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Emerging technologies in bioremediation

Genoremediation

Genetic modifications in plants for the increased expression of metal chelators, metal transporters, metallothioneins, and phytochelatins are known as genoremediation. The main classes of metal chelators are phytochelatins (PCs) and metallothioneins (MT), between which only metallothioneins are direct products of gene expression. Model plants of tobacco (Nicotiana glauca and Nicotiana tabacwn) were either showing greater accumulation of Cd and Pb. or were tolerant to Cd depending on the origin of the transgeue (Huang et al. 2012, Chen et al. 2015). Phytochelatins gene from Populus tomentosa reduced cadmium translocation to the aerial parts of tobacco (Chen et al. 2015), while phytochelatins gene from aquatic macrophyte ceratophyllum demersum enhanced cadmium translocation (Shukla et al. 2013). To overcome this issue, co-transformation could be an efficient way to produce potential hyperaccumulator. Guo et al. (2012) transformed A. thaliana with two genes such as PCS (responsible for phytochelatin synthesis) and YCF1, ABC metal transporter and plants showing co-tolerance to Cd and As with increased bioaccumulation in vacuoles. Zhao et al. (2014) proved that co-transformation with Phytochelatins (PCS) and glutamyl cysteine synthetase (GCS) genes was beneficial than transformation with PCS gene alone because of overexpression of GCS enhanced PCS activity, resulting in higher production of phytochela this.

Recently, transgenic plants developed for PCBs phytoremediation has been evaluated by several researchers. Extracellular oxidative enzymes like lignin peroxidase (Lip), manganese-dependent peroxidase (Mnp) and laccase (Lac) produced by Basidiomycetes (white rot fungus) were found to be important in degradation of PCBs. Genes produced by Phaenerochete chrysoporium responsible for generation of Lip, Mnp and Lac enzymes have been introduced into the DNA of Arabidopsis thaliana to make a transgenic species that potentially degrade PCBs (Sonoki et al. 2007). Francova et al. (2003) developed transgenic tobacco plants (Nicotiana tabacum) by inserting a gene which is responsible for 2,3-dihydroxybiphenyl ring cleavage, bphC, front the PCB degrader Comamottas testosteroni.

Electro-assisted bioremediation

The bioconversion of a wide range of groundwater contaminants such as hydrocarbons, aromatic organics into inorganic substances like nitrate, sulfate and heavy metals is promoted by electrochemical approaches. The combination of bioremediation and electrochemical stimulation (electrokinetic enhancement) has significantly enhanced the degradation of contaminant. An accelerated bioreduction of perchloroetlieue (PCE) and further cathodic oxidation of the intermediates can be achieved by the use of in situ generated hydrogen and oxygen from water electrolysis, respectively, and complete mineralization of PCE in groundwater occurs (Lolmer and Tiehm 2009). An energy efficiency improvement can be achieved by establishing a direct delivery of electrons between the electrodes and degrading bacteria (Leitao et al. 2015). The incorporation of EDTA enhancement and bioaugmentation improved electrokinetics process, resulting in

92.7 percent lead (Pb), 64.3 percent copper (Cu) and 45.9 percent zinc (Zn) removal simultaneously with low power consumptions, as reported by Lee and Kim (2010). Similar strategies have also been implied to remediate arsenite (As) from the soil by combining anaerobic bioleacliing and electrokinetics, yielding 66.5 percent As removal within 16 days compared to 17.3 percent removal in sole bioleaching system (Lee et al. 2009).

These electrochemical reduction and precipitation methods could be applied to release most heavy metal species including copper, chromium, cadmium, mercury, uranium, and vanadium (Huang et al. 2011, Williams et al. 2010). Many electro-active bacteria such as Geobacter species could efficiently catalyze metal reduction that are mostly present in groundwater (Williams et al.

2010). Hao et al. (2015) reported remediation of vanadium significantly with a removal efficiency of 93.6 percent within 12 hours under accelerated microbial vanadium reduction in groundwater via bioelectrocliemical simulation. Biosorption and precipitation of metals such as Cd (II) and Zn (II), with in situ formed sulfide at the alkaline cathode, are also essential mechanisms for their removal (Abouraclied et al. 2014, Varia et al. 2014). The bio-electrochemical process can also be used to improve nitrate removal efficiency from groundwater.

 
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