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7 Use of Wetland Plants in Bioaccumulation of Heavy Metals 119

microorganisms. Further, Pb influences child’s s nervous system, slowing down
nerve responses affecting learning abilities and behavior. Again, Hg when released
into the environment, is retained in the soil in the form of complexes of toxic ionic
2+
mercury (Hg ), which may subsequently be converted into methylmercury and is
likely to be accumulated within different organisms (Ke et al. 2001; Brim et al.
2000). Methylmercury poisoning mainly affects the brain (similar to lead) of
children, even causes detrimental consequences at embryonic stages entering
through placenta. While, Cd is toxic and mostly affects kidneys, resulting in kidney
dysfunction and increased excretion of proteins in urine (proteinuria). However, Cr
differs from Cd, Pb, and Hg by being essential in form of Cr(III) to humans and
animals. Widespread effect is caused by exposure to chromium (especially Cr(VI)
compounds) which are generally considered the most toxic (assumed to cause
cancer) (Shanker et al. 2005).

7.3 Heavy Metal Remediation and Ecosystem Restoration

Heavy metals are natural trace components of the aquatic environment, but back-
ground levels in the environment have increased especially in areas where indus-
trial, agricultural, and mining activities are widespread (Bryan and Langston 1992).
Heavy metals released into the environment from different sources as direct input or
surface runoffs find their way into aquatic systems and consequently, aquatic
organisms may be exposed to elevated levels of heavy metals (Kalay and Canli
2000). Heavy metals may affect organisms directly by accumulating in their body
or indirectly by transferring to the next trophic level of the food chain.
Schaller et al. (2011) reported that by the end of the 1980s, only mining activities
damaged approximately 9,300 km of streams and rivers and 72,000 ha of lakes and
wetlands worldwide. Different water bodies worldwide receive more than 180
million tonnes of perilous mine wastes (more than 1.5 times of all the municipal
waste dumped in US landfills in 2009), discarded by the mining companies every
year (Earthworks and mining watch 2012; USEPA 2009a). This poses serious threat
of heavy metal and different chemical contamination of vital water bodies. Usually
metals in soil may be linked with different fractions. It may be as free metal ions
2+
2+
3+
(e.g., Cd ,Zn ,Cr ) and soluble metal complexes, adsorbed form to other
0

inorganic soil constituents, complexes with soil organic matter (CdCl 3 , CdSO 4 ,
+
ZnCl etc.), associated as a structure of silicate minerals, linked with mobile
organic or inorganic colloidal substances or precipitated such as oxides,
hydroxides, and carbonates (Tessier et al. 1979; Lasat 2000). Therefore, the con-
centration of a metal in the solution of soil or sediment is the sum totals of various
fractions like concentrations of free ion of the metal plus soluble organic and
inorganic metal complexes plus the metals associated with movable materials.
Inorganic and organic ligands are the main components of soluble metals
3

2
2
complexes. Inorganic ligands (e.g. SO 4 ,Cl ,OH ,PO 4 ,NO 3 , and CO 3 )

and their metal complexes in soil are well characterized. Soil organic ligands vary