6 Source and Fate of Perchlorate in the Environment: A Grave …
147
6.7.1.1 Natural Biodegradation
Natural biodegradation is an in situ biodegradation (ISB) process which is used
to reduce contaminant at source (Faris and Vlassopoulus 2003). Though, in many
environmental conditions, the stable nature of ClO 4
− makes its remediation difficult,
still natural biodegradation of ClO 4
− is possible under certain conditions and in the
presence of respiring microorganisms, available in different environments such as
freshwater of ponds and rivers as well as wastewaters, soils and sediments (Rice et al.
1996; Van Ginkel et al. 1995). Many bacterial isolates such as Dechloromonas and
Azospira species have the capacity to reduce ClO 4
− to chloride for cell respiration
(Raj and Muruganandam 2012). If ClO 4
− respiring microorganisms exist naturally
in the system and can compete for the existing sources of organic matter, then only
in situ remediation of ClO 4
− contaminated site is possible. Otherwise, if nutrients
are added to the contaminated sites, ClO 4
− reduction may be enhanced. Microbial
reduction of perchlorate progresses in the following steps:
ClO
−
4 (aq) → ClO
−
3(aq) → ClO
−
2 (aq) → Cl
−
(aq) + O 2(g) . Perchlorate reductase catalyses reduction of ClO
−
4 (aq) → ClO
−
3 (aq) → ClO
−
2(aq) , while chlorate dismutase catalyses ClO
−
2(aq) → Cl
−
(aq) + O 2 (g) (Nadaraja et al. 2013; Ye et al. 2012). Therefore,
bio-reduction or natural biodegradation can successfully remove perchlorate, and
this technology can be applied for treatment of wastewater in large-scale treatment
plants (Kumarathilaka et al. 2016).
6.7.1.2 Phytoremediation
Another important in situ mechanism for treating ClO 4
− is phytoremediation where
vegetations like French tarragon, cottonwood, willow, etc., are used successfully
to remove ClO 4
− . Study reveals that willows, under hydroponic conditions, can
degrade ClO 4
− from 10,000 µg/L to below detection limit within 53 days (Nzengung
et al. 1999). Some vascular plant species such as sweetgum (Liquidambar styraciflua), black willow (Salix nigra), pickleweed (Allenrolfea occidentalis), smartweed
(Polygonum punctatum), water-lily (Nymphaea odorata) and duckmeat (Spirodela
polyrrhiza) also have the capacity to remediate ClO 4
− (Susarla et al. 2000) from
water (Fig. 6.1).
6.7.1.3 Bioreactor
Along with already discussed in situ ClO 4
− treatment technology, some ex situ
treatment systems are also developed which can degrade ClO 4
− from contaminated
water. Bioreactor is one of them, in which microorganisms like certain bacteria
degrade ClO 4
− from contaminated groundwater and surface water, soil, etc., under
certain anaerobic conditions. In addition to the electron sink, these organisms require
carbon sources such as ethanol, methanol or acetic acid for their growth (Coates et al.
147
6.7.1.1 Natural Biodegradation
Natural biodegradation is an in situ biodegradation (ISB) process which is used
to reduce contaminant at source (Faris and Vlassopoulus 2003). Though, in many
environmental conditions, the stable nature of ClO 4
− makes its remediation difficult,
still natural biodegradation of ClO 4
− is possible under certain conditions and in the
presence of respiring microorganisms, available in different environments such as
freshwater of ponds and rivers as well as wastewaters, soils and sediments (Rice et al.
1996; Van Ginkel et al. 1995). Many bacterial isolates such as Dechloromonas and
Azospira species have the capacity to reduce ClO 4
− to chloride for cell respiration
(Raj and Muruganandam 2012). If ClO 4
− respiring microorganisms exist naturally
in the system and can compete for the existing sources of organic matter, then only
in situ remediation of ClO 4
− contaminated site is possible. Otherwise, if nutrients
are added to the contaminated sites, ClO 4
− reduction may be enhanced. Microbial
reduction of perchlorate progresses in the following steps:
ClO
−
4 (aq) → ClO
−
3(aq) → ClO
−
2 (aq) → Cl
−
(aq) + O 2(g) . Perchlorate reductase catalyses reduction of ClO
−
4 (aq) → ClO
−
3 (aq) → ClO
−
2(aq) , while chlorate dismutase catalyses ClO
−
2(aq) → Cl
−
(aq) + O 2 (g) (Nadaraja et al. 2013; Ye et al. 2012). Therefore,
bio-reduction or natural biodegradation can successfully remove perchlorate, and
this technology can be applied for treatment of wastewater in large-scale treatment
plants (Kumarathilaka et al. 2016).
6.7.1.2 Phytoremediation
Another important in situ mechanism for treating ClO 4
− is phytoremediation where
vegetations like French tarragon, cottonwood, willow, etc., are used successfully
to remove ClO 4
− . Study reveals that willows, under hydroponic conditions, can
degrade ClO 4
− from 10,000 µg/L to below detection limit within 53 days (Nzengung
et al. 1999). Some vascular plant species such as sweetgum (Liquidambar styraciflua), black willow (Salix nigra), pickleweed (Allenrolfea occidentalis), smartweed
(Polygonum punctatum), water-lily (Nymphaea odorata) and duckmeat (Spirodela
polyrrhiza) also have the capacity to remediate ClO 4
− (Susarla et al. 2000) from
water (Fig. 6.1).
6.7.1.3 Bioreactor
Along with already discussed in situ ClO 4
− treatment technology, some ex situ
treatment systems are also developed which can degrade ClO 4
− from contaminated
water. Bioreactor is one of them, in which microorganisms like certain bacteria
degrade ClO 4
− from contaminated groundwater and surface water, soil, etc., under
certain anaerobic conditions. In addition to the electron sink, these organisms require
carbon sources such as ethanol, methanol or acetic acid for their growth (Coates et al.