Page 152 - Soil and water contamination, 2nd edition

Heavy metals 139

In most soils, selenium concentrations have been estimated to vary between 0.01 and
-1
0.2 mg kg . Selenium-enriched natural soils can be found in areas with Se-bearing rocks.
Soils may also become naturally enriched in selenium as a result of seepage and subsequent
evaporation of groundwater originating from Se-bearing aquifers. This typically occurs in
semi-arid regions and results in alkaline and saline soils. In North America, selenium is found
in high concentrations across the Great Plains (in particular Wyoming and South Dakota)
and Canadian Prairies, westward to the Pacific and south into Mexico. Selenium also occurs
in alkaline soils in certain localities in Colombia, Ireland, Israel, South Africa, and China.
High soil salinity and pH promote the adsorption of selenite and selenate onto clays and
sequioxides. Selenite is adsorbed much more strongly than selenate, so selenate is the major
bioavailable form of selenium. Some soil anions, such as phosphate, compete with selenium
anions for adsorption site and so increase selenium concentrations in the soil solution and
bioavailability.
In pristine surface waters, typical concentrations of dissolved selenium range from 0.07
-1
to 0.19 μg l (Luoma and Rainbow, 2008). As a result of its geochemical behaviour in rocks
and soils, in natural waters, selenium is primarily present in an anionic speciation. In surface
waters, selenate can be taken up by plants –including phytoplankton– or can be reduced
by microorganisms to particulate forms in sediments. In plants, selenite is transformed to
organo-selenide. When organo-selenium is released it can be oxidised to selenite. Further
oxidation back to selenate is extremely slow and therefore only little selenite is reconverted
into selenate. Dissolved selenite and organo-selenium are bioavailable to animals, but the
uptake rate is very slow. The biogeochemical cycling in aquatic ecosystems leads to the
predominance of organo-selenium and selenite, which would not be expected based upon
chemical-thermodynamic predictions alone (Luoma and Rainbow, 2008).
Selenium bioaccumulates in aquatic habitats, which leads to concentrations being much
higher in organisms than in the surrounding water. Zooplankton can concentrate organo-
selenium compounds over 200 000 times. Inorganic selenium bioaccumulates more readily
in phytoplankton than in zooplankton. Phytoplankton can concentrate inorganic selenium
by a factor of 3000. Further concentration through biomagnification may occur along the
food chain.
Trace amounts of selenium are necessary for cellular function in animals and humans.
Unlike animals, plants do not appear to require selenium for survival. The toxicity of
elemental selenium and most metallic selenides is relatively low, because of their relatively
low solubility and bioavailability. By contrast, the more oxidised species of selenates
and selenites are very toxic, having an oxidant mode of action similar to that of arsenite.
In humans, acute and fatal toxicities may occur after accidental or deliberate ingestion of
high doses of selenium. Poisoning due to excessive intake of selenium is also referred to as
selenosis. Long-term exposure to smaller doses of selenium in food and water may result in
chronic selenosis. The most common symptoms of selenosis are hair and nail brittleness and
loss. Other symptoms may include gastrointestinal disorders, fatigue, skin rashes, a garlic
breath odour, and neurological damage.
The major pathway to animals is ingestion of plants or sediments. Organic forms of
selenium found in grain, cereals, and forage crops constitute a major source of dietary intake
of selenium. In general, there is a wide variation in the selenium concentration in plants and
grains. Because plants do not appear to require selenium, the incorporation of selenium into
plants depends largely on the selenium concentration in soil. Other, non-vegetable food
sources of selenium include organ meats, seafood, and muscle meats.
Anthropogenic sources of selenium include the mining and smelting of sulphide ores and
coal burning. After coal burning, selenium is concentrated in the remaining ash and so ash
disposal sites may be an important local source of selenium contamination. Mining activities
often cause a massively increased exposure of selenium-bearing pyrite to the atmosphere.

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