PHYTOREMEDIATION

OUTLINE Definition Importance of phytoremediation Mechanism behind phytoremediation process Post phase of phytoremediation Types of plants used Advantages and Limitations Role of genetics Conclusion References

DEFINITION• Phytoremediation is an emerging technology that uses various

plants to degrade, extract, contain, or immobilize contaminants from soil and water.

• The term phytoremediation (phyton = plant(in Greek) and remediare = correct evil(in Latin)) is relatively new, coined in 1991.

• Phytoremediation takes the advantage of the unique and selective uptake capabilities of plant root systems, together with the translocation, bioaccumulation, and contaminant degradation abilities of the entire plant body for the remediation process. This technology is environmental friendly and potentially cost effective.

ImportanceSince the beginning of the industrial pollution, contamination of the biosphere with toxic trace metals has further increased and become a world-wide public health concern. Heavy metal pollution has been “globalized” by large scale use, and atmospheric transport mechanisms.

Contamination by heavy metals is a critical contributor to soil and water quality degradation, as well as to human and animal health because they bio-accumulate in the food chain.

Classical heavy metal remediation comprises the removal of the contaminated soil followed by disposal on landfills or immobilization treatment of the contaminated soil. This is costly. Phytoremediation, on the other hand, is a cheaper technique that combines several methods to remediate soil contaminated with heavy metals, and other toxic pollutants.

MECHANISMS OF PHYTOREMEDIATION

• Phytoextraction• Phytostabilization• Phytotransformation• Phytostimulation• Phytovolatilization• Rhizofiltration

Overview

1.PHYTOEXTRACTIONDefinition : The uptake of contaminants by plant roots and

movement of these contaminants from roots to the

above part of plants - by absorbing, concentrating and

precipitating the contaminants.

1.PHYTOEXTRACTION (cont.)Two ways for Phytoextraction: Natural andassisted• Natural: where plants naturally takeup contaminants from the soil - unassisted• Assisted: use of chelating agents,microbes and plant hormones to mobilizeand accelerate contaminant uptake.

Works well on metals such as lead, cadmium, copper, nickel etc. Uptake of contaminants also acceleratedby use of hyper accumulators

e.g. Thlaspi caerulescens

1.PHYTOEXTRACTION (cont.)Advantages:

• Cost is fairly inexpensive compared to conventional methods.• Contaminant permanently removed from soil.• Amount of waste material that must be disposed of is decreased

up to 95%.• In some cases, contaminant can be recycled.

Limitations:

• Metal bioavailability within the rhizosphere.• Rate of metal uptake by roots.• Proportion of metal “fixed” within the roots.• Cellular tolerance to toxic metals.

2.phytostabilizationDefinition: Refers to the immobilization of

contaminants in the soil through:○ absorption and

accumulation by roots,○ precipitation within the

roots.

• Eventually, the mobility of the contaminant is reduced, migration to groundwater is prevented and thus bioavailability of metal into food chain is reduced.

• Phyto-stimulation is also called Rhizodegradation or plant-assisted bioremediation.

2.Phytostabilization (cont.)Advantages:

• No disposal of hazardous material / biomass is required.• Very effective when rapid immobilization is needed to preserve

ground and surface waters.

Disadvantages:

• Contaminant remain in soil.• Application of extensive fertilisation / soil amendments.• Mandatory monitoring required.

3.PhytotransformationDefinition: Also known as phytodegradation, it is the breakdown of contaminants taken up by plants by metabolic processes within the plant.

• Remediate some organic contaminants, such as chlorinated solvents, herbicides, and munitions.

• It can address contaminants in soil, sediment, or groundwater.

Advantages:

• Both economically and environmentally friendly.

Disadvantages:

• Requires more than one growing season to be efficient.

• Soil must be less than 3 ft., in depth and groundwater within 10 ft., of the surface.

• Contaminants may still re-enter the food chain through animals or insects that eat plant material.

3.Phytotransformation (cont.)

4.PhytostimulationDefinition: Breakdown of contaminants within the plant root zone, or rhizosphere.

• Carried out by bacteria or other microorganisms flourishing in the rhizosphere.

• Microbes in rhizosphere transform contaminant to non toxic product.

• Works well in the removal of petroleum hydrocarbons.

Advantages:

• in situ practice resulting in no disturbance.• No removal of contaminated materials.• Complete mineralisation of the contaminant can occur.• Low installation and maintenance cost.

Disadvantages:

• Development of extensive root zone required- takes time• Root depth limited due to physical structure of soil• Organic matter from plant may be used as a C source

instead of contaminant -> decrease amount of contaminant biodegradation

4.Phytostimulation (cont.)

5.Phytovolatilization

Definition: Involves plants taking up contaminants from soil, transforming them into volatile forms and transpiring them into atmosphere.

• Works on organic compounds and heavy metal contaminants, TCE as well.

• Mercury is the primary metal contaminant that this process has been used for.

5.Phytovolatilization (cont.)Advantage:

• The contaminant, mercuric ion, may be transformed into a less toxic substance (i.e., elemental Hg).

Disadvantage:

• The mercury released into the atmosphere is likely to be recycled by precipitation and then re-deposited back into lakes and oceans, repeating the production of methyl-mercury by anaerobic bacteria.

6.RhizofiltrationDefinition: Adsorption or precipitation onto plant roots or absorption of contaminants in the solution surrounding the root zone.

• Used to remediate extracted groundwater, surface water, and waste water with low contaminants.

• Compared to Phytoextraction, here the plants are used to address the groundwater rather than soil.

Advantages:

• Ability to use both terrestrial and aquatic plants for either in situ and ex situ applications.

• Contaminants do not have to be translocated into shoots.

Disadvantages:

• Constant need to adjust pH.• Plants may first need to be grown in greenhouse / nursery.• There is periodic harvesting and plant disposal.• Tank design should be well engineered.

6.Rhizofiltration (cont.)

Post- phase of phytoremediation• After the plant grows by phytoremediation, the shoot of the plant

containing the contaminants is harvested to recover the metal.

EXTRACTION OF METALS FROM SHOOT OF THE PLANT

Types of plants used1. Indian mustard (Brassica juncea

L.) : Brassicaceae species are really useful to accumulate certain metals while producing high quantities of biomass in the process, and Indian mustard is the star of this group. It can remove three times more Cd than others, reduce 28% of Pb, up to 48% of Se, and it is effective against Zn, Hg and Cu as well. However, what is unknown is that Indian mustard removed radioactive Cs137 from Chernobyl.

Types of plants used(CONT.)2. Willow (Salix species). (White Willow):Their roots have demonstrated viability, accumulating lower levels of heavy metals than Brassicaceae, and they deal with Cd, Ni and Pb, and work even in mixed heavy metals like diesel fuel polluted sites.

3. Poplar tree (Populus deltoides). (Populus deltoides W. Bartram ex Marshall eastern cottonwood) : Their secret lies in the naturally well-designed root system which take up large quantities of water.

4. Indian grass (Sorghastrum nutans) (Sorghastrum nutans (L.) Nash) : It has power to detoxify common agro-chemical residues such as well-known pesticides and herbicides related to atrazine and metalochlor. This is capable to remediate petroleum hydrocarbons. The list includes other grasses like Common buffalo grass or Western wheatgrass, leading the ranking.5. Sunflower (Helianthus Annuus L.) (Helianthus annuus L. common sunflower) : sunflowers reduce different PAH level from soil, in an effective way, but what is really surprising is how varied range of contaminants they can accumulate.Heavy metals such as Pb, Zn, N, P, K, Cd, Cu or Mn, seem to be its food.

Types of plants used(CONT.)

Types of plants used(CONT.)

Advantages• In situ and passive technique• Uses solar energy and is low cost• Has reduced environmental impact and contributes to the

landscape improvement• High acceptance by the public• Provides habitat for animal life• Reduction in dispersal of dust and contaminants by wind• Reduction of surface runoff• Reduction of leaching and mobilization of contaminants in soil• Harvesting of the plants or organs that have accumulated metals is

easy to accomplish with existing technology• The harvested biomass can be economically valuable• Plant process more easily controlled than those of microorganisms

Limitations• Limited to shallow soils or where contamination is localized to the

surface (< 5 m)• Still under development and therefore not accepted by many regulatory

agencies• There is little knowledge of farming, genetics, reproduction and diseases

of phytoremediating plants• Metal concentrations in the soil can be toxic and lethal to plants• Generally, plants are selective in metal remediation• Treatment slower than the traditional physico-chemical techniques• Contamination may spread through the food chain if accumulator plants

are ingested by animals• Efficient phytoremediating plants may not adapt to climatic and

environmental conditions at contaminated sites• If the plants release compounds to increase the mobility of the metals,

these can be leached into groundwater• The area to be decontaminated must be large enough to allow

application of cultivation techniques

Genetic engineering is a powerful method for enhancing natural phytoremediation capabilities, or for introducing new capabilities into plants Possibly, the most spectacular application of biotechnology for environmental restoration has been the bioengineering of plants capable of volatilizing mercury from soil contaminated with methyl-mercury.

Methyl-mercury, a strong neurotoxic agents, is biosynthesized in Hg contaminated soils. To detoxify this toxin, transgenic plants (Arabidopsis and tobacco) were engineered to express bacterial genes merB and merA. In these modified plants, merB catalyzes the protonolysis of the carbon-mercury bond with the generation of Hg2+, a less mobile mercury species. Subsequently, MerA converts Hg(II) to Hg (0) a less toxic, volatile element which is released into the atmosphere .

THE ROLE OF GENETICS

•Phytoremediation is an emerging technology that employs the use of higher plants for the cleanup of contaminated environments.

• The main advantage of Phytoextraction is environmental friendliness. Traditional methods which are used for cleaning up heavy metal contaminated soil disrupt soil structure and reduce soil productivity, whereas Phytoextraction can clean up the soil without causing any kind of harm to soil quality.

• Another benefit of Phytoextraction is that it is less expensive than any other clean up process and the possibility of the recovery and re-use of valuable metals .

• The possibility of using biotechnology to improve the efficiency of phytoremediation processes makes even better than any other existing methods.

CONCLUSION

REFERENCES1. Environmental Science & Technology. 1998. Phytoremediation; forecasting. Environmental Science & Technology. Vol. 32, issue 17, p.399A.2. Etim, E.E. (2012). Phytoremediation and its mechanisms: A review. International Journal of Environmental and Bioenergy 2(3), 120 - 136. http://modernscientificpress.com/Journals/ViewArticle.aspx? gkN1Z6Pb60HNQPymfPQlZIsaO1oMajYkT5i8/SIthV/i1509l3XqlgX4XSDiXBec [Date acccessed: 09/03/2014]3. Phytoremediation ppt on slideshare http://www.igece. org/WRKY/BrachyWRKY/WRKY/IMG/Rhizofiltration.jpg [Date accessed: 12/03/14]4. Online site of landarchs. http://landarchs.com/5-best-plants-for-phytoremediation/

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