Selenium in coals

At NBS, the neutron activation method with combustion separation step applied to the determination of mercury and selenium in coal has been modified and extended to analysis for arsenic, zinc, and cadmium by Orvini et al. (14). 

Shah, P., Strezov, V., Stevanov, C. and Nelson, P.F. (2007) Speciation of arsenic and selenium in coal combustion... 

ASTM D-4606. Standard Test Method for Determination of Arsenic and Selenium in Coal by the Hydride Generation/Atomic Absorption Method. 

Andren AW, Klein DH. 1975. Selenium in coal-fired steam plant emissions. Environ Sci Technol 9 856-858. 

Scalp involvement can be treated with twice-daily topical corticosteroids in conjunction with a shampoo containing selenium sulfide, coal tar, or salicylic acid to help soften and remove scales. 

Selenium sulphide, coal tar, ketoconazole and salicylic acid are agents that can be used in dandruff. Permethrin is an insecticide indicated in the eradication of head lice. Permethrin is available as alcoholic or aqueous lotions. 

In summary, coal-fired power plants appear not to be the major source of most enriched elements on particles In urban areas, despite the great attention devoted to mechanisms by which those elements become preferentially attached to fine particles (e.g.. Refs. 18, 34). However, the detailed studies of processes In coal-fired plants are of considerable value, as the fundamentals should be applicable to other kinds of combustion sources. Furthermore, It may be necessary to use this fundamental approach to develop methods for predicting the source compositions for coal-fired power plants that have not been measured. Selenium Is much... 

The selection of the correct dissolution step in coal decomposition is vital in determining trace elements. Such elements as copper and nickel can easily be picked up as contaminants from the laboratory environment or reagents. Other elements such as mercury and selenium can be lost in the dissolution step. The dissolution procedure involving the least exposure to contamination without potential loss of volatile components should be used in for each trace element. 

The Determination of Selenium. The most difficult trace element to determine in coal by wet chemical methods is selenium. Two alternative dissolution techniques can be used—H. L. Rooks combustion method (7) and the oxygen bomb combustion method (4). Also, two alternative analytical methods can be used—the hydride evolution method (5) and the graphite furnace method. 

It is apparent from Table IV that trace elements determined by the x-ray fluorescence method are limited to those occurring in whole coals at concentrations of at least a few parts per million. Elements such as selenium, mercury, and antimony, which are generally present in whole coal at levels below 1 ppm, cannot be determined by this method. The major elements in coal, hydrogen, carbon, oxygen, and nitrogen, cannot be determined by x-ray fluorescence, but this should not inhibit the use of the method for trace and minor element determinations. 

In November 1971, NBS issued Standard Reference Material (SRM) 1630, Mercury in Coal, with a provisionally certified mercury content of 0.13 ppm. Later the provisional value of 2.1 ppm selenium in SRM 1630 was issued. 

For a trace element concentration to be certified by NBS, it must be determined by at least two independent methods, the results of which must agree within a small experimental error range of 1% to 10%, depending on the nature of the sample and the concentration level of the element. Such accuracy in determining some trace elements for certification of coal SRM is achieved most easily by NAA with radiochemical separation. Scientists at NBS have extensively tested a neutron activation method that involves a combustion separation procedure on coal as well as on several other matrices to be certified as standard reference materials. The procedures they have thus developed to determine mercury (12), selenium (13), and arsenic, zinc, and cadmium (14) are outlined in a following section on methods for determining specific elements in coal. 

Selenium. In 1969, Pillay et al. (22) determined the selenium content of 86 coals from various coal-producing areas of the United States by NAA combined with extractive selenium distillation. 

Radiochemical yields are quantitative. The relative standard deviation of a measurement is usually better than 10%. Analysis of SRM 1630 gave 2.0 =b 0.13 ppm selenium, and results for the NBS-EPA round-robin coal and fly ash samples agreed within experimental error with the probable certified values of selenium in those samples. The selenium concentrations of 101 coals analyzed by the above method range from 0.45 to 7.7 ppm and have a median value of 1.9 ppm (16). 

At NBS, the neutron activation with combustion separation method used for determining mercury in coal (12) was further investigated for determining selenium by Rook (13). The same procedure is used except that the sample is heated finally to 1000°C. Mercuric oxide is also used as carrier for the selenium because selenium oxides are difficult to dissolve in mineral acids. The mercuric selenide formed carries the selenium effectively, and, as it is soluble in nitric acid, the dissolution procedure developed for the mercury separation can be used so that mercury and selenium can be determined in the same sample. 

Finally, the trace constituents become important factors when substantial amounts of coal are burned. Selenium, which is present at approximately 2 ppm in coal, could be released at a rate of 210 tons/yr, assuming 210 million tons of coal consumed and a 50% escape rate during combustion (6). Significant discharges could also be expected for Hg, F, As, and perhaps Sb, Zn, and Cu. 

Precise and accurate trace element assays on coal are extremely difficult and require constant vigilance to avoid errors that can be introduced both in the laboratory and in the coal-handling procedures. In the future this program will extend the present studies to coals from different parts of the country, study additional trace elements in coal such as arsenic, selenium, beryllium, and others, and apply the developed techniques to more power plants. 

L6pez-Ant6n, M.A., Dfaz-Somoano, M., Fierro, J.L.G. and Martfnez-Tarazona, M.R. (2007) Retention of arsenic and selenium compounds present in coal combustion and gasification flue gases using activated carbons. Fuel Processing... 

He, B., Liang, L. and Jiang, G. (2002) Distributions of arsenic and selenium in selected Chinese coal mines. Science of the Total Environment, 296(1-3), 19-26. 

Arsenic and selenium occur in coal to the extent of several parts per million and on combustion of the coal, varying quantities of these elements are released or retained in the ash, depending largely on the conditions under which the combustion takes place and on the nature of the coal ash. 

Selenium in rainfall is derived principally from earth-surface volatilization, volcanic sources, fossil-fuel combustion (especially coal), and the incineration of municipal wastes. Few determinations of selenium in atmospheric precipitation have been reported, but concentrations are usually very low. Hashimoto and Winchester (1967) found concentrations in the range 0.04-1.4 p,gL (Table 9). 

As noted above, the principal natural sources of selenium in water are likely to be sulfides or metal oxides containing adsorbed selenium, especially Se(IV). Coal can be an additional primary source of selenium either directly through oxidation or indirectly via atmospheric precipitation following combustion. Selenium is readily oxidized during the weathering of minerals. Seleniferous ground-water areas such as those in the USA and Pakistan... 

The contents of some trace elements in the continental crust, shales, soils, bituminous coals and plankton are given in Table 1.1 to provide some perspective when considering other aspects of these elements. In each of these situations, organic matter is associated with the elements to a greater or a lesser degree. This is not usually very marked with crustal rocks except shales, but may be a major factor for some elements in surface soils and coals. The data in Table 1.1 show that, for some elements, e.g. beryllium, cadmium, cobalt and molybdenum, the contents of the various reservoirs are similar, while for others, there may be enrichments relative to the crust, e.g. boron and sulfur in many shales, soils and coals, mercury, nickel and selenium in many shales, and germanium in some coals. 

Peterson, P.J. and Butler, G.W., 1967. Significance of selenocystathionine in an Australian selenium-accumulating plant, Neptunia amplexicaulis. Nature, 213 599—600. Porritt, R.E. and Swaine, D.J., 1976. Mercury and selenium in some Australian coals and fly-ash. Preprint, Institute of Fuel Conference Sydney, Paper 18, 9 pp. 

More commonly, selenium enters the air from burning coal or oil. Most of the selenium in air is bound to fly ash and to suspended particles. The elemental selenium that may be present in fossil fuels forms selenium dioxide during combustion (burning). Selenium dioxide can then form selenious acid with water or sweat. Selenium anhydride is released... 

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