What is Phytoremediation? Advantages and Disadvantages of Phytoremediation Types of Phytoremediation The Science - How does Phytoremediation Work? Harvest and Disposal of Plants Biotechnology and Phytoremediation Current Research Areas in Phytoremediation Phytoremediation and Natural Resources Sustainable Development and Phytoremediation Bibliography

What is Phytoremediation?

Phytoremediation is the use of a plant's natural ability to contain, degrade, or remove toxic chemicals and pollutants from soil or water. It can be used to clean up metals, pesticides, solvents, explosives, crude oil, and contaminants that may leak from landfill sites (called leachates). The term phytoremediation is a combination of two words – phyto, which means plant, and remediation, which means to remedy.

Scientists are investigating phytoremediation's potential by using plants such as sunflower, ragweed, cabbage and geranium, as well as other less known species. The plants are often used in combination with other traditional technologies for cleaning up contaminated sites because of the phytoremediation's limitations (see below).

Advantages and Disadvantages of Phytoremediation

Advantages

• Environmentally friendly, cost-effective, and aesthetically pleasing.
• Metals absorbed by the plants may be extracted from harvested plant biomass and then recycled. Phytoremediation can be used to clean up a large variety of contaminants.
• May reduce the entry of contaminants into the environment by preventing their leakage into the groundwater systems.

Disadvantages

• Relies on natural cycle of plants and therefore takes time.
• Phytoremediation works best when the contamination is within reach of the plant roots, typically three to six feet underground for herbaceous plants and 10 to 15 feet for trees.
• Some plants absorb a lot of poisonous metals, making them a potential risk to the food chain if animals feed upon them.

Types of Phytoremediation

There are five basic types of phytoremediation, categorized by the type of material they remediate – metals or organic chemicals.

For metal contamination:

• Phytoextraction -- In this process, the plant absorbs metal contaminants through the roots and moves them into the upper portions of the plant (stems and the leaves). Hyperaccumulators are often used in this process for their enhanced capacity to extract contaminants from their surrounding environment. The plants are then harvested and discarded appropriately. It may also be possible to recover the original metals from the plants.
  
  
• Rhizofiltration -- This method applies specifically to surface and ground water remediation. In this process, contaminants are extracted either directly by absorption through the roots, or indirectly through root adsorption, meaning that contaminants are attracted to and held by the roots. Plants that have been grown in clean water are transplanted to the contaminated water site. When the roots become saturated with the contaminants, they are harvested and new ones are planted. The use of constructed wetlands to treat wastewater and landfill leachate are examples of rhizofiltration.
  
  
• Phytostabilization -- Plants are used to contain soil and water contaminants or reduce their mobility through the surrounding environment. This is accomplished by contaminant absorption or adsorption through the plant roots, or by decreasing soil erosion and wind-blown dust. For metals, which do not biodegrade, this is an effective method for preventing their spread to ground or surface water.

For organic chemicals:

• Phytotransformation -- In this process, the plant absorbs and breaks down organic chemicals in contaminated soil and groundwater through its metabolic processes.
  
  
• Rhizosphere Bioremediation (or Plant Assisted Bioremediation/Rhizodegradation) -- Plants stimulate the growth of microorganisms in the area around their roots (called the rhizosphere) through the release of natural substances. These microorganisms, such as yeast, fungi, and bacteria, degrade contaminants through their metabolic processes.

The Science – How does Phytoremediation Work?

Phytoremediation takes advantage of a plant's natural ability to absorb, accumulate, or metabolize contaminants from the soil or other media in which it grows. Interactions between these plants and microorganisms that live in the soil can also contribute to phytoremediation.

As part of normal growth and development, plants naturally absorb and metabolize certain organic chemicals and metals. Some metals are essential for plant growth, including zinc, copper, and iron. Some plants, classified as hyperaccumulators, absorb more metal than other plants, including metals that do not appear necessary for plant function.

Harvest and Disposal of Plants

When the plants have absorbed and accumulated contaminants, they can be harvested and discarded.

If organic chemical contaminants are degraded into molecules like water and carbon dioxide, the plants may not require any special method of disposal.

Controlled incineration is the most common method used to dispose plants that have absorbed large amounts of contamination. This process produces ashes, which can then be discarded at appropriate waste sites. For plants that have absorbed metals, controlled incineration produces ashes with a high metal content. Researchers are working on methods to recover the original metals from these ashes.

Since phytoremediation is a technology still in the early stages of development, many disposal and metal recovery methods are still being explored. These methods include sun-drying, composting, and leaching.

Biotechnology and Phytoremediation

Many different genes are suspected to be involved in regulating metal absorption in plants. These genes control the solubility of metals in the soil surrounding the roots, as well as the transport proteins that move metals into root cells and up into the plant's shoots.

Improved knowledge of the genes, molecular mechanisms, and roles microorganisms play in governing phytoremediation will allow scientists to better apply plants to the clean-up of contaminated sites.

Current Research Areas in Phytoremediation

Researchers are examining the exact mechanisms surrounding metal transport in plants, and why some plants can absorb and tolerate high amounts of toxic metals while others cannot. Identified genes are being cloned, and certain plants are being genetically modified to tolerate metal contamination. In some cases, plants are being genetically modified with bacterial genes. For example, researchers at the University of Georgia genetically modified yellow poplar trees with a gene from a mercury-resistant bacteria. In the lab, the trees thrived in mercury-heavy soils. The researchers are further studying the trees in a greenhouse setting.

Scientists are also studying biodiversity at metal-contaminated sites across Canada. This work may determine whether seed banks can be established from these wild species, and whether they can be successfully grown in greenhouses. This could eventually lead to the creation of an inventory of plants that could be used at other metal-contaminated sites.

Phytoremediation and Natural Resources

The use of phytoremediation technology is being considered in various industries, including mining and oil.

In 1997, workers at Saskatchewan Federal Co-operatives Limited planted 203 poplars on a test site near Kelvington in an abandoned gas station that contained underground petroleum pollution. Balsam poplars were chosen for the experiment because they were hardy, fast-growing and readily available in Saskatchewan. An adult poplar can take in up to 120 litres of water per day.

Their experiment proved that the trees could survive in areas with high levels of petroleum contamination. They will measure the amount of petroleum left in the soil and expect to see results in 10 years.

Sustainable Development and Phytoremediation

In order for a technology to be sustainable, it should be economically viable and environmentally compatible. Phytoremediation uses the existing capabilities of plants and the systems they support to clean up soil and water. It is more cost-effective than traditional remediation methods for contaminated soil, which involve digging up the entire contaminated area and taking it away to another location for chemical treatment, incineration, or burial. Phytoremediation takes less labour and does not disturb the natural surroundings of the contamination site. Although phytoremediation takes time, it is a good way to make use of naturally existing resources.

Bibliography

Belz, Kelly E. "Phytoremediation." Soil and groundwater pollution, Civil Engineering Dept. Virginia Tech. 1994. www.ce.vt.edu/program_areas...al/teach/gwprimer/phyto/phyto.html. 10 December 2001.

Currie, Bennie M. "Flower Power: Using Plants to Remove Contaminants." Brownsfield News, April 1999. www.bugsatwork.com/sswm/little.htm. 20 September 2001.

West, Bridget. "Phytoremediation: A Natural Approach for Toxic Clean Up." Information Technology Services, California Institute of Technology. www.its.caltech.edu/~sciwrite/2000-01/westb.htm. 10 December 2001.

Absorbing Possibilities: Phytoremediation. Innovations, volume 103, Number 12, December 1995. Environmental Health Information Services, National Health Institute ephnet1.niehs.hih.gov/docs/1995/103-12/innovations.html. 27 November 2001.

Plants Clean Up Contaminated Sites. Science and the Environment Bulletin. March/April 1999. Environment Canada www.ec.gc.ca/science/sandemar99/article1_e.html. 2 January 2002.

Phytoremediation: Using Plants to Clean Up Pollution. The AgBiotech Infosource, Issue 44 March 1999.

A Citizen's Guide to Phytoremediation. Technology Innovations Office, United States Environmental Protection Agency, August 1998 clu-in.org/download/citizens/citphyto.pdf

Schnoor, Jerald L. Phytoremediation. Technology Evaluation Report, GWRTAC Series, October 1997.

Black, Harvey. "Phytoremediation: A Growing Field with Some Concerns." The Scientist, March 1, 1999 www.the-scientist.com/yr1999/mar/black_p1_990301.html. 27 November 2001

Phytoremediation. Ecological Engineering Group, Sustainable Strategies. www.ecological-engineering.com/phytorem.html 20 September 2001.

Phytoremediation: Using Plants to Clean Up Soils. Agricultural Research Service, United States Department of Agriculture www.ars.usda.gov/is/ar/archives/jun00/soil0600.htm 10 December 2001.

Miller, Ralinda R. Phytoremediation. Technology Overview Report, GWRTAC Series, October 1996.

Engineering trees that can clean up. Warnell School of Forest Resources.