5.2.1 Metal Mobility in the Rhizosphere
Rhizosphere plays a major in the wetland treatment system. In sediments, wherein
reducing condition the metal pollutants (Fe and Mn) are generally associated with
carbonates and sulfides, the oxidation process aids in remobilizing these pollutants
resolving the metals in wetland sediment and accumulating in surface water (Lacerda
et al. 1993; Weis and Weis 2004). The combination of acute microbial activity with
oxygen in wetland sediments creates anaerobic and aerobic zones, so that both
reduction and oxidation reactions take place together in soil and plant interface
(Sobolewski 1999). Also the microbial symbionts such as mycorrhizae present in
wetland can affect the accumulation of metals (Weis and Weis 2004) by increasing
the surface area absorption by root hairs and providing an interface between soil and
root hairs.
5.2.2 Metal Uptake by Plants
The uptake and accumulation of metals by plants is element and plant specific. They
are generally accumulated initially in root tissues and then in shoots. There are
various factors that influence metal accumulation like temperature, metal concentrations, pH, and nutrient levels in the wetland surrounding. Zn can be accumulated in
leaves, while lead accumulates in roots and shoots (Weis and Weis 2004). The heavy
metals accumulated can be essential micronutrients such as Zn, Mn, Ni, and Cu and
nonessential toxic heavy metals, such as Cd, Pb, As, and Hg (Kabata-Pendias and
Pendias 2001; Papoyan et al. 2007). Besides, pH, temperature, and other factors,
such as sediment organic matter content, microbial biomass, grain size, ions, and
nutrients, may also affect the metal uptake process by wetland plants (Dong et al.
2007; Reboreda and Caçador 2007).
5.3 Chemical Processes
There chemical processes that cause the heavy metal removal in wetlands are
described below.
5.3.1 Sorption
It is considered the most important and efficient process in wetland soils/sediments,
for chemical removal of contaminants by two processes: (1) short-term retention and
(2) long-term immobilization. In this process ions are transferred from the solution
phase (water) to the solid phase (soil/sediment) by adsorption and precipitation
reactions (Sheoran and Sheoran 2006). The capacity of soils/sediments (cation
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Rhizosphere plays a major in the wetland treatment system. In sediments, wherein
reducing condition the metal pollutants (Fe and Mn) are generally associated with
carbonates and sulfides, the oxidation process aids in remobilizing these pollutants
resolving the metals in wetland sediment and accumulating in surface water (Lacerda
et al. 1993; Weis and Weis 2004). The combination of acute microbial activity with
oxygen in wetland sediments creates anaerobic and aerobic zones, so that both
reduction and oxidation reactions take place together in soil and plant interface
(Sobolewski 1999). Also the microbial symbionts such as mycorrhizae present in
wetland can affect the accumulation of metals (Weis and Weis 2004) by increasing
the surface area absorption by root hairs and providing an interface between soil and
root hairs.
5.2.2 Metal Uptake by Plants
The uptake and accumulation of metals by plants is element and plant specific. They
are generally accumulated initially in root tissues and then in shoots. There are
various factors that influence metal accumulation like temperature, metal concentrations, pH, and nutrient levels in the wetland surrounding. Zn can be accumulated in
leaves, while lead accumulates in roots and shoots (Weis and Weis 2004). The heavy
metals accumulated can be essential micronutrients such as Zn, Mn, Ni, and Cu and
nonessential toxic heavy metals, such as Cd, Pb, As, and Hg (Kabata-Pendias and
Pendias 2001; Papoyan et al. 2007). Besides, pH, temperature, and other factors,
such as sediment organic matter content, microbial biomass, grain size, ions, and
nutrients, may also affect the metal uptake process by wetland plants (Dong et al.
2007; Reboreda and Caçador 2007).
5.3 Chemical Processes
There chemical processes that cause the heavy metal removal in wetlands are
described below.
5.3.1 Sorption
It is considered the most important and efficient process in wetland soils/sediments,
for chemical removal of contaminants by two processes: (1) short-term retention and
(2) long-term immobilization. In this process ions are transferred from the solution
phase (water) to the solid phase (soil/sediment) by adsorption and precipitation
reactions (Sheoran and Sheoran 2006). The capacity of soils/sediments (cation
11 Toxic Metals in Industrial Wastewaters and Phytoremediation Using. . .
273