Selenium volatilisation

De Souza MP, Pilon-Smits EAH, Terry N (1999) The physiology and biochemistry of selenium volatilisation by plants. In Ruskin I, Ensley BD (eds) Phytoremediation of toxic metals. Wiley, New York... 

Roasting occurs between temperatures of 530—650°C. Virtually no volatilisation of selenium or tellurium takes place during roasting. Conversion of both elements to the hexavalent form is complete. 

Sulfation Roasting. Acid roasting technology (Fig. 2) rehes on differences in the volatiUty of the tetravalent oxides of selenium and tellurium at roasting temperatures of 500—600°C to selectively volatilise selenium from slimes. Acid roasting uses sulfuric acid as the oxidant for the conversion of selenium/selenides and tellurium/teUurides to their respective tetravalent oxides. Typical oxidation reactions are as foUow ... 

Many analytical methods depend on the conversion of the tellurium in the sample to teUurous acid, H2Te02. Should teUurous acid precipitate on dilution, it can be redissolved with hydrochloric acid. Although tellurium is not as readily volatile as selenium, precautions should be taken to prevent the volatilisation of tellurium when halogen or hydrohaUde media are used during sample decomposition. 

The oxidation of teUurium(IV) by permanganate as an analytical method has been studied in some detail (26). The sample is dissolved in 1 1 nitric-sulfuric acid mixture addition of potassium bisulfate and repeated fuming with sulfuric acid volatilises the selenium. The tellurite is dissolved in 10 vol % sulfuric acid, followed by threefold dilution with water and titration with potassium permanganate ... 

The decopperized slimes are then roasted in the presence of sulfuric acid or sulfur dioxide and oxygen to volatilise the selenium as an oxide. 

Gaseous and volatilised analytes can also be easily determined by FAAS and ETAAS. For example, the determination of several elements by the formation of covalent volatile hydrides e.g. arsenic, selenium) and cold vapour generation (mercury and cadmium) is feasible with good analytical sensitivity (see Section 1.4.1.1). 

J. R. Castillo, New sensitive determination of selenium by bromide volatilisation inductively coupled plasma atomic emission spectrometry,... 

A. Lopez-Molinero, R. Gimenez, P. Otal, A. Callizo, P. Chamorro and J. R. Castillo, New sensitive determination of selenium by bromide volatilisation inductively coupled plasma atomic emission spectrometry, J. Anal. At. Spectrom., 17, 2002, 352-357. 

A method involving electrometric titration depends on the volumetric reduction of selenious acid to selenium by means of titanium sulphate. The method is rapid and accurate5 if the solution is in cold concentrated hydrochloric acid saturated with sodium chloride. The presence of the latter is important, for it ensures rapid and uniform coagulation of the selenium hydrosol and increases the sharpness in the change of voltage at the end-point. The use of the hydrochloric acid in the cold eliminates the otherwise almost inevitable loss of selenium by volatilisation. Under these conditions any tellurium which may be present is unreduced and only has the effect of modifying the nature of the end-point.6... 

Estimation of Selenium in Sulphide Minerals.s—In various sulphite-cellulose manufactories difficulties have occurred which have been traced to the presence of selenium in the pyrites used for burning. Part of the selenium remains in the burnt pyrites and part volatilises with the sulphur dioxide. 20 to 30 grams of pyrites are dissolved in hydrochloric acid (dens.=1-19) and potassium chlorate. Zinc is added to reduce the iron to the ferrous condition more hydrochloric acid is then added, the solution boiled and stannous chloride added to precipitate selenium. Since the selenium may contain arsenic, it is collected on an asbestos filter, dissolved in potassium cyanide and reprecipitated using hydrogen chloride and sulphur dioxide. The element may then be estimated by the iodometric method described below. In order to determine the relative proportion of volatile to non-volatile selenium, the pyrites may be roasted in a current of oxygen. After this treatment the contents of the tube are dissolved in warm potassium cyanide and the selenium reprecipitated and estimated in the ordinary way. 

De la Calle Guntinas et al. [769] volatilised selenium from natural water samples by reaction with sodium tetraethylborate and measured the volatilised selenium by gas chromatography microwave-induced plasma atomic emission spectrometry. The detection limit for a 5mL sample was 8ppt. 

Ferrous selenide was first obtained by Divers and Shimidzu 9 by heating a mixture of wrought-iron filings and selenium in a graphite pot. When the mixture had grown hot, a considerable portion of the selenium had volatilised and a further portion was added which combined with the iron with very little loss, causing vivid ignition and complete liquefaction. The product thus obtained closely resembles iron sulphide. It is not very sensitive to acids, but when warmed with... 

When precipitated, selenium appears as a red powder, which, when heated, melts, and on cooling forms a brittle mass, nearly black, but transmitting red light in thin plates. When more strongly heated it volatilises, yielding a yellow gas, which condenses into a crystalline dark red powder on a cold surface. 

Volatilisation is the microbial biochemical process that methylates heavy metals. Several methyl-metal complexes have significant vapour pressures at room temperature. In this way metals methylated by the microbes just boil ofF. The best known metals to be treated in this fashion are mercury, selenium, tellurium, arsenic and tin. Much of the biomethylation pathway is unknown it appears to be very complex [26]. As a result of the complexity of the volatilisation process it has not been used in effluent treatment and is not discussed further herein. 

This technique makes use of a property that these elements exhibit, i.e., the formation of covalent, gaseous hydrides that are not stable at high temperatures. Antimony, arsenic, bismuth, selenium, tellurium, and tin (and to a lesser degree germanium and lead) are volatilised by the addition of a reducing agent like sodium tetrahydroborate(III) to an... 

Selenium is normally separated by volatilisation during sintering or smelting operations and predominantly reports to fumes. Any present in lead bullion can be separated in caustic slags from the Harris process, but is relatively minor and of no real consequence. 

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