Chemistry Reference
In-Depth Information
2. Heavy metal
This is a member of an ill-defined subset of an element that exhibit metallic properties,
which would mainly include the transition metals, some metalloids, lanthanides and
actinides. Many different definitions have been proposed - some based on density, some
based on atomic number or atomic weight and some on chemical properties or toxicity
(Purseglove, 1977)
The primary anthropogenic sources of heavy metals are point sources such as mines,
foundries, smelters, coal-burning power plants, and other sources such as combustion of
wastes, and vehicle emissions. Some investigations carried out have shown that heavy metal
accumulation in vegetables may pose a direct threat to human health.
Vegetables ingest metals by absorbing them from contaminated soil as well from deposits
on different parts of the vegetable exposed to the metal from polluted environments. Many
heavy metals act as biological poisons even at parts per billion (ppb) levels.
The distribution of heavy metals (such as iron, copper, manganese, lead, chromium and
zinc) in leaves, stems and roots of fluted pumpkin (
Telfeiria occidentalis
) were investigated at
three different street roads in Cross River State of Nigeria (Edem et al., 2009). The streets
were Afokang, Anantigha and Eneobong. The result showed that, the concentrations of Fe,
Mn, Pb and Cr were highest in the leaves.
Also, Afokang Street with the heaviest vehicular traffic out of the three streets recorded the
highest concentration of Pb in the leaves of the plants. This may be attributed to the high
level of exhaust emissions from vehicles plying the road.
Ademoronti (1986) in his findings on a study carried out in Benin, Nigeria, showed that lead
deposits on bark of trees were found to vary according to traffic volumes; areas of high
traffic volume recorded higher concentrations of lead. Further studies have revealed that
lead does not readily accumulate in the fruiting part of vegetables and crops (e.g., corn,
beans, tomatoes, apples and berries); a higher concentration of lead is most likely to be
found in leafy vegetables and on the surface of roots crops.
According to Ademoronti (1986), plants accumulate minerals essential for their growth from
the environment and also accumulate metals such as cadmium (Cd) and chromium (Cr)
which have no known direct benefit to the plants (Brook and Robinson 1989). From
Ademoroti's (1995) findings, cadmium is present in low concentrations in vegetable leaves,
entering the human body through diet. The mechanism of trace metals' movement within
plants was investigated by Walsh (1971). He found that most metals become chelated by a
relatively simple agent in xylem sap.
The iron content of normal plant tissue varies with the plant species, but it is usually
between the range of 20 - 200mg/kg dry matter (Walsh, 1971). The cobalt content of a normal
plant is usually within the range of 0.01 - 1.00mg/kg dry matter (Walsh, 1971). Lead is toxic
to crops at concentration range of 3 -20 ppm depending on the plant species; and to animals
at a concentration of 1mg/day (Bowen, 1979). Zinc is an essential mineral involved in
