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Toxic Metals. Elements shown in bold outline are known to be essential for human metabolism. About 1/3 of all known enzymes require the presence of a metal ion for them to exhibit biochemical activity. Toxic metals.

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Elements shown in bold outline are known to be essential for human metabolism.

  • About 1/3 of all known enzymes require the presence of a metal ion for them to exhibit biochemical activity.
toxic metals1
Toxic metals
  • Some metals are essential for the normal development and well-being of humans and other creatures.
    • They include Na, K, Ca, Mg, Cu, Cr, and Zn.
  • Other metals are toxic even when present in an organism at very low concentrations.
    • Examples: Antimony, Arsenic, Barium, Beryllium, Cadmium, Lead, Mercury, Nickel, Selenium, and thallium
    • (Arsenic and selenium are not metals but are often listed with them).
    • The toxic metals can cause brain damage, kidney and liver disorders, and bone damage.
biogeochemical cycle of metal species
Biogeochemical cycle of metal species
  • All metals cycle naturally through the environment.
  • Sources: weathering of rocks
  • Transportation: uptake and processing by plants and microorganisms.
    • Plants living in soils rich in metal species have evolved protective mechanisms.
    • These plants can be used in remediation of soils contaminated with metals.
  • Human intervention have greatly perturbed the natural biological cycles of the metals.
    • Mining and metallurgy
    • Coal burning
  • Metals can not be broken down into simpler, less toxic forms. As a result, they persist in the environment and bioaccumulate through food chain.
hexavalent chromium
Hexavalent Chromium
  • Cr (VI) is a carcinogen
  • Sources
    • Steel-making, cutting, grinding, and welding
    • Spills and residues of electroplating baths
    • Chromate emissions from cooling towers (used to inhibit corrosion)
chemical basis for the toxicity of hg cd and pb
Chemical basis for the toxicity of Hg, Cd, and Pb
  • All three are “soft” Lewis acids, i.e., with large polarizability.
  • They have strong affinity for soft Lewis bases, such as the sulfhydryl side chain of cysteine residue.
  • The heavy metals likely exert their toxic effects by tying up critical cysteine residues in proteins.

Lewis acid is a species that can form a covalent bond by accepting an electron pair from another species.

Lewis base is a species that can form a covalent bond by donating an electron pair to another species


Cystein residues provide structurally important cross-linking sites through formation of disulfide bridges. Heavy metals interfere with formation of such bridges.

  • When ingested, Hg0, Hg22+, and Hg2+ are not toxic.
  • Harmful forms: methylmercury ion (CH3)Hg+ regardless of the route of exposure, Hg vapor when inhaled.
  • (CH3)Hg+ is produced from mercury ions by methanogens living in sediments.
  • (CH3)Hg+ is soluble in water and bioaccumulates in the aquatic ecosystem food chain (in protein-laden tissues).
  • (CH3)Hg+ binds with protein and peptide sulfhydryl groups.
  • It is dangerous to eat large quantities of very big ocean fish such as tuna and swordfish!
mercury sources related to human activities
Mercury sources related to human activities
  • Coal-burning (Hg0, Hg2+, Hg bound to particulate)
  • Chlor-alkali plants

Hg used as cathode for collecting metallic Na

  • Use in electrical devices and equipment, such as batteries, switches, lamps
  • Use in extracting gold or silver from ores.
  • Dental amalgam fillings
  • Use of complexes of phenylmercury as paint preservatives.
  • Organomercurials used as fungicides for seed treatment
hg pollution long range transportation
Hg pollution: long-range transportation
  • It has been discovered that fish have elevated Hg levels even in lakes remote from any local sources. evidence for the long-range transportation of Hg compounds.
  • The two volatile forms of Hg, Hgo and dimethyl mercury, are responsible via air transportation.
  • Both Hgo and dimethyl mercury are formed from other Hg species (Hg2+ and CH3Hg+) through bacteria actions.
  • Acidic pH favors the formation of CH3Hg+ over (CH3)2Hg lake acidification would increase mercury toxification
hg pollution chemistry
Hg pollution chemistry

Lake acidification would increase mecury toxification.

mercury poisoning tragedies
Mercury Poisoning tragedies
  • Minamata, Japan,1950s: A polyvinyl chloride plant released large quantities of Hg compounds (used as a catalyst) into Minamata Bay. Many residents who relied heavily on fish from the bay for their diet became sick. 52 people died and many suffered numbness, impaired vision, paralysis, and brain damage.
  • Iraq, 1972: Wheat grain treated with a methyl mercury fungicide was imported as seeds for planting. The warning label was in a language the Iraqis did not understand. The wheat was used to make bread. 500 people died from eating the bread.
  • Cd is chemically similar to Zn.
    • It is always found in association with Zn in Earth’s crust, obtained as a side-product of Zn mining and extraction.
    • It is always present as a contaminant in zinc products. (e.g. galvanized steel).
    • It is actively taken by many plants since Zn is an essential nutrients.

Cd intake by human beings is mainly through food crops which take up Cd from Cd-laden soils.


Jinzu vally, Japan Ouch-ouch disease, irrigation water from a river contaminated with waste water from a zinc mining and smelting plant

An Ouch-ouch disease victim

source of cd pollution
Source of Cd pollution
  • Airborne sources
    • Coal burning
    • Incineration
    • Cement plants
  • Phosphate fertilizer: Cd is a natural constituent of phosphate ore.
  • Antagonistic (mutually resistant) to the essential elements Ca, Fe, I, and possibly Cu.
    • Interfere with the incorporation of Fe into porphyrin precursors of haem, producing anaemia.
    • The metabolism of Pb closely resembles that of Ca in many aspects.
    • Impair uptake of I by thyroid.
source of lead pollution
Source of lead pollution
  • Pb-containing paints
    • PbCrO4 provides the yellow striping on roads and for school buses,
    • Pb3O4 is the base for the corrosion-resistant red paints on metal structures
    • Pb3(OH)2(CO3)2 is a white paint, which was widely used as the base of indoor paints
  • Leaded gasoline
    • Tetraethyl lead is used as gasoline additive to improve octane rating

Food and direct ingestion of dust account for most of the average lead intake by human beings.