Cadmium
Cadmium is
a lustrous, silver-white, ductile, very malleable metal. Its surface has a
bluish tinge and the metal is soft enough to be cut with a knife, but it
tarnishes in air. It is soluble in acids but not in alkalis. It is similar in
many respects to zinc
but it
forms more complex compounds.
Applications
About
three-fourths of cadmium is used in Ni-Cd
batteries, most of the remaining one-fourth is used mainly for pigments,
coatings and plating, and as stabilizers for plastics. Cadium
has been used particularly to electroplate steel where a film of cadmium only
0.05 mm thick will provide complete protection against the sea. Cadmium has
the ability to absorb neutrons, so it is used as a barrier to control nuclear
fission.
Cadmium
in the environment
Cadmium can
mainly be found in the earth's crust. It always occurs in combination with zinc.
Cadmium also consists in the industries as an inevitable by-product of zinc, lead and
copper extraction. After being applied it enters the environment mainly through the
ground, because it is found in manures and pesticides.
Naturally a
very large amount of cadmium is released into the environment, about 25,000
tons a year. About half of this cadmium is released into rivers through
weathering of rocks and some cadmium is released into air through forest
fires and volcanoes. The rest of the cadmium is released through human
activities, such as manufacturing.
No cadmium
ore is mined for the metal, because more than enough is produced as a
byproduct of the smelting of zinc from its ore, sphelerite
(ZnS), in which CdS is a
significant impurity, making up as much as 3%. Consequently, the main mining
areas are those associated with zinc. World production is around 14.000 tonnes per year, the main
producing country is
Health Effects of Cadmium
Human uptake of cadmium takes place mainly
through food. Foodstuffs that are rich in cadmium can greatly increase the
cadmium concentration in human bodies. Examples are liver, mushrooms,
shellfish, mussels, cocoa powder and dried seaweed.
An exposure to significantly higher cadmium levels occurs when people
smoke. Tobacco smoke transports cadmium into the lungs. Blood will
transport it through the rest of the body where it can increase effects by potentiating cadmium that is already present from
cadmium-rich food.
Other high exposures can occur with people who live near hazardous waste
sites or factories that release cadmium into the air and people that work
in the metal refinery industry. When people breathe in cadmium it can
severely damage the lungs. This may even cause death.
Cadmium is first transported to the liver through the blood. There, it is
bond to proteins to form complexes that are transported to the kidneys.
Cadmium accumulates in kidneys, where it damages filtering mechanisms. This
causes the excretion of essential proteins and sugars from the body and
further kidney damage. It takes a very long time before cadmium that has
accumulated in kidneys is excreted from a human body.
Other health effects that can be caused by cadmium are:
- Diarrhoea, stomach pains and severe vomiting
- Bone fracture
- Reproductive failure and possibly even infertility
- Damage to the central nervous system
- Damage to the immune system
- Psychological disorders
- Possibly DNA damage or cancer development
Environmental Effects of Cadmium
Cadmium waste streams from the industries
mainly end up in soils. The causes of these waste streams are for instance
zinc production, phosphate ore implication and bio industrial manure.
Cadmium waste streams may also enter the air through (household) waste
combustion and burning of fossil fuels. Because of regulations only little
cadmium now enters the water through disposal of wastewater from
households or industries.
Another important source of cadmium emission is the production of
artificial phosphate fertilizers. Part of the cadmium ends up in the soil
after the fertilizer is applied on farmland and the rest of the cadmium
ends up in surface waters when waste from fertilizer productions is dumped
by production companies.
Cadmium can be transported over great distances when it is absorbed by
sludge. This cadmium-rich sludge can pollute surface waters as well as
soils.
Cadmium strongly adsorbs to organic matter in soils.
When cadmium is
present in soils it can be extremely dangerous, as the uptake through food
will increase. Soils that are acidified enhance the cadmium uptake by
plants. This is a potential danger to the animals that are dependent upon
the plants for survival. Cadmium can accumulate in their bodies, especially
when they eat multiple plants. Cows may have large amounts of cadmium in
their kidneys due to this.
Earthworms and other essential soil organisms are extremely susceptive to
cadmium poisoning. They can die at very low concentrations and this has
consequences for the soil structure. When cadmium concentrations in soils
are high they can influence soil processes of microrganisms
and threat the whole soil ecosystem.
In aquatic ecosystems cadmium can bio accumulate in mussels, oysters,
shrimps, lobsters and fish. The susceptibility to
cadmium can vary greatly between aquatic organisms. Salt-water organisms
are known to be more resistant to cadmium poisoning than freshwater
organisms.
Animals eating or drinking cadmium sometimes get high blood-pressures,
liver disease and nerve or brain damage.