Metals, also toxicity

The metal itself, having an appreciable vapour pressure, is also toxic, and produces headaches, tremors, inflammation of the bladder and loss of memory. The best documented case is that of Alfred Stock (p. 151) whose constant use of mercury in the vacuum lines employed in his studies of boron and silicon hydrides, caused him to suffer for many years. The cause was eventually recognized and it is largely due to Stock s publication in 1926 of details of his experiences that the need for care and adequate ventilation is now fully appreciated. 

Species may differ by oxidation state for example, manganese(II) and (IV) iron(II) and (III) and chromium(III) and (VI). Oxidation state is influenced by the redox potential. Mobility is affected because oxidation state influences precipitation-dissolution reactions and also toxicity in the case of heavy metals. 

The ability of bacteria to accumulate toxic metals also varies with cell age. Shuttleworth and Unz (1993) reported that one-day-old cells of Thiothrix strain Al accumulated considerably less Ni or Zn than its 2-5 day old... 

The term heavy metals includes various elements. Some of them, being nutrients, have essential biochemical functions (Fe, Cu, Zn, Mo, Mn, et al.), whereas these functions are unknown for others (Cd, As, Pb, Hg), and these elements are considered as toxics. One should mention that even the micronutrients consumed by living organisms are required only at microrates and in the greater doses they are also toxic. 

The trace metals listed in Table 11.2 (with the inclusion of Sn) are of particular concern as they are toxic at low concentrations. For historical reasons, these elements are commonly referred to as the heavy metals. The degree to which the heavy metals cause toxic effects is dependent on their concentration, chemical speciation, and other environmental conditions, such as temperature. As illustrated in Table 28.6, the type and physiological state of the target organism are also important as these fectors determine the degree to which internal metabolic processes can detoxify or eliminate the pollutant. 

Carraher and coworkers employed the last two processes to recover the uranyl ion. The uranyl ion is the natural water-soluble form of uranium oxide. It is also toxic, acting as a heavy metal toxin. Through the use of salts of dicarboxylic acids and poly(acrylic acid), the uranyl ion was removed to 10 M with the resulting product much less toxic and convertible to uranium oxide by heating. 

As regards the nature of their action on a human organism, tellurium and its compounds are similar to the inorganic compounds of selenium and arsenic. Hydrogen telluride is the most toxic. Tel-lurium(IV) oxide and aqueous solutions of the salts of tellurous and telluric acids are also toxic. Only tellurium ( metallic ) is not toxic if it gets into an organism. 

Literally hundreds of complex equilibria like this can be combined to model what happens to metals in aqueous systems. Numerous speciation models exist for this application that include all of the necessary equilibrium constants. Several of these models include surface complexation reactions that take place at the particle-water interface. Unlike the partitioning of hydrophobic organic contaminants into organic carbon, metals actually form ionic and covalent bonds with surface ligands such as sulfhydryl groups on metal sulfides and oxide groups on the hydrous oxides of manganese and iron. Metals also can be biotransformed to more toxic species (e.g., conversion of elemental mercury to methyl-mercury by anaerobic bacteria), less toxic species (oxidation of tributyl tin to elemental tin), or temporarily immobilized (e.g., via microbial reduction of sulfate to sulfide, which then precipitates as an insoluble metal sulfide mineral). 

Salinity affects microbial activity, in part, because it controls water availability. The higher the salinity, the more energy an organism must expend to maintain a favorable osmotic balance. Salts, of course, have effects on living organisms beyond water availability. For example, salts can be both a source of essential nutrients as well as a source of toxic heavy metals. Also, sulfate salts appear to be more favorable for life than chloride salts see the discussion in Sect. 5.1.2 (Aqueous Saline Environments). However, in this section on salinity, the focus will be on salinity as a control on water availability. 

Thiol-functionalized mesoporous silicas not only serve as a basis for sulfonic acid-functionalized phases, but can be used for several other applications, for instance as adsorbents. Thiol-functionalized silicas can also be prepared as small nanoparticles with spherical morphologies by employing modified Stober reaction conditions.174 The high affinity of these compounds for thiophilic heavy metals, especially toxic Hg2+ ions, has been demonstrated by several authors.175-178... 

A Note about Chelation Therapies. Chelation therapies are used to prevent or treat metal-induced toxicities. They are often used in acute poisoning scenarios, but can also be used to assess exposure. One of the major challenges in the management of chelation therapies is the tendency for chelating agents to interact with essential metals, particularly calcium and zinc. Chelation therapies should only be administered by a physician due to the potential to disrupt essential metal functions. The Food and Drug Administration does not regulate dietary supplements, and several do it yourself chelation therapies are available. These are not advisable. 

Extraction and manufacture of Zn, Pb, and Cu ores can give pollution of the environment with Cd derivatives. Cd is present also in industrial sludge and waste water. A source of Cd release is the burning of fossil fuels and the incineration of rubbish. Fertilizers can contain variable amount of Cd. Toxicological characteristics and environmental fate of Cd resemble those of Pb and Hg (see Metal Ion Toxicity). In aqueous environment Cd + shows a relative mobility it depends on pH, presence of organic molecules, and... 

Zinc efflux is mediated by a zinc exporter known as ZntA (Zn + transport or tolerance), a membrane protein which was identified through studies of bacterial strains that were hypersensitive to zinc and cadmium. Sequence inspection revealed that ZntA was a member of the family of cation transport P-type ATPases, a major family of ion-translocating membrane proteins in which ATPase activity in one portion of the protein is used to phophorylate an aspartate within a highly conserved amino acid sequence, DKTG, in another portion of the protein. The cysteine rich N-terminus of these soft metal transport proteins contains several metal-binding sites. How the chemical energy released by ATP hydrolysis results in metal ion transport is not yet known, in part because there is only partial information about the structures of these proteins. The bacterial zinc exporter also pumps cadmium and lead and is therefore also involved in protection from heavy metal toxicity (see Metal Ion Toxicity). 

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