Phytoremediation is a bioremediation process that is cost-effective, eco-friendly and solar energy driven in situ process that uses various kinds of plants associated with microorganisms to clean, stabilize and transfer the pollutants in the contaminated site. Plants can be successfully used for cleaning radionuclides and pollutants of both organic and inorganic origin (Ali et al. 2013). Phytoremediation includes different techniques like phytoextraction, phytovolatilization, phytofiltration, phytodegradation and phytostabilization (Alkorta et al. 2004). The first process among them is phytoextraction, which is uptake of the different contaminants, translocating them into shoots and storing them in the tissues of shoots (Sekara et al. 2005). The next important process is phytofiltration, which is uptake of the contaminants by different plant parts like roots (rhizofiltration), seedlings (blastofiltration) or by excising plant shoots (caulofiltration) to reduce their movement into the groundwater (Mesjasz-Przybylowicz et al. 2004). Other processes are phytostabilization and phytoimmobilizatiou. These processes reduce the bioavailability of the heavy metals so that it can prevent the contamination in the ground water and migration to the food chain (Erakhmmeu et al. 2007). The immobilization happens in the same way as sorption, precipitation, the formation of organic complex and transformation of the redox state but all these processes happen in the rhizosphere controlled by the plants and related microorganisms (Barcelo et al. 2003). The organic pollutants are degraded by the root exudates through the process of phytostimulation. The contaminants inside the plants are metabolized by the enzymes (oxygenaes, dehalogenases, etc.), which is a completely different process from the immobilization process of microorganisms (Vishnoi and Srivastava 2008). The volatile metals like Hg and Se are not completely immobilized by the plants, they rather get converted into different forms (from solid to gas) and are released into the atmosphere (Karami and Shamsuddin 2010). This process is known as phytovolatilization. This process can remove the contaminants temporarily from the soils as this process only allows the transformation of the heavy metals front one medium (soil and water) to another medium (air), so they can change the medium anytime. All these processes are shown briefly in the Figure 2.
Among the plants known for phytoremediation, hyperaccumulator is very popular due to its usefulness. The criteria for hyperaccumulation vary with the types of pollutant heavy metals like 100 mg kg'1 for Cd and 1000 mg kg"1 for Cu, Co, Cr, and Pb. The shoot to soil ratio of the metal should be higher than 1 in the case of hyperaccumulators (Baker et al. 1994). The hyperaccumulators should have a high tolerance towards the heavy metal concentration in their biomass (Prasad and Freitas 2003). There are some plants known for hyperaccumulation of heavy metals like Arabidopsis halleri and Solanumnigram L. for Cd accumulation (Wei et al. 2005), Populous deltoids for Hg accumulation (Che et al. 2003), Brassica juncea and Astragalusbisulcatus for Se (Bitther et al.
2012), Populuscanescens for Zn, etc. However, hyperaccumulator plants are very few, very slow
Figure 2. Different processes of phytoremediation for controlling heavy metal pollution.
growing, and low in biomass yield which restricts their application where quick remediation is needed (Xiao et al. 2010). These restrictions can be handled by applying growth-promoting rhizobacteria or arbuscular mycorrhiza (Wei et al. 2003). The root exudates containing carbohydrates, flavonoids, amino acids, etc. can accelerate the microbial activity, which produces an enzyme named ACC deaminase that reduces the of ethylene level in the soil, promoting an environment for healthier root development of the plants (Kuiper et al. 2004, Glick et al. 1998). Kluweraascorbate SUD 165 is a Ni-resistant bacteria which lowers the level of ethylene and promotes the growth of Brasicacampestris (Burd et al. 1998).
7. Biotechnology in bioremediation
Recombinant DNA technology has been used to alter the genetic materials of microorganisms or plants to create genetically modified microorganisms or plants, more efficient and specific than the previous versions (Sayler and Ripp 2000). They are important as then ability to sustain in the adverse condition is more than the normal strain, the development of “microbial biosensor” is possible which can be used to detect the contamination accurately in a short periods of time, and many microorganisms associated with plants can increase the rate of bioremediation by increasing the rate of phytochelation and degradation of the metals (Divya et al. 2011). Genetically modified E. coli and Moreaxella sp. can accumulate 25 times more Cd and Hg than their wild type expressing a gene phytochelatin 20 on the cell surface (Bae et al. 2001, 2003). Following genetic modification,
P. fluorescens expresses Phytochelatin synthase (PCS) and E. coli expresses Hg2+ transporter w'hich increases removal of Ni and Hg, respectively (Zhao et al. 2005, Lopez et al. 2002, Sriprang et al. 2003). The problem is that the genetically engineered microorganisms face competition with the native microorganisms for survival (Wu et al. 2006).
The main problem with phytoremediatiou is the accumulation of the heavy metals and their metabolites within the tissues of the plants w'hich is harmful to the plants and after the death of the plants again there is a chance to reenter into the atmosphere. If these plants are genetically modified with the genes of those bacteria w'hich are capable of degrading heavy metals, then the metals could be degraded inside the plant tissues. The plants will be made capable of producing different metal chelators such as metallothineins and phytochetains, which will help the plant to uptake and accumulate more heavy metals from the soil (Ruis and Daniell 2009). High biomass yielding plants like poplar, willow, and jatropha can be used as heavy metal accumulators as well as the plants that can be used for energy production. But if these plants are burned after heavy metal accumulation, it will release the metals to the atmosphere which will transfer the problem from soil to air. Poplar trees are genetically modified to synthesize mercuric reductase and Y-gltamylsysteine, which increases the Hg, Cd and Cu accumulation and degradation inside the plant tissues, respectively, to ensure the production of healthy biomass for further application (Bittsanszkya et al. 2005, Gullner et al. 2001. Abhilash et al. 2012).
Mostly, the contaminated sites have multiple pollutants, which are very difficult for the plants to control efficiently. The rhizospliere needs some energy to fight with this situation. Biostimulation is a technique to increase the microbial activity in the rhizospliere by the addition of growth stimulants. Bioaugmentation is the addition of selected and cultured microorganisms to the rhizospliere to remove the contamination.
8. Nano technology in bioremediation
Nanoparticles are more effective than the microorganisms as they can cover a huge area and can also reduce the processing tune. The enhancement of the microbial activity by applying nauoparticles for the removal of the contaminants is called “nanobioremediation”. Different biological cells are used for the preparation of the nano-particles because of their small size, cells can be easily genetically modified and they can be easily cultured under controlled conditions. Different polymers and magnetosomes are used as these macromolecules are easily converted to nanostructures. Different proteins can be used like virus-like protein (VLP) and tailored metal particles (Sarikaya et al. 2003). US Department of Energy (DOE) has taken an initiative to clean the radioactive waste by using a radioactive-resistant organism Deinococcus radiodurans (Brim et al. 2000, Smith et al. 1998).
Development and industrialization are synonymous. They are unstoppable and have an adverse effect on the environment. The conventional methods have failed to control the situation, and are not user-friendly. Bioremediation is the best hope left in our hands, though it has some restrictions. Several organisms are out there that cannot break the toxic contaminants successfully. Many of them cannot tolerate the adverse environmental condition. In order to improve these situations, scientists should be encouraged to find out other successful techniques like biotechnology. GEMs are more efficient than their wild type. Their outer protein membranes are modified by biotechnology so that they can adsorb more toxic metals. They are more element-specific and they are also able to break the contaminants into metabolites. Bioremediation will be the future of pollution control, but it must make sure that it is completely non-toxic to the environment.
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