exchange capacity) to retain these cations in wastewater, such as ammonium ion
(NH4 +), Cu2+, Cd2+, Pb2+, Zn2+, and Ni2+, increases with substrates like clay
colloids and organic matter (Locke et al. 1997). The metal ion adsorption depends
upon the medium which indirectly affects the metal solubility and binding capacity
onto medium surface. This process is suitable for Cu that predominantly exists as an
organic compound and carbonates in leachates of road dust, Mn, Zn, and Cd that
exist in the form of ions and carbonate.
5.3.2 Precipitation and Coprecipitation
These two mechanisms are majorly used for removal of heavy metals in wetlands by
sedimentation-precipitation-adsorption phenomena (Yao and Gao 2007). For example, Pb, As, Cu, Zn, and Cd can be coprecipitated with pyrite releasing insoluble
sulfides available to biota (Morse 1994). Acidic conditions are needed for removal of
cationic metals such as As, Sb, and Se; Fe facilitates adsorption and alkaline
conditions for cationic metals like Cu, Cd, Zn, and Ni for coprecipitation.
5.3.3 Oxidation and Hydrolysis of Metals
The sediments sometimes act as a source of pollutants when there is a decrease in pH
or redox potential and toxic metals may be released from anoxic sediments causing
drying/aeration of the wetland sediment with exposure to air (van den Berg et al. 1999;
Wilson and Chang 2000; Hartley and Dickinson 2010). The degradation rate of
organic matter increases with high oxidization rate of metal sulfides due to increase
of redox potential in sediment, accelerating the rate of heavy metals adsorption. For
example, in case of a stable Cd compound, an increase of redox potential in sediment
causes a decrease in its quantity from 65% to 30% will form a more labile mobile
(Zoumis et al. 2001; Kelderman and Osman 2007; Peng et al. 2009). Acidithiobacillus
ferrooxidans can catalyze the oxidation of ferrous to ferric iron under acidic conditions. The reaction can be expressed as: 4Fe
2+ + O 2 + 4H + ! 4Fe
3+ + 2H 2 O.
There will be a decrease in sediment pH due H+ ions release into pore water
sediment and affect the iron hydroxide solubility and cause a secondary release of
heavy metals (shown below in reaction) (Kusel 2003; Hartley and Dickinson 2010).
Some of this release material will be re-adsorbed onto the mobile binding compounds: 4FeS 2 + 15O 2 + 14H 2 O 4Fe(OH)
3 + 8SO 4
2À + (Fe(OH)
3 + 3H + Fe
3+ +
3H 2 O) 16H+.
5.3.4 Metal Carbonates and Sulfides
The heavy metals may be removed in the form of carbonates with their hydroxides as
the suspended particles in wetland adsorb heavy metal carbonates and hydroxides
both forming their components. Most of the metal cations may combine with CO32–
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M. Rastogi and M. Nandal
(NH4 +), Cu2+, Cd2+, Pb2+, Zn2+, and Ni2+, increases with substrates like clay
colloids and organic matter (Locke et al. 1997). The metal ion adsorption depends
upon the medium which indirectly affects the metal solubility and binding capacity
onto medium surface. This process is suitable for Cu that predominantly exists as an
organic compound and carbonates in leachates of road dust, Mn, Zn, and Cd that
exist in the form of ions and carbonate.
5.3.2 Precipitation and Coprecipitation
These two mechanisms are majorly used for removal of heavy metals in wetlands by
sedimentation-precipitation-adsorption phenomena (Yao and Gao 2007). For example, Pb, As, Cu, Zn, and Cd can be coprecipitated with pyrite releasing insoluble
sulfides available to biota (Morse 1994). Acidic conditions are needed for removal of
cationic metals such as As, Sb, and Se; Fe facilitates adsorption and alkaline
conditions for cationic metals like Cu, Cd, Zn, and Ni for coprecipitation.
5.3.3 Oxidation and Hydrolysis of Metals
The sediments sometimes act as a source of pollutants when there is a decrease in pH
or redox potential and toxic metals may be released from anoxic sediments causing
drying/aeration of the wetland sediment with exposure to air (van den Berg et al. 1999;
Wilson and Chang 2000; Hartley and Dickinson 2010). The degradation rate of
organic matter increases with high oxidization rate of metal sulfides due to increase
of redox potential in sediment, accelerating the rate of heavy metals adsorption. For
example, in case of a stable Cd compound, an increase of redox potential in sediment
causes a decrease in its quantity from 65% to 30% will form a more labile mobile
(Zoumis et al. 2001; Kelderman and Osman 2007; Peng et al. 2009). Acidithiobacillus
ferrooxidans can catalyze the oxidation of ferrous to ferric iron under acidic conditions. The reaction can be expressed as: 4Fe
2+ + O 2 + 4H + ! 4Fe
3+ + 2H 2 O.
There will be a decrease in sediment pH due H+ ions release into pore water
sediment and affect the iron hydroxide solubility and cause a secondary release of
heavy metals (shown below in reaction) (Kusel 2003; Hartley and Dickinson 2010).
Some of this release material will be re-adsorbed onto the mobile binding compounds: 4FeS 2 + 15O 2 + 14H 2 O 4Fe(OH)
3 + 8SO 4
2À + (Fe(OH)
3 + 3H + Fe
3+ +
3H 2 O) 16H+.
5.3.4 Metal Carbonates and Sulfides
The heavy metals may be removed in the form of carbonates with their hydroxides as
the suspended particles in wetland adsorb heavy metal carbonates and hydroxides
both forming their components. Most of the metal cations may combine with CO32–
274
M. Rastogi and M. Nandal