Phytoremediation is the use of living green plants for in situ risk reduction and/or removal of contaminants from contaminated soil, water, sediments, and air. Specially selected or engineered plants are used in the process. Risk reduction can be through a process of removal, degradation of, or containment of a contaminant or a combination of any of these factors. Phytoremediation is an energy efficient, aesthically pleasing method of remediating sites with low to moderate levels of contamination and it can be used in conjuction with other more traditional remedial methods as a finishing step to the remedial process.

One of the main advantages of phytoremediation is that of its relatively low cost compared to other remedial methods such as excavation. The cost of phytoremediation has been estimated as $25 - $100 per ton of soil, and $0.60 - $6.00 per 1000 gallons of polluted water with remediation of organics being cheaoer than remediation of metals. In many cases phytoremediation has been found to be less than half the price of alternative methods. Phytoremediation also offers a permanent in situ remediation rather than simply translocating the problem. However phytoremediation is not without its faults, it is a process which is dependent on the depth of the roots and the tolerance of the plant to the contaminant. Exposure of animals to plants which act as hyperaccumulators can also be a concern to environmentalists as herbivorous animals may accumulate contaminate particles in their tissues which could in turn affect a whole food web.

How Does It Work?

Phytoremediation is actually a genneric term for several ways in which plants can be used to clean up contaminated soils and water. Plants may break down or degrade organic pollutants, or remove and stabilize metal contaminants. This may be done through one of or a combination of the methods described in the next chapter. The methods used to phytoremediate metal contaminants are slightly different to those used to remediate sites polluted with organic contaminants.

Metal	Organic
Phytoextraction	Phytodegradation
Rhizofiltration	Rhizodegradation
Phytostabilisation	Phytovolatilisation

Methods of Phytoremediation

Phytoremediation of metal contaminated sites

Phytoextraction (Phytoaccumulation)

Phytoextraction is the name given to the process where plant roots uptake metal contaminants from the soil and translocate them to their above soil tissues. As different plant have different abilities to uptake and withstand high levels of pollutants many different plants may be used. This is of particular importance on sites that have been polluted with more than one type of metal contaminant. Hyperaccumulator plant species (species which absorb higher amounts of pollutants than most other species) are used on may sites due to their tolerance of relatively extreme levels of pollution.
Once the plants have grown and absorbed the metal pollutants they are harvested and disposed of safely. This process is repeated several times to reduce contamination to acceptable levels. In some cases it is possible to recycle the metals through a process known as phytomining, though this is usually reserved for use with precious metals. Metal compounds that have been successfully phytoextracted include zinc, copper, and nickel, but there is promising research being completed on lead and chromium absorbing plants.

Rhizofiltration

Rhizofiltration is similar in concept to Phytoextraction but is concerned with the remediation of contaminated groundwater rather than the remediation of polluted soils. The contaminants are either adsorbed onto the root surface or are absorbed by the plant roots. Plants used for rhizoliltration are not planted directly in situ but are acclimated to the pollutant first. Plants are hydroponically grown in clean water rather than soil, until a large root system has developed. Once a large root system is in place the water supply is substituted for a polluted water supply to acclimatise the plant. Afer the plants become acclimatised they are planted in the polluted area where the roots uptake the polluted water and the contaminants along with it. As the roots become saturated they are harvested and disposed of safely. Repeated treatments of the site can reduce pollution to suitable levels as was exemplified in Chernobyl where sunflowers were grown in radioactively contaminated pools.

Phytostabilisation

Phytostabilisation is the use of certain plants to immobilise soil and water contaminants. Contaminant are absorbed and accumulated by roots, adsorbed onto the roots, or precipitated in the rhizosphere. This reduces or even prevents the mobility of the contaminants preventing migration into the groundwater or air, and also reduces the bioavailibility of the contaminant thus preventing spread through the food chain. This technique can alos be used to re-establish a plant community on sites that have been denuded due to the high levels of metal contamination. Once a community of tolerant species has been established the potential for wind erosion (and thus spread of the pollutant) is reduced and leaching of the soil contaminants is also reduced.

Phytoremediation of organic polluted sites

Phytodegradation (Phytotransformation)

Phytodegradation is the degradation or breakdown of organic contaminants by internal and external metabolic processes driven by the plant. Ex planta metabolic processes hydrolyse organic compounds into smaller units that can be absorbed by the plant. Some contaminants can be absorbed by the plant and are then broken down by plant enzymes. These smaller pollutant molecules may then be used as metabolites by the plant as it grows, thus becoming incorporated into the plant tissues. Plant enzymes have been identified that breakdown ammunition wastes, chlorinated solvents such as TCE (Trichloroethane), and others which degrade organic herbicides.

Rhizodegradation

Rhizodegradation (also called enhanced rhizosphere biodegradation, phytostimulation, and plant assisted bioremediation) is the breakdown of organic contaminants in the soil by soil dwelling microbes which is enhanced by the rhizosphere's presence. Certain soil dwelling microbes digest organic pollutants such as fuels and solvents, producing harmless pproducts through a process known as Bioremediation. Plant root exudates such as sugars, alcohols, and organic acids act as carbohydrate sources for the soil microflora and enhance microbial growth and activity. Some of these compound may also act as chemotactic signals for certain microbes. The plant roots also loosen the soil and transport water to the rhizosphere thus additionaly enhancing microbial activity.

Phytovolatilization

Phytovolatilization is the process where plants uptake contaminaints which are water soluble and release them into the atmosphere as they transpire the water. The contaminant may become modified along the way, as the water travels along the plant's vascular system from the roots to the leaves, whereby the contaminants evaporate or volatilize into the air surrounding the plant. There are varying degrees of success with plants as phytovolatilizers with one study showing poplar trees to volatilize up to 90% of the TCE they absorb.

Hydraulic control of Pollutants

Hydraulic control is the term given to the use of plants to control the migration of subsurface water through the rapid upltake of large volumes of water by the plants. The plants are effectively acting as natural hydraulic pumps which when a dense root network has been established near the water table can transpire up to 300 gallons of water per day. This fact has been utilised to decrease the migration of contaminants from surface water into the groundwater (below the water table) and drinking water supplies. There are two such uses for plants:

Riparian corridors

Riparian corridors and buffer strips are the applications of many aspects of phytoremediation along the banks of a river or the edges of groundwater plumes. Pytodegradation, phytovolatilization, and rhizodegradation are used to control the spread of contaminants and to remediate polluted sites. Riparian strips refer to these uses along the banks of rivers and streams, whereas buffer strips are the use of such applications along the perimeter of landfills.

Vegetative cover

Vegetative cover is the name given to the use of plants as a cover or cap growing over landfill sites. The standard caps for such sites are usually plastic or clay. Plants used in this manner are not only more aesthically pleasing they may also help to control erosion, leaching of contaminants, and may also help to degrade the underlying landfill.

Where has Phytoremediation Been Used?
As it is a relatively new technology phytoremediation is still mostly in it's testing stages and as such has not been used in many places as a full scale application. However it has bee tested successfully in many places around the world for many different contaminants. This table shows the extent of testing across some sites in the USA

Location	Application	Pollutant	Medium	plant(s)
Ogden, UT	Phytoextraction & Rhizodegradation	Petroleum & Hydrocarbons	Soil & Groundwater	Alfalfa, poplar, juniper, fescue
Anderson, ST	Phytostabilisation	Heavy Metals	Soil	Hybrid poplar, grasses
Ashtabula, OH	Rhizofiltration	Radionuclides	Groundwater	Sunflowers
Upton, NY	Phytoextraction	Radionuclides	Soil	Indian mustard, cabbage
Milan, TN	Phytodegradation	Expolsives waste	Groundwater	Duckweed, parrotfeather
Amana, IA	Riparian corridor, phytodegradation	Nitrates	Groundwater	Hybrid poplar

Pro's & Con's of Phytoremediation
As with most new technologies phytoremediation has many pro's and cons. When compared to other more traditional methods of environmental remediation it becomes clearer what the detailed advantages and disadvantages actually are.

Advantages of phytoremediation compared to classical remediation

It is more economically viable using the same tools and supplies as agriculture
It is less disruptive to the environment and does not involve waiting for new plant communities to recolonise the site
Disposal sites are not needed
It is more likely to be accepted by the public as it is more aesthetically pleasing then traditonal methods
It avoids excavation and transport of polluted media thus reducing the risk of spreading the contamination
It has the potential to treat sites polluted with more than one type of pollutant

Disadvantages of phytoremediation compared to classical remediation

It is dependant on the growing conditions required by the plant (ie climate, geology, altitude, temperature)
Large scale operations require access to agricultural equpment and knowledge
Success is dependant on the tolerance of the plant to the pollutant
Contaminants collected in senescing tissues may be released back into the environment in autumn
Contaminants may be collected in woody tissues used as fuel
Time taken to remediate sites far exceeds that of other technologies
Contaminant solubility may be increased leading to greater environmental damage and the possibility of leaching

The low cost of phytoremediation (up to 1000 times cheaper than excavation and reburial) is the main advantage of phytoremediation, however many of the pro's and cons of phytoremediation applications depend greatly on the location of the polluted site, the contaminants in question, and the application of phytoremediation.
Phytoremediation & Biotechnology
The first goal in phytoremediation is to find a plant species which is resistant to or tolerates a particular contaminant with a view to maximising it's potential for phytoremediation. Resistant plants are usually located growing on soils with underlying metal ores or on the boundary of polluted sites. Once a tolerant plant species has been selected traditional breeding methods are used to optimize the tolerance of a species to a particular contaminant. Agricultural methods such as the application of fertilisers, chelators, and pH adjusters can be utilised to further improve the potential for phytoremediation.
Genetic modification offers a new hope for phytoremediation as GM approaches can be used to overexpress the enzymes involved in the existing plant metabolic pathways or to introduce new pathways into plants. Richard Meagher and colleagues introduced a new pathway into Arabidopsis to detoxify methylmercury, a common form of environmental pollutant to elemental mercury which can be volatilised by the plant.

The genes originated in gram-negative bacteria
MerB encodes a protein organomercurial lyase converts methylmercury to ionic mercury
MerA encodes mercuric reductase, which reduces ionic mercury to the elemental form
Arabidopsis plants were transformed with either MerA or MerB coupled with a consitutive 35S promoter
The MerA plants were more tolerant to ionic mercury, volatilised elemental mercury, and were unaffected in their tolerance of methylmercury
The MerB Plants were significantly more tolerant to methylmercury and other organomercurials and could also convert mthylmercury to ionic mercury which is approximately 100 times less toxic to plants
MerA MerB double transgenics were produced in an F2 generation. These plants not only showed a greater resistance to organic mercury when compared to the MerA, MerB, and wildtype plants but also capable of volatilising mercury when supplied with methylmercury.
The same MerA/MerB inserts have been used in other plant species including tobacco(Nicotiana tabacum), yellow poplar(Liriodendron tulipifera).
Wetland species (bulrush and cat-tail) and water tolerant trees (willow and poplar) have also been targetted for transformation.

Risk Assessment
The use of phytoremediation in the field is subject to many environmental concerns, especially in the light of the recent pulic hysteria about the release of GM crops into the environment. Even if non GM strains of plants are used there are still many concerns:

It is unknown what ecological effects hyperaccumulator plants may have if ingested by animals
Fallout from senescing tissues in autumn may also re-enter the food chain
Do volatilized contaminants remain at 'safe' levels in the atmosphere
Exposure of the ecosystem to contaminants is prolonged as phytoremediation is a relatively slow process

However there are other issues that affect the risk assesment for the use of transgenic organisms as phytoremediators. Not only do such organisms have the same risks as witld type remediators but they also have the same risks as releasing any GM organism into the field have:

The potential genetic pollution of native species
Potential for the gene to recombine with other genes possibly leading to the hyperaccumulation of non-contaminant compounds
Reporter/marker genes may also escape into the environment
The GM plants may revert to a wild type genotype

Glossary
This section is intended as a basic glossary for some of the more technical terms used in these pages. For a more comprehensive glossary check out the links section

ADAPTATION: Changes in an organism or population through which they become more suited for living in the current environment

BIOACCUMULATION: The intracellular accumulation of environmental pollutants by a living oragnism

BIOAVAILABILITY: The availability of chemicals to degradative microorganisms

BIODEGRADATION: The breakdown of organic substances by microorganisms

BIOREMEDIATION: The process whereby living organisms degrade or convert organic contaminants

BIOTRANSFORMATION: The metabolic alteration to the chemical structure of a compound by a living organism or enzyme

ORGANIC PUMP: The uptake of vast quantities of water by plant roots whereby the water is transpired by the plant into the atmosphere.

PHYTODEGRADATION: The process where plants are able to metabolically degrade organic pollutants

PHYTOEXTRACTION: The use of plants to extract contaminants from the environment

PHYTOMINING: Use of plants to extract metal compounds of high economic value

PHYTOREMEDIATION: Use of plants to remediate polluted soil and/or groundwater

PHYTOSTABILISATION: Use of plants to reduce bioavailability and migration of contaminants

PHYOTVOLATILISATION: The use of plants to volatilise pollutants from polluted soils and water

RHIZOFILTRATION: The uptake of contaminants by the roots of plants which are immersed in water

RHIZOSPHERE: The soil area immediatley surrounding the plant root surface. Typically up to a few millimetres from the root surface

Useful Links These are some of the useful websites I used when creating this site:

Sizzling Hot Jalfrezi: A useful site to self teach HTML with a really good tutorials section

Hexadecimal Colour Conversions: This site allows you to calculate the hexadecimal code for almost any colour

A Citizens guide to phytoremediation: The american EPA's guide to phtoremediation

The Bioremediation and Phytoremediation Glossary: A useful glossary relating to Bio- and phytoremediation