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13.2.1.3 Organic Matter Content

The efficiency of phytoextraction is often linked to humic content in contaminated
media (Wanga et al. 2010). The organic matter of soils has a great influence on
metal mobility and bioavailability due to the tendency of metals to bind with humic
compounds in both the solid and solution phases in soil. The formation of soluble
complexes with organic matter, in particular the fulvic fraction, is responsible for
increasing the metal content of soil solutions. However, higher molecular weight
humic acids can greatly reduce heavy metal bioavailability due to the strength of
the linkages. Both complexation and adsorption mechanisms are involved in the
linking of metals by organic matter, thus including inner-sphere reactions and ion
exchanges (Pezzarossa and Petruzzelli 2001). Negatively charged functional groups
(phenol, carboxyl, amino groups, etc.) are essential in metals that are retained by
organic matter. The increase in these functional groups during humification
increases the stability of metal organic complexes, which also show a greater
stability at higher pH values.

13.2.1.4 Cation Exchange Capacity

The density of negative charges on the surfaces of soil colloids is determined by the
type of clay and the amount of organic colloids present in the soil. The negative
surface charges may be pH dependent or permanent. To maintain electro neutrality,
they are balanced reversibly by equal amounts of cations from the soil solution.
Weak electrostatic bonds link cations to soil surfaces, and heavy metals can easily
substitute alkaline cations on these surfaces by exchange reactions. In addition,
specific adsorption promotes the retention of heavy metals, also by partially cova-
lent bonds, although major alkaline cations are present in soil solutions at much
greater concentrations. This can drastically reduce the possibility of plants absorb-
ing inorganic contaminants.

13.2.1.5 Redox Potential

Reduction–oxidation reactions in soils are controlled by redox potential (Eh). High
levels of Eh are encountered in dry, well-aerated soils, while soils with a high
content of organic matter or subject to waterlogging tend to have low Eh values.
Plant-induced reductions of the redox potential and low Eh values can promote the
solubility of some metals such as arsenic (Cherlatchka and Cambier 2000), thus
increasing metal phytoextraction. This can be ascribed to the dissolution of Fe–Mn
oxyhydroxides under reducing conditions, thus resulting in the release of adsorbed
metals. However, under anaerobic conditions, the solubility of heavy metals could
decrease when sulfides are formed from sulfates thus diminishing plant uptake.