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Aluminum [Al, 27]

1.1 INTRODUCTION
Aluminum (Al) is a metal of the group 13 in the periodic table of elements. It is
the third most abundant element in the Earth’s crust, occurring at about 8%, and up
to 5% in soils. It reveals lithophile properties, and is common in both igneous and
sedimentary rocks. In coal, its contents vary from 1.5% to >10%. Due to its low
content, but important functions in plants and humans, it is included in the trace
elements group.
Aluminum occurs mainly at 3+ oxidation stage and is a common component
of several minerals, especially silicates. It reveals amphoteric properties and thus
may react with both alkaline and acid compounds. Minerals composed only of Al
are boehmite, γ-AlOOH; diaspore, α-AlOOH; hydrargilite/gibbsite, γ-Al(OH) 3 ; and
corundum, Al 2 O 3 . The host minerals of Al are feldspars, micas, and all layer silicates. Under special crystal system, Al 2 O 3 may be gemstones (e.g., sapphire or ruby,
where some Al +
3 is replaced by Cr
+
3 ).
World production of Al in 2010 was 41,400 kt (USGS 2011). The highest production was in China, 16,800 kt. A great proportion of Al, especially in Europe
and the United States, is obtained from old scraps and wastes. The most common Al ores are bauxites, which mainly include Al minerals such as gibbsite,
boehmite, and diaspore, very often occur in a mixture with two Fe oxides (goethite
and hematite).
Due to the versatile properties of Al, it is used in various industrial sectors such
as metallurgy, construction, electricity, chemistry, and transportation and packing
(especially food products). It is used for several chemical treatments, such as water
purification, sugar refining, wood preservations, leather tanning, and several other
processes.
1.2 SOILS
Aluminum is an abundant element in soils and a main component of several common
minerals. Its content is inherited from parent rocks, and its species and distribution
in soil profiles are governed by soil properties, of which soil pH is the most significant. Only easily mobile and exchangeable fractions of Al play a significant role in
soil properties, among which the most important are pH and soluble organic matter
(SOM). Very acidic soils, at pH < 5, contain mainly Al 3+ ; at pH 5–7 Al(OH) 2
+ and
Al(OH) 2+ predominate; and at pH > 6 Al(OH) 4– dominates. Rainwater containing
sulfur (mainly SO 2 ) stimulates the formation of Al(OH)SO 4 , which is very mobile
in soils. Drainage water from acidic soils may contain Al up to 250 μg/L, whereas
those from other soils is about 50 μg/L. Mobile Al species in acidic soils can be
taken up by plants, which creates a chemical stress in plants.
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