Volatile toxic elements

The most successful class of active ingredient for both oxidation and reduction is that of the noble metals silver, gold, ruthenium, rhodium, palladium, osmium, iridium, and platinum. Platinum and palladium readily oxidize carbon monoxide, all the hydrocarbons except methane, and the partially oxygenated organic compounds such as aldehydes and alcohols. Under reducing conditions, platinum can convert NO to N2 and to NH3. Platinum and palladium are used in small quantities as promoters for less active base metal oxide catalysts. Platinum is also a candidate for simultaneous oxidation and reduction when the oxidant/re-ductant ratio is within 1% of stoichiometry. The other four elements of the platinum family are in short supply. Ruthenium produces the least NH3 concentration in NO reduction in comparison with other catalysts, but it forms volatile toxic oxides. 

With respect to Cr a distinction should be made between Cr(III), which is the common oxidation state in the soils, being rather immobile and so toxic, and Cr(VI), which is very mobile and very toxic. With respect to Hg, the situation is even more complex, due to the occurrence of mercuric mercury (Hg2+), mercurous mercury (Hg2+), elemental mercury (Hg°) and organic mercury species, such as methyl mercury, (CH3)2Hg (see Section 18.5). Furthermore, volatilization of elemental mercury and organic mercury species is common. A description of these... 

Combustion of coal and other fossil fuels is a major source in the envi- ronment of trace elements that are hazards to human health. Toxic elements such as Hg, As, Sb, F, Se, and T1 are volatilized during coal combustion and are emitted directly into the atmosphere or concentrated in the fly ash (1, 2, 3). Most elements in coal occur at only parts per million levels, but large tonnages of coal are consumed each year in the United States. In addition, coal conversion processes, which could vastly increase coal use are now being considered seriously. The fate of trace elements during these processes is largely unknown. 

One of the problems associated with coal gasification is that in coal, many of the elements of the periodic table can be found in minor concentrations. An element of emerging concern is arsenic, which may be present in concentrations on the order of 1-10 ppmw in coal. Toxic elements are of no concern when they end up bound in the slag or in stable chemical compounds. The problem with arsenic is that under reducing conditions it forms the volatile compound AsHs. It is a known poison for... 

Oxidation of sulfur entities of metal sulfides to obtain energy is an example of direct dissolving action under aerobic conditions (Kurek, 2002). When oxidized metal compounds [e.g., Fe(III), Mn(IV), As(V)] act as electron acceptors, anaerobic respiration becomes an example of direct dissolving action under anaerobic conditions (Ahmann et al., 1994 Ehrlich, 2002). Volatilization of metals and metalloids or biomethylated metals and metalloid compounds from the soil into the atmosphere can be a mechanism of detoxification of toxic elements such as Hg, As, and Se for microorganisms (Gadd, 1993). 

The gas chromatograph may be interfaced with atomic spectroscopic instruments for specific element detection. This powerful combination is useful for speci-ation of different forms of toxic elements in the environment. For example, a helium microwave induced plasma atomic emission detector (AED) has been used to detect volatile methyl and ethyl derivatives of mercury in fish, separated by GC. Also, gas chromatographs are interfaced to inductively coupled plasma-mass spectrometers (ICP-MS) in which atomic isotopic species from the plasma are introduced into a mass spectrometer (see Section 20.10 for a description of mass spectrometry), for very sensitive simultaneous detection of species of several elements. 

Thiuram Sulfides. These compounds, (8) and (9), are an important class of accelerator. Thiurams are produced by the oxidation of sodium dithiocarbamates. The di- and polysulfides can donate one or more atoms of sulfur from their molecular stmcture for vulcanization. The use of these compounds at relatively high levels with litde or no elemental sulfur provides articles with improved heat resistance. The short-chain (methyl and ethyl) thiurams and dithiocarbamates ate priced 2/kg. Producers have introduced ultra-accelerators based on longer-chain and branched-chain amines that are less volatile and less toxic. This development is also motivated by a desire to rninirnize airborne nitrosamines. 

The compounds of Se, Te and Po should all be treated as potentially toxic. Volatile compounds such as H2Se, H2Te and organo derivatives are particularly dangerous and maxirnum permissible limits for air-bome concentrations are 0.1 mg m (cf. lOmgm for HCN). The elements are taken up by the kidneys, spleen and liver, and even in minute concentrations cause headache, nausea and irritation of mucous membrane. 

The aP4 white form is the most volatile, reactive (and highly toxic) and thermodynamically the least stable solid modification. The slow oxidation of the vapour over the solid results in the phosphorescence characteristic of this element. By heating above 180°C, white phosphorus is transformed into red phosphorus which... 

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