HANDBOOK OFCHEMICAL RISK ASSESSMENT Health Hazards to Humans, Plants, and Animals ( VOLUME 1 ) - PART 7 (end ) potx

CHAPTER 31 Selenium 31.1 INTRODUCTION Selenium poisoning is an ancient and well-documented disease (Rosenfeld and Beath 1964) Signs of it were reported among domestic livestock by Marco Polo in western China near the borders of Turkestan and Tibet in about the year 1295; among livestock, chickens, and children in Colombia, South America, by Father Pedro Simon in 1560; among human adults in Irapuato, Mexico, in about 1764; and among horses of the U.S Cavalry in South Dakota in 1857 and again in 1893 (Rosenfeld and Beath 1964) In 1907/08, more than 15,000 sheep died in a region north of Medicine Bow, Wyoming, after grazing on seleniferous plants The incidents have continued, and recent technical literature abounds with isolated examples of selenosis among domestic animals and wildlife Selenium (Se) was first identified as an element in 1817 by the Swedish chemist Berzelius It is now firmly established that selenium is beneficial or essential in amounts from trace to µg/kg (ppb) concentrations for humans and some plants and animals, but toxic at some concentrations present in the environment (Rosenfeld and Beath 1964) Selenium deficiency was reported among cattle grazing in the Florida Everglades, which showed evidence of anemia, slow growth, and reduced fertility (Morris et al 1984) Selenium deficiency has been demonstrated in Atlantic salmon, Salmo salar (Lorentzen et al 1994), in various species of deer in Florida and Washington (Hein et al 1994; McDowell et al 1995), and in free-ranging ungulates in Washington state, including moose (Alces alces) and bighorn sheep (Ovis canadensis) (Hein et al 1994) Conversely, calves of Indian buffaloes died of selenium poisoning after eating rice husks grown in naturally seleniferous soils (Prasad et al 1982) Adverse effects of excess selenium are reported on reproduction of cattle, monkeys, sheep, swine, rats, and hamsters, including fetal and maternal death, and a dramatic increase in developmental abnormalities (Domingo 1994) Severe reproductive and developmental abnormalities were observed in aquatic birds nesting at selenium-contaminated irrigation drainwater ponds in the San Joaquin Valley, California (Ohlendorf et al 1986, 1986a, 1987, 1989, 1990; Hoffman et al 1988; Schuler et al 1990; Besser et al 1993; Lemly 1996b) Accumulation of more than mg Se/kg dry weight in fish gonads is the probable cause of reduced reproduction and subsequent species disappearances in Belews Lake, North Carolina, and the endangered razorback sucker (Xyrauchen texanus) from the Green River, Utah, in 1991 (Cumbie and Van Horn 1978; Hamilton and Waddell 1994; Waddell and May 1995) Selenium has been the subject of many reviews (Rosenfeld and Beath 1964; Frost 1972; Sandholm 1973; Zingaro and Cooper 1974; Frost and Ingvoldstad 1975; Anonymous 1975; National Academy of Sciences [NAS] 1976; Harr 1978; U.S Environmental Protection Agency [USEPA] 1980, 1987; Lo and Sandi 1980; Shamberger 1981; Wilber 1980, 1983; Fishbein 1977, 1983; National Research Council [NRC] 1983; Reddy and Massaro 1983; Eisler 1985; Lemly and Smith © 2000 by CRC Press LLC 1987; Ohlendorf 1989; Hodson 1990; Goede 1993; Lemly 1993, 1996a, 1996b; Heinz 1996; U.S Public Health Service [USPHS] 1996) These authorities agree that selenium is widely distributed in nature, being especially abundant with sulfide minerals of various metals, such as iron, lead, and copper The major source of environmental selenium is the weathering of natural rock The amount of selenium entering the atmosphere as a result of anthropogenic activities is estimated to be 3500 metric tons annually, of which most is attributed to combustion of coal and the irrigation of highselenium soils for crop production However, aside from highly localized contamination, the contribution of selenium by human activities is small in comparison with that attributable to natural sources Collectively, all authorities agree that selenium may favorably or adversely affect growth, survival, and reproduction of algae and higher plants, bacteria and yeasts, crustaceans, molluscs, insects, fish, birds, and mammals (including humans) Most acknowledge that sensitivity to selenium and its compounds is extremely variable in all classes of organisms and, except for some instances of selenium deficiency or of selenosis, metabolic pathways and modes of action are imperfectly understood For example, selenium indicator plants can accumulate selenium to concentrations of thousands of parts per million (mg/kg) without ill effects In these plants, selenium promotes growth; whereas in crop plants, accumulations as low as 25 to 50 mg/kg may be toxic Thus, plants and waters high in selenium are considered potentially hazardous to livestock and to aquatic life and other natural resources in seleniferous zones 31.2 ENVIRONMENTAL CHEMISTRY Selenium is characterized by an atomic weight of 78.96, an atomic number of 34, a melting point of 271°C, a boiling point of 685°C, and a density of 4.26 to 4.79 Chemical properties, uses, and environmental persistence of selenium were documented by a number of researchers whose works constitute the major source material for this section: Rosenfeld and Beath (1964); Bowen (1966); Lakin (1973); Stadtman (1974, 1977); Frost and Ingvoldstad (1975); Chau et al (1976); Harr (1978); Wilber (1980, 1983); Zieve and Peterson (1981); Robberecht and Von Grieken (1982); Cappon and Smith (1982); Nriagu and Wong (1983); Eisler (1985); USPHS (1996) There was general agreement on four points Selenium chemistry is complex, and additional research is warranted on chemical and biochemical transformations among valence states, allotropic forms, and isomers of selenium Selenium metabolism and degradation are significantly modified by interaction with heavy metals, agricultural chemicals, microorganisms, and a variety of physicochemical factors Anthropogenic activities (including fossil fuel combustion and metal smelting) and naturally seleniferous areas pose the greatest hazards to fish and wildlife Selenium deficiency is not as well documented as selenium poisoning, but may be equally significant Selenium chemistry is complex (Rosenfeld and Beath 1964; Harr 1978; Wilber 1983; Porcella et al 1991; Wiedmeyer and May 1993; Besser et al 1994; USPHS 1996) In nature, selenium exists: as six stable isotopes (Se-74, -76, -77, -78, -80, and -82), of which Se-80 and -78 are the most common, accounting for 50% and 23.5%, respectively; in three allotropic forms; and in five valence states Changes in the valence state of selenium from –2 (hydrogen selenide) through (elemental selenium), +2 (selenium dioxide), +4 (selenite), and +6 (selenate) are associated with its geologic distribution, redistribution, and use Soluble selenates occur in alkaline soils, are slowly reduced to selenites, and are then readily taken up by plants and converted into organoselenium compounds, including selenomethionine, selenocysteine, dimethyl selenide, and dimethyl diselenide In drinking water, selenates represent the dominant chemical species Selenites are less soluble than the corresponding selenates and are easily reduced to elemental selenium In seawater, selenites are the dominant chemical species under some conditions (Cappon and Smith 1981) Selenium dioxide is formed by combustion of elemental selenium present in fossil fuels or rubbish © 2000 by CRC Press LLC Selenium is the most strongly enriched element in coal, being present as an organoselenium compound, a chelated species, or as an adsorbed element On combustion of fossil fuels, the sulfur dioxide formed reduces the selenium to elemental selenium Elemental selenium is insoluble and largely unavailable to the biosphere, although it is still capable of satisfying metabolic nutritional requirements Hydrogen selenide is highly toxic (at to µg/L in air), unstable, acidic, and irritative Selenides of mercury, silver, copper, and cadmium are very insoluble, although their insolubility may be the basis for the reported detoxification of methylmercury by dietary selenite, and for the decreased heavy metal toxicity associated with selenite Metallic selenides are thus biologically important in sequestering both Se and heavy metals in a largely unavailable form In areas of acid or neutral soils, the amount of biologically available selenium should steadily decline The decline may be accelerated by active agricultural or industrial practices In dry areas, with alkaline soils and oxidizing conditions, elemental selenium and selenides in rocks and volcanic soils may oxidize sufficiently to maintain the availability of biologically active selenium Concentrations of selenium in water are a function of selenium levels in the drainage system and of water pH In Colorado, for example, streams with pH 6.1 to 6.9 usually contain 0.5 µg/m in the vicinity of selenium production plants, and these were at least 500 times higher than in a control area (Table 31.1) Table 31.1 Selenium Concentrations in Nonbiological Materials Sample and Unit of Concentration Concentration Referencea 0.05 0.2 0.08 Up to 0.05 0.6 8.0 0.34–4.8 3.4 (0.5–10.7) 1–10 500–1650 2 2 4, 5, 15 15 0.22 0.35–0.75 0.2–14.5 0.12–0.44 10