The suitability of fast-growing trees such as willows has also been examined or
wastewater treatment in constructed wetlands.
4.2 Role of Microbes in Removal of Toxic Elements from
Industrial Wastewater
In wetland soils, the microorganisms accumulate nutrients, but in organic contaminants, removal of the metabolic functions is more critical. Mostly the bacteria of soil
consume the carbonaceous material as an energy source and convert it into methane
in anaerobic environment and carbon dioxide in aerobic environment. In the removal
of inorganic nitrogen, the microbial metabolism is very important. Feasible environment for microorganisms is the area around plant root called rhizosphere. The
root surface is covered with bacteria, and growing roots may transport bacteria
through the soil (Trapp and Karlson 2001). Predominantly, the rhizosphere bacteria
are gram-negative; this may be due to their ability to consume proficiently growth
substrates accessible in the rhizosphere and to handle with contaminated surroundings due to the occurrence of detoxifying enzymes (Chaudhry et al. 2005). The most
commonly originated genera in rhizosphere comprise of Pseudomonas, Azotobacter,
and Rhizobium (Hoagland and Williams 1985).
Symbiotic bacterial are involved in nitrogen fixation provide protection against
pathogens and contaminants treatment it also improve plant nutrition and growth
(Mastretta et al. 2009). The key mechanisms involved in growth-promoting of
bacteria are associated with the production of hormones and siderophores. Microbial
siderophores can work together with heavy metals, decreasing their toxicity or
increasing metal pools and accumulation by roots (Lemanceau et al. 2009). Roots
also live in symbiosis, and their mycelia are also covered with bacteria. Fine feeder
roots of Phragmites australis or Juncus effusus are also sites for colonization by
Arbuscular mycorrhizal fungi (Oliveira et al. 2001). Arbuscular mycorrhizal fungi
can enhance plant tolerance to environmental stresses and against metals (Leyval
et al. 2002).
Microorganism-mediated oxidation: Oxygen is mainly transported to the rhizosphere by macrophytes. This together with the accomplishment of nitrifying bacteria
such as Nitrosomonas spp. and Nitrobacter spp. allows ammoniacal N removal from
the soil atmosphere. Additionally, oxidizing soil situations encourage development
of iron oxides, hydroxides, and oxyhydroxides and subsequently effect in metal
removal by coprecipitation (Lee and Scholz 2007). Maintaining aerobic conditions
also promotes the action of bacteria such as Thiobacillus spp. Moreover bacteria also
participate in oxidation of sulfides to sulfites and then to sulfates because sulfides
take part in metal coprecipitation in the substrate oxidation which may lead to metal
mobilization (Murray-Gulde et al. 2005).
Microorganism-mediated reduction: Under reducing conditions, Desulfovibrio spp.
sulfate-reducing bacteria play an important role in the conversion of sulfates to sulfites
and consequently to sulfides (Murray-Gulde et al. 2005). Then these form insoluble
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S. Khan et al.
wastewater treatment in constructed wetlands.
4.2 Role of Microbes in Removal of Toxic Elements from
Industrial Wastewater
In wetland soils, the microorganisms accumulate nutrients, but in organic contaminants, removal of the metabolic functions is more critical. Mostly the bacteria of soil
consume the carbonaceous material as an energy source and convert it into methane
in anaerobic environment and carbon dioxide in aerobic environment. In the removal
of inorganic nitrogen, the microbial metabolism is very important. Feasible environment for microorganisms is the area around plant root called rhizosphere. The
root surface is covered with bacteria, and growing roots may transport bacteria
through the soil (Trapp and Karlson 2001). Predominantly, the rhizosphere bacteria
are gram-negative; this may be due to their ability to consume proficiently growth
substrates accessible in the rhizosphere and to handle with contaminated surroundings due to the occurrence of detoxifying enzymes (Chaudhry et al. 2005). The most
commonly originated genera in rhizosphere comprise of Pseudomonas, Azotobacter,
and Rhizobium (Hoagland and Williams 1985).
Symbiotic bacterial are involved in nitrogen fixation provide protection against
pathogens and contaminants treatment it also improve plant nutrition and growth
(Mastretta et al. 2009). The key mechanisms involved in growth-promoting of
bacteria are associated with the production of hormones and siderophores. Microbial
siderophores can work together with heavy metals, decreasing their toxicity or
increasing metal pools and accumulation by roots (Lemanceau et al. 2009). Roots
also live in symbiosis, and their mycelia are also covered with bacteria. Fine feeder
roots of Phragmites australis or Juncus effusus are also sites for colonization by
Arbuscular mycorrhizal fungi (Oliveira et al. 2001). Arbuscular mycorrhizal fungi
can enhance plant tolerance to environmental stresses and against metals (Leyval
et al. 2002).
Microorganism-mediated oxidation: Oxygen is mainly transported to the rhizosphere by macrophytes. This together with the accomplishment of nitrifying bacteria
such as Nitrosomonas spp. and Nitrobacter spp. allows ammoniacal N removal from
the soil atmosphere. Additionally, oxidizing soil situations encourage development
of iron oxides, hydroxides, and oxyhydroxides and subsequently effect in metal
removal by coprecipitation (Lee and Scholz 2007). Maintaining aerobic conditions
also promotes the action of bacteria such as Thiobacillus spp. Moreover bacteria also
participate in oxidation of sulfides to sulfites and then to sulfates because sulfides
take part in metal coprecipitation in the substrate oxidation which may lead to metal
mobilization (Murray-Gulde et al. 2005).
Microorganism-mediated reduction: Under reducing conditions, Desulfovibrio spp.
sulfate-reducing bacteria play an important role in the conversion of sulfates to sulfites
and consequently to sulfides (Murray-Gulde et al. 2005). Then these form insoluble
140
S. Khan et al.