Aluminum [Al, 27]
3
in tap water—2.47 μg/L (Birke et al. 2010). Contribution of Al in drinking water to
the total oral exposure is calculated to be about 4%.
1.4 AIR
Aluminum contents in air (Table 1.1) from remote regions are estimated by
Reimann and de Caritat (1998) at the range of 46–70  ng/m 3 , whereas in urban
regions it is 150–1300 ng/m 3 , and the maximum may be up to 3500 ng/m 3 . Natural
sources of Al are wind-blown earth dust and volcanic eruptions. Industrial emissions contain various Al compounds: Al 2 O 3 , AlF 3 , and Na 3 AlF 6 . It is estimated that
the Al industry releases about 0.2 kg Al per 1 t of Al mined (Kabata-Pendias and
Mukherjee 2007).
1.5 PLANTS
Aluminum is a common constituent of all plants. Its content may vary highly,
depending on both soil factors and plant species. Physiological functions of Al in
plants are not described; however, there are some evidences that its low levels may
have a beneficial effect on plant growth. The most significant problem is associated
with the Al toxicity, which is considered to be one of the major factors limiting plant
growth on acid minerals in soils (Matsumoto et al. 2001).
Plant species and even cultivars differ considerably in their uptake ability, translocation, and toleration of the excess Al. All these processes are related to the cell
walls and plasma membrane properties. Citrates excluded in the rhizosphere protect
plants against increased Al uptake. In acidic soils, however, citrates are absorbed by
oxide minerals and thus the phytoavailability of Al is increased (Hashimoto 2007).
The most significant mechanisms of Al tolerance depend on (1) exclusion of Al in the
root–soil interface; (2) plant-induced pH barrier in the rhizosphere; (3) Al immobilization at cell walls and binding by proteins; and (4) activities of enzymes. According
to Barker and Pilbaem (2007) Al is one of the most important factors that limit plant
growth.
Complex Al toxicity in plants is reflected in several interactions, mainly in the
reduction of the uptake of nutrients such as P, Ca, Mg, K, and N. Addition of these
elements to soils, especially of Ca and Mg, may reduce the Al toxicity. Al toxicity
may be associated with increased levels of Fe and Mn, and also other cations, which
are easily available to plants grown on acidic soils.
The highest Al contents are noticed in clovers (85–3470 mg/kg) and grasses
(10–3410 mg/kg). Mean Al contents in cereal grains are within the range 30–70 mg/
kg, with the highest value for rye grains. Among vegetables, the highest Al content
(about 100  mg/kg) has been found in spinach leaves. The lowest Al contents are
noticed in various fruits, up to 15 mg/kg in orange fruits. Contents of Al in the most
tea leaves are very high (average 2969 mg/kg), since tea bushes grow on very acidic
soils (Houba and Uittenbogaard 1994). Concentrations of Al in mature tea leaves
correlated significantly with exchangeable Al in soils. Levels of Al in mature leaves
and young shoots were reduced by the application of large amounts of N fertilizers
(Ruan et al. 2006).