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Bioremediation for Sustainable Environmental Cleanup

9.5.2 Immobilization or Phytostabilization

Immobilization refers to the vegetation’s ability to maintain contaminated soils and sediments

in place, as well as the immobilization of harmful pollutants in soils (Mukhopadhyay and Maiti

2010). In-situ inactivation or phyto immobilization are other terms for phytostabilization. Sorption,

precipitation, complexation and metal valence reduction are strategies that can help plants to

stabilize metals (Ghosh and Singh 2005). An in-situ remediation process is a suitable option for

metal remediation (Jadia and Fulekar 2009). Root-zone microbiology and chemistry, as well as

changes in the soil environment or contaminant chemistry, all, contribute to phytostabilization. Plant

root exudates or CO2 generation can modify the pH of the soil, that has a bearing on metal ion

transport. Phytostabilization can affect metal solubility and mobility, as well as organic compound

dissociation. Metals can be converted into an insoluble oxidation state from a soluble state in a

plant-affected soil environment (Salt et al. 1995). Plants can also help to prevent metal-contaminated

soil from eroding. Plants having elevated transpiration rates, like forage plants, grasses, reeds and

sedges, can help with phytostabilization that could be used for metal remediation. The approach of

utilizing trees, such as the densely rooted and perennial, in combination may be a good mix (Berti

and Cunningham 2000).

9.5.3 Phytovolatilization

Toxic metals, including mercury, selenium and arsenic, are capable of being biomethylated to

produce volatile substances that are discharged into the atmosphere. Phytovolatilization is the

mechanism which is involved in the reduction of pollutants via the transpiration of plants. The plant

absorbs the contaminant that is present in the water, passes through it, or undergoes transformation

there, then is released into the atmosphere. Water passes through the plant’s internal transport

mechanism circulating from roots up to the leaves, where the inorganics get evaporated or volatilized

and consequently released in the air enclosing the plant, potentially modifying the contaminant.

Using phytovolatilization and phytoextraction to remove metals from commercial projects is a

realistic option (Sakakibara et al. 2010). Tritium (3H), a radioactive isotope of hydrogen, has been

successfully phytovolatilized; it decomposes into a stable form of helium, having a half-life of about

12 yr. HMs can be absorbed by several plants, including Arabidopsis thaliana, Chara canescens and

Brassica juncea, converting them into their gaseous forms within plants, and then releasing them

back into the environment (Ghosh and Singh 2005). Dimethylselenides and dimethyldiselenides

are produced by plants (i.e., Brassica juncea and Arabidopsis thaliana), which are the volatile

forms of volatile Se when grown on a high Se medium. Similarly, data from a study on heavy

metal volatilization revealed that P. vittata is quite efficient at volatilizing Arsenic (As), as had been

documented by its removal by almost 90% of total intake from As-affected soils in a greenhouse

with subtropical conditions (Sakakibara et al. 2010). In contrast to the other ways of cleanup,

after toxins have been removed via volatilization, they cannot be stopped from spreading to other

areas. Similar occurrences of soil remediation based on volatilization have been recorded in many

other publications (Tangahu et al. 2011, Conesa et al. 2012). Although it is well recognized that

microbes dispense a significant function in the Se volatilization from soil systems (Karlson and

Frankenberger 1989), it was investigated that plants can fulfil the same job. B. juncea has been

recognized as a useful source for extracting Se from soils (Bauelos and Meek 1990, Baualos et al.

1993). The Se volatilization into methyl selenates has been hypothesized as a dominant mechanism

for plant Se elimination (Zayed and Terry 1994, Terry et al. 1992). Non-volatile methyl selenate

derivatives accumulate in the leaf of some plants, allowing them to extract Se from the soil. In the

Se accumulator Astragalus bisculatus, the enzyme that serves in producing methyl selenocysteine

was isolated and described (Neuhierl and Bock 1996).