Toxic metalloid

Unlike selenium there is no required biological role for tellurium in bacteria or plants that has been determined however, this may ultimately not be the whole story.111 Selenium was only viewed as a toxic metalloid with no necessary role for metabolism until at least the 1950s see above. While tellurite is less soluble than tellurate in aqueous solution, in general tellurite is probably more toxic to most organisms.190 The non Te-resistant wild type E. coli bacterium (Gramnegative) has MICs of 1 to 3 ppm for tellurite and tellurate.144,191,192 Tellurite is used to enrich and select for Staphylococcus aureus.169,193,194... 

In the United States and many other countries, the toxic metalloid arsenic is not allowed in drinking water at levels greater than 10 pg/L, a level that has been exceeded by some water supplies used in the past. Arsenic usually is present in water as arsenic(V), primarily H2ASO4 and HASO4. Groundwater supplies containing the more toxic As(III) form have to be treated with chlorine, ozone. 

Arsenic. Can this Toxic Metalloid Sustain Life ... 

In the past, removing metal and metalloid contaminants from soil has been impossible, and site clean-up has meant excavation and disposal in a secure landfill. An exciting new approach to this problem is phytoextraction, where plants are used to extract contaminants from the soil and harvested. Immobilization and Toxicity-Minimization. 

Arsenic is another element with different bioavailabiUty in its different redox states. Arsenic is not known to be an essential nutrient for eukaryotes, but arsenate (As(V)) and arsenite (As(III)) are toxic, with the latter being rather more so, at least to mammals. Nevertheless, some microorganisms grow at the expense of reducing arsenate to arsenite (81), while others are able to reduce these species to more reduced forms. In this case it is known that the element can be immobilized as an insoluble polymetallic sulfide by sulfate reducing bacteria, presumably adventitiously due to the production of hydrogen sulfide (82). Indeed many contaminant metal and metalloid ions can be immobilized as metal sulfides by sulfate reducing bacteria. 

B. Venugopal and T. D. Luckey, Metal Toxicity in Mammals Chemical Toxicity of Metals and Metalloids, Vol. 2, Plenum Press, New York, 1978. 

Control of metalloid content in natural objects, foodstuff and pharmaceuticals is an important task for modern analytical chemistry. Determination of elements such as Arsenic is necessary for evaluation of object toxicity, since their content in environment may exceed MCL (maximum contaminant level), posing hazard to human health. Elements such as Selenium in definite doses are healthy, but in greater quantities they produce toxic effect. 

The most common toxic metals in industrial use are cadmium, chromium, lead, silver, and mercury less commonly used are arsenic, selenium (both metalloids), and barium. Cadmium, a metal commonly used in alloys and myriads of other industrial uses, is fairly mobile in the environment and is responsible for many maladies including renal failure and a degenerative bone disease called "ITA ITA" disease. Chromium, most often found in plating wastes, is also environmentally mobile and is most toxic in the Cr valence state. Lead has been historically used as a component of an antiknock compound in gasoline and, along with chromium (as lead chromate), in paint and pigments. 

There is concern over the toxicity of a nnmber of metals and metalloids, and their oxyanions. As for antibiotics, the genes for resistance are often plasmid-bome. There are several mechanisms that may operate—redaction, methylation, efflnx, and the synthesis of metal-binding metallothioneins. The following text illustrates aspects of these mechanisms. 

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. 

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... 

Many of the chemicals used in the semiconductor industries are highly toxic. For example, germanium-halogen compounds are extremely toxic, both as a powder and in a gaseous state. Precautions should be taken when working with germanium as with similar metalloids from group 14 (IVA). 

In view of the above, it is not surprising that significant efforts have been dedicated to the problem of generating synthetic chloride receptors. Indeed, a number of such systems are known. Unfortunately, many of them are hydrophilic polycations and, accordingly, suffer from low solubility in organic media. " " Others are metal- or metalloid-derived systems. The clinical utility of these is clouded by metal toxicity. ° The sapphyrins, being both wholly organic and... 

Occupational and environmental poisoning with metals, metalloids, and metal compounds is a major health problem. Exposure in the workplace is found in many industries, and exposure in the home and elsewhere in the nonoccupational environment is widespread. The classic metal poisons (arsenic, lead, and mercury) continue to be widely used. (Treatment of their toxicities is discussed in Chapter 57.) Occupational exposure and poisoning due to beryllium, cadmium, manganese, and uranium are relatively new occupational problems, which present new and previously unaddressed problems. 

---