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Phytovolatilization

Phytovolatilization is defined as the technique by which the plants transform/volatilize the contaminants into volatile forms such as mercury- or arsenic-containing compounds and transpire them into the atmosphere (USEPA 2000). In this process, the plants take up toxic contaminants in water soluble form, which get translocated from the roots to the leaves along with water through plant’s vascular system. The plant would then convert them to less toxic forms and release them into the atmosphere through plant transpiration mechanism along with water. This technique is relevant in the remediation of soils contaminated with metalloids like Se. Hg and As. Among these metalloids, phytovolatilization of Se has been given the most attention to date (Тепу et al. 1992, McGrath 1998), as this element is a serious problem in many parts of the world where there are areas of Se-rich soil (Brooks 1998). The release of volatile Se compounds (Dimethyl selenide) from higher plants was first reported by Lewis et al. (1966). The members of Brassicaceae (Cmciferae) are capable of releasing up to 40 g Se ha-1 day1 as various gaseous compounds (Тепу et al. 1992) and some aquatic plants, such as cattail (Typhalatifolio L.), are also good for Se phytoremediation (Pilon-Smits et al. 1999). Volatile Se compounds, such as dimethyl selenide, are 1/600 to 1/500 as toxic as inorganic fonns of Se found in the soil (De Souza et al. 2000). Similarly, the mercury ion is transformed into less toxic elemental mercury and lost to the atmosphere. There has been a considerable effort in recent years to insert bacterial Hg ion reductase genes into plants for the purpose of Hg phytovolatilization (Heaton et al. 1998, Rugli et al. 1998, Bizily et al. 1999). Unlike plants that are being used for Se volatilization, those which volatilize Hg are genetically modified organisms. Arabidopsis thaliana L. and tobacco (Nicotiancitabacum L.) have been genetically modified with bacterial organomercuriallyase (MerB) and mercuric reductase (MerA) genes. These plants absorb elemental Hg(II) and methyl mercury (MeHg) from the soil and release volatile Hg(O) from the leaves into the atmosphere (Heaton et al. 1998). The phytovolatilization of Se and Hg into the atmosphere has several advantages. The volatilization of Se and Hg is a permanent solution for the contaminated site because the inorganic forms of these elements are removed and the gaseous species are also not likely to be redeposited at or near the site (Atkinson et al. 1990). Furthermore, sites that utilize this technology may not require much management after the original planting. Hence, this remediation method has the added benefits of minimal site disturbance, less erosion, and no need to dispose of contaminated plant material (Rugli et al. 2000). Unlike other remediation techniques, once contaminants have been removed via volatilization, there is a loss of control over their migration to other areas. However, its use is limited by the fact that it does not remove the pollutant completely; only it is transferred from one segment (soil) to another (atmosphere) from where it can be redeposited. Hence, phytovolatilization technique would not be a better option for remediation of contaminated sites near population centers or at places with unique meteorological conditions which promote the rapid deposition of volatile compounds (Heaton et al. 1998, Rugh et al.

2000). Therefore, this technique is the most controversial of all the phytoremediation technologies (Padmavathiarnma and Li 2007). Despite the controversy surrounding phytovolatilization, this technique is a promising tool for the remediation of Se and Hg contaminated soils and also for the removal of organic contaminants.

Phytostabilization

Phytostabilization, also known as phytorestoration, is another subset of phytoremediation technology wliich aims at reducing the mobility of contaminants within the vadose zone through accumulation by roots or immobilization within the rhizospliere by establishing a plant cover on the surface of the contaminated sites (Bolan et al. 2011). It creates a vegetative cap for the long-term stabilization and containment of the contaminant. The plant canopy serves to reduce eolian dispersion, whereas plant roots prevent water erosion, immobilize metals by adsorption or accumulation, and provide a rhizospliere wherein metals precipitate and stabilize. The mobility of contaminants is reduced by the accumulation of contaminants by plant roots, absorption onto roots, or precipitation within the root zone. The immobilization of metals will be accomplished by decreasing wind-blown dirt, minimizing soil erosion, and reducing contaminant solubility or bioavailability to the food chain. It provides hydraulic control, which suppresses the vertical migration of contaminants into the groundwater, and physically and chemically immobilizes contaminants by root sorption and by chemical fixation with various soil amendments (Berti and Cunningham 2000, Flathman and Lanza 1998, Salt 1995, Schnoor 2000). This technique of phytostabilization is particularly attractive when other methods to clean areas are not feasible. Unlike phytoextraction, or hyperaccumulation of metals into shoot/root tissues (Ernst 2005), phytostabilization primarily focitses on sequestration of the metals within the rhizospliere but not in plant tissues. Consequently, metals become less bioavailable and livestock, wildlife, and human exposure is reduced (Cunningham et al. 1995, Wong

2003). However, since the contaminants are left in place, the site requires regular monitoring to ensure that the optimal stabilizing conditions are maintained.

Plants suitable for phytostabilization

Plants chosen for phytostabilization ideally should have the following characteristics: they must be metallophytes (metal-tolerant plants) but excluders, i.e., the metal tolerant plants that do not accumulate or limit metal accumulation to root tissues. The lack of appreciable metals in shoot tissue also eliminates the necessity of treating harvested shoot residue as hazardous waste (Flatlmian and Lanza 1998). Also, the plants should be native to the area, have high production of root biomass with the ability to immobilize contaminants and the ability to hold contaminants in the roots, should have the ability to tolerate soil conditions, then establishment and maintenance under field conditions should be easy and should have the ability to self-propagate, and they should have rapid growth to provide adequate ground coverage. Ideally, the plants that are suitable for phytostabilization should have bioconcentration factor (BCF) or accumulation factor (AF) (total element concentration in shoot tissue/total element concentration in mine tailings) and translocation factor (TF) or shoofiroot (S:R) ratio (total element concentration in shoot tissue/total element concentration in the root tissue) should be < 1 (Brooks 1998). In addition to the metal accumulation ratios, several metal concentration guidelines, like soil plant toxicity levels, which can provide a guide in evaluating metal tolerance and the plant leaf tissue toxicity limits, which can help assess the long-term potential for plant establishment (Munshower 1994, Mulvey and Elliott 2000, Kataba-Pendias and Pendias

2001), can be used to evaluate metal toxicity issues that may arise during phytostabilization.

Phytostabilization is useful at sites with shallow contamination and where contamination is relatively low. The efficiency of phytostabilization depends on the plant and soil amendment used. Enzymes and proteins secreted by plant roots into adjacent soil results in immobilization and precipitation of the contaminants in soil or on root surface. Phytostabilization can be enhanced by using soil amendments that are effective in the immobilization of metal(loid)s, but may need to be periodically reapplied to maintain their effectiveness. Soil amendments used in phytostabilization techniques favors inactivate heavy metals, which prevents plant metal uptake (Marques et al. 2009). The best soil amendments are those that are easy to handle, safe to workers who apply them, easy to produce, inexpensive and most importantly are not toxic to plants (Marques et al. 2008). Most of the times, organic amendments are used because of their low cost and the other benefits they provide such as provision of nutrients for plant growth and improvement of soil physical properties (Marques et al. 2009). Marques et al. (2008) showed that Zn percolation through the soil reduced by 80% after application of manure or compost to polluted soils on which Solanwnmgrum was grown. Other amendments that can be used for phytostabilization include phosphates, lime, biosolids, and litter (Adriano et al. 2004). Plants that accumulate heavy metals in the roots and in the root zone typically are effective at depths of up to 24 niches (Blaylock et al. 1995). It does not produce secondary waste that needs treatment and is best suited at sites having fine-textured soils with high organic matter content (Berti and Cunningham 2000). However, phytostabilization is not a permanent solution because the heavy metals remain in the soil; only their movement is limited. Actually, it is a management strategy for stabilizing (inactivating) potentially toxic contaminants (Vangronsveld et al. 2009). Though phytoremediation has shown promising results as an innovative clean-up technology, it is still at an infant stage. Intensive pilot scale research work is needed to manage post-harvest stages of this remediation technology.

 
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