Examples of Toxic Metals

The corrosive effects to be considered (mainly simple corrosion of metals) are, as would be expected from the edible nature of foodstuffs which are not excessively either acidic or basic but which may contain sulphur, less severe than those often encountered with inedible materials containing reactive substances. The importance of corrosive efiects where foodstuffs are concerned lies not so much in the action of the foodstuffs on the metal involved as in the resultant metal contamination of the foodstuff itself, which may give rise to off-flavours, in the acceleration of other undesirable changes (by the Maillard reaction for example), and in the possible formation of toxic metallic salts. Metal ions generally have threshold values of content for incipient taste effect in different liquid foodstuffs. Except in the case of the manufacture of fruit juices and pickles, process plant failure through corrosion must be rare. Nevertheless all foodstuffs, particularly liquid ones, should be regarded as potentially corrosive and capable of metal pick-up which may be undesirable. 

The widespread use of many metals such as silver, cadmium, copper, mercury, nickel, lead, and zinc has resulted in their accumulation in the environment. Sediments are often the repositories of toxic metals (e.g.. Table 15-2). For example, copper is used as an anti-biofouling agent in marine paints and many harbor sediments contain markedly elevated levels of copper. 

Hazardous waste burning incinerators, cement kilns, and LWAKs do not follow a tiered approach to regulate the release of toxic metals into the atmosphere. The MACT rule finalized numerical emission standards for three categories of metals mercury, low-volatile metals (arsenic, beryllium, and chromium), and semivolatile metals (lead and cadmium). Units must meet emission standards for the amount of metals emitted. For example, a new cement kiln must meet an emission limit of 120pg/m3 of mercury, 54pg/m3 of low-volatile metals, and 180 pg/m3 of semivolatile metals. 

Emissions of actually and potentially dangerous toxic elements may influence the human and ecosystem health on local, regional and global scales. Accumulation of toxic metals may be in soils, waters, bottom sediments and biota. For example, the accumulation of heavy metals in the upper layers of bottom sediments and glaciers occurring during the 20th century is shown in many recent studies. 

Steps have been taken by the World Health Organization (WHO) and the U.S. Environmental Protection Agency (EPA) to reduce the amount of toxic metal ions in the environment. For example, large concentrations of lead have been shown to be lethal to humans. The maximum amount of lead tolerated in drinking water according to the WHO and the EPA, is 0.05 mg/l(9l) and 0.5 mg/I,(92) respectively. For this reason, innovative techniques to measure low concentrations of metal ions are emerging. 

The term heavy metal refers to any metallic chemical element that has a relatively high density (nsnally specific density of more than 5 g/mL) and is toxic or poisonous at low concentrations. Examples of heavy metals include arsenic (As), cadmium (Cd), chromium (Cr), mercury (Hg), lead (Pb), and thallium (Tl). The sources, uses, and environmental effects of several exemplary specific metals are discussed briefly here. 

Biopiles have some potential limitations. For example, certain chemicals such as polychlorinated biphenyls and other hydrocarbons are resistant to biodegradation. In addition, high concentrations of toxic metals, such as lead, copper, and mercury, may limit treatment using biopiles. 

Mercury is one of the most significant examples of toxic heavy metal pollution. Anthropogenic sources of mercury include those associated with its use in chlor-alkali, paint, agriculture, pharmaceutical, and paper and pulp industries. 

Nervous System. The nervous system is also a common target of toxic metals particularly, organic metal compounds (see Chapter 16). For example, methylmercury, because it is lipid soluble, readily crosses the blood-brain barrier and enters the nervous system. By contrast, inorganic mercury compounds, which are more water soluble, are less likely to enter the nervous system and are primarily nephrotoxicants. Likewise organic lead compounds are mainly neurotoxicants, whereas the first site of inorganic lead is enzyme inhibition (e.g., enzymes involved in heme synthesis). 

In living systems that use metal ions in several places, transport of metal ions is an important process, for which efficient systems are in operation. Well-known examples are transferrin for iron transport in humans, albumin for copper transport, and ferritin for iron storage. In addition to these natural transporting proteins, nature makes use of other systems to remove excess of toxic metal ions. 

Schubauer-Berigan, M.K., Amato, J.R., Ankley, G.T., Baker, S.E., Burkhard, L.P., Dierkes, J.R., Jenson, J.J., Lukasewycz, M.T. and Norberg-King, T.J. (1993) The behaviour and identification of toxic metals in complex mixtures examples from effluent and sediment pore water toxicity identification evaluations, Archives of Environmental Contamination and Toxicology 24, 298-306. 

Other examples of transition metal carbonyls are nickel tetracarbonyl, Ni(CO)4, a colorless, toxic, flammable liquid that boils at 43°C and chromium hexacarbonyl, Cr(CO)6, a colorless crystal that sublimes readily. Ni(CO)4 is tetrahedral and Cr(CO)6 is octahedral. 

The MIPs are particularly versatile materials, especially as the number and types of templates that have been successfully imprinted increase. For example, imprinted polymers have been prepared using metal ions as templates. ° Potential applications for these polymeric ionophores include the remediation of toxic metals from the environment, recognition elements for ion sensors for medical diagnostics, catalysis, and... 

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