Metalloids accumulation

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). 

As can be seen in Fig. 3.67, the corrosion resistance of amorphous alloys changes with the addition of metalloids, and the beneficial effect of a metaU loid in enhancing corrosion resistance based on passivation decreases in the order phosphorus, carbon, silicon, boron (Fig. 3.72). This is attributed partly to the difference in the speed of accumulation of passivating elements due to active dissolution prior to passivation... 

Metalls and metalloids are characterized by special ecochemical features. They are not biodegradable, but undergo a biochemical cycle during which transformations into more or less toxic species occur. They are accumulated by organisms and cause increased toxic effects in mammals and man after long term exposure [55]. 

Numerous commercial dyes are metal chelate complexes. These metals form pollutants which must be eliminated. One of the strongest points in favour of electrochemical reduction/removal of metal ions and metal complexes - the metal ions and weakly complexed ions form the toxic species - and of the metals from the metal-complex dye is that they are eliminated from the solution into the most favorable form as pure metal, either as films or powders. Polyvalent metals and metalloids can be transferred by reduction or oxidation treatment to one valency, or regenerated to the state before use, e.g. Ti(III)/Ti(IV), Sn(II)/Sn(IV), Ce(III)/Ce(IV), Cr(III)/Cr(VI), and can be recycled to the chemical process. Finally, they can be changed to a valence state better suited for separation, for instance, for accumulation on ion exchangers, etc. Parallel to the... 

The metals of most concern are the heavy metals, especially cadmium, lead, and mercury. Although it is a metalloid with characteristics of both metals and nonmetals, arsenic is commonly classified as a heavy metal for a discussion of its toxicity. Though not particularly toxic, zinc is abundant and may reach toxic levels in some cases. For example, zinc accumulates in sewage sludge and crop productivity has been lowered on land fertilized with sludge because of zinc accumulation. Copper may be toxic to plants. Aluminum, a natural constituent of soil, may be leached from soil by polluted acidic rainwater and reach levels that are toxic to plants. Other metals that may be of concern because of their toxicides include chromium, cobalt, iron, nickel, and vanadium. Radium, a radioactive alpha particle-emitting metal, can be very toxic at even very low levels in water or food. 

Another illustration of the potential in harnessing plant life for soil remediation is the finding20 that the fern, Pier is vittata, when grown in soil containing 6 ppm arsenic, hyper-accumulated 755 ppm of this metalloid in its fronds in only two weeks. When Pier is vittata was grown in artificially contaminated soil (1500 ppm As), the fronds took in 15,861 ppm As in the same two-week time frame. Similarly, research in both the United States and the United Kingdom has demonstrated the potential of using plants from the family Brassicacae in the remediation of soils heavily contaminated with zinc, cadmium, nickel, lead, and selenium.21... 

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. 

Methylmercury species (or other metal alkyls) may be produced at a rate fastother organisms. They may accumulate in organisms. Metals and metalloids that have been reported to be biomethylated in minor amounts include Ge, Sn, As, Se, Te, Pd, Pt, Au, Hg, T1 and probably Pb. Anthropogenic pollution by alkylated metal ions is certainly more significant than biomethylation. Methylated tin species have been observed in near-shore polluted waters. For data on As, Sb, Se, and Te see the recent review by Andreae (1986a). 

The aquatic migration coefficient by Perelman describes not the migration ability of components on the whole but only their capability to accumulate in water. Perelman series (Table 3.5) show that metalloids, especially halogens, are noticeably easier to transfer to water than metals and that is why they possess higher migration mobility. 

The deposition and accumulation of fly ash downwind from coal-combustion sites is a concern because it may be significantly enriched in potentially toxic trace elements, including lead (Pb) and arsenic (As), compared to the burned coal (Coles et al, 1979 Eary et al, 1990 Hower et al, 1999 Kaakinen et al, 1975). Other elements such as zinc (Zn) and germanium (Ge), of less environmental concern, may also be enriched in fly ash. The relatively high concentrations of As in fly ash reflect partly its presence in pyrite in coal from the Appalachian Basin (Goldhaber et al, 2002). More importantly, the concentration of these metals and metalloids occurs during the combustion process itself. A suite of elements including As, Ca, Cr, Cu, Ga, Mo, Ni, Pb, Sb, Se, V, and Zn is enriched in the fine fraction of coal fly ash (Coles et al, 1979), because of vaporization in the furnace and subsequent condensation or absorption onto ash particles (Kaakinen et al, 1975). 

The biochemistry reported above outlines the necessary physical parameters for the transfer of CH to positively charged metal ions in the aqueous environment. Under aerobic conditions it is clear that the heavy metals Hg, Pb and the metalloid Se are more soluble and available to react in an acidic environment. These toxic elements are biomethylated under acidic conditions to give methylated products which accumulate in aquatic predators. The process of methylmercury uptake by lake biota is extremely rapid. 

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