine by selective ion electrode analysis. Other analytical techniques applied to coal include X-ray fluorescence spectroscopy and atomic absorption spectroscopy. For a recent discussion of chemical analysis of coal, see ref. 17. The concentration of an element in coal has been used as a gauge of the element's potential environmental and health impacts. Clearly, knowing the concentration of a potentially toxic element in coal will help determine if use of the coal might present a health risk. Furthermore, knowing the vertical and lateral distribution of the element in a coal deposit will allow for intelligent decisions regarding mining all or parts of the deposit. However, the concentration of an element by itself provides only a partial measure of the element's potential impacts. The modes of occurrence and textural relations of the element are important parameters that may help in designing procedures to reduce human exposure to the element. The element's modes of occurrence (chemical form) may help to predict the element's behavior during coal cleaning, combustion, weathering, and leaching (18). For example, arsenic associated with sulfide minerals in coal may be retained in the fly ash and bottom ash of the combusted coal, whereas organically bound arsenic may be preferentially volatilized (19). In addition, modes of occurrence information are necessary for developing efficient procedures for physically removing toxic elements prior to coal combustion (20). The low concentrations and the dispersed nature of many trace elements in coal makes determining their modes of occurrence a challenge.

7 Trace Elements in Coal 203 The modes of occurrence of an element can be inferred from indirect evidence, such as from the analysis of density separates. It is assumed that those elements associated with the inorganic constituents (minerals) will be concentrated in the heavier-specific-gravity fractions and those elements associated with the organic constituents (macerals) will be concentrated in the lighter-specific-gravity fractions. Other indirect evidence includes statistical correlation with other elements and behavior of an element during heating and leaching experiments (21). Preferably, the modes of occurrence of elements in coal should be determined directly, using microbeam instruments such as scanning electron microscopy with energy dispersive detectors, electron microprobe analyzers, X-ray mineralogical analysis (21), and X-ray absorption finestructure spectroscopy (22). CONCLUSIONS A better knowledge of coal quality characteristics may help to minimize some of the health problems caused by coal. Information on the concentrations and distributions of potentially toxic elements in coal will help to avoid those coal deposits or zones within coal deposits having undesirably high concentrations of toxic compounds. Information on the modes of occurrence of potentially toxic elements and the textural relations of the minerals and the organic components in which they occur may help to anticipate the behavior of the potentially toxic components during coal cleaning, combustion, weathering, and leaching. Coal quality characterization, therefore, offers coal scientists and biomedical researchers opportunities to contribute to improve public health in many parts of the world. REFERENCES 1. D. C. Adriano, Trace Elements in the Terrestrial Environment, Springer-Verlag, New York (1986). 2. Energy Information Agency, Coal [online]. Available at ieo97/coal.html (file last modified 5/1/97). 3. U.S. Statutes at Large, Public Law , Provisions for attainment and maintenance of national ambient air quality standards, 101st Congress, 2nd Session, 104, Part 4, pp (1990). 4. P. Singh-Mahendra, R. M. Singh, and D. Chandra, Environmental and health problems due to geochemical alterations associated with trace elements in coals, Ghugus Coalfield, Wardha Valley, Maharashtra, Quart. J. Geol. Min. Metall. Soc. India 57, (1987). 5. V. Bencko and K. Symon, Health aspects of burning coal with a high arsenic content. II. Hearing aspects of burning coal with a high arsenic content, Environ. Res. 13, (1977). 6. V. Bencko, Health aspects of burning coal with a high arsenic content: the Central Slovakia experience, in Arsenic: Exposure and Health Effects, C. O. Abernathy and R. L. Calderon, eds., Chapman & Hall, New York, pp (1997).

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