increased by the electrokinetic field (EKF) and thereby lower the pH of contaminated
soil. Furthermore, they observed that EKF significantly enhanced the
bioaccumulation of As and Cs in plant roots and shoots and, thus, enhanced
phytoremediation efficiency. However, the optimization of electrical parameters
such as electrical field intensity, current application mode, distance between the
electrodes, stimulation period, and their effect on the mobility and bioavailability of
HMs are the associated key challenges (Mao et al. 2016). Further, the application of
electrokinetic-phytoremediation for the mixed contaminants (organic and inorganic)
is also not reported so far.
2.5 Constructed Wetlands
Constructed wetlands (CWs) are the eco-technological option for the treatment and
purification of HM-rich wastewaters. These are the man-engineered systems
constructed to utilize the natural processes of aquatic macrophytes with their associated microbial assemblages for wastewater treatment within a more controlled
environment (Stottmeister et al. 2003; Khan et al. 2009). CWs are mainly vegetated
with different wetland plants with high biomass, fast growth rate, and metal accumulation capacity such as Phragmites australis, Typha latifolia, Canna indica,
Stenotaphrum secundatum, Scirpus americanus, Scirpus acutus, Iris pseudacorus,
etc. for metal-rich wastewater treatment (Bharagava et al. 2017c). CWs have been
proven to be successful in the removal of a variety of organic and inorganic
pollutants such as metals, nutrients, fecal indicator bacteria, and pathogens and a
wide range of micro-pollutants, such as pharmaceutical and personal care products
(Zhang et al. 2015). However, the pollutant removal efficiency of CWs mainly
depends on wastewater treatment rate, organic loading rate, hydrologic regime,
hydraulic retention time, operational mode, and vegetation type (Zhang et al.
2015). The application of CWs in pollutants’ removal from wastewaters has been
recently reviewed by many workers (see Vymazal 2010; Zhang et al. 2015;
Bharagava et al. 2017a, b, c).
For instance, phytoremediation potential of Pennisetum purpureum, Brachiaria
decumbens, and Phragmites australis in CWs has been reported for the
phytoremediation (phytoextraction) of Cr from TWW (Mant et al. 2004). Calheiros
et al. (2007) has been also reported the phytoremediation potentials of Canna indica,
Typha latifolia, P. australis, Stenotaphrum secundatum and Iris pseudacorus in
CWs for the treatment of TWW under two different hydraulic loading rates at 3 and
6 cm/day. It was found that only P. australis and T. latifolia were able to establish
successfully. Further, they also evaluated Arundo donax and Sarcocornia fruticosa
in two series of horizontal subsurface flow CWs that are used to treat TWW received
from a conventional biological treatment plant and reported the removal of COD
(51 and 80%) and BOD 5 (53 and 90%) for COD inlet (68–425 mg L
À1 ) and for
BOD 5 inlet (16–220 mg L
À1 ) (Calheiros et al. 2012).
CWs may provide many ecological and economic benefits such as require low
capital investment for construction, low electricity for operation, and less
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G. Saxena et al.
soil. Furthermore, they observed that EKF significantly enhanced the
bioaccumulation of As and Cs in plant roots and shoots and, thus, enhanced
phytoremediation efficiency. However, the optimization of electrical parameters
such as electrical field intensity, current application mode, distance between the
electrodes, stimulation period, and their effect on the mobility and bioavailability of
HMs are the associated key challenges (Mao et al. 2016). Further, the application of
electrokinetic-phytoremediation for the mixed contaminants (organic and inorganic)
is also not reported so far.
2.5 Constructed Wetlands
Constructed wetlands (CWs) are the eco-technological option for the treatment and
purification of HM-rich wastewaters. These are the man-engineered systems
constructed to utilize the natural processes of aquatic macrophytes with their associated microbial assemblages for wastewater treatment within a more controlled
environment (Stottmeister et al. 2003; Khan et al. 2009). CWs are mainly vegetated
with different wetland plants with high biomass, fast growth rate, and metal accumulation capacity such as Phragmites australis, Typha latifolia, Canna indica,
Stenotaphrum secundatum, Scirpus americanus, Scirpus acutus, Iris pseudacorus,
etc. for metal-rich wastewater treatment (Bharagava et al. 2017c). CWs have been
proven to be successful in the removal of a variety of organic and inorganic
pollutants such as metals, nutrients, fecal indicator bacteria, and pathogens and a
wide range of micro-pollutants, such as pharmaceutical and personal care products
(Zhang et al. 2015). However, the pollutant removal efficiency of CWs mainly
depends on wastewater treatment rate, organic loading rate, hydrologic regime,
hydraulic retention time, operational mode, and vegetation type (Zhang et al.
2015). The application of CWs in pollutants’ removal from wastewaters has been
recently reviewed by many workers (see Vymazal 2010; Zhang et al. 2015;
Bharagava et al. 2017a, b, c).
For instance, phytoremediation potential of Pennisetum purpureum, Brachiaria
decumbens, and Phragmites australis in CWs has been reported for the
phytoremediation (phytoextraction) of Cr from TWW (Mant et al. 2004). Calheiros
et al. (2007) has been also reported the phytoremediation potentials of Canna indica,
Typha latifolia, P. australis, Stenotaphrum secundatum and Iris pseudacorus in
CWs for the treatment of TWW under two different hydraulic loading rates at 3 and
6 cm/day. It was found that only P. australis and T. latifolia were able to establish
successfully. Further, they also evaluated Arundo donax and Sarcocornia fruticosa
in two series of horizontal subsurface flow CWs that are used to treat TWW received
from a conventional biological treatment plant and reported the removal of COD
(51 and 80%) and BOD 5 (53 and 90%) for COD inlet (68–425 mg L
À1 ) and for
BOD 5 inlet (16–220 mg L
À1 ) (Calheiros et al. 2012).
CWs may provide many ecological and economic benefits such as require low
capital investment for construction, low electricity for operation, and less
118
G. Saxena et al.