Recent Research
Occurrence of selenium in the coal power industry
Frank E. Huggins, Constance L. Senior, Paul Chu, Ken Ladwig, and Gerald P Huffman, Selenium and Arsenic Speciation in Fly Ash from Full-Scale Coal-Burning Utility Plants, Environmental Science & Technology, Volume 41 Number 9, April 2007, pp 3284-3289.
X-ray absorption fine structure spectroscopy has been used to determine directly the oxidation states and speciation of selenium and arsenic in 10 fly ash samples collected from full-scale utility plants. Such information is needed to assess the health risk posed by these elements in fly ash and to understand their behavior during combustion and in fly ash disposal options, such as sequestration in tailings ponds. Selenium is found predominantly as Se(IV) in selenite (SeO3 2-) species, whereas arsenic is found predominantly as As(V) in arsenate (AsO4 3-) species. Two distinct types of selenite and arsenate spectra were observed depending upon whether the fly ash was derived from eastern U.S. bituminous (Fe-rich) coals or from western subbituminous or lignite (Ca-rich) coals. Similar spectral details were observed for both arsenic and selenium in the two different types of fly ash, suggesting that the postcombustion behavior and capture of both of these elements are likely controlled by the same dominant element or phase in each type of fly ash.
The journal is available at libraries and the article may be purchased on-line at: http://pubs.acs.org/cgi-bin/abstract.cgi/esthag/2007/41/i09/abs/es062069y.html
Ya.E. Yudovich, M.P. Ketris, Selenium in Coal: A review, International Journal of Coal Geology, Volume 67, 2006, pp 112-126.
The World average Se content in coals (coal Clarke of Se) for hard coals and brown coals are respectively 1.6±0.1 and 1.0± 0.15 ppm. On an ash basis, these contents are greatly increased and are 9.9±0.7 and 7.6±0.6 ppm, respectively. Therefore, Se is a very coalphile element: it has strong affinity to coal matter — organic and (or) inorganic but is certainly authigenic. The “coal affinity” of Se is like that for Ge and S. Both organic (Seorg) and inorganic selenium (Semin) can exist in coal. In addition, Se can occur not only as a chemical-bound form, but also in sorbed (acid leachable) selenate form in the oxidized coals. The sulfidic form of Se is represented by isomorph Se in pyrite and some more rare sulfides, and the selenidic form by clausthalite PbSe. “Organic” Se may be present as both Se-organic compounds and elemental Se° dissiminated through organic matter. There are some preliminary data that cleaning of high-sulfur coals, very effective for S, is less effective for Se due to enhanced contribution of the Seorg form (vs. Sorg). There are two genetic types of the Se-accumulations in coal: “reducing” and “oxidizing”. In the first type, Se is enriched in highsulfur coals, concentrating in sulfide phases. These accumulations are, in general, syngenetic, and may be epigenetic only if there are abundant hydrothermal sulfides of Fe, Cu, As, Pb. In coals of the second type, Se is enriched in the bed oxidation zones. Such coals are (or were) located in the areas with arid climate and enhanced Se content in water. Se concentrates in coal as a reduction or sorption geochemical barrier, probably, mostly as Se° in oxidized organic matter and partly pyrite. This type is characterized by oxidizing zonality and close paragenesis of Se with U, Fe, Mo, V, and Pb. These Se accumulations are mostly epigenetic. A study of Se in coal is evidently insufficient, and further detailed studies are needed, especially regarding Se forms (modes of occurrence).
The journal is available at libraries and the article may be purchased on-line at: http://www.sciencedirect.com/science?_ob...
Ken Ladwig, Field Leachate Characterization at Coal Combustion Product Management Facilities, presentation at Flue Gas Desulfurization (FGD) By-Products At Coal Mines And Responses To The National Academy Of Sciences Final Report "Managing Coal Combustion Residues In Mines,” a Technical Interactive Forum sponsored by the Mid-Continent Regional Office, U.S. Office of Surface Mining Reclamation and Enforcement, U.S. Department of the Interior, November 15, 2006.
In this slide presentation, the author, who is with the Electric Power Research Institute, reports the results of a study to broadly characterize solid waste leachate from field samples at a wide variety of coal combustion product (CCP) management sites and to determine the chemical form (speciation) of arsenic (As), Chromium (Cr), Mercury (Hg) and selenium (Se) in leachate samples. For fly ash, the report shows that 25% of the samples exceeded the maximum contaminant level (MCL) for antimony, arsenic, cadmium and selenium in solid wastes (EPA Toxic Characteristic Leaching Procedure) and these leachates would be classified as hazardous wastes. For flue gas desulfurization by-products, the report shows that 25% of the samples exceeded the MCL for arsenic, cadmium and selenium.
The presentation, as well as the other presentations at this conference, are available at the following link: http://www.mcrcc.osmre.gov/PDF/...