Page 101 - Plant-Based Remediation Processes

90 M. Mleczek et al.

5.4 Phytoremediation Potential of Most Popular Plants

Heavy metals are conventionally defined as elements with metallic properties
(ductility, conductivity, stability as cations, ligand specificity, etc.) and atomic
number >20. The most common heavy metal contaminants are Cd, Cr, Cu, Hg,
Pb, Co, Ni, and Zn. Metalloids are chemical elements with properties that are in
between or a mixture of those of metals and non-metals and which are considered to
be difficult to classify unambiguously as either a metal or a non-metal (B, Si, Ge,
As, Sb, and Te). Both metals and metalloids are natural components in soil, but high
levels resulting from industrial activities, such as mining and smelting of metallif-
erous ores, electroplating, gas exhaust, energy and fuel production, fertiliser and
pesticide application, and generation of municipal waste (Kabata-Pendias and
Pendias 1999), are the most serious environmental problems.
Some heavy metals and metalloids, such as As, Cd, Hg, or Pb, are not essential,
since they do not perform any known physiological function in plants. Others,
e.g. Co, Cu, Fe, Mn, Mo, Ni, and Zn, are essential elements required for normal
growth and metabolism of plants. Heavy metal phytotoxicity may result from
alterations of numerous physiological processes caused at the cellular/molecular
level by inactivating enzymes, blocking functional groups of metabolically impor-
tant molecules, displacing or substituting for essential elements, and disrupting
membrane integrity. A rather common consequence of heavy metal poisoning is
the enhanced production of reactive oxygen species (ROS) due to interference with
electron transport activities, especially those of chloroplast membranes (Pagliano
et al. 2006; La Rocca et al. 2009; Rascioa and Navari-Izzo 2011). This increase in
ROS exposes cells to oxidative stress, leading to lipid peroxidation, biological
macromolecule deterioration, membrane dismantling, ion leakage, and DNA-strand
cleavage (Rascioa and Navari-Izzo 2011).
Plants respond with a series of defence mechanisms that control uptake,
accumulation, and translocation of these dangerous elements and detoxify them
by excluding the free ionic forms from the cytoplasm. Although all plants may
extract toxic elements from soil, only some plants species may survive, grow, and
reproduce under heavy metal/trace element contamination. What is interesting and
very important is that these plants tolerate high concentrations of heavy metals,
which are highly toxic for other species of plants. The identification of metal-
accumulating plants has increased interest due to their use in remediation methods
of contaminated soil. The technology of phytoremediation using hyperaccumulator
plants to remove metals and contaminations from soils, sediments, and water by
absorbing metals from soil, followed by their transport and accumulation in
shoots, is called phytoextraction (Padmavathiamma and Li 2007; Van Nevel
et al. 2007). The term hyperaccumulator was first applied during accumulation
of nickel in Sebertia acuminata (Jaffre et al. 1976). The first definition of
hyperaccumulator was plants that can accumulate more than 1,000 mg kg 1 Ni
dry weight (dw) in their shoots (Brooks et al. 1977). Now hyperaccumulators are
called species capable of exceptional accumulation of any kind of heavy metal in