Introduction to Contaminated Site Management - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 22

1.3.3.1 Metals and Metalloids

'Metals and metalloids' is an important group of soil contaminants, since they are
very often found in soils practically everywhere in the world. Metals (from the
Greek word metallon ) usually are characterised by a hard, malleable and shiny
appearance, mostly solid at room temperature, with a high density and a high melt-
ing point and a good conductance of electricity and heat. Since metals readily lose
electrons, they easily form positive ions (cations) in soils and, hence, have metallic
bonds between metal atoms and ionic bonds with non-metals. Examples of the latter
often found in soils are salts of metals and anions such as chloride (Cl − ) and carbon-
ate (CO 3 2 − ). Different from most other contaminants, metals are elements included
in the periodic table. Often the term 'heavy metals' is used for a sub-group of metals
that are very often found at contaminated sites, although according to Duffus ( 2002 )
the term is controversial and archaic. Very important representatives with regard to
soil contamination are cadmium (Cd), copper (Cu), lead (Pb), zinc (Zn), mercury
(Hg), and selenium (Se).
Except for metals, this category of contaminants also includes the metalloids (or
semi-metals), representatives of which, such as arsenic (As) and boron (B), are also
often found in soils. These metalloids can be considered as transitional elements
between metals and non-metals.
It is important to realise that metals and metalloids do not have a specific
exposure affinity. The exposure depends on many factors, among them the form
(speciation) in which the contaminant is available in the soil. This speciation fol-
lows from a combination of specific contaminant characteristics, soil properties and
other chemical elements present. Many different speciations are found in soils for
each metal. In this respect, a Soil Quality Standard for a specific metal could be con-
sidered as a lump standard for a whole group of chemicals which have the presence
of the metal in common.
A general phenomenon in many residential areas is the presence of lead in soil,
mainly due to the former use of tetraethyl lead in gasoline as an anti-knock agent
(e.g., Wong and Xiang ( 2004 ), who measured elevated lead concentrations due
to traffic activities in Hong Kong, China). This generally is a long-lasting major
problem, since lead can result in retardation of the brain development of young
children and is relatively immobile and will stay in the soil for decades or even
centuries.
Another common problem is the presence of cadmium at agricultural sites or
vegetable gardens (e.g., Wong et al. ( 2002 ) who measured enriched cadmium con-
centrations in crops, paddy and natural soils in the Pearl River Delta, one of
the most developed regions in China; heavy metal enrichment was most signi-
ficant in the crop soils, which might be attributed to the use of agrochemicals).
Cadmium, which is often found in soils, due to atmospheric deposition from
smelters or the application of fertilizers, is easily taken up by vegetables and
can induce kidney dysfunction and several types of cancer at relatively low

exposures.