Environmental Engineering Reference
In-Depth Information
heavy metals such as chromium, lead, cadmium, arsenic, copper, iron,
manganese, nickel, mercury and cobalt, among others. h e amount and
the number of metals present in any wastewater are related directly to the
operations carried out in an industry. For example, tanneries discharge
chromium in wastewater; copper, chromium, zinc, and cadmium are
widely generated from metal plating; the production of electrical equip-
ment and mining, smelting, and fossil fuel combustion contribute to mer-
cury pollution; and lead is generated from a number of industrial and
mining sources. In most wastewaters, the concentration of heavy metals
present is much larger than the safe permissible limits and, therefore, they
need to be removed. Table 11.1 summarizes the anthropogenic sources of
heavy metals in the environment.
Arsenic is ubiquitous and ranks 20th in natural abundance, compris-
ing about 0.00005% of the Earth's crust, 14th in seawater, and 12th in the
human body [14]. Its concentration in most rocks ranges from 0.5 to 2.5 mg/
kg, though higher concentrations are found in i ner-grained argillaceous
sediments and phosphorites. Since its isolation in 1250 A.D. by Albertus
Magnus [15], this element has been a continuous center of controversy.
Arsenic is mobilized by natural weathering reactions, biological activity,
geochemical reactions, volcanic emissions and other anthropogenic activ-
ities. Soil erosion and leaching contribute to 612×10
8
and 2380×10
8
g/year
of arsenic, respectively, in dissolved and suspended forms in the oceans.
Most environmental arsenic problems are the result of mobilization under
natural conditions. However, mining activities, combustion of fossil fuels,
use of arsenic pesticides, herbicides, and crop desiccants and use of arsenic
additives to livestock feed create additional impacts [14, 16].
Mercury is a highly toxic element that is found both naturally and as
an introduced contaminant in the environment. Although its potential for
toxicity in highly contaminated areas such as Minamata Bay, Japan, in the
1950s and 1960s, and San Francisco Bay, USA, is well documented [17, 18,
19, 20, 21, 22, 23, 24, 25], research has shown that mercury can be a threat
to the health of people and wildlife in many environments that are not
obviously polluted. h e risk is determined by the likelihood of exposure,
the form of mercury present, and the geochemical and ecological factors
that inl uence how mercury moves and changes form in the environment.
Cadmium is a naturally occurring metallic element, one of the com-
ponents of the Earth's crust, and present everywhere in our environment.
However, in certain forms and in high concentrations, cadmium may pro-
duce adverse human health and environmental ef ects. Cadmium emis-
sions to the environment may arise from both natural and anthropogenic
or man-made sources. Estimates of the proportion of total cadmium
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