Bioremediation is the use of microorganisms to remove pollutants from contaminated areas of land, water or air. Phytoremediation is a form of bioremediation in which plants are used in addition to, or instead of, microorganisms.
Superfund, an environmental program created by the Environmental Protection Agency (EPA) to address abandoned hazardous waste sites, was established by the Comprehensive Environmental Response, Compensation and Liability Act of 1980 (CERCLA). This legislation allows the EPA to clean up these sites and to compel responsible parties, such as various industries and corporations, to perform cleanups or to reimburse the government for cleanups performed by the EPA.
A variety of remediation methods are used to clean up these, and other, contaminated sites. For soil remediation, there are biological, chemical and physical treatments of various kinds, as well as immobilization and thermal methods. For groundwater, there is air-stripping, free-radical reactions, incineration, and specific contaminant removal methods for contaminants such as mercury. A comprehensive assessment is done at each Superfund site to determine what kinds of remediation efforts will be most effective in cleaning up a given site.
Phytoremediation, which is a relatively new technology, has been tested nationwide at over 200 Superfund sites. The term “phytoremediation” itself is also a somewhat new one, coined in 1991. The most favorable sites for the use of phytoremediation are those in which there are low concentrations of contaminants over large areas, and at shallow depths, because its results are limited to the surface area and depth occupied by the roots of the chosen plant.
Phytoremediation projects can be classified according to the fate of the contaminant: i.e, degradation, extraction, containment, hydraulic control, or a combination of these. They can also be classified based upon the mechanisms involved, such as volatilization of contaminants from plants to the air, control of runoff, erosion, or infiltration by vegetative covers, or others.
It is believed that some plants may be able to take in toxic compounds and in trying to utilize the available nutrients, the plants actually detoxify the compounds. This seems to work best with more volatile compounds. The EPA has several sites in which poplars are being used, such as the Oregon Poplar site, located in Clackamas, Oregon. The contaminants at the site were VOCs (Volatile Organic Compounds), likely found there because of illegal dumping activities. The trees were planted on the site in 1998 to remediate the contaminated ground water. Later, tissue samples taken from four trees indicated that the trees were actively removing VOCs from the groundwater and soil. Poplars, chosen once again for their rapid growth and high transpiration rates, were also used at a munitions disposal site known as the J-Field located near the Aberdeen Proving Ground in Harford County, Maryland. 183 poplars were planted over an area of about an acre in 1996. The VOCs that were to be removed have been detected in the tree’s leaves as well as in gas and water samples expelled by the trees. Further investigation is ongoing, but it has been estimated that within 30 years, contaminants at J-Field may be reduced up to 85%.
This process, also known as phytomining, is often used when the contaminants are toxic metals. It is done by planting a crop of a species that is known to accumulate contaminants in the shoots and leaves of the plants. Then, both the crop and the contaminants are harvested, and must be transported for disposal or recycling. Since the contaminants are concentrated during the process, it has the advantage of leaving a much smaller mass of contaminants to be disposed of than does the more traditional excavation and landfilling. Extraction is also sometimes called “biomining” and can be done to mine valuable heavy metals from contaminated soil, with the goal of obtaining the metal’s ores to be used or sold for profit.
Containment and Immobilization
Using containment in phytoremediation involves binding the contaminants to the soil, or within the plant’s roots. Vegetative cover can also be used as a living cap to prevent leaching of contaminants from a site, such as a landfill, into the nearby soil or groundwater. This plant cover is also more esthetically pleasing than the more standard plastic or clay landfill cap.
The amazing water uptake capacity of plants can be used to decontaminate subsurface groundwater. When a dense network of roots is created by planting near the water table, the plants can act as natural hydraulic pumps, and will decrease the migration of contaminants from surface water into the groundwater and drinking water supplies. The EPA is conducting research in this area at several sites, including the SITE (Superfund Innovative Technology Evaluation Program) projects at Ogden, Utah and Ft. Worth, Texas.
Like all technologies, the use of phytoremediation has both advantages and disadvantages. One of the key elements to its successful use is a thorough and knowledgeable evaluation of the site to be remediated. Because it is a relatively new technology, there is not yet much hard data on the total costs of the various ongoing projects at this time. However, intelligent cost estimates have been made by various researchers including David Glass (1998), who has estimated that total costs for some phytoremediation applications will be 50% to 80% lower than alternatives. Table 2-4 presented in this link compares example costs of phytoremediation treatments with conventional treatments. Of course, the cost will depend upon the type of contaminant being remediated and the matrix in which it is found.
But phytoremediation does have other advantages besides simple cost savings. It can be used to treat sites with more than one type of pollutant. It is far less disruptive to the environment than more traditional excavation methods, and is more esthetically pleasing. There is much less of a need for disposal sites for most phytoremediation projects. While it may take more time than traditional methods, it is, in many ways, a better long-term solution, because it utilizes the natural properties of plants which can be grown in order to become a useful, beautiful and nurturing part of the environment for years to come.