Page 283 - Plant-Based Remediation Processes

13 Phytoremediation Towards the Future: Focus on Bioavailable Contaminants 277

13.2.1.6 Iron and Manganese Oxides

Hydrous Fe and Mn oxides are particularly effective in influencing metal solubility
in relatively oxidizing conditions. They are important in reducing metal
concentrations in soil solutions by both specific adsorption reactions and precipita-
tion. Although Mn oxides are typically less abundant in soils than Fe oxides, they
are particularly involved in sorption reactions with heavy metals. Mn oxides also
adsorb heavy metals more strongly, thus reducing their mobility and thus reducing
phytoextraction efficiency. Under reduced conditions, on the other hand, the disso-
lution of Fe and Mn oxides/hydroxides can release adsorbed Arsenic, and
phytoextraction is promoted (Fitz and Wenzel 2002). When a chelating agent
such as EDTA is used to increase metal solubility (Pb), this can also promote the
dissolution of oxy-hydroxides, thus also promoting the uptake of different inorganic
elements such as arsenic (Pedron et al. 2010).

13.2.1.7 Other Factors

There are a number of other factors that may affect the solubility of metals in soils
and in turn phytoextraction efficiency. Temperature, which influences the decom-
position of organic matter, can modify the mobilization of organometal complexes
and consequently plant uptake. An increase in the ionic strength of soil solutions
reduces the sorption of heavy metals by soil surfaces, due to the increased compe-
tition from alkaline metals (Petruzzelli and Pezzarossa 2003). Similar effects also
derive from the simultaneous presence of many heavy metals in soil solutions, and
these metals compete for the same sorption sites. This increases mobility in
contaminated soils due to the saturation of adsorption sites. The living phase of
soil is also of great importance in determining metal solubility, which is dependent
to some extent both on microbial and on root activity. In the rhizosphere, microbial
consortia are able to mobilize metals by changes in the rhizosphere pH. Plants can
increase metal solubility following the release in the exudates both of protons,
which increase the acidity, and of organic substances which act as complexing
agents. Microbial biomass may promote the removal of heavy metals from soil
solutions by precipitation as sulfides and by sorption processes on new available
surfaces characterized by organic functional groups (Wenzel 2009).

13.3 Bioavailability with a View to Phytoextraction

Depending on the soil’s properties, metals are distributed in soil in different pools
of availability to plants. In phytoextraction, only metals in soil solutions will be
available for plant uptake. This amount should be considered in terms of both
intensity and capacity. Intensity identifies the concentration of metals in a soil