Selenium excess and deficiency have been established as the cause of various health problems in man and animals. Combustion of fossil fuels, especially coal, may be a major source of the anthropogenic introduction of selenium in the environment. Coal is enriched in selenium relative to selenium's concentration in most other rocks and relative to selenium in the Earth's crust.Data from almost 9,000 coal samples have been used to determine the concentration and distribution of selenium in US coals. The geometric mean concentration of selenium in US coal is 1.7 ppm. The highest mean selenium value (geometric mean 4.7 ppm) is in the Texas Region. Atlantic Coast (Virginia and North Carolina) and Alaska coals have the lowest geometric means (0.2 and 0.42 ppm, respectively). All western coal regions have mean selenium concentrations of less than 2.0 ppm. In contrast, all coal basins east of the Rocky Mountains (except for several small basins in Rhode Island, Virginia, and North Carolina) have mean selenium values of 1.9 or greater.Generally, variations in selenium concentration do not correlate with variations in ash yield, pyritic sulphur, or organic sulphur concentrations. This may be the result of multiple sources of selenium; however, in some non-marine basins with restricted sources of selenium, selenium has positive correlations with other coal quality parameters.Selenium occurs in several forms in coal but appears to be chiefly associated with the organic fraction, probably substituting for organic sulphur. Other important forms of selenium in coal are selenium-bearing pyrite, selenium-bearing galena, and lead selenide (clausthalite). Water-soluble and ion-exchangeable selenium also have been reported.
The concentration of arsenic (As) in U.S. coal is significant in coal cleaning, coal utilization, and environmental considerations. Arsenic is significant because of its potential toxicity for plants and animals. This chapter examines concentrations and modes of occurrence of arsenic in U.S. coals. The data used in this study are from more than 5,000 determinations of As in coal samples, analyzed on an as-received basis using wet chemical and instrumental neutron activation analysis (INAA).Variation in As content was related to coal provinces and coal regions, coal rank, geologic age, sulfur and ash contents, heating values, and stratigraphic and lateral variation. The As content of foreign coal, roof-and floor-rock and coal partings, and coal wastes is discussed. The mode of occurrence of As is reviewed. It appears that the bulk of arsenic in coal is in sulfide minerals, primarily iron sulfides. Much of the arsenic-bearing sulfides may be epigenetic in origin, or the arsenic may have been emplaced by reaction of pyrite with arsenic-bearing mineralogic solutions. Low levels of arsenic (<5 ppm) may be organically associated.Analysis of the As data indicated the following. (1) Appalachian and Western Interior coals have the highest As content; Fort Union and Wind River regions have the least. (2) There is no systematic relation between As content and coal rank, coal deposit age, and total sulfur content; however, there is a sympathetic relation with pyritic sulfur content. (3) Arsenic concentration is highest in coal with heating values between 12,000 and 13,000 Btu/lb. (4) Stratigraphic profiles of As content for four areas show wide variations, with samples from western Kentucky displaying the least variation and samples from the southern Appalachians the most. Pyritic sulfur-content variations generally parallel those of As content. (5) Arsenic distribution maps of a small area in the Western Kentucky coal field generally show As content increasing from the basin toward the margin. (6) Roof and floor rocks contain similar amounts of As as in U.S. coal; arsenic content in U.S. coal is much less than that in coal wastes. (7) Arsenic enrichment factors reveal little difference on the basis of geologic age and coal rank regardless of whether average shale or the crustal averages are used for comparison. There are significant differences in enrichment factor values between some coal provinces or regions.Coleman, S. L., and Bragg, L. J., 1990, Distribution and mode of occurrence of arsenic in coal, in Chyi, L.
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