Geologic Distribution: Selenium was first noted by Villanova in the 14th century, but was discovered by Berzelius in 1818 as an impurity in copper pyrites 1. Selenium occurs in coal primarily within host minerals, most within commonly occurring pyrite 2. Isolated small selenium concentrations in pyrite were observed by scanning electron microscopy (SEM) 3. Finkelman, using SEM, found occurrences of PbSe (probably the rare mineral clausthalite) in Appalachian coals 4. An unpublished study at the WVGES using SEM found selenium in PbSe and/or clausthalite-chalcopyrite solid solution in 12 of 24 coal samples studied, mainly in upper Kanawha Formation coals. Selenium in West Virginia coals averaged 4.20 ppm, and did not statistically correlate with ash yield, total sulfur, pyritic sulfur or other trace elements except only slightly with thorium (Th). Coal beds of the Allegheny and upper Kanawha Formations of the Middle Pennsylvanian exhibit the highest selenium contents. The coals show a generally decreasing trend in selenium content from the upper Middle Pennsylvanian to the Lower Pennsylvanian. Coals of the Upper Pennsylvanian exhibit the lowest selenium abundances. Coals containing the highest selenium contents are in a region of south central WV where Allegheny and upper Kanawha coals containing the most selenium are mined, but the association of highest whole coal Se with these coals remains unexplained.
Economic Impact: Selenium in coal is not economically important. Selenium is a valuable element with applications in electronics, in photovoltaic cells used in photographic light meters, solar cells, semiconductors and rectifiers 1. Selenium is also used in stainless steel and in Xerography 1.
Environmental Impact: Selenium compounds are five times as toxic as arsenic 5. The major environmental concern of selenium in coal is through burning coal in arid parts of the US and world where it becomes concentrated in the soil and seleniferous (selenium accumulating) plants sometimes to toxic levels 5. Selenium is not an environmental problem in moist regions like the Eastern U.S. where concentrations average 0.2 ppm in normal soils 6. Coal combustion contributes a high proportion of Se in the atmosphere 7. Selenium has a low boiling point of 680°C and during the combustion of coal, selenium is vaporized and highly fractionated, with only 0.3% incorporated into slag, 68% incorporated in fly ash and 32% emitted in the vapor phase to the atmosphere. These data were derived for coal combustion rates using pre 1975 technology, and based upon a single steam generation plant in Memphis, Tennessee to estimate that coal combustion contributed 1.5-2.5 times the amount of Selenium into the environment as natural weathering of Se-containing rocks 7. The low boiling point of also means that the selenium should condense shortly after being emitted and could effect the environment near the power plant 7.
References: Considerable information about Se is available on
the World Wide Web at URL 8.
1. CRC Handbook of Chemistry and Physics
2. Swaine, D.J., (1990)
3. Minkin, J.A. et al, (1984)
4. Finkelman, R.B., (1981)
5. Bauer, Francie, (1997)
6. Rosenfeld, I. and O. A. Beath, (1964)
7. Andren, A.W., Klein, D.H. and Talmi, Y., (1975)
8. O. A. Beath Searchable Selenium Database at the University of Wyoming
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