Page 210 - Soil Degradation, Conservation and Remediation

6.2 Sources of Soil Pollutants 199

present. Mercurous and mercuric mercury are more stable under oxidizing conditions.
When mildly reducing conditions exist, organic or inorganic Hg may be reduced to
elemental Hg, which may then be converted to alkylated forms by biotic or abiotic
processes. Mercury is most toxic in its alkylated forms which are soluble in water
and volatile in air (Smith et al. 1995 ).
Almost all commercial production of mercury comes from the sulfi de ore,
cinnabar, which results from hydrothermal mineralization associated with volcanic
activity. Mercury abundance in the earth’s crust is very low being in the range of
−1
0.02–0.06 mg kg , although it is likely to be more concentrated in argillaceous
sediments and in coal (Kabata-Pendias and Mukherjee 2007 ). The total mercury
−1
concentrations in soils of UK were reported to range from 0.07 to 1.22 mg kg with
−1
a mean value of 0.13 mg kg . Urban soils were found to contain higher total
−1
mercury concentrations ranging from 0.07 to 1.53 mg kg , with a mean of
−1
0.35 mg kg (Environment Agency 2007 ).
Mercury readily forms amalgams with sodium and zinc. Mercury forms inorganic
compounds in both the Hg (I) and Hg (II) valent states. A large number of organo-
mercury compounds are also known and can be synthesized by the action of sodium
amalgam or Grignard reagent and HgCl 2 with halogenated hydrocarbons (Greenwood
and Earnshaw 1997 ). Mercury is most commonly encountered in the environment
in elemental form, as inorganic mercuric compounds or as monomethylmercury
compounds with the general formula, CH 3 HgX. The most important source of
mercury is the naturally occurring mineral cinnabar (HgS). Monomethylated mer-
cury compounds are most likely to be found in soil as a result of natural microbial
transformation of inorganic mercury.
Elemental Hg is stable in the soil environment. It has a strong tendency to form
−
2−
−
complexes with other anions (such as Cl , OH , and S ) and humic matter. The mercu-
2+
ric cation (Hg ) is rarely found in soil solution under natural conditions and the major
fraction is bound in soil minerals or adsorbed either to inorganic mineral surfaces or to
organic matter. Yin et al. ( 1996 ) observed in experimental studies that soil adsorption

decreased significantly above pH 5 as a result of increasing amounts of dissolved
organic matter and the tendency for mercury to complex strongly to organic carbon.
Inorganic mercury can be methylated by abiotic and microbial processes in soil
systems and is the primary source of methylmercury compounds in soil (ATSDR
1999 ). In surface soils, about 1–3 % of total mercury is in the methylated form with
the rest predominantly as mercuric compounds (Kabata-Pendias and Mukherjee
2007 ). Dimethylmercury is a highly toxic and volatile compound and is readily lost
from soil to air. Monomethylated mercury compounds (CH 3 HgX) are also volatile,
and due to their relatively high mobility compared with inorganic forms, they are
the most important mercury species for environmental pollution (ATSDR 1999 ).

Chromium

Chromium is a transition metal of group VIB in the periodic table with the following
−3
properties: atomic number 24, atomic mass 52, density 7.19 g cm , melting point
1,875 °C, and boiling point 2,665 °C. It is one of the less common elements and