Atmosphere selenium

Incineration of rubber tires, paper, and municipal waste is an additional source of atmospheric selenium. Hashimoto et al. (1970) reported selenium concentrations in rubber tires to be 1.3 mg/kg. Seventy different kinds of paper have been found to contain selenium (West 1967). Combustion of municipal solid waste results in stack emissions ranging from 0.00098 to 0.00216 pounds (0.44-0.98 g) of selenium per ton of refuse (Johnson 1970). 

The amount of selenium contributed to the air by other sources is not known. Microbial action within the soil may also contribute selenium to the air (Fishbein 1983). Selenium biomethylation volatilizes about 3,000 tons of selenium per year into the atmosphere, which eventually returns to earth in rainfall (NAS 1976a). Volcanic gas is suspected to be the major natural source of atmospheric selenium. Certain plants metabolize inorganic selenium compounds to volatile selenium in the forms of dimethyl selenide (Lewis et al. 1971) and dimethyl diselenide (Evans et al. 1968). Animals are also capable of volatilizing selenium and releasing dimethyl selenide in expired air (Schultz and Lewis 1940). 

Fly ash settling ponds (which contain high concentrations of selenium) and hazardous waste sites where selenium compounds were disposed of in the past are potential sources of atmospheric selenium through fugitive dust emissions. Selenium emissions from these potential sources have not been quantified. 

Ross HB. 1990. Biogeochemical cycling of atmospheric selenium. Met Speciation Environ NATO ASI Ser., Ser G Vol 23 523-543. 

A small fraction of selenium may exist in the gaseous state in the atmosphere (Table 7.2). However, the majority of atmospheric selenium is expected to be present in particulate form (NAS 1976). Removal of atmospheric selenium occurs primarily through wet and dry deposition and its association with sub-micrometer aerosols (Mosher and Duce 1983, NAS 1976, Jickells etal. 1984). Selenium is emitted naturally into the atmosphere mainly in the form as dimethylselenide (Jiang et al. 1983). Major producers of dimethylselenide are plants and soil bacteria (Terry et al. 2000). Through bacterial and phytovolatilization substantial amounts of selenium may be removed from selenium-contaminated soil. Inor-... 

Although selenium levels are low in natural waters, fluvial action is important in selenium transport. Bectine and Goldberg [54] estimated that 8,000 tons of selenium are deposited annually in the oceans. The average concentration of selenium in seawater is 0.09 ppb, due to loss by precipitation of selenite-metal hy-drozies. Analysis of rainwater for selenium content [21, 25] have shown a range of 0.04 to 1.4 ppb, and atmospheric selenium in rainwater is correlated to industrial emissions [21]. 

Bina Selenides. Most biaary selenides are formed by beating selenium ia the presence of the element, reduction of selenites or selenates with carbon or hydrogen, and double decomposition of heavy-metal salts ia aqueous solution or suspension with a soluble selenide salt, eg, Na2Se or (NH 2S [66455-76-3]. Atmospheric oxygen oxidizes the selenides more rapidly than the corresponding sulfides and more slowly than the teUurides. Selenides of the alkah, alkaline-earth metals, and lanthanum elements are water soluble and readily hydrolyzed. Heavy-metal selenides are iasoluble ia water. Polyselenides form when selenium reacts with alkah metals dissolved ia hquid ammonia. Metal (M) hydrogen selenides of the M HSe type are known. Some heavy-metal selenides show important and useful electric, photoelectric, photo-optical, and semiconductor properties. Ferroselenium and nickel selenide are made by sintering a mixture of selenium and metal powder. 

Selenium trioxide, SeO, is white, crystalline, and hygroscopic. It can be prepared by the action of sulfur trioxide on potassium selenate or of phosphorous pentoxide on selenic acid. It forms selenic acid when dissolved in water. The pure trioxide is soluble in a number of organic solvents. A solution in Hquid sulfur dioxide is a selenonating agent. It is stable in very dry atmospheres at room temperature and on heating it decomposes first to selenium pentoxide [12293-89-9] and then to selenium dioxide. 

Purification. Tellurium can be purified by distillation at ambient pressure in a hydrogen atmosphere. However, because of its high boiling point, tellurium is also distilled at low pressures. Heavy metal (iron, tin, lead, antimony, and bismuth) impurities remain in the still residue, although selenium is effectively removed if hydrogen distillation is used (21). 

Zinc (76ppm of the earth s crust) is about as abundant as rubidium (78 ppm) and slightly more abundant than copper (68 ppm). Cadmium (0.16 ppm) is similar to antimony (0.2 ppm) it is twice as abundant as mercury (0.08 ppm), which is itself as abundant as silver (0.08 ppm) and close to selenium (0.05 ppm). These elements are chalcophiles (p. 648) and so, in the reducing atmosphere prevailing when the earth s crust solidified, they separated out in the sulfide phase, and their most important ores are therefore sulfides. Subsequently, as rocks were weathered, zinc was leached out to be precipitated as carbonate, silicate or phosphate. 

When a solid compound possesses a relatively high vapor pressure below its melting point, it may be possible to purify it by sublimation. Selenium dioxide, for example, is easily purified prior to use by sublimation at atmospheric pressure (Chapter 1, Section XI). More commonly, the method of choice is sublimation at reduced pressure, which allows more ready evaporation of solids with limited volatility. A convenient vacuum sublimation apparatus is shown in Fig. A3.19. The impure sample is placed in the... 

A mixture of 290 mg of the 16,21-diacetate of 6a,9a-difluoro-16o --hYdroxY-hYdrocortisone, 30 cc of t-butanol, 0.5 cc of pyridine and 150 mg of selenium dioxide was refluxed for 53 hours under an atmosphere of nitrogen and cooled ethyl acetate was added and filtered through celite the solvent was evaporated to dryness under reduced pressure, the residue... 

A procedure involving (a) the deposition of nearly stoichiometric films of copper and indium on suitable substrates using vacuum evaporation or electrodeposition and (b) the heat treatment of Cu-In films in a hydrogen-selenium atmosphere at temperatures above 630 °C was reported to yield large grain (several mm in size), stoichiometric thin films of chalcopyrite CIS with a preferred 112 orientation [167]. 

Mixtures with sulfur are unstable, and may ignite some hours after preparation, depending on the state of subdivision and atmospheric humidity [1], Selenium reacts violently with aqueous solutions of the oxidant [2],... 

Interaction of selenium and potassium is exothermic and ignition occurs, or, with excess potassium, a slight explosion. Tellurium and potassium become incandescent when warmed in a hydrogen atmosphere to prevent aerobic oxidation. 

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