Poisoning with heavy metals

Another method of increasing the catalyst s selectivity is its poisoning with heavy metals, like lead. This was effective with a Pd catalyst in the hydrogenation of cyclopen-tadiene, 1,3-cyclohexadiene and cyclooctatriene143. 

Table 22.2 Changes in metabolic activity caused by poisoning with heavy metal salts and after therapeutic treatment...

Some TCA masks have been presented as chelated TCA. Chelation is a medical therapy that aims to detoxify the body of harmful minerals and metals. Chemically, chelation is the process by which an organic substance (the chelator) binds metal ions (iron, copper, lead, calcium, etc.) into inactive, non-toxic and water-soluble complexes that are easily eliminated in the urine. Intravenous chelation therapy (e.g. with ethylenediamine tetra acetic acid, EDTA) is often used to treat poisoning with heavy metals, including lead. The use of the term chelation therefore seems inappropriate as far as TCA is concerned, and has no chemical basis since TCA is not a metal. The little information available states that chelation reduces the speed of penetration of TCA and therefore its depth of action. Might what we call chelation be partial inactivation of the TCA The directions of use for Accu Peel state that a process called chelation allows the TCA to reach an even depth at the same time as using lower concentrations of TCA . Might chelation, on the contrary, be a process that activates the TCA But how can TCA be activated ... 

Toxic heavy metals and ions, eg, Pb, Hg, Bi, Sn, Zn, Cd, Cu, and Fe, may form alloys with catalytic metals (24). Materials such as metallic lead, ziac, and arsenic react irreversibly with precious metals and make the surface unavailable for catalytic reactions. Poisoning by heavy metals ordinarily destroys the activity of a precious-metal catalyst (8). 

These cracking and H-addition processes also require catalysts, and a major engineering achievement of the 1970s was the development of hydroprocessing catalysts, in particular cobalt molybdate on alumina catalysts. The active catalysts are metal sulfides, which are resistant to sulfur poisoning. One of the major tasks was the design of porous pellet catalysts with wide pore structures that are not rapidly poisoned by heavy metals. 

Fig. 22.1 Erythrocytes of rats with sub-chronic poisoning by heavy metal salts, animal group 2 (a), and after treatment, animal group 3 (b). Light microscopy magnification x 40...

To reduce excess salivation associated with heavy metal poisoning or parkinsonism Neurology... 

Finally, herbal products imported from China may be contaminated with pesticides or with heavy metals added during the manufacturing process. Contaminated Chinese herbals have led to cases of arsenic, lead, mercury, thallium, and cadmium poisoning. Similarly, investigations of traditional Indian Ayurvedic remedies have shown them to sometimes contain dangerous levels of lead, zinc, mercury, arsenic, aluminum, and tin. 

Iron is widely used in over-the-counter vitamin preparations and is a leading cause of childhood poisoning deaths. As few as 10-12 prenatal multivitamins with iron may cause serious illness in a small child. Poisoning with other metals (lead, mercury, arsenic) is also important, especially in industry. See Chapter 33 Agents Used in Anemias Hematopoietic Growth Factors and Chapter 58 Heavy Metal Intoxication Chelators for detailed discussions of poisoning by iron and other metals. 

Crosslinking of protein monolayers by mercuric ion (MacRitchie, 1970) and silicic acid (Minones et al., 1973) has been reported. These studies are relevant to poisoning by heavy-metal ions and to silicosis, effects that seem likely to result from attack on the cell membrane proteins. Crosslinking by mercuric ion was detected by a spectacular increase in surface viscosity and a decrease in compressibility when a number of proteins (BSA, insulin, ovalbumin, and hemoglobin) were spread on 0.001 M mercuric chloride solution. Poly-DL-alanine was unaffected whereas poly-L-lysine and poly-L-glutamic acid were affected in a similar manner to the proteins, indicating that mercuric ion interacts with the ionizable carboxyl and amino groups on the protein side—chains. Silicic acid similarly caused protein monolayers... 

The iodate is a poison potassium iodide, however, is used in foodstuffs. Thus the iodate must be completely removed frequently by a final reduction with carbon. After re-solution in water, further purification is carried out before recrystallization. Iron, barium, carbonate, and hydrogen sulfide are used to effect precipitation of sulfates and heavy metals. 

Preconditioning for Particulates Heavy particulate loading of the inlet gas with dust, grease, oils, or other aerosols can be very dam-aging to the pore structure of the filter bed, resulting in an eventual pressure-drop increase. Oils and heavy metals that are deposited on the filter bed can be poisonous to the microorganisms that live within the biofilm. Particulate APC equipment such as fabric filters and venturi scrubbers are generally adequate for this level of particulate removal. 

Germanium and Sn are non-toxic (like C and Si). Lead is now recognized as a heavy-metal poison it acts by complexing with oxo-groups... 

Catalysts can be poisoned, or inactivated. A common cause of such poisoning is the adsorption of a molecule so tightly to the catalyst that it seals the surface of the catalyst against further reaction. Some heavy metals, especially lead, are very potent poisons for heterogeneous catalysts, which is why lead-free gasoline must be used in engines fitted with catalytic converters. The elimination of... 

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