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
? 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.
Maximum permissible levels for metals in
drinking water (USEPA standards)
Metal Max,Level
(ppb)
Metal Max,Level
(ppb)
Antimony (Sb) 6 Lead 50
Arsenic 50 Mercury 2
Barium 2000 Nickel 100
Beryllium 4 Selenium 50
Cadmium 5 Thallium 2
Chromium 100
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
? 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
? 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.
c ys te in e
N H 2
C
C O O H
C H 2 S HH
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.
Mercury
? 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
? 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
? 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
Lake acidification would increase mecury toxification.
Hg,Biogeochemical cycle
Mercury Poisoning tragedies
1,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.
2,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.
Cadmium
? 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
? Airborne sources
– Coal burning
– Incineration
– Cement plants
? Phosphate fertilizer,Cd is a natural constituent of
phosphate ore.
Lead
? 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
? 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.
? 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
? 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.
Maximum permissible levels for metals in
drinking water (USEPA standards)
Metal Max,Level
(ppb)
Metal Max,Level
(ppb)
Antimony (Sb) 6 Lead 50
Arsenic 50 Mercury 2
Barium 2000 Nickel 100
Beryllium 4 Selenium 50
Cadmium 5 Thallium 2
Chromium 100
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
? 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
? 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.
c ys te in e
N H 2
C
C O O H
C H 2 S HH
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.
Mercury
? 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
? 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
? 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
Lake acidification would increase mecury toxification.
Hg,Biogeochemical cycle
Mercury Poisoning tragedies
1,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.
2,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.
Cadmium
? 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
? Airborne sources
– Coal burning
– Incineration
– Cement plants
? Phosphate fertilizer,Cd is a natural constituent of
phosphate ore.
Lead
? 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
? 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.
