(Sheoran and Sheoran 2006). The amounts and forms of Fe in solution strongly affect
removal of metal. Fe (II) is soluble and characterizes an important bioavailable
fraction. Under aerobic condition, it can be oxidized to Fe (III) in conjunction with
Hþ ion consumption. Fe (III) can precipitate to produce oxides, hydroxides, and
oxyhydroxides (Jonsson and Lovgren 2006). Fe (II) can coprecipitate with other
metals such as Cd, Ni, Zn, and Cu (Matagi 1998). Iron oxides have a particularly
strong affinity for cations with a similar size compared to Fe (III) and Fe (II), e.g., Cd,
Zn, Ni, and Cu (Dorman et al. 2009). The other metalloids like arsenic (As) can be
removed by adsorbing against amorphous iron hydroxides or by coprecipitating with
iron oxyhydroxides in water column (Manning et al. 1998). Metals can also form
insoluble compounds through reduction. Under chemically reducing conditions (Eh <
50 mV), sulfates can be reduced to sulfides. These can combine with various elements,
i.e., As, Hg, Se, and Zn, to coprecipitate in relatively insoluble forms (Murray-Gulde
et al. 2005). A constructed wetland based on a matrix with exclusively reducing
conditions, however, cannot be efficient. These conditions promote huge ion discharge, mainly of Mn and Fe, into the water by reduction of the oxides and
oxyhydroxides confined in the substrate (Goulet and Pick 2001).
Metal Carbonates Metal carbonates can be formed from metals though; carbonates are not as much constant than sulfides and play a vital role in initial tricking of
metals (Sheoran and Sheoran 2006). For the removal of Pb and Ni carbonates,
precipitation is generally effective. According to Maine et al. (2006), the external
wastewater structure having high carbonates, pH, and calcium concentrations preferred the metal retaining in the sediment. Metals are removed and adsorbed to
carbonates in wastewater.
pH The pH plays a vital role in affecting the effectiveness of metal removal in
wetlands. Proton production takes place due to the conversion of ammonium into
nitrites during nitrification. In promoting the nitrification process, these ions of
hydrogen are normalized by the ions of bicarbonates because oxygen is released by
the macrophytes. The pH decreased because the protons produced due to nitrification
may not be able to neutralize by HCO 3 ions (Lee and Scholz 2007). To promote
adsorption and removal of oxyanions, for example, As, Sb, and Se, iron
coprecipitation must occur under acidic conditions (Sheoran and Sheoran 2006).
Conversely, alkaline environments are essential to encourage coprecipitation of cationic metals, such as Cd, Cu, Ni, and Zn. The constructed wetland efficiency can be
reduced by metal removal as a result of high nitrification rate (Lee and Scholz 2007).
4.1 Plant’s Role in Removal of Toxic Elements from
Industrial Wastewater
The option of using plants in constructed wetlands is an important concern because
the plants need to survive in possible lethal effects of wastewater and against its
136
S. Khan et al.
removal of metal. Fe (II) is soluble and characterizes an important bioavailable
fraction. Under aerobic condition, it can be oxidized to Fe (III) in conjunction with
Hþ ion consumption. Fe (III) can precipitate to produce oxides, hydroxides, and
oxyhydroxides (Jonsson and Lovgren 2006). Fe (II) can coprecipitate with other
metals such as Cd, Ni, Zn, and Cu (Matagi 1998). Iron oxides have a particularly
strong affinity for cations with a similar size compared to Fe (III) and Fe (II), e.g., Cd,
Zn, Ni, and Cu (Dorman et al. 2009). The other metalloids like arsenic (As) can be
removed by adsorbing against amorphous iron hydroxides or by coprecipitating with
iron oxyhydroxides in water column (Manning et al. 1998). Metals can also form
insoluble compounds through reduction. Under chemically reducing conditions (Eh <
50 mV), sulfates can be reduced to sulfides. These can combine with various elements,
i.e., As, Hg, Se, and Zn, to coprecipitate in relatively insoluble forms (Murray-Gulde
et al. 2005). A constructed wetland based on a matrix with exclusively reducing
conditions, however, cannot be efficient. These conditions promote huge ion discharge, mainly of Mn and Fe, into the water by reduction of the oxides and
oxyhydroxides confined in the substrate (Goulet and Pick 2001).
Metal Carbonates Metal carbonates can be formed from metals though; carbonates are not as much constant than sulfides and play a vital role in initial tricking of
metals (Sheoran and Sheoran 2006). For the removal of Pb and Ni carbonates,
precipitation is generally effective. According to Maine et al. (2006), the external
wastewater structure having high carbonates, pH, and calcium concentrations preferred the metal retaining in the sediment. Metals are removed and adsorbed to
carbonates in wastewater.
pH The pH plays a vital role in affecting the effectiveness of metal removal in
wetlands. Proton production takes place due to the conversion of ammonium into
nitrites during nitrification. In promoting the nitrification process, these ions of
hydrogen are normalized by the ions of bicarbonates because oxygen is released by
the macrophytes. The pH decreased because the protons produced due to nitrification
may not be able to neutralize by HCO 3 ions (Lee and Scholz 2007). To promote
adsorption and removal of oxyanions, for example, As, Sb, and Se, iron
coprecipitation must occur under acidic conditions (Sheoran and Sheoran 2006).
Conversely, alkaline environments are essential to encourage coprecipitation of cationic metals, such as Cd, Cu, Ni, and Zn. The constructed wetland efficiency can be
reduced by metal removal as a result of high nitrification rate (Lee and Scholz 2007).
4.1 Plant’s Role in Removal of Toxic Elements from
Industrial Wastewater
The option of using plants in constructed wetlands is an important concern because
the plants need to survive in possible lethal effects of wastewater and against its
136
S. Khan et al.