Volatile organic selenium compounds

Organic selenium compounds and siUceous materials (rock, ore, concentrates) are fused with mixtures of sodium carbonate and various oxidants, eg, sodium peroxide, potassium nitrate, or potassium persulfate. For volatile compounds, this fusion is performed in a bomb or a closed system microwave digestion vessel. An oxidizing fusion usually converts selenium into Se(VI) rather than Se(IV). 

Selenate is the most mobile form of Se. Selenium becomes biologically unavailable by reduction to elemental Se or by formation of metal selenides or Se-sulfides. Inorganic Se compounds can be converted to volatile organic Se such as dimethyl selenide or dimethyl diselenide by... 

Importantly, it also occurs naturally in several oxidation states and is, therefore, redox sensitive. Methylation and hydride formation are important, and sulfur and iron compounds play an important role in the cycling of selenium. Microbiological volatilization of organic selenium, particularly dimethyl selenide, is known to be an important factor in the loss of selenium from some selenium-rich soils and waters (Frankenberger and Arshad, 2001 Oremland, 1994). Phytoplankton can also promote the production of gaseous selenium compounds in the marine environment (Amouroux et aL, 2001). 

The exclusion of selenium from the proteins of accumulator plants is thought to be the basis for their selenium tolerance. Their selenium metabolism is based mainly on water-soluble nonprotein forms such as selenium methylselenomethionine (Jacobs, 1989). The garlic odor characteristic of selenium-accumulator plants reflects the volatile organic compounds dimethylselenide and dime-thyldiselenide. Plants can suffer selenium toxicity as a result of selenium competition with essential metabolites for biochemical sites, replacement of essential ions by selenium, mainly major cations, selenate occupation of the sites of essential groups such as phosphate and nitrate, or selenium substimtion in essential sulfur compounds. 

The volatile selenium compounds that partition into the atmosphere include the inorganic compounds selenium dioxide and hydrogen selenide and the organic compounds dimethyl selenide and dimethyl diselenide. Hydrogen selenide is highly reactive in air and is rapidly oxidized to elemental selenium and water (NAS 1976a), but the other compounds can persist in air. 

Many of the basic analytical methods used for determining selenium in biological media are also used for determining selenium levels in soil, water, and air. Precautions in the collection and storage of environmental samples, however, are necessary to prevent loss of the volatile selenium compounds to the air. The destruction of organic matter before selenium measurement is also often necessary. 

Both oxidation and methylation are microbial transformations involved in the redistribution and global cycling of arsenic. Oxidation involves the conversion of toxic arsenite to less toxic arsenate. Bacterial methylation of inorganic arsenic under anaerobic conditions may be a mechanism of arsenic detoxification. Fungi also transform inorganic and organic arsenic compounds into volatile methylar-sines. However, unlike methylated selenium which is nontoxic, the volatile arsine... 

Typically, the emulsion globules are 0.1-2 mm in diameter, and the encapsulated, aqueous droplets are 0.1-5 fim in diameter. A typical emulsion formulation for selenium extraction would consist of a high concentration of aqueous solution of NaCl dispersed in a low volatility, non-polar hydrocarbon like kerosene, and stabilized with an oil-soluble surfactant. The organic phase also contains a complexing agent for the selenium compounds. In the ELM process, a selenium anion first diffuses to the surface of the oil globule where, depending on the ionic... 

Gas chromatography has, of course, been used extensively in the analysis of many types of organic compounds with boiling points up to about 250°C, also to the analysis of organic compounds of lead, mercury, selenium, tin, manganese and silicon. Derivitisation of these compounds to produce compounds sufficiently volatile to be amenable to gas chromatography is frequently practised. Gas chromatography has also been applied to the determination of arsenic, antimony, selenium, tin, beryllium and aluminium and the common anions such as sulphate, nitrate, phosphate, sulphide, cyanide and thiocyanate. 

Phytovolatilization involves the use of plants and plant-associated soil microbes to take up contaminants from the soil, transform them into volatile forms, and release them into the atmosphere (Lin, 2008). Phytovolatilization occurs as growing trees and other plants take up water and the organic and inorganic contaminants. Metalloids, such as selenium. As, and tin, can be methylated to volatile compounds or mercury that can be biologically transformed to elemental Hg. Phytovolatilization has been primarily used for the removal of mercury and selenium. 

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