Page 203 - Soil Degradation, Conservation and Remediation

192 6 Soil Pollution

6.2.12.1 Chemical Behavior of Heavy Metals
Soil properties that affect the behavior of heavy metals in soil include depth, slope,

texture, porosity, pore size distribution, infiltration, permeability, microbial population
and diversity, organic matter content, cation exchange capacity, soil pH, redox
potential, and temperature. Heavy metals enter into a variety of reactions, as soil
conditions permit, with the physical and chemical components of soil. These
reactions affect the mobility, bioavailability, distribution, and retention of heavy
metals in soil. The most common heavy metals found at contaminated sites, in order
of abundance are Pb, Cr, As, Zn, Cd, Cu, and Hg (USEPA 1996 ). Those metals
are important since they are capable of decreasing crop production due to the risk
of bioaccumulation and biomagnification in the food chain. There’s also the risk of

superficial and groundwater contamination. Once in the soil, heavy metals are
adsorbed by initial fast reactions (minutes, hours), followed by slow adsorption
reactions (days, years) and are, therefore, redistributed into different chemical forms
with varying bioavailability, mobility, and toxicity (Shiowatana et al. 2001 ; Buekers
2007 ). This distribution is believed to be controlled by reactions of heavy metals in
soils such as (1) mineral precipitation and dissolution; (2) ion exchange, adsorption,
and desorption; (3) aqueous complexation; (4) biological immobilization and
mobilization; and (5) plant uptake (Levy et al. 1992 ).
Heavy metals in soil may be found in one or more of the following forms
(Aydinalp and Marinova 2003 ):

(a) Dissolved (in soil solution)
(b) Exchangeable (on organic and inorganic colloidal surfaces)

(c) As structural components of the lattices of soil minerals

(d) As insoluble precipitates with other soil components as in other metal oxides

and carbonates

The first two forms are mobile or labile, while the other two are currently unavailable
and potentially available in the longer term. The easily soluble fractions are of the
greatest interest, as their highest mobility, bioavailability, or toxicity can infl uence
the quality of environment. Extractants including distilled water and several salt
solutions (KCl, CaCl 2 , NH 4 OAc, NH 4 NO 3 , EDTA, and DTPA) have been used to
extract the mobile or bioavailable forms of heavy metals with single extraction
procedures (Karczewska et al. 1998 ). The mobility of metals is strongly affected by
soil properties and may vary significantly in relation to changes in soil pH and organic

carbon content. Other soil properties, such as cation exchange capacity (CEC), organic
matter content, quantity and type of clay minerals, the content of the oxides of iron
(Fe), aluminum (Al), and manganese (Mn), and the redox potential determine the
soil’s ability to retain and immobilize heavy metals.
Mobility of heavy metals in soil depends greatly on soil pH. Fuller ( 1977 )
considered that in acidic soils (pH 4.2–6.6) the elements Cd, Ni, and Zn are highly
mobile, Cr is moderately mobile, and Cu and Pb are practically immobile, and in
neutral to alkaline soils (pH 6.7–7.8), Cr is highly mobile, Cd and Zn are moder-
ately mobile, and Ni is immobile. Humic substances form metal–organic complexes
which influence metal mobilization in soils. Using gel permeation chromatography,