Metals/metalloids volatilization

The main mechanisms involved in the removal of metals/metalloids by activated sludge consist of precipitation, adsorption of soluble metal/metaUoid species on the extracellular pol5tmerie substances (EPS), uptake of soluble metal forms by the cells (i.e., bioaccumulation) and for specific metals, volatilization (Brown and Lester, 1979 Kelly et ah, 2004). Figure 12.2 summarizes the most prominent mechanisms. The contribution of each mechanism depends on the type and eon-centration of metals and metalloids, the presence of competing cations, the ionie strength of the liquid phase of the mixed liquor, the MLSS concentration and the SRT. 

The conversion of specific metals/metalloids to volatile forms is due to microbial action. This process can take place for the elements that can be converted to organometallic forms through methylation. These elements include As, Hg, Pb, Se and Te. The methylation in wastewater treatment plants is usually limited (Goldstone et al., 1990 Santos and Judd, 2010). 

Metals and metalloids that form alkyl compounds, eg, methylmercury and methylarsenic acid, tributjltin, deserve special concern because these compounds are volatile and accumulate in cells they are poisonous to the central nervous system of higher organisms. Because methylmercury or other metal alkyls may be produced at a rate faster than it is degraded by other organisms, it may accumulate in higher organisms such as fish. Hg species are also reduced to elementary Hg which is soluble in water but lost by volatilization to the atmosphere (40). 

Methylation of both metals and metalloids has been observed for both fungi and bacteria. These metabolites may, however, be toxic to higher biota as a result of their volatility. The Minamata syndrome represents the classic example of the toxicity of forms of methylated Hg to man, even though the formation of Hg(CH3)2 was probably the result of both biotic and abiotic reactions. 

In situations like these the method of measurement of the element (or elements) of interest can be tailored specifically to suit the particular needs. As a matter of fact, for a few metals like mercury and tin and the metalloids arsenic, selenium, antimony, and tellurium special methods of sample preparation are usually required (29,30,31,32,33). The reason special precautions and special methods are required for the elements listed above, and for certain others, is that they either are readily reduced to volatile forms, react chemically to form volatile compounds, or are influenced in their analysis by the matrices usually present. ... 

Methods for several metals or metalloids involve conversion to a volatile form. Arsenic, antimony, and selenium can be reduced to their volatile hydrides, AsH3, SbH3, and H2Se, repectively, which can be determined by atomic absorption or other means. Mercury is reduced to volatile mercury metal, which is evolved from solution and measured by cold vapor atomic absorption. 

Metals and metalloids that form alkyl compounds (e.g., methylmercury) deserve special concern, because these compounds are volatile and accumulate in cells they are poisonous to the central nervous system of higher organisms. 

The native species of most metals and metalloids, such as mercury, lead, tin, arsenic, and selenium, are generally present as ionic forms in sample matrices. For GC-based coupling techniques, these compounds need to be extracted from the sample matrix and to be converted to volatile and thermally stable derivatives. Frequently, the derivatives are then concentrated by cry-otrapping or extracting into an organic solvent prior to injection onto a GC column [1]. 

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). 

Natural particles suspended in the air can be transported to regions far from their sources. This is important for transporting many metals and metalloids in the ecosystem. A few metals and metalloids, most notably Hg, As, and Se, can exist not only in the solid and liquid phases but also as gases in ambient environments. The loss of Hg from the aqueous phase can result from reduction of Hg " " to Hg and alkylation to form methyl- or dimethylmercury. Through microbial activity, the methylated forms can be converted to Hg, which is more volatile and less toxic. Microbial mediation can also transform several other trace elements (e.g., As, Se) to organometallic compounds (Gadd, 1993). These volatile organometallic compounds can dominate the transport of these trace elements in local environments. However, bacterial mediation of alkylation of metals such as Hg is influenced substantially by Hg speciation. Mineral colloids vary in their ability to affect the bioavailability and methylation of Hg(II) in aqueous systems... 

Feld MANN J, Grumping R and Hirner AV (1994) Determination of volatile metal and metalloid compounds in gases from domestic waste deposits with GC/ICP-MS. Fresenius J Anal Chem 350 228-234. 

However, apart from a few exceptional cases this method is, for obvious technical reasons, restricted to alloys made of relatively volatile metals and metalloids, that is, those boiling below 1000-llOO C at 760 mm. This is because distillation at higher temperatures is quite difficult in practice. (However, much less volatile... 

J Feldmann, AV Hirner. Occurrence of volatile metal and metalloid species in landfill and sewage gases. Intern J Environ Anal Chem 60 339-359, 1995. 

The presence of heavy metals and other elements may inhibit or enhance microbiological transformations of arsenic in soil systems. It was observed that presence of phosphate and selenate causes inhibition of methylated evolution of arsenic (5,46,56). Frankenberger (57) studied the effect of 21 trace elements for their activation or inhibition on methylated arsine production by a Penicillium sp. from MMAA. Metals and metalloids at an elemental concentration of 0, 0.1, 1, 10, 100, and 1000 pM were tested for their influence on arsenic volatilization by the Penicillium sp. The effect of trace elements varied considerably depending on the speciation and concentration. At the lower elemental concentrations (0.1 and 1 pM), the metals and metalloids that stimulated arsenic volatilization were... 

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