Page 151 - Soil and water contamination, 2nd edition

138 Soil and Water Contamination

may be the result of evaporative concentration or mobilisation from the aquifer material.
Nevertheless, high arsenic concentrations in groundwater are not necessarily related to high
arsenic concentrations in the source rocks. In sedimentary aquifers in arid inland basins,
the elevated concentrations are probably linked to the desorption of arsenate from iron and
other oxides under oxic , alkaline conditions, whereas in young alluvial and deltaic aquifers,
such as in Bangladesh, arsenic mobilisation occurs under strongly reducing conditions
(Smedley and Kinniburgh, 2001). Such conditions are produced by the microbially mediated
decomposition of buried peat deposits, which induces both the reduction of arsenate to
arsenite (less strongly adsorbed by the ferric oxyhydroxides ) and the reductive dissolution of
ferric oxyhydroxides, thereby releasing the adsorbed load of arsenic to groundwater (Smedley
and Kinniburgh, 2001; McArthur et al., 2001).
Anthropogenic sources of arsenic include mining activities, metal smelters, combustion
of fossil fuels, pesticide applications, household products, and waste disposal. About 40
percent of the anthropogenic emissions are derived from the smelting of copper and other
metals that releases inorganic arsenic into the atmosphere. Low levels of arsenic are found
in most fossil fuels (oil, coal, gasoline), so the burning of these materials results in inorganic
arsenic emissions into the air. Fuel combustion accounts for approximately 20 percent of
the anthropogenic emissions. Arsenic has been used as a component of several pesticides .
Some products, mostly weed killers, contain organic arsenic as the active ingredient, whereas
other pesticides used to control weeds, insects, or rodents, or for wood preservation, contain
inorganic arsenic. In the past, inorganic arsenic was also contained in household products
such as paints, dyes, medicines, and rat poisons. These products are, however, no longer in
general use. As a result of the disposal of these agricultural and domestic products, some
waste disposal sites contain large quantities of arsenic and may be an important local source
of soil and water pollution by arsenic.

7.10 SELENIUM

Selenium is a member of the sulphur group of non-metallic elements. Despite officially
being a non-metal, selenium is sometimes considered to be a metalloid because it shares
physical, chemical, biological, and toxic properties with heavy metals. In its pure form,
selenium occurs as metallic grey to black hexagonal crystals, but in nature it is commonly
found in sulphide minerals (e.g. pyrite), where it partly replaces sulphur, or combined
with silver, copper, lead, and nickel minerals. Selenium occurs naturally in five oxidation
2-
states: 2-, 0, 2+, 4+, and 6+. The primary species are selenate (SeO or Se[VI]), selenite
4
2-
(SeO , or Se[IV]) and organo-selenide (Se(II); e.g. selenomethionine). The chemical
3
properties of selenium are similar to sulphur. Consequently, selenium plays a role analogous
to that of sulphur in organic compounds. Selenium combines with metals and many non-
metals directly or in aqueous solution. It reacts with oxygen to form a number of oxides, the
most stable of which is selenium dioxide.
Selenium is found naturally in igneous rocks, volcanic deposits, ore deposits, and in
sedimentary rocks such as sandstone, carbonaceous siltstones and shales. The average
-1
occurrence of selenium in the Earth’s continental crust is 0.12 mg kg (see Table 1.1). It
is typically found in marine, carbonaceous (organic-rich) shale formations, in which
selenium is primarily hosted by organic matter and pyrite. Se-rich shales can contain bulk
Se concentrations up to 9.1%. Weathering usually transforms most inorganic Se into more
oxidised species (i.e. selenite and selenate), which are very soluble in water. Therefore,
the release of selenium during weathering is largely controlled by the oxidation of pyrite.
Furthermore, pyrite oxidation promotes selenium release from other pools by the associated
release of acidity (Matamoros-Veloza et al., 2011).

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