Environmental selenium and its significance

FUNDAMENTALAND APPLIEDTOXICOLOGY 3:411-419 (1983)

Environmental Selenium and Its Significance LAWRENCE FISHBEIN, PH.D.

Department of Health and Human Services, Food and Drug Administration, National Center for Toxicology Research, Jefferson, AR 72079

ABSTRACT

Environmental Selenium and its Significance. Fishbein, L. (1953). Fundam. AppL ToxicoL 3:411-419. Selenium occupies a unique position in regards to continuing conflicting aspects of its toxicological and nutritional significance. This review focuses on the environmental sources and sinks, transport and alterations of selenium. Both natural and anthropogenic sources of selenium are considered with primary emphasis on the occurrence and significance of selenium in soils, air, water, pla~ts and foods. The patterns and trends of application of selenium are also highlighled in order to focus on the the potential populations at risk. INTRODUCTION There is increasing recognition'tnat selenium is an import'ant metalloid with industrial, environmental, biological and toxicological significance. The toxicology of seJenium and its compounds, often conflicting and controversial, is of considerable continuing interest for a variety of reasons including: 1) the long-established selenium poisoning of domestic animals foraging on seleniferous plants (Molson and Olson, 1974); 2) disorders in humans and animals resulting from selenium deficiencies (Cooper and Glover, 1974; Underwood, 1971 ); 3) the nutritional essentiality of the element (Burk, 1976; Flohe, 1973; Frost, 1972; Frost and Lish, 1975; Schwarz, 1976; Schwarz and Foltz, 1957); 4) the protective effect of selenium against metal toxicity and the metabolic interaction between selenium and vitamin E and other antioxidants (Ganther, 1978; Parizek, 1971; Ridlington and Whanger, 1981); 5) the reported carcinogenicity (Nelson, et al., 1943; Schloeder, 1967; Volgarev and Tscherkes, 1967; Whanger and Weswig, 1969; Wolff and Oehme, 1974) and 6) anticarcinogenicity (Harr and Muth, 1972; Jacobs, 1980; Shamberger, 1970; Shamberger and Willis, 1971) including a decrease in the incidence of hepatic tumors in rodents induced by 3'-methyl4-dimethylaminoazobenzene (Daoud and Griffin, 1980) or 2acetylaminofluorene (Harr, et al., 1973), of colon tumors induced by methylazoxymethanol acetate and 1, 2-dimetl~ylhydrazine (Jacobs, et al., 1977) and azomethane (Soullier, et al., 1981 ) and of mammary tumors induced by 7, 12-dimethylbenz(a)anthracene (Medina and Shephard, 1981; Welsch, et al., 1981). In addition, enhancement of DNA repair processes (Russell, et al., 1980) and immunological mechanisms (Spallholtz, eta/., 1973a, b) as well as inverse relationships between the dietary intake of selenium and cancer mortality in debated environmental studies nave all been reported (IARC, 1975; Schrauzer, eta/., 1977, Shamberger and Frost, 1969; Shamberger, et al., 1971 ). It is generally acknowledged that one of the most dangerous and pernicious forms of pollution arises from the potential mobilization of a spectrum of toxic trace metals and metalloids

in our environment. The primary objective of this overview is to highlight germane aspects of the more recent knowledge of the environmental sources and sinks, transport and alterations of selenium. This information is considered vital for the better assessment of the scope of exposure to this important element. It is extremely important to know the sources (both natural and anthropogenic) and fate of selenium in the environment and, hence, mechanisms involved in its transport and transformation are viewed as integral. Recent general reviews on selenium which are particularly germane include; history, occurence, and properties (Cooper, et al., 1974), occurrence in the environment via natural and industrial activities and distribution, cycling and transport behavior (Allaway, et aL, 1967; Glover, et el., 1979; ,Johnson, 1976; Lakin, 1973; Newland, 1982; Olson, 1967; Shamberger, 1981 ), and occurrence in foods (Lo and Sandi, 1980) and in water (Shamberger, 1980). OCCURENCE Selenium belongs to the V1B group of the periodic system located between sulfur and tellurium and possesses chemical and physical properties which are intermediate between metal and nonmetal. Figure 1 illustrates the interrelationships between selenium, sulfur, arsenic, and phosphorus and the inorganic and organic forms of selenium. Changes in the valence state of selenium from 2 , through O, 2', 4' and 6' are associated with its geologic distribution, redistribution and use (Harr, 1978). It is one of the widely distributed elements, estimated to be the 69th most abundant element in the earth's crust at an average estimated content of 0.05 - 0.09 mg/kg (Glover, et aL, 1979), approximating that of cadmium and antimony (Fleischer, 1954) and ranking above molybdenum, silver, mercury and uranium (Goldschrnidt, 1954; Turekian and Wedepohl, 1961). Much of the selenium in the earth's crust occurs in association with sulfide minerals or as selenides of silver, copper, lead, mercury and nickel or other metals. Hence, the sources of selenium minerals are both numerous and varied (e.g., selenium is recognized as a major constituent in at least 22 selenides, 6 sulfosalts, 1 oxide, 4 selenites and 1 selenate and as a minor constituent of at least 24 sulfides and tellurides (Luttrell, 1959)). Although the selenium of commerce is ultimately mined from mixtures of these minerals, most can not be considered as ore mineral since selenium is produced from minerals mined for their other constituents (e.g., copper, lead, silver, etc.). selenium is mainly concentrated in sulfide minerals such as galena, chalcopyrite, arsenopyrite, sphalente, pyrite, marcasite, and pyrrhotite (Coleman and Delevaux, 1957). The highest selenium contents (to about 1%) of sulfides are associated with uranium ores in sandstone-type deposits in the western

Copyright 1983, 5nciety of Toxicology

Fundamental and Applitd Toxicology

f3J 9-10/83

411

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I'IG. 1. Inh'rrel.~ti~)nships between selenium, sulfur, .~rsenic, ,~nd pl~.~phorus, and thu inorganic and org,mic form~ of sulenium. United States. Concentrations as high as 1000/Jg/g selenium are not uncommon in the immediate vicinity of sandstone-type uranium deposits and other similar metal deposits in sedimentary rocks (Shamberger. 1981 ). Occurrences of high concentrations of selenium in hydrothermal ore are known to be widespread. Among the best known are those associated with epithermal gold, silver, antimony and mercury deposits (Cooper, et al., 1974). Selenium also occurs in geochemically significant concentrations (as much as 120 ppm) in volcanic rocks in various parts of the western United States. SOILS The occurrence of selenium in soils is of particular importance due to its recognized toxic effects on cattle foraging on plants which can accumulate the element from seleniferous soils. Additionally, deficiency diseases are prevalent in animals on a selenium-deficient diet, primarily animals grazing in areas in which the soils are especially low in selenium (Cooper, et a/.. 1974; Cooper and Glover, 1974; Molson and Olson, 1974). Selenium in soils is related to bGth the whole geochemical cycle of the element and the solution chemistry of the element. Important factors include the selenium content of host rocks. redox potentials, pH and the nature of the drainage waters (Coopcr eta/., 1974; Shamberger, 1981 ). Hence, selenium in the soil (Cooper, et a/., 1974; Rosenfeld and Beath, 1964; Shamberger, 1981) may arise from factors including: 1)rock formations and outcrops, 2) formations lying below the soil mantle, 3) decomposition via the weathering of the host rock and subsequent transport of selenium by wind or surface waters, 4)enrichment of the soil with seleniurn resulting from mining operations, and 5) the combustion of fossil fuel s (Lakin and Davidson, 1967). Figure 2 illustrates a skeletal geochemical cycle of selenium, from molten rocks to igneous rocks or the atmosphere, which can be converted to sediments, or the hydrosphere, where the element can be taken up by plants and soils (Shamberger, 1981}. The distribution of selenium in soils is primarily in weathered products of sedimentary formations containing selenium (Newland, 1982). Since sedimentary rocks cover more than three quarters of the land surface of the •earth, they are the principal parent material of agricultural soils. For example, estimates ranging from 0 . 0 8 p g / g t e l p g / g are higher than the estimates of 0,05 -0.09 for the earth's crust. The selenium Content of sandstone 4|2

and carbonate rocks ranges from 0.05 - 1 /~g/g and 0.O to 2.0 pg/g, respectively (Shamberger, 1981). tt i~ important to note that selenium is abundant in coal from runny sources ranging from 0.1 to 4 pg/g. Hence, the burning of seleniferous coals introduces selenium into the atmosphere which is ultimately redistributed to the ear~'h's surface (Lakin and Davidson. 1967: Shamberger, 1981). The selenium in soils may occur in many forms: e,g., as selenites, selenates, elemental selenium and as selenium in association with pyrite and other minerals. "Additionally, in seleniferous soils a significant amount of selenium is present in the form of soluble organic or inorganic compounds as a result of the decay of seleniferous plant materials (Cooper, el aL. 1974). Soil levels of selenium globally are known to vary greatly with commonly found values between 1 and 10 ppm (Glover, et al., 1979). The content of most soils was estimated by Swaine (1955) to be between O.1 and 0.2 ppm and by Sindeeva (1964) to be around O.01 ppm. The analysis of several thousand soil samples in the United States indicated that the majority of seleniferous soils contained on the average less than 2 ppm, with a maximum concentration of some soil samples being less than 100 ppm (Rosenfeld and Beath, 1964). In the U.S., the highest soil levels are found in areas of the West and Midwest, particularly in soils derived from Cretaceous beds (Lakin. 1961 b), The total selenium content of soils and rocks in 18 countries is shown in Table 1 (Robberrecht, et aL, 1982). Th'us. in soil, concentrations may range from O.1 ppm in selenium-deficient areas, such as found in New Zealand (Wells, 1967) to 1200 mg/kg in seleniferous areas in Ireland (Fleming. 1962; Shamberger, 1981). The corresponding values for plants grown in these two extreme conditions have been reported to be 0.01 and 10 000 ppm (Johnson. 1976). The accumulation of selenium by certain plants, particularly in regions of the western and southwestern United States, is well documented (Cooper, et aL. 1974; Glover, et aL, 1979; Molson and Olson, 1974; Rosenfeld and Beath, 1964). There are 24 species and varieties of the plantAstragalus and some species of Machaeranthera, Haplopappus and Stan/eya which are known as "'primary seleniun i,ndicators" and grow only in soil containing high levels of available selenium and can accumulate up to 100-10000 ppm. Consumption of such plants by livestock is associated with the acute disease syndrome of "blind staggers': (Cooper, et al., 1974; Glover, et al., 1979; Molson and Olson, 1974). While "secondary" selenium absorbir~g plants (e.g., Aster and Atriplex species)can concentrate selenium up to 50-500 ppm, many weeds and most crop plants, grains and grasses can contain up to 30 ppm of selenium. Consumption of seleniferous crop plants is associated in livestock with the chronic disease syndrome "alkali disease". In those areas of the world where the soil contains low levels of selenium available for uptake by plants, selenium deficiency diseases have been noted in several species of farm animals (e.g., white muscle disease in sheep, calves and horses, hepatests dieteticia in swine and exudative diathesis in chickens). In some countries, selenium as sodium selenite or selenate is added at a level of O. 1 - 0.2 mg selenium/kg of feed to prevent va rious deficiency diseases in swine and poultry (Glover, et aL, 1979; National Academy of Sciences, 1976). The selenium uptake by plants is influenced by a number of factors including: total level of selenium in the soil, soil type, pH. colloidal content, concentration of organic material, time of contact and the oxidation-reduction potentials in the r0ot-soil environment (GisseI-Nielsen, 1971; GisseI-Nielsen and Hamdv, Fundam. Appl. Toxicol. (3)

September/October, 1983

S E L E N I U M - T O X I N O R PANACEA

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1978: Lakin, 1961b; Lo and Sandi, 1980; Rnbberrecht, et el., 1982). in poorly aerated acid soils (pH 4.5 6.5) selenium is usually bound as a basic ferric selenite of very low solubility while in well aeraled alkaline soils selenium is readily oxidized to water-soluble selenite or selenate and available to plants (Glover. et eL, 1979; Lo and Sandi, 1980). For example, the reaction 2H2SeO.~ + O~ -- 2H.,SeO4 occurs more readily in an alkaline environment (Shamberger, 1981). Hence, selenates are more readily absorbed than selenites, and calcium selenate more readily absorbed than sodium selenate (Cooper, et al., 1974; Glover, et al., 1979; Molson and Olson, 1974). Figure 3 illustrates the generalized chemistry of selenium in soils and weathering sediments (Allaway, 1968). Most selenium occurs as both insoluble elemental and selenide forms which in oxidizing environments is converted to selenites and selenates (both of which are soluble) (Allaway, 1968). These soluble oxyanions are washed into waterways or leached backdown to reducing zones where they again become insoluble. However, in arid areas with alkaline soils, selenate is a predominant and stable form (L~kin, 1961b), and elsewhere both selenate and selenite are easily reduced. Selenite forms have a strong affinity for metal hydroxides and are adsorbedonto insoluble complexes under acidic soil conditions or in water (Howard, 1973; Newland, 1982). WATER An average value of 0.09/~g/L was found for selenium in the principal oceans with a marked, but not consistent, variation in depth at several locations in the eastern Pacific Ocear, (Schutz and Turekiam, 1965) while values on the o,rder of 4 p g / L have been no~ed in samples from the North Sea and coas~; of Japan (Cooper et al., 1974). It was noted that considering the amount of selenium carried into the oceans, a sub:;tantial removal of the element from aqueous solution by coprecipitation w i t h basic oxides (e.g., hydrous ferric oxide) must occur, followed by the incorporation of these precipitates into marine sediments (Cooper, et el., 1974). Selenium levels in ground and surface waters may range from 0.1 to about 400 # g / L (Lindberg, 1968; Morette and Divin, 1965i Scott and Voegeli, 1961). However, i t should be noted that, depending on geological factors, ground water may reach concentrations up to 6000/~g/L (Glover, et al.; 1979). The selenium content in surface waters is to a large extent influenced by the pH. For example, selenium contents are h i g h in acid waters having a pH of 2,4 to 3.0 and in weakly alkaline waters h a v i n g a Fundamental and Applied Toxicology

(3) 9q0/83

pH of 7.4 to 8 . 0 . However, in acid solutions, when ferric iron is not hydrolyzed, selenium is present as the selenite ion but is precipitated with ferric hydroxide in solutions above a pH of 3.5. In alkaline aerated waters (pH of about 8) selenite is oxidized to the selenate ion and becomes mobile (e.g., increasing in concentration in water up to 10-400 #g/L)(GIover, et el., 1979; Sharnberger, 1980). Selenium is generally a minor constituent in potable water with concentrations ranging from O. 1 to 100 # g / L (Davis Snd DeWiest, 1966) with a safe upper limit for selenium in drinking water considered to be 10/~g/L (U.S. Dept. Health, Education & Welfare, 1962). However, there are areas in the United States that have substantial amounts of selenium in the drinking water (Shamberger, 1980). In general, selenium in the drinking water woL, Id not add substantively to the daily intake of selenium. However, according to Shamberger (1980), since selenium may be an important factor in heart disease, the amounts of selenium in the drinking water should be considered in relation to epidemiological studies with calcium, magnesium, sodium, heavy metals and the degree of hardness~. FOODS Although early studies in the 1930's disclosed that the selenium content in foods produced in some highly seleniferous farming areas of South Dakota were generally high (e.g., milk, eggs, bread, meats and vegetables contained up to 1.2, 10, 1,8 and 17 ppm, respectively) (Byers, 1935; Smith and Westfall, 1937), wheat and grain produced in other seleniferous areas, including part of the Canadian prairie provinces, were reported to have average concentrations of 0.4 to 1.0 ppm (Lakin and Byers, 1941; Lo and Sandi, 1980). More recent surveys indicated that the selenium content of U.S. feeds was 0.O1 - 0.70 ppm in wheat and 0.38 ppm in oats (National Academy of Sciences, 1971 ). It should be noted that seleniferous grains have been recognized in a number of countries besides the United States and TABLE 1 Total Selenium Content of Soils and Rocks in Different Countries (based on Robberrecht, 1982) Conc. R a n g e ( p p m )

Country

0.005- 9.9 0.008 - 35.8 0.01

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0.02 - 0.63

Icela nd

0.04 - 6.0

Canada

0.04 - 0.27

Belgi.um

0.08 - 1.21

Turkey

0.1

- 4.0

New Zealand

0.14 - 1.5

Denmark

0.16 - 0.97

Sweden

0.16 - 7.35

Norway

0.18 - 0.85

Egypt

0.22 - 0.42

Scotland

0,3 - 3.5

Mexico

0.3 ' 3:7

Ireland

0.4- i.2

Japan

1.5 -7~0

England

2 -i2

PUertO Rico

413

that it is most difficult to obtoin a selenium-free diet. The argument has been advanced that in view of the nutrient value of traces of selenium in the animal diet a selenium-free diet can be considered nutritionally undesirable (Cooper and Glover, 1974), Recent total diet studies reported the following selenium levels: 0.07 ppm for the dairy products group 0.20(0.10-0.40) ppm for the meats, fish and poultry group; 0.24 (0.10.0.40) ppm for grains and cereal products group and trace amounts in the other t~ood groups (Johnson and Manske, 1977; Lo and Sandi, 1980; Mahaffey, et al., 1975; Manske and Johnson, 1977), The .estimated normal dietary daily intake of selenium for humans in most parts of the world ranges from 4 - 35/~g/person in infants to 60 - 300 p g / p e f s o n in adults (Lo and Sandi, 1980; Kazantis, 1981 ). Based on lack of definitive toxic effects on man of selenium of food origin and considering the normal levels in foods, Lo and Sandi (1980) estimated that 500 p g / m a n / day may be regarded as a maximum tolerable level. AMBIENT AIR Although atmosphoric concentrations of selenium usually are of the order of a few nanograms per cubic meter (Gordon, et eL, 1973; Harrison, et aL, 1971 ; John, et el,, 1973; Pierson, et el., 1973), it should be well noted that point-source emissions, primarily from the combustion of fossil fuels, may contribute to the local air pollution by selenium (Cooper, et el., 1974; Glover, et eL, 1979). Fossil fuels may contain 1 to 10 ppm of selenium (Harr, 1978). The selenium present in 120 fossil fuel 3amples, both coal and oil, in the U.S. have been determined by Pillay, et aL (1969) employing neutron activation. In the majority of the coal samples from 20 states, the selenium ranged between 1 and 5 ppm with an average value of 3.2 ppm. An average of 0.17 ppm selenium (0.06 - 0.35 ppm) was found in the crude oil samples (mainly from Texas). Estimates of the annual release of selenium from the combustion of fossil fuels in the United States vary. For example, in one report the annual release from the combustion of fossil fuels was estimated at 1500 tons. in addition, industrial losses were estimated at 2700 tons and municipal waste at 360 tons. Of these 4600 tons of selenium released in fuel, industrial, and refuse processing, about 25% is in atmospheric emissions and the balance is in the ash (Harr, 1978). Pillay, et al. (1969) estimated the annual release of selenium from the combustion of coal and oil in ti~e U.S. to be about 4000 tons. This figure is nearly six times the 1964 production of selenium in the who(e of North America and four times the world production for the same year. Selenium dioxide (2+)is formed by combustion of elemental selenium present in fossil fuels or rubbish (Weiss, et hi., 1971). Generally sulfur dioxide is formed concurrently and rapidly reduces the seJenium to elemental selenium, which is not available to plants or biosystems.

The emission of selenium to the atmosphere in the U.S. in 1969 was estimated at 986 tons (Davis, 1972). The emissions that resulted from the combustion of coal were about 65% of the total emissions while those due to the manufacture of glass were nearly 21%, Emissions from metallurgical processing of nonferrous metals and the combustion of fuels other than coal were 9 and 7%, respectively, while all other emissions were less than 1% of the total, Projections of annual air emissions in the United States of selenium from stati0nary sources (primarily combustion of fossi! fuels) through the period 1969-1971, 1978 and 1983 (assuming no changes in processes or control 414

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FIG. 3. Generalized chemistry of selenium in soils and weathering sediments (based on Newland, 1982). technology) (in tons/year) were: 900, 1240, and 1560. respectively (Flinn and Reimers, 1974; Lee and Duffield, 1977). Other possible sources of selenium in the atmosphere include the incineration of paper and rubber tires. West (1967) found 70 different kinds of paper to contain selenium, the content varying widely with the type of paper. Typical newsprint has 4 ppm selenium; rice paper, 5.6 ppm; mimeograph paper, 0.7 ppm; bond paper, 1.4 ppm; and paper towels, 1.2 ppm. In addition, selenium was found in relatively large quantities (e.g., about 10 ppm) in cigarette paper. Olson and Frost (1970) found the selenium content of a number of papers (less than 0.05 ppm) and tobacco to be somewhat lower than that reported earlier by West (le67). Pipe and cigarette tobacco contained a little more selenium that the papers, ranging from 0.03 to 0.'t"3 ppm while cigar tobaccos contained somewhat more, ranging from 0.33 to 1.01 ppm. Levels of selenium found in rubber tires were reported to average 1.33 ppm ( 0 . 7 0 . 2 . 0 0 ppm) (Hashimoto, e tal., 1970). Johnson (1970) described the determination of selenium in a range of solid waste materials (~.g., garbage and trash} including incinerator stack emissions, particulate emission following combustion. Approximately 70 percent of the almost 1 billion pounds of solid waste collected daily in the United States is paper. Stack emission values of pounds of s e l e n i u m / ton refuse from municipal solid waste ranged from 2.16 x 10 ;~ to 9.80 x 1fit. Shendriker and West (1973) reported that smoke from open trash burning contained an amount of selenium proportional to that of fuel. As much as 273 g/rn a of selenium dioxide was found when dry wood chips were burned and the lowest selenium concentration was found in general trash. There is a paucity of data relative to levels of selenium in the ambient air near industrial point sources. Aii" concentrations of about 500 n g / m 3 selenium were found in the immediate vicinity of two plants processing anode mud from copper refining and at a distance of 2 km from the plants, the selenium concentrations did not exceed 70 n g / m 3 (Selyankina and Alekseeva, 1970). Air concentrations of selenium between 7 and 50 # g / m 3 (Kinnigkeit, 1962) and 40-400 p.g/m 3 (Glover, 1967) were reported in selenium rectifier plants. While absorption of selenium through the lungs may be of importance in occupational exposure, lack of reliable data does not allow a quantitative evaluation of this route of absorption (Glover, et eL, 1979). Fundam. AppL ToxicoL (3)

September/October, 1983

SELENIUM - TOXIN OR PANACEA

The TLV-I"WA for selenium compounds (as selenium) is 0.2 m g / m a, P R O D U C T I O N A N D USE

Selenium is obtained primarily as a byproduct of copper refining. More than 90% of the U.S. output and more than 80% of the world's production is derived from the anode mud deposited during eletrolytic refining of copper. All commercial processes for the production of selenium may be considered as modifications or combinations of three fundamental methods, viz., smelting with soda ash, roasting with soda ash and roasting with sulfuric acicL The association of selenium with sulfur in copper sulfide minerals has resulted in the world production of selenium being related largely to those countries processing such minerals in significant tonnages, viz., Canada, Japat~, the United States and the U.S.S.R. Smaller quantities of selenium are produced in Belgium, Chile, Finland, Peru, Sweden, Federal Republic of Germany, Yugoslavia and Zambia (Cooper, eta/., 1974). Copper anodes contain an average of 0.05% selenium or 1 pound of selenium per ton of copper, but actual recovery of selenium ranges from 0.4 to 0.75 pounds/ton of electrolytic copper produced (Ageton, 1970). Selenium production in the western world from 1964 to 1973 averaged 1042 tons (National Academy of Sciences, 1976). The production of seleTABLE 2 Some S e l e n i u m C o m p o u n d s and T h e i r Uses Compound

Use

Selenium

Rectifiers, photuelectric cells, blasting caps, in xerography, stainless steel; dehydrogenation-catalyst.

Sodium selenate Na.,SeO4

As insecticide; in glass manufacture; in medicinals to control animal diseases.

Sodium selenite Na=SeO4

In glass manufacture; ,as soil ,additive for selenium deficient areas.

Selenium diethyldit hioca rba mate

Fungicide; vulcanizing agent.

Selenium disulfide SeSz

In veterinary medicine,

Selenium dioxide SeO2

Catalyst for oxidation, hydrogenation or dehydrogenation of organic compounds.

Selenium monosulfide SeS

In veterinary medicine.

Selenium hexafluoride SeF6

As gaseous electric insulator.

Selenium oxychloride SeOCl.,

Solvent for sulfur, selenium, tellurium, rubher, bakelite, gums, resins, glue, asphalt and otlaer materials.

Aluminum selenide AIzSe3

Preparation of hydrogen selenide for semiconductors.

Ammonium selenite (NH4)zSeO3

Manufacture of red gla~s.

Cadmium selenide

Photoconductors, photoelectric cells, rectifiers.

Cupric selenate CuSeO4

In coloring copper and copper alloys.

Tungsten diselenide WSe2

in lubricants.

Fundamental and Applied Toxicology

(3) 9-10/83

nium in 1976 was only slightly higher at 1087 tons. The U.S. 0reduced 184 tons and imported the remaining portion of the 453 tons used from Canada (42%), Japan (27%), and others (Newland, 1982). World trade in selenium is very closely allied to world trade in copper ores, concentrates, matte and blister because of the natural occurrence of selenium with t!~ese copper-bearing materials. Although the largest source of selen:um by far is via the electrolytic refining of blister copper, there are several other important potential sources. For example, as noted earlier, the average content of selenium in coal is at least 1.5 ppm (Lakin and Davidson, 1968). Since coal resources in the U.S. alone are estimated to be in t~,e order of 1 580 000 million tons, their estimated selenium resource would be about 2.3 million tons if the selenium could be recovered. Secondary sources and recovery of selenium include the following sources: factory scrap generated during manufaclure of selenium rectifiers, burned-out rectifiers, spent catalysts, and used xerography copying cylinders. It was estimated that industrial recovery of selenium from old scrap sources in 1968 totaled at least 30 000 pounds. Applications of selenium in the chemical industry are normally considered dissipative. Selenium per se is marketed as either commercial grade in powder form containing from 97.00 to 99.94 percent selenium or high-purity grade (for electronic applications) in pellets, sticks and rectifier shot containing from 99.95 to 99.99 + percent selenium. The composition of high purity selenium is best expressed in terms of the impurity levels, since there are a number of critical impurities which bear ari important relation to the end use of selenium. These include mercury, tellurium, arsenic and chlorine in rectifier applications (Cooper, et a/., 1974. Selenium is widely employed in industrial products and processes with the glass industry (fiat-glass and pressed or blown glass and glassware) using approx!mately 27% of the total consumption (Glover, et aL, 1979; Newland, 1982). Selenium is used by glass makers to counteract coloration due to iron oxides. From 0.02 to 0.3 pounds of elemental selenium, sodium selenate, barium selenite or sodium selenite per ton of glass is required. From 1 to 50 pounds of selenium per ton of glass yields a ruby-red glass, varieties of which are employed in tableware, light filters, traffic and other signal lenses and infrared equipment. Additionally, in excess of 100 000 pounds of selenium per year is us'ed to produce "black " glass that is used as an outer surface in many modern office buildings. Electronic uses of selenium (in high purity form) (accounting for approximately 23% of its use) are related principally to its semiconductor and photoelectric characteristics. Selenium is used in the manufacture of semiconductors, rectifiers, specialty transformers, thermo-elements, photoelectric photocells and xerographic" materials (Glover, et eL, 1979; Newland, 1982). Approximately 23% of the total demand for selenium is for use in duplicating machines. A dry photographic copying process, such as xerography, employs selenium-coated metal cylinders from which the photographic image is transferred by static electricity (Dessauer and Clark, 1965). Approximately 4 pounds of selenium is required for each 100 square feet of effective copying surface and replacement of the selenium unit may be required after 50 000 to 500 000 copies have been reproduced. The manufacture of inorganic pigments, principally cadmium sulfoselenide, accounts for about 14% Of the annual 415

conuumption of selenium. These pigments possess considerable light stability plus resistance to heat, chemical attack and weathering and are used in plastics, paints, enamels, inks, rubber and ceramics (Glover, et al., 1979; Nazarenko and Ermakov, 1972; Newland, 1982), The remainder of the selenium (amounting to approximately 13%) is used in a broad spectrum of applications. Compounds of selenium are used as accelerators and vulcanizing agents in rubber production to promote heat, oxidation and abrasion resistance and to increase the resilience of rubber. Selenium added to stainless steel improves casting, forging and machining characteriatics, without reducing corrosion resistance, Selenides of refractory metals such as MeSa=, NbS.~and WSe= have been effective as high temperature vacuum-stable solid lubricants. Numerous organic reactions including oxidation, hydrogenation, isomerization and polymerization are catalyzed by selenium and its compounds such as selenium dioxide. Selenium catalysts have been used commercially in catalytic liquid phase oxidations and in the isomerization of unsaturated oils (Cooper, et al., 1974). Elemental selenium imparcs exceptional antioxidant properties to printing ink, mineral oils, transformer oils and vegetable oils, and nondrying properties to linseed, oiticica and tung oils. Selenium oxychloride is a powerful solvent that has been employed as a paint and varnish remover and as a solvent for rubber resins, glue and other organic substances (Glover, et al.. 1979; Newland, 1982). Selenium compounds, also find applications in lubricating oils and in extreme-pressure lubricants through their antioxidant and antigalling properties; in insecticides, parasiticid~s, bactericides and herbicides; in photographic pi~otosensitizers and toners; in mercury vapor detectors, fireproofing agents, insect repellents, phosphorescents and luminescents. Selenium also is used extensively as a reducing or oxidizing agent in delay-action blasting caps, and to help control micro cracking in electroplated chromium. Since the establishment of selenium as an essential element, selenium has been extensively used as a feed additive for livestock. Small amounts (0.1-2 ppm) have been added to feeds since 1974 when the Food and Drug Administration (FDA) lifted the ban on selenium (National Academy of Sciences, 1976; Newland, 1982). Selenium (principally in the form sodium selenate)solutions have been used to control certain animal diseases in sheep, cattle and pigs. Human medical uses in the U.S. have been restricted to topical applications for the treatment of dandruff and seborrhea dermatitis of the scalp. These compounds are in the form of selenium mono- and disulfide with Selsun Blue® (1% mixture) and Selsun Red® (2,5 mixture) as the major products (Newland, 1982). Additionally, limited use of radioactive selenium (~Seselenomethionine) as a diagnostic scanning and labeling agent for malignant tumors of various types is being investigated since these compounds concentrate principally in the liver, pancreas and other or§ans whichhave been difficult to study with X-rays (National Academy of Sciences, 1976; Newland, 1982; Potchen, 1963), Table 2 lists some selenium compounds and their uses. Although there is little reliable information on occupational exposure levels, acute poisoning through inhalation may occur in industry via exposure to selenium dust and fumes, 4!6

selenium dioxide and hydrogen ~,~elenide. The most common effects observed for respiratory occupational exposure to selenium and its compounds if~C,;Iude, irrit,~tion of mucous membranes of the eyes, throat ~I@ lungs while skin damage can result from a direct contact exI,~nsure(Glover, et al., 1979). T R A N S P O R T B~I~fA VIOR

Although, as noted earlier, selenium levels are low in natural waters, fluvial action is important in selenium transport (Bectine and Goldberg, 1971; Newland, 1982). It has been estimated that approximately 8000 tons of selenium are deposited annually in the oceans. There is increasing evidence which emphasizes the importance of microbial transformations of selenium in the cycling of this element (Zieve and Paterson, 1~,~81).Studies of biological selenium conversions demonstral~J that microorganisms can metabolize selenium (Chau, et al.. 1976). For example, several types of aquatic bacteria converted selene-compounds to volatile methylated forms (Chau, eta/., 1976). Experiments with some plants, fungi, bacteria and rats have demonstrated their ability t O synthesize volatile selenium compounds from inorganic selenium salts, or from several organo-selenium compounds (Cox and Alexander, 1974; Fleming and Alexander, 1972; Lewis. et al., 1966). Additionally. ~he volatilization of selenium from soils amended with organic or inorganic selenium has also been reported (Abu-Erreish, et aL, 1968; Doran and Alexander, 1977; Francis. at,a~., 1974; Reamer and Zoller, 1980). Dimethyl selenide was shown to be the most Soils Acid-poo¢ly 111111 ,I HIIII refill q

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FIG. 4. Chemical and biochemical changes in selenium possibly involved in its movement from soil :through plants and animals (based on Newland, 1982). F u n d a m . A p p L Toxicol. (3)

Sep:ember/October, 1983

SELENIUM - TOXIN OR PANACEA predominant volatile organoselenium compound formed (Francis, et al., 1974), although dimethyldiselenide and dimethylselenone or methylmethylselenite have also been reported (Reamer and Zoller, 1980). it should also be noLsd that fungi belonging to several genera and at least one bacterium grown in culture are able to synthesize demethylselenide from inorganic selenium precursors (Barkes and Fleming, 1974; Challenger, 1945; Challenger and Charlton, 1947; Cox and Alexander, 1974). Zieve and Peterson (1981) ~n recent studies (under laboratory conditions) involvir~g factors influencing the volatilization of selenium-75 from a soil amended with sodium selenite, reported that volatilization was dependent upon microbiological activity, temperature, moisture, time, concentration of water-soluble selenium and season of the year when the soil sample was collected. For example, soil collected in the spring, and assayed under standard conditions, evolved substantially more selenium than that collected in the summer, autumn or winter. Estimates of the low-temperature volatilization of selenium from soil indicated that this source could approach the calculated value for anthropogenic release. Hence, this emphasized the relative importance of microbial transformations of selenium in the cycling of this element. For example, in estimating the output of selenium into the atmosphere from soil on a global scale, the following values were calculated from the data of Zieve and Peterson (1991). If the maximum spring rate of selenium volatilization of 0.35 x 10 4/~g/day cm ~ was maintained throughout the year, then 172 x lOSg selenium per year would have been released to the atmosphere taking the surface of land less ice to be 1.33 x 10 =~ cm ~ (Garrels, et al., 1975). In contrast, the lower rate of volatiliza. tion during autumn would result in an annual input into the atmosphere of 24 x 10 s g selenium per year. It had been previously calculated that the anthropogenic emission of selenium from fossil fuel combustion and the roasting of sulfide ores contributes 119.8 x 10Sg selenium per year (Mackenzie, eta/., 1979). Hence, the studies of Zieve and Peterson (1981) would suggest that the amount of microbiological volatilization of selenium could approach the anthropogenic flux later. The chemical and biochemical changes in selenium possibly involved in its movement from soil through plants and animals are depicted in Figure 4 (Newland, 1982). In nature, all valence forms of selenium are known to exist with the specific forms dependent on the solubility and oxidationreduction reactions which are possible in the environment (Newland, 1982). As noted earlier, many plants and algae convert selenium into volatile compounds such as methyl and dimethyl selehides. This may be a mechanism for reduction of selenium to tolerable non-toxic levels (Newland, 1982; Shrift, etal., 1961 ). Some accumulator plants have a strong odor (Beeson, 1961 ) cause by methyl and dime~hyl selenides. A pathway for reduction of selenate' to selenite, which is the form which reacts to form selenium containing amino acids, was suggested by Nissen and Benson (I 964) to proceed as follows: Se04 sulratoadenyhransferase adenosine-5'-phosphoselenate redueto.se l I~ystent

SeOa2REFERENCES Abu-Erreish, G.M., Whit'ehead, E.I. and Olson, E. (1968). Evolution of Volatile Selenium from Soil. Soil. ScL 106:415-420. Fundamental and Applied Toxicology

(3) 9-10/83:

Ageton, R.W. (1970). Selenium. In Mineral Facts and Prob/en:s. p. 713-721, U.S. Dept. Interior Bur. Mines Bull. No. 650. Allaway, W.H. (1968). Control of the Environmental Levels of Selenium. In Trace Suh.wance,~ in l:'nviromnental Heahh - IL (D,D. Hemphil[, ed.), Proceedings of University of Missouri's 2nd Annual Conference Trace Substances in Environmental Health. University of Missouri, Columbia, MO. Allaway, W,H., Cary, E,E, and Ehllng, C,F. (1907), Cycling of Low Levels of Soils, Plants and Animals. In Seleniunt it: Biomedichw, (O.H. Muth, ed.), pp. 273-296, Avi. Publ. Col., Westport, CT. Anonymous (1979). Selenium in the Heart of China. Lancet 2:889. Barkes, L. :RndFleming, R.W. (1974). Production of Dimethylselenide Gas from Inorganic Selenium by Eleven Soil Fungi. Bull. Environ. Contanl. 7b.v&'oL 12:308-311. Bectine, K.K. and Goldberg, E.D. (1971). Fossil Fuel Combustion and the Major Sedimentary Cycle. Sci,'m.e 173:233-235. Beeson, K.C. (1961). Occurrrence and Significance of Selenium in Plants. in Agricultural llandbook 200. U.S. Dept. of Agriculture, Washington, DC, pp. 33-40. Burk, R.F. (197b). Selenium. In Nutrition Reviews: Present Knowledge ha Nutrition. 4th Ed., The Nutrition Foundatio1~, Inc., Washington, DC, pp. 310-316. Byers, H.G. (1935). Selenium Occurrence in Certain Soils in the United States with a Discussion of Related Topics. U.S. Dept. Agriculture TeNt. l]ull. No. 482, Washington, DC. Challenger, F. (1945). Biological Methylation. Chem. Rev. 36: 315-361. Challenger, £. and Charlton, P. (1947). Studies on Biological Methylation. X. The Fission of the Mona-and Disulfide Links by Moulds. J. Ou,nh Sue. pp. 421-424. Chau, Y.K., Wang, P.T.S., Silverberg, B.A., Luxon, P.L. and Bengert, G.A. (1976). Methylation of Selenium in the Aquatic Environment. Science 192:1130-1131. Coleman, R.G. and Delevaux, M.H. (1957). Occurrence of Selenium in Sulfide from Sedimentary Rocks in the Western United States. Econ. Geol. 52:499-502. Cooper, W.C. and Glarer, J.R. (1974). The Toxicology of Sc/enium and Its Compounds. in Selenium. (R.A: Zingaro and W.C. Cooper, eds.), pp. 654-674, Van Nostrund, NY. Cooper, W.C., Bennett, K,G. and Croxton, F.C. (1974). The History, Occurrence and Properties of Selenium. In Selenhml. (R.A. Zingaro and W.C. Cooper, eds.), pp. 1-30, Van Nostrund, NY. Cox, D.P. and Alexander, M. (1974). Factors Affecting Trimethyla rsine and Dimethylselenide Formation by Canidida humicola..I. Microbial. Ecol. 1:136-144. Daoud, A.H. and Griffin, A.C, (1980). Effect of Retinoic Acid, Butylated Hydroxytolffene, Selenium and Sorbic Acid on Azodye.Hepatocarcinogenesis. Cancer Lett. 9:299-304. Davis, S.N. and DeWiest, R.J.M. (1966). Hydrogeology., John Wiley & Sons, NY. Davis, W.E. (1972). National Inventory of Sources and Emissions. Boron, Copper, Selenium and Zinc 1969. Selenium Section. U.S. National'Tech. in Form Serv., PB Rept. No. 219679, pp. 57, Springfield, VA. Dessauer, J.H. and Clark, H.B. (1965). Xerography and Related Processes, pp. 520, Focal Press, NY. Doran, J.W. and Alexander, M. (1977). Microbial Formation of Volatile Selenium Compounds in Soils. Soil. Sci. Sac. Am. J. 41:70-73. Fleischer, M. (1954). The Abundance and Distribution of Chemical Elements in the Earth's Crust. J. Chem. Ed. 31:446-451. Fleming, G.A. (1962), Selenium in Irish Soils and Plants. Soil Sci. 94:28-35.

Fleming, R.W. and Alexander, M. (1972). Dimethylselenide and Dimethyltelluride Formation by a Strain of Penicillium. AppL Microbiol. 24:424-441. Flinn, J.E. and Reimers, R.S. (1974). Development of Predictions of Future Problems. EPA Report No. 600]3-74-005,. pp.: 36-38, Washington, DC. 417

Flohe, L., Gunzler, W.A. and Schock, H.H. (1973). Glutathione Peroxidase: A Selenoenzyme. Fd. Eur. Svc. ia,tt. 32:132-134. Francis, A.]., Auxbury, J.M. and Alexander, M. (1974). Evolution of Dimethylselenide from Soils. Appl. Microbial. 28:248-250. Frost, D.V. (I972). The Two Faces of Selenium-Can Selenophobia Be Cured;' CRC Crit. Rev. Toxicol. 1:.167-514. Frost, D.V. and Lish, P.M. (1975). Selenium in Biology. Annu. Rev. Pharmacol. 15:259-284. Ganther, H.E. 0978). Modification of Methylmercury Toxicity and Metabolism by Selenium and Vitamin E: Possible Mechanisms. I-nviron. Ilhh. Persp. 25:71-76. Garrels, R.M., Mackenzie, F,'I'. and Hunt, C. (1975). Chemical Cycles and the Global Environment. In Assessing tluman htfluettces, pp. 206, William Kaufman I'ubl., Los Altos. GisseI-Nielsen, G. (1971 ). Influence of pl| and Texture of the Soil on Plant Uptake of Added Selenium. J. Agr. l.bod Chem. 19:1165-11o7. GisseI-Nielsen, G. and Hamdy, A.A. (1978). Plant Uptake of Selenium and Se-values in Different Soils. Z. lylanzen,'rnaehr. Both,nktL 141:67-75. Glarer, I.R. (I 967). Selenium and Its Indust rial Toxicology. ,.Imz. Occlq~. llyg. 10:3-14. Glarer, J., Levander, O., Parizek, I. and Vouk, V. (1979). Selenium. In llandbook on the Toxit'ologr v f 3h,taZr. (L. Friberg, G.F. Nordberg and V.B. Vouk, eds.), pp. 555-557, Elsevier/NorthHolland Biomedical Press, Amsterdam. Goldschmidt, V.M. (1954). Geochemistry., Oxford University Pres% London. Gordon, G.E., /.oiler, W.H. and Gladuly, E.S. (1973). In Trace Substan~a,s h~t':nvironmental th,alth, Vol. 7, (D.D. I'temphill, ed.), pp. IO7-173, University of Missouri, Columbia, MO. Harr, I.R. (1978). Biological Effects of Selenium. In lbxicity of lh,av.r Metals in the Environment. Part L (F.W. Oehme, ed.), pp. 393-426, Marcel Dekker, NY. Harr, I.R. and Muth, O.H. (1972). Selenium Poisoning in Domestic Animals and Its Relationship to Man. Clin. Toxicol. 5:175-186. Hart, J.R., Exon, J.H., Weswig, P.H. and Whanger, P.D. (1973). Relalionship of Dietary Selenium Concentration, Chemical Induction and Tissue Concentration of Selenium in Rats. Clin. Toxh'ol. 6:487-495. Harrison, P.R., Rahn, K.A., Dams, R., Robbins, J.A., Winchester, J.W., Brar, S.S. and Nelson, D.M. (1971). Area Wide Trace Metal Concentrations Measured by Multielement Neutron Activation Analysis - One Day Study in North-West Indiana. J. AO. Polha. Control Assoc. 21:563-570. Hashimoto, Y., Hwang, J.T. and Yanagisawa, S. (1970). Possible Source of Atmospheric Pollution oF Selenium. Environ. Sci. Technol. 4:157~158. Howard, I.H. (1973). Control of Geochemical Behavior of Selenium in Natural Waters by Absorption on Hydrous Ferric Oxides. In Trace Substances in Enviromnental Health it., (D.D. Hemphill, ed.), Proceedings, University of Missouri's 7th Annual Conference Trace Substances in Environmental Health, pp. 485-496, University of Missouri, Columbia, MO. IARC (1975). Monographs on the Evaluation of Carcinogenic Risk of Chemicals to Man. Vol. 9. Selenium and Seleniun! Compounds, pp. 245-260, International Agency for Research on Cancer, Lyon, lacobs, M.M. (1980). Effects of Selenium on Chemical Carcinogens. Prey. Med. 9:362-367. Jacobs, M.M., Jansson, B. and Griffin, A.C. (1977). Ir.hibitory Effects of Selenium on 1,2-Dimethylhydrazine and Methyl Azoxymethanol Acetate Induction of Colon Tumors, Cancer Lett. 2:133-137. John, W., Kalfer, R., Rahm, K. and Weslowski, l.l. (1973). Trace Element Concentrations in Aerosols from the San Francisco Bay Area. Atmos. Environ. 7:107-118. Johnson, C.M. (1976). Selenium in the Environment. Residue Rex,. 62:102-130. 418

Johnson, C.M., Asher, C.J. and Brayer, T.C. (1967). SymposiumSelenium. In Biomedicine, (O.H. Muth, ed.), p. 57, Avi Publ. Co., Westport, CT. Johnson, H. (1970). Determination of Selenium in Solid Waste. Environ. Sci. Technol. 4:850-853. Johnson, R.D. and Manske, D.D. (1977). Pesticides in Food and Feed: Pesticide and Other Chemical Residue in Total Diet Samples. Pestle. Monit. J. "/1:116-131. Kazantis, G. (1981). Role of Cobalt, iron, Lead, Manganese, Mer,=ury, Platinum, Selenium, and Titanium in Carcinogenesis. Environ. ilhh. Persp. 40:143-161. Kinnigkeit, G. (1962). Untersuchungen Selenextonierter Arbeiter eines Gleichrichter Werks. Z. Ge.mmte ltrg. 8:350-362. iakir~, H.W. (1961a). Geochemistry of SeJenium in Relation to Agriculture. In Agricuhure llandbook 200., pp. 3-12, U.S. Dept. of Agriculture, Washington, DC. Lakin, ||.W. (1961b). Seleniun Content in Soils. In Agrieuhure Ilandbook 2(1(I., pp. 27-34° U.S. Dept. of Agriculture, W:lshington, DC. Lakin, H.W. (1973). Selenium in Our Environment. In Trace Eh'ments in the EnvironnwnL, (E.L. Kothny, ed.), pp. 96-111, Advances Chemistry Series. No. 123, American Chemical Society, Washington, DC. Lakin, H.W. and Byers, H.G. (1941). Selenium i,a Wheat and Wheat Products. Cereal Chem. 18:73-78. Lakin, H.W. and Davidson, D.F. (1907). The Relationship of the Geochemistry of Selenium to Its Occurrence in Soils. In 3]rmposium-Seleniuim in Biomedical., (O.H. Muth, ed.), p. 27, Avi Publ. Co., Westport, CT. Lakin, H.W. and Davidsor~, D.F. (t968). Relationship of the Geochemistry of Selenium to Its Occurrence in Soils. U.S. Geo. Survt:r Pr¢~ Paper No. 820., pp. 573-576. Lee, R.E., Jr. and Duffield, F.V. ( 1977J. Sources of Environmentally Important Metals in the Atmosphere. Adv. Chem. Scr. 172:140. Lewis, B.G., Johnson, C.M. and Deiwiche, C.C. (I 966). Release of Volatile Selenium Compounds by Plants: Collection Procedures and Preliminary Observations. J. Agr. Food Chenl. 14:638-640.

Lindberg, P. (1968). Selenium Determination in Plant and Animal Material ,and in Water. Acta. Vet. Scand. Suppl. 23:48. Lo, M.T. and Sandi, E. (1980). Selenium: Occurrence in Foods and Its Toxicological Significance - A Review. J. Environ. Pathol. Toxicol. 4:193-218. Luttrell, G.W. (1959). Bibliography of Iron Ore Resources in the World. U.S. Geol. Surver Bull. No. 1019-M, pp.867-972. Mackenzie, F.T., Lantzy, R.J. and Paterson, V. (1979). Global Trace Metal Cycles and Predictions. Math. Geol. 2:99-142. Mahaffey, K.R., Corneliussen, P.E., lelinek, C.F. and Fiorino, J.A. (1975). Heavy Metal Exposure from Foods. Environ. Hhh. Persp. 12:63-69. Manske, D.D. and Johnson, R.D. (1977). Residue in Food and Feed: Pes'ticide and Other Chemical Residue in Total Diet Samples. Pestic. Monit. J. 10:134-148. Medina, D. and Shephard, F. (1981). Selenium-mediated Inhibition of 7, 12-Dimethylbenz(a)anthracene-inducdd Mouze Mammary Tumorigenesis. Carchmgenesis. 2:451-455. Molson, A.L. and Oison, O.E. (1974). Selenium in Agriculture. In Selenium., (R.A. Zingaro and W.C. Cooper, eds.), pp. 675-707, Vati Nostrund, NY. Morette, A. and Divin, J.P. (1965). Determination of Selenium in Water. Ann. Pharm. Ft. 23:169-178. National Academy of Sciences (1971)..4tins o f Nutritional Data on United States and Canadian Feeds. Washington, DC. National Academy of Sciences (1976). Selenium. p. 203, Washington, DC. Nazerenko, l.l. and Ermakov, A.N. (1972). Anal.vtieat Chemistry o f Selenium and Tellurium,, Hoisted Press, NY.

Fundam. Appl. Toxicol. (3)

September/October. 1983

SELENIUM - TOXIN OR PANACEA Nelson, A.A., Fitzhugh, O.G. and Calvery, H.O. (1943). Liver Tumors Following Cirrhosis Caused by Selenium in Rats. Cancer Res. 3:230-230. Newland, L.W. (1982). Selenium. in Ilandbook o f Environmental Chemislr.r. t"oL 3. Part B. Anlropogenic CompountA., (0.

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