Phytoremediation—Using Plants to Clean up Polluted Soil
(Conservation Currents, Northern Virginia Soil and Water Conservation District, Feb 2004)
Polluted soil poses a severe problem for both ecosystem health and land development. Because soil lies at the confluence of many natural systems, soil pollution can be spread to other parts of the natural environment. Groundwater, for instance, percolates through the soil and can carry the soil pollutants into streams, rivers, wells and drinking water. Erosion can create the same problem. Plants growing on polluted soil may contain harmful levels of pollutants themselves, and this can be passed on to the animals and people that eat them. Dust blown from polluted soil can be inhaled directly by passersby. Additionally, in an urban setting such as Fairfax County, polluted soil makes valuable open land unusable for parks, recreation or commercial development.
Despite the benefits of cleaning polluted soil, remediation often never takes place because of the cost and effort of the work. Both soil minerals and soil pollutants carry small electric charges that can cause each to bond with each other, thus making polluted soil very hard to clean. Additionally, soil is a dense medium. This causes excavation of polluted soil for off site treatment or disposal to be very expensive because of the time, labor and heavy machinery necessary to do the job. Therefore, cheaper on-site, or in-situ, remediation techniques have been the focus of much attention and research lately. One of the most interesting and promising of these in-situ techniques is phytoremediation.
Phytoremediation is the use of specialized plants to clean up polluted soil. While most plants exposed to high levels of soil toxins will be injured or die, scientists have discovered that certain plants are resistant, and an even smaller group actually thrive. Both groups of plants are of interest to researchers, but the thriving plants show a particular potential for remediation because it has been shown that some of them actually transport and accumulate extremely high levels of soil pollutants within their bodies. They are therefore aptly named hyper-accumulators.
Hyper-accumulators already are being used throughout the country to help clean up heavy metal polluted soil. Heavy metals are some of the most stubborn soil pollutants. They can bond very tightly to soil particles, and they cannot be broken down by microbial processes. Most heavy metals are also essential plant nutrients, so plants have the ability to take up the metals and transport them throughout their bodies. However, on polluted soil, the levels of heavy metals are often hundreds of times greater than normal, and this overexposure is toxic to the vast majority of plants. Hyper-accumulators, on the other hand, actually prefer these high concentrations. Essentially, hyper-accumulators are acting as natural vacuum cleaners, sucking pollutants out of the soil and depositing them in their above ground leaves and shoots. Removing the metals is as simple as pruning or cutting the hyper-accumulators above ground mass, not excavating tons of soil.
Resistant, but not hyper-accumulating, plants also have a role in phytoremediation. Organic toxins, those that contain carbon such as the hydrocarbons found in gasoline and other fuels, can be broken down by microbial processes. Plants play a key role in determining the size and health of soil microbial populations. All plant roots secrete organic materials that can be used as food for microbes, and this creates a healthier, larger, more diverse and active microbial population, which in turn causes a faster breakdown of pollutants. Resistant plants can thrive on sites that are often too toxic for other plants to grow. They in turn give the microbial processes the boost they need to remove organic pollution more quickly from the soil.
Both forms of phytoremediation have the added benefit of not disturbing the soil. While excavation is an effective way to get rid of pollution, it removes the organic matter rich topsoil and, because of the use of heavy machinery, compacts the soil that is left behind. Phytoremediation does not degrade the physical or chemical health of the soil. Actually, it creates a more fertile soil. Soil organic matter is increased as a result of root secretions and falling stems and leaves, and the roots create pores through which water and oxygen can flow. Additionally, few would argue that a dusty excavation site is more aesthetically pleasing than a nicely planted field.
There are, however, many limitations to phytoremediation. It is a slow process that may take many growing seasons before an adequate reduction of pollution is seen, whereas soil excavation and treatment cleans up the site quickly. Also, hyper-accumulators can be a pollution hazard themselves. For instance, animals can eat the metal rich hyper-accumulators and cause the toxins to enter the food chain. If the concentration of metals in the plants is thought to be high enough to cause toxicity, there must be a way to segregate the plants from humans and wildlife, which may not be an easy task. Additionally, phytoremediation is in its infancy, and its effectiveness in cleaning up various toxins compared to conventional means of treatment is not always known. However, with more research and practice, the practicality of using phytoremediation should increase.
The growing popularity of phytoremediation can be seen in the number of businesses beginning to appear in urban areas of the United States that are involved in the field. In New Jersey, a company named Phytotech is using mustard greens to remove lead from polluted yards in Boston and hydroponically grown sunflowers to take radioactive metals out of the water surrounding the Chernobyl nuclear power plant. Right here in Fairfax County, Edenspace™ Systems Corp. of Chantilly has more than two dozen current and future contracts to utilize phytoremediation in the field, and they even sell an arsenic accumulating Edenfern® that can be planted in your backyard today.