Environmental Engineering Reference
In-Depth Information
Although the existent valence states for arsenic are −3, 0, +3 (arsenite), and +5 (arsenate),
arsenite and arsenate are the more common forms of arsenic found in nature. Arsenic is
a toxic element, and a regulatory limit of 50 μg/L in groundwater (aquifers) for drink-
ing water has been adopted in many countries and regulatory agencies. In the United
States, this limit was lowered to 10 μg/L for all water systems in 2006. Ingestion of arsenic
for a period, for example, in the use of arsenic-contaminated waters in some regions of
the world, can lead to serious health problems, e.g., mortality from hypertensive heart
disease traceable to ingestion of arsenic-contaminated drinking water (Lewis et al., 1999)
and arsenic-associated skin lesions of keratosis and hyperpigmentation (Mazumder et al.,
1998). Similarly, inhalation of arsenic dust generated in ore reining processes can also
lead to serious health problems, e.g., nasal septal perforation, and pulmonary insuficiency
(USEPA, 1984).
2.4.1.2 Cadmium (Cd)
Cadmium can be found in nature as greenockite (cadmium sulide, CdS) or otavite (cad-
mium carbonate, CdCO
3
) and is usually associated with zinc, lead, or copper in sulide
form. The two major groups using cadmium include (1) cadmium as a iller, alloy, or active
constituent for an industrial product, e.g., nickel-cadmium batteries, enamels, fungicides,
phosphatic fertilizers, motor oil, solders, paints, plastics, and (2) cadmium as a coating or
plating material, e.g., steel plating, metal coatings. The presence of Cd as a pollutant in the
geoenvironment can be traced to:
• Non-point sources associated with the use of fungicides and fertilizers.
• Deposition of Cd particles in the atmosphere because of mining activities and
burning of coal and other Cd-containing wastes.
• Speciic sources such as industrial discharges and wastes and municipal wastes
where the products manufactured and consumed include Cd as a iller, alloy, or
active constituent.
From the viewpoint of human health effects and requirements, cadmium is consid-
ered to be a nonessential element. The USEPA Toxicity Characteristic Leaching Procedure
(TCLP) regulatory level for Cd is 1.0 mg/L (roughly equivalent to 1 ppm). The EPA speci-
ies a threshold limit of 5 ppb for drinking water and the Food and Drug Administration
(FDA) speciies a limit of 15 ppm of Cd in food coloring (ATSDR, 1999). Accumulation of Cd
in the liver and kidney from oral ingestion can lead to distress to these organs.
2.4.1.3 Chromium (Cr)
Chromium is found naturally as chromite (ferrous chromic oxide, FeCr
2
O
4
) and crocoisite
(lead chromate, PbCrO
4
) minerals. It is an essential element in human nutrition. The three
common valence states for chromium are 0, +3, and +6, i.e., chromium(0), chromium(III),
chromium(VI). Chromium(III) is found naturally in the environment, whereas compounds
of chromium are generally with chromium(VI). Trivalent chromium (chromium(III)) is
stable and is considered to be relatively non toxic. Cr(III) can form various stable inert
complexes:
Cr HO
−
. Meanwhile,
hexavalent chromium (chromium (VI)) is highly toxic and is considered to be a carcinogen.
Oxidation of the trivalent chromium to the hexavalent chromium anions chromate (CrO
4
)
2
3+
, Cr(H
2
O)
5
(OH)
2+
, Cr(H
2
O)
3
(OH)
3
, and
Cr HO OH
(
(
)(
)
2
4
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