Page 211 - Soil Degradation, Conservation and Remediation

200 6 Soil Pollution

does not occur naturally in elemental form. Chromium is mined as a primary ore
product in the form of the mineral chromite, FeCr 2 O 4 . Major sources of
Cr-contamination include releases from electroplating processes and the disposal
of Cr containing wastes (Smith et al. 1995 ). Chromium (VI) is the form of Cr com-
monly found at contaminated sites. Chromium can also occur in the + III oxidation
state, depending on pH and redox conditions. Chromium (VI) is the dominant form
of Cr in shallow aquifers where aerobic conditions exist. Chromium (VI) can be
2−
2+
reduced to Cr (III) by soil organic matter, S and Fe ions under anaerobic
conditions often encountered in deeper groundwater. Major Cr (VI) species include
2−
2−
chromate (CrO 4 ) and dichromate (Cr 2 O 7 ) which precipitate readily in the presence
2+
2+
+
of metal cations (especially Ba , Pb , and Ag ). Chromate and dichromate also
adsorb on soil surfaces, especially iron and aluminum oxides. Chromium (III) is the
3+
−
dominant form of Cr at low pH (<4). Cr forms solution complexes with NH 3 , OH ,
−
−
−
2−
Cl , F , CN , SO 4 , and soluble organic ligands. Chromium (VI) is the more toxic
form of chromium and is also more mobile. Chromium (III) mobility is decreased
by adsorption to clays and oxide minerals below pH 5 and low solubility above pH 5
due to the formation of Cr(OH) 3 (s) (Chrostowski et al. 1991 ). Chromium mobility
depends on sorption characteristics of the soil, including clay content, iron oxide
content, and the amount of organic matter present. Chromium can be transported
by surface runoff to surface waters in its soluble or precipitated form. Soluble
and un- adsorbed chromium complexes can leach from soil into groundwater. The
leachability of Cr (VI) increases as soil pH increases.
Nickel
Nickel has atomic number 28 and atomic weight 58.69. In low pH regions, the metal
exists in the form of the nickelous ion, Ni(II). It precipitates as nickelous hydroxide,
Ni(OH) 2 , in neutral to slightly alkaline solutions. The precipitate dissolves in acid
solutions, forming Ni (III), and in very alkaline conditions, it forms nickelite ion,
HNiO 2 , that is soluble in water. In very oxidizing and alkaline conditions, nickel
exists in form of the stable nickelo-nickelic oxide, Ni 3 O 4 , that is soluble in acid
solutions. Other nickel oxides such as nickelic oxide, Ni 2 O 3 , and nickel peroxide,
NiO 2 , are unstable in alkaline solutions and decompose by giving off oxygen. In
2+
acidic regions, however, these solids dissolve, producing Ni . Nickel occurs in the
environment only at very low levels and is essential in small doses, but it can be
dangerous when the maximum tolerable amounts are exceeded (Wuana and
Okieimen 2011 ). This can cause various kinds of cancer on different sites within the

bodies of animals, mainly of those that live near refineries. The major sources of
nickel contamination in the soil are metal plating industries, combustion of fossil
fuels, and nickel mining and electroplating. In acidic soils, however, Ni becomes
more mobile and often leaches down to the adjacent groundwater. Microorganisms
can also suffer from Ni toxicity.