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Arsenic in Coal

Sources of Arsenic
Arsenic is a persistent toxin that occurs naturally in trace amounts in rocks, sediments, and coal. Small amounts of arsenic may be present in ground water or, less commonly, surface waters, especially where there is a nearby source of arsenic. The most widespread natural source of arsenic is pyrite, a common mineral composed of iron and sulfur, which can contain a small amount of arsenic in its structure in place of sulfur. Arsenic occurs in some ground-water aquifers due to chemical oxidation of pyrite or to reduction (the opposite of oxidation) of iron oxide minerals in the aquifer. Other sources of arsenic include past industrial activities, application of arsenic as a pesticide, and drainage from abandoned mine lands that contain pyrite. Prolonged consumption of drinking water from wells that greatly exceed arsenic health standards is the most serious arsenic-related health hazard in the United States and throughout the world.

Arsenic in bituminous coal occurs primarily in pyrite and, to a lesser extent, in organic portions of the coal. A small fraction of this arsenic is emitted during coal combustion. This Fact Sheet provides information on the arsenic content of U.S. coals, how arsenic occurs, and its behavior during mining, coal preparation, and coal combustion and in postcombustion beneficial uses.

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Arsenic in U.S. Coal
All coals contain some arsenic. The U.S. Geological Survey (USGS) maintains an extensive database of over 7,000 analyses of U.S. coals. Data from this compilation indicate that the average arsenic concentration for U.S. coal is about 24 parts per million (ppm; Bragg and others, 1998; fig. 1). There is considerable variation by coal basin, ranging from an average low of 1.4 ppm in the Raton Mesa Basin of Colorado and New Mexico to a high of 71 ppm in the southern (Alabama-Tennessee) portion of the Appalachian Basin (table 1).

Figure 1. Histogram showing arsenic distribution in U.S. coal samples (data from Bragg and others, 1998). Arsenic (in parts per million, ppm) is expressed on a remnant moisture whole coal basis. Forty samples having arsenic values exceeding 300 ppm are excluded to make the sample distribution easier to view but are included in statistics given in table 1.

Many Eastern U.S. coals are subjected to a cleaning process prior to use in coal-burning power stations. Coal cleaning is primarily intended to reduce the sulfur content of these coals, but in many cases arsenic contents are reduced as well. As a result, for coals that are cleaned, the arsenic concentration is, on average, lower than equivalent in-ground arsenic contents.

Table 1. Comparison of average arsenic content in U.S. coals by basin and calculated average powerplant input loadings of arsenic.
[Data from Bragg and others, 1998. ppm, parts per million; Btu, British thermal unit; lb, pound]

Coal basin Appalachian, Northern Appalachian, Central Appalachian, Southern Eastern Interior Fort Union Green River Gulf Coast Pennsylvania Anthracite Powder River Raton Mesa San Juan River Uinta Western Interior Wind River
U.S. Department of the Interior U.S. Geological Survey

Median 16 7.8 29 10 4.2 1.2 2.2 3.2 1.6 0.99 0.92 0.7 14 2.4

Arsenic (ppm) Mean 28 22 71 19 8.5 4.8 3.2 8.1 4.2 1.4 2.5 1.5 21 7

Number of samples 1,607 1,742 974 289 280 391 141 51 602 40 185 249 286 41

Calorific value (Btu/lb) Median Mean Number of samples 12,570 13,360 12,850 11,510 6,340 9,950 6,440 12,860 8,050 12,500 9,380 11,290 11,320 9,630 12,440 13,210 12,760 11,450 6,410 9,560 6,470 12,530 8,080 12,300 9,640 10,820 11,420 9,570 1,500 1,643 968 255 257 264 110 39 486 34 169 222 261 41

Arsenic input loadings (in 103 lbs per 1012 Btu) Median Mean Number of samples 1.3 0.6 2.2 0.92 0.7 0.13 0.34 0.25 0.2 0.073 0.095 0.074 1.2 0.25 2.3 1.7 5.8 1.7 1.4 0.44 0.54 0.79 0.5 0.1 0.26 0.14 1.9 0.75 1,500 1,643 968 255 257 264 110 39 486 34 169 222 261 41
Fact Sheet 2005–3152 February 2006

Printed on recycled paper


Mode of Occurrence
The mode of occurrence of arsenic is important in determining its relative mobility during mining, combustion, and storage of coal for electric power generation; disposal or use of coal combustion byproducts; and leaching of coal beds in contact with ground water. Iron sulfides such as pyrite and marcasite are common inorganic constituents of coal, composing anywhere from a negligible amount to about 5 percent by weight. Other than iron and sulfur, arsenic is generally the most abundant element in pyrite and marcasite, but its concentration is highly variable (fig. 2). In bituminous coals, pyrite generally accounts for the single largest fraction of arsenic, with lesser fractions in the organic portion of the coal and in other inorganic forms (fig. 3). Lower rank coals (lignite and subbituminous) generally have a larger proportion of their arsenic in the organic portion. Some coals have unusually high levels of arsenic, independent of coal rank. This enrichment is thought to result from past geologic interaction of coal beds with metal-enriched fluids, similar to the process

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Figure 3. Selective leaching results for 11 U.S. bituminous coal samples. Plot shows that arsenic leached by nitric acid (yellow sub-bars) is the dominant form in these samples. Nitric acid is known to dissolve pyrite. The fraction of arsenic not leached (100 percent minus total bar height) represents organic-associated arsenic not removed by the leaching process. Arsenic present in all other forms is shown as “other.” Modified from Palmer and others, 1998.

by which metallic ore deposits are formed (Goldhaber and others, 2003).

abandoned mines or undisturbed pyrite-rich strata, however, decomposition of pyrite in coal and rock can lead to dispersal of arsenic to the environment. For example, in the Warrior Coal Field of Alabama, stream sediments in areas that have been mined have higher arsenic concentrations than stream sediments outside the coal-mining areas (Goldhaber and others, 2001). With exposure to moisture and atmospheric oxygen, arsenic in pyrite is rapidly converted to an oxidized form known as arsenate that is readily leached by precipitation and ground water (Huggins and others, 2002). The presence of arsenic in pyrite is thought to destabilize it, possibly hastening its decomposition to iron oxyhydroxide minerals. Because pyrite in coal oxidizes rapidly, coal storage facilities and waste materials from coal washing operations are potential sources of arsenic mobilization.

Arsenic in Waste from Coal Mining, Processing, and Storage
Mining operations in the United States generally must meet water-quality standards for discharge of mine water. In some

Arsenic in Coal Combustion Products
USGS studies indicate that, during coal combustion in modern coal-fired utilities, 90–100 percent of arsenic is

B A Figure 2. Elemental maps and microprobe analysis points showing distribution of arsenic (As) in pyrite in two Alabama coal samples. Maps are falsecolor images using a “thermal” intensity scale. A, Pyrite from a bituminous coal sampled in Cullman County, Alabama. Arsenic concentration at the points shown ranges from less than the detection limit (0.01 weight percent) to 2.6 weight percent. Image by Allan Kolker, USGS. B, Pyrite collected from a fracture in bituminous coal from Walker County, Alabama. Image shows zoning of arsenic content ranging up to nearly 4.5 weight percent. Image by Rob Koeppen, USGS; from Goldhaber and others (2003).

captured in coal combustion byproducts (Brownfield and others, 1999). Coal combustion byproducts such as fly ash and bottom ash likely contain multiple forms of arsenic. Some researchers have concluded that arsenic in coal ash is incorporated into iron-rich glass and crystalline silicate minerals that constitute the bulk of the solid ash. A more widely held view is that arsenic in flue gases combines with calcium oxide in the hot gases to form calcium arsenate (Ca3(AsO4)2) that is concentrated on the surfaces of coal ash particles. Coal combustion byproducts are widely used in a variety of commercial applications. These include cement and concrete, aggregates, structural fill, and even reclamation of abandoned coal mines. Such uses for coal ash are permitted by its designation as nonhazardous waste under the Resource Conservation and Recovery Act. Standardized tests, including the U.S. Environmental Protection Agency’s (USEPA) Toxicity Characteristic Leaching Procedure, show that harmful substances such as arsenic are not readily leached from these materials under simulated environmental conditions. Additional information on coal combustion products is given in Kalyoncu and Olson (2001).

Health Impact
In the United States, human exposure to arsenic primarily occurs in rural areas, through consumption of water from wells that greatly exceed the arsenic drinking water standard (10 micrograms per liter) specified by the USEPA. In the vast majority of cases, this exposure is unrelated to coal or emissions from coal-burning power-

plants. More direct exposure to arsenic from coal occurs where coal is used for domestic purposes. In the United States, such exposure is very limited. Domestic coal use is far more common in countries such as China, India, and South Africa. Health effects from arsenic have been documented where extraordinarily arsenic-rich coals were used to dry foods that were later consumed (Finkelman and others, 2002; fig. 4).

Technological Impact
Coal-fired utilities have been required to reduce emissions of nitrous oxides (NOx) to meet air quality standards. To comply with these new emissions regulations, many existing coal-fired powerplants and new installations are equipped with selective catalytic reduction (SCR) technology. SCR works by injecting ammonia into the flue gas stream. The ammonia passes over a catalyst that reduces the nitrous oxides (NOx) in the flue gas to elemental nitrogen and water. Although SCR is very effective in controlling NOx emissions, the effectiveness of the catalyst is greatly reduced if there are elevated concentrations of arsenic and selenium present in the flue gas stream. As a result, the amounts of these two elements in utility feed coal are a potential concern to utilities (Rigby and others, 2000) and may be monitored closely.

Figure 4. Interior view of residence in southwestern Guizhou Province, China, where arsenic-rich coal and coal briquettes are used to dry crops (chili peppers) that are later consumed, resulting in arsenic toxicity. Note the lack of ventilation in this dwelling. Photograph by Harvey Belkin, USGS.

Arsenic in bituminous coals is present primarily within the mineral pyrite. Pyrite occurs in small amounts in many rock types and sediments and coal. Pyrite and its weathering products are potential sources of arsenic in ground water and, more locally, in surface water. During coal combustion in modern coal-fired utilities, most of the arsenic is captured in coal combustion byproducts, which are considered nonhazardous waste, on the basis of acceptable performance in standardized leaching tests. Arsenic exposure in the United States primarily results from consumption of drinking water from private wells that exceed drinking water standards. Documented health effects from arsenic in coal are known elsewhere in the world in cases where unusually arsenic-rich coal is used domestically.

Goldhaber, M.B., Irwin, E., Atkins, B., Lee, L., Black, D.D., Zappia, H., Hatch, J.R., Pashin, J.C., Barwick, L.H., Cartwright, W.E., Sanzolone, R., Ruppert, L., Kolker, A., and Finkelman, R.B., 2001, Arsenic in stream sediments of northern Alabama: U.S. Geological Survey Miscellaneous Field Studies Map MF–2357, 1 sheet. (Available on the Web at mf/2001/mf-2357/.) Goldhaber, M.B., Lee, R.C., Hatch, J.R., Pashin, J.C., and Treworgy, J., 2003, Role of large scale fluid-flow in subsurface arsenic enrichment, in Welch, A.H., and Stollenwerk, K.G., eds., Arsenic in ground water—Geochemistry and occurrence: Boston, Kluwer Academic Publishers, p. 127–164. Huggins, F.E., Huffman, G.P., Kolker, Allan, Mroczkowski, S.J., Palmer, C.A., and Finkelman, R.B., 2002, Combined application of XAFS spectroscopy and sequential leaching for determination of arsenic speciation in coal: Energy and Fuels, v. 16, no. 5, p. 1167–1172.

Kalyoncu, R.S., and Olson, D.W., 2001, Coal combustion products: U.S. Geological Survey Fact Sheet FS–076–01, 4 p. (Available on the Web at Palmer, C.A., Mroczkowski, S J., Finkelman, R.B., and Crowley, S.S., 1998, The use of sequential leaching to quantify the modes of occurrence of elements in coals: Proceedings of the Fifteenth Annual International Pittsburgh Coal Conference, 28 p., CD-ROM. Rigby, K., Johnson, R., Neufort, R., Pajonk, G., Hums, E., Klatt, A., and Sigling, R., 2000, SCR catalyst design issues and operating experience—Coals with high arsenic concentrations and coals from the Powder River Basin: Proceedings of the 2000 International Joint Power Generation Conference, Miami Beach, Fla., July 23–26, 2000, 8 p. By Allan Kolker, Curtis A. Palmer, Linda J. Bragg, and Joseph E. Bunnell

References Cited
Bragg, L.J., Oman, J.K., Tewalt, S.J., Oman, C.L., Rega, N.H., Washington, P.M., and Finkelman, R.B., 1998, U.S. Geological Survey Coal Quality (COALQUAL) Database, version 2.0: U.S. Geological Survey Open-File Report 97–134, CD-ROM. (Available on the Web at of/1997/of97-134/.) Brownfield, M.E., Affolter, R.H., Cathcart, J.D., O’Connor, J.T., and Brownfield, I.K., 1999, Characterization of feed coal and coal combustion products from power plants in Indiana and Kentucky: Proceedings of the 24th International Technical Conference on Coal Utilization and Fuel Systems, Clearwater, Fla., p. 989–1000. Finkelman, R.B., Orem, William, Castranova, Vincent, Tatu, C.A., Belkin, H.E., Zheng, Baoshan, Lerch, H.E., Maharaj, S.V., and Bates, A.L., 2002, Health impacts of coal and coal use—Possible solutions: International Journal of Coal Geology, v. 50, p. 425–443.

Arsenic in coal and modes of occurrence Allan Kolker (703) 648-6418 Selective leaching studies Curtis A. Palmer (703) 648-6185 The coal quality database Linda J. Bragg (703) 648-6451 Health effects from arsenic exposure Joseph E. Bunnell (703) 648-6497 U.S. Geological Survey 956 National Center Reston, VA 20192