Edouard Bastarache Inc. on sat 11 oct 03
"And where does it come from, and
how is it mined and processed?
Thanks,
Paul Lewing, Seattle"
1-CADMIUM EMISSIONS
Cadmium emissions arise from two major source categories,
natural sources and man-made or anthropogenic sources.
Emissions occur to the three major compartments of the
environment - air, water and soil, but there may be
considerable transfer between the three compartments after
initial deposition. Emissions to air are considered more mobile
than those to water which in turn are considered more mobile
than those to soils.
2.1-Natural Cadmium Emissions
Even though the average cadmium concentration in the earth's
crust is generally placed between 0.1 and 0.5 ppm, much
higher levels may accumulate in sedimentary rocks, and marine
phosphates and phosphorites have been reported to contain
levels as high as 500 ppm (Cook and Morrow 1995, WHO
1992).
Weathering and erosion of parent rocks result in the transport
by rivers of large quantities, recently estimated at 15,000 metric
tonnes (mt) per annum, of cadmium to the world's oceans
(WHO 1992, OECD 1994).
Volcanic activity is also a major natural source of cadmium
release to the atmosphere, and estimates on the amount have
been placed as high as 820 mt per year (WHO 1992, OECD
1994, Nriagu 1980, Nriagu 1989).
Forest fires have also been reported as a natural source of
cadmium air emissions, with estimates from 1 to 70 mt emitted
to the atmosphere each year (Nriagu 1980).
The problem is distinguishing what are the expected natural levels
of the substances, what is from human sources, and what, if any,
are risks to the environment.
2.2-Example of the Influence of Soil Geochemistry:
In north central Saskatchewan and eastern and central Manitoba,
low cadmium and arsenic concentrations in soils reflect the
southward glacial transport of cadmium-poor debris derived
from the Canadian Shield to the north. In contrast, high
cadmium and arsenic concentrations in southwestern Manitoba
soils reflect glacial incorporation of trace-element-rich shale
bedrock forming the Manitoba Escarpment (Garrett and
Thorleifson 1999). In both areas, the geochemical expression
of the source is really more extensive in soil than in bedrock
as the result of glacial transport.
Where elements are readily dissolved, there can be close
compositional linkages between surficial materials and water.
3-Andropogenic Sources
3.1-Where does Cd come from?
Cadmium is a by-product of zinc, lead, and copper mining.
Its major feedstock, sphalerite (ZnS), contains only 0.25%
cadmium. Because Zn is produced in large quantities
(8 million metric tons in 1999), substantial amounts of cadmium
are produced as a by-product.This cadmium by-product can
be put to beneficial use in many products, or it can be discharged
into the environment. When the market does not absorb the
Cd generated by metal smelters/refiners, it is cemented and
buried, stored for future use, or disposed of in landfills as
hazardous waste.
3.2-Cadmium-Containing vs. Non-Cadmium Containing Products
Man-made cadmium emissions arise either from the manufacture,
use and disposal of products intentionally utilising cadmium, or
from the presence of cadmium as a natural but not functional
impurity in non-cadmium containing products. In the former
category of cadmium-containing products are included:
· Nickel-Cadmium Batteries
· Cadmium Pigmented Plastics, Ceramics, Glasses, Paints and Enamels
· Cadmium Stabilised Polyvinylchloride (PVC) Products
· Cadmium Coated Ferrous and Non-ferrous Products
· Cadmium Alloys
· Cadmium Electronic Compounds
In the latter category of non-cadmium containing products are included:
· Non-ferrous Metals and Alloys of Zinc, Lead and Copper
· Iron and Steel
· Fossil Fuels (Coal, Oil, Gas, Peat and Wood)
· Cement
· Phosphate Fertilisers
3.3-Sources of Cd exposure to humans :
Cadmium is released into the environment from phosphate
fertilizers, burning fuels, mining and metal processing
operations, natural sources, cement production, and
disposing of metal products. Releases from disposed
cadmium products, including NiCd batteries, are minor
contributors to human exposures because Cd is
encapsulated in their structure. Most human cadmium
exposure comes from ingestion of food, and most of
that stems from the uptake of cadmium by plants from
fertilizers, sewage sludge, manure, and atmospheric
deposition. Van Assche (1998) has developed a model
for human exposure to cadmium and allocated this exposure
to these sources. The assumptions and the data inputs
for the model are based on actual data from Belgium and
the European Community (ERL, 1990; OECD, 1994).
The model estimates of the relative importance of various
cadmium sources to human exposure are shown in Table 1.
Table 1. Sources and Relative Contributions of Cd Exposure
to Humans (in Europe)
Phosphate fertilizers 41.3 %
Fossil fuel combustion 22.0 %
Iron and steel production 16.7 %
Natural sources 8.0 %
Non-ferrous metals 6.3 %
Cement production 2.5 %
Cadmium products 2.5 %
Incineration 1.0 %
Later,
"Ils sont fous ces Quebecois"
Edouard Bastarache
Irreductible Quebecois
Indomitable Quebeker
Sorel-Tracy
Quebec
edouardb@sorel-tracy.qc.ca
http://sorel-tracy.qc.ca/~edouardb/
http://perso.wanadoo.fr/smart2000/index.htm
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