After reading this article you will learn about:- 1. Distribution of Lead 2. Emissions of Lead 3. Toxicity 4. Transformation 5. Biochemical Effects.
Contents:
- Distribution of Lead
- Emissions of Lead
- Toxicity of Lead
- Transformation of Lead
- Biochemical Effects of Lead
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1. Distribution of Lead:
Lead (Pb) is widely distributed in the rocks and soils of the earth’s crust, although the mean concentration is only about 16 ppm. Lead is mined primarily from deposits of the mineral galena or lead sulphide (PbS) Since metallic lead can be separated from PbS by heating to temperatures easily achieved by burning wood or charcoal (PbS + O2 heat → SO2 + Pb), it -was not difficult for early civilizations to extract lead.
Though there are a number of instances of lead use in domestic purposes since time immemorial, but the principal uses of lead today are of industrial nature (Table 25.7).
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2. Emissions of Lead:
There is good evidence that lead emissions to the environment have been greatly increased on a global scale as a result of human activities. Analysis of marine sediments has indicated that millions of years ago the flux of Pb to the oceans was about 13,000 mt. per year, whereas river runoff alone contribute above 240,000 mt. of Pb to the ocean each year. Lead is also deposited on the ice sheets through precipitation.
Lead released to the atmosphere from Pb smelting, refining of lead scrap and burning leaded gas subsequently falls out onto crops. It will contaminate or accumulate in food grains too. Some agricultural practices, including the ploughing and irrigation of fields and the use of lead pesticides, contribute to lead concentrations in water and soil. Lead pipes used to transport water pose an obvious water contamination problem.
The problem of air contamination by Pb in urban area is primarily derived from automobile exhaust fume. The typical Pb concentration in urban air is about 1.3 Hg/m3 in contrast to rural air which is 0.05 Mg/m3. Another major source, of global lead contamination has been the use of lead additives in gasoline.
3. Toxicity of Lead:
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Lead enters the human body primarily via inhalation and through the ingestion of contaminated food & water. Absorption of inhaled Pb is about 37% in lung and 25- 30% and 8% via gastrointestinal tract in children and adults, respectively.
Once absorbed into the blood stream, lead is transported to all parts of the body and incorporated into the tissues, particularly in bones, by replacing calcium. The highest, concentrations of organic lead are often found in the brain and liver. Small amount of lead can be excreted out by feces, urine and sweat.
Lead is known to disrupt several enzymes and is also involved in blockage of haemoglobin biosynthesis. Thus, anaemia is developed.
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It damages nervous system—both nerve fibers and brain—and causes paralysis of the organs. Kidney damage characterised by atrophy of the renal tubules is a well- established effect of lead-poisoning. The damage is associated with elevated levels of amino acids, sugar and phosphate in the urine.
With respect to human health, in general, the greatest danger of lead poisoning is undoubtedly to young children, particularly those living in urban areas. Neurological damage caused by the poisoning of such children may be permanent and can result in impaired physical as well as mental development.
The toxicity of lead to aquatic organisms has not been examined thoroughly. However, chronic toxicity found in marine organisms is about 100 ppb of Pb. (EPA, 1972, 1975).
An average estimated of lead consumption from various sources is given in Table 25.8:
4. Transformation of Lead:
Recent evidence indicates that lead may also be methylated by bacteria like Pseudomonas, and Flavobacterium.
5. Biochemical Effects of Lead:
Lead is a relatively abundant metal in nature, occurring in lead minerals. In the atmosphere it is relatively more abundant than other toxic metals. By far the major source of airborne Pb is the combustion of leaded petrol/gasoline. Pb is added in the form of tetra-alkyl lead, primarily Pb(CH,)4 and Pb(C2H5)4, together with the scavengers 1, 2-dichloroethane and 1, 2-dibromoethane. In common with other particulate pollutants, Pb is removed from the atmosphere by wet and dry deposition processes. As a result, street dusts and roadside soils become enriched with Pb, with concentrations typically of the order of 1,000-4,000 mg kg-1 on busy streets.
It may be noted that most of the Pb intake by a typical city dweller is from diet (about 200-300 pg per day), air and water adding a further 10-15pg per day each. Of this total intake, 200 µg of Pb is excreted while 25µg is stored in the bones each day.
The major biochemical effect of Pb is its interference with heme-synthesis, which leads to haematological damage. Pb inhibits several of the key enzymes involved in the overall process of haeme- synthesis whereby the metabolic intermediates accumulate. One such intermediate is delta-amino levulinic acid. An important phase of haeme-synthesis is the conversion of delta-aminolevulinic acid to porphobilinogen.
Pb inhibits the ALA-dehydrase enzyme (I) so that it cannot proceed further to form prophobilinogen (II) The overall effect is the disruption of the synthesis of hemoglobin as well as other respiratory pigments, such as cytochromes, which require heme. Finally, Pb does not permit utilisation of O*and glucose for life-sustaining energy production.
This interference can be detected at a lead level-ih the blood of about 0 3 ppm. The detection of (I) provides a sensitive test for Pb in the body. At higher levels of Pb in the blood (at > 0.8 ppm) there will be symptoms of anaemia due to the deficiency of haemoglobin. Elevated Pb levels (0.5-0.8 ppm) in the blood cause kidney dysfunction and. finally, brain damage.
Due to the chemical of pb2+ with Ca 2+ epositories for Pb accumulated by the body. Subsequently, this Pb may be remobilized along with phosphates from the bones which exert a toxic effect when transported to soft tissues.
Lead poisoning can be cured by treatment with chelating agents which strongly bind Pb2+ Thus calcium chelate in solution is fed to the victim of lead poisoning. Pb2– displaces Ca2 + from the chelate and the resulting Pb2 + chelate is rapidly excreted in the urine. Three typical Pb chelates are shown above.