The natural power of certain plants to absorb toxins is proving to be an effective and inexpensive way to purify polluted soil and water. Some plants store toxins in their tissues while others simply hold contaminated soil in place, reducing the dispersal of pollutants into the atmosphere or to other sites.

The process known as phytoremediation is persuading scientists that plants can help solve one of society’s most vexing environmental problems. What is phytoremediation? Phyto: to bring forth using plants Remediation: to correct a problem. Put together, the words describe an emerging technology using green plants to clean up a contaminated environment.

As phytoremediation has been increasingly recognized, the technology has been applied both in situ (right where the contamination has taken place) and ex situ (by excavating contaminated soil, sediment, sludge, groundwater, surface water, or wastewater and then remedying it).

Scientists do not know exactly how phytoremediation works but the results of over a decade of research are convincing:

• Cattails and bulrushes in a wetland can remove 70% of the selenium from wastewater.

• Ferns growing on a contaminated site contain arsenic levels 200 times higher than the soil where they are growing.

• Sunflowers accumulate levels of radioactive substances several thousand times higher than the concentration in the water.

History

Although it is now increasingly being applied for environmental mediation, phytoremediation is not a new technology. Roman civilization reportedly used eucalyptus trees to de-water saturated soils more than two thousand years ago. Phytoremediation for environmental cleanup began in the late 1970s and early ‘80s, and application of this technology increased dramatically in the late 1980s and early ‘90s. Phytoremediation was first publicized as an environmental cleanup technology for agricultural contaminants such as pesticides and excess plant nutrients such as nitrate, ammonia, and phosphate, although the principles of phytoremediation have been applied in the wastewater industry for many years. This remedial technology has been used in hundreds of cases worldwide. It is low-cost and versatile, and in some cases it has better public support than other methods for cleaning up a contaminated site.

Uses

The U.S. Environmental Protection Agency has identified six broad applications of phytoremediation. Some of these include translocating the contamination to the roots, trunk, or limbs to be more easily dealt with. Other methods bring the contaminant to those same areas to be degraded by the plant, while still other applications contain the contaminant within the immediate groundwater instead of letting it flow off site. Another use is as a self-containing vegetation cap over a landfill.

Phytoremediation has been successful where growers once drained their leftover irrigation water into evaporation ponds to prevent minerals from flowing into rivers or soaking into the aquifer. In place of evaporation ponds, researchers planted hybrid poplars along the edges of a field of crops. The trees take up excess irrigation water that would have drained into the ponds, along with the salts, boron, and selenium in the water. Whether the trees eventually sequester the salts in their root systems or throughout their structure is still being studied.

Currently Used Vegetation

As the technology expands, more types of plants are being used for phytoremediation for both organic and inorganic contaminants. Early efforts utilized hybrid poplar because it is a fast-growing, water loving tree. Its physiologic and genetic characteristics are well-known from its use in the pulp and paper industry and from biomass fuel research. Now grasses and other trees are also being used in phytoremediation. Well over 400 different plants are known to have the ability to remove harmful products from affected areas.

Hybrid Poplars

Poplars offer the most promising phytoremediation possibilities.

• Arbuscular mycorrhizae attached to the roots of cloned poplar can be selected for phytoremediation of soils contaminated with heavy metals.

• Genetically altered cottonwood trees suck mercury from contaminated soil. Some of the mercury is expected to vaporize into the air while most is stored in the tree. After several years of growth, the trees can be cut down and incinerated.

• New trichloroethylene tests, found improved rates of uptake from solutions of chloroform, the byproduct of disinfecting drinking water; carbon tetrachloride, a solvent; and vinyl chloride, a substance used to make plastics. The poplars produce the enzymes that breaks down trichloroethylene, C₂HCl₃, into chloride ions and recombine the carbon and hydrogen with oxygen to produce water and carbon dioxide.

• Other species of cottonwood capture chlorinated solvents.

• Biodegradation of nitro-substituted explosives occurs with a phytosymbiotic Methylobacterium species associated with poplar tissues (Populus deltoides x nigra DN34).

Other Examples

The following is a short list of many other plants used in phytoremediation:

• The Chinese Ladder fern Pteris vittata, also known as the brake fern, is a highly efficient accumulator of arsenic. P. vittata grows rapidly and can absorb up to 2% of its weight in arsenic. It can extract arsenic from soil even where the level is low, for example 6 ppm, which is normal for many soils. When grown on soil with 100 ppm arsenic, not only did it absorb more arsenic, but it grew 40% larger than normal.

• Houseplants such as spider plants can effectively remove benzene, formaldehyde, CO, and nitrogen oxides (undesirable products of burning tobacco, carpets, and wood) from indoor office air.

• Researchers are using transgenic Indian mustard plants to soak up dangerously high selenium deposits.

• Researchers are engineering trees to retain more carbon and thus combat global warming.

• Alfalfa absorbs petroleum from contaminated soils and cleans the soils with the addition of oxygen.

• Willow trees clean up petroleum-contaminated ground water.

• Mercury-binding peptides have been found in Chromolaena odorata, a fast-growing perennial shrub native to South America.

• Aquatic and wetland plant species such as duckweed are used for phytoremediation of explosives-contaminated groundwater.

• Certain grasses take up heavy metals.

• Indian mustard captures lead.

Benefits

Until recently, decontamination of toxic waste sites required digging up the polluted soil and hauling to landfills equipped to handle the contamination, or sometimes chemicals were applied to neutralize the pollutant. These treatments are expensive and disruptive to the ecosystem and the neighborhood. Plants offer a more appealing option.

• Plants are inexpensive, solar powered, attractive, and environmentally friendly.

• Advantages of phytoremediation include its low capital cost, which is generally about one-third to one-fifth that of more conventional technologies. In addition, phytoremediation tends to have low costs for ongoing operation and maintenance, although it should not be construed as maintenance-free.

• Phytoremediation is attractive for non-point-source contamination, such as nitrates and pesticides in agricultural settings and parking lot runoff in urban areas.

• Some plant species can also reduce the net infiltration of surface water, which minimizes the potential for leaching of contaminants into groundwater.

Scientists at the University of Washington say that genetically engineered poplar plants being grown in a laboratory were able to take as much as 91% of trichloroethylene out of a liquid solution. Unaltered plants removed 3%. The poplar trees were also able to break down the pollutant into harmless byproducts at rates 100 times that of the control plants.

Phytoremediation has been accepted by the public, since it is environmentally compatible and can improve the long-term aesthetics of a site. Phytoremediation can be used as a single-treatment technology, or it can be coupled with more aggressive conventional technologies. For example, contaminated soils from a site can be excavated and treated in engineered phytoremediation treatment units, rather than thermally treated or taken off-site and disposed of in a landfill. Contaminated groundwater can also be pumped from a site using conventional methods and used as irrigation for trees or grasses to capture the contaminants.

Limitations

Despite the benefits of phytoremediation, there are some disadvantages to the technology that makes it unsuitable or undesirable for some applications. Phytoremediation is a long-term remedial technology at most sites, with treatment times on the order of several years. In addition, the technology can be directly implemented only where the contaminants are present at depths within about 20 feet of the land surface. If vegetation is used for the purpose of extracting groundwater, the contaminants must be located within a few feet of the water table surface.

Plants have adapted to grow in some of the most inhospitable conditions known to exist. However, phytoremediation will not be successful if soil conditions or contaminant characteristics or concentrations prove to be phytotoxic. Phytoremediation of metals poses special considerations that can make its use impracticable at times. For example, the consequences of transferring contamination from soil or groundwater into plants that can enter the food chain must be considered, particularly for heavy metals such as lead and cadmium which have detrimental effects on human health.

Using phytoremediation often hasn’t made sense given the timetables required by regulatory agencies at remediation sites. Commercial use of poplars require federal regulatory approval and monitoring, and regulations are becoming increasingly strict for transgenic plants intended for biopharmaceutical or industrial purposes, including phytoremediation. Federal regulations do not allow the commercial growing of transgenic trees. Poplars are fast growing and can grow for several years without flowering, at which time they could be harvested to prevent seeds from generating. Unlike some other kinds of trees, branches of the hybrid poplar being studied do not take root in soils when branches fall to the ground.

Engineering Considerations

Phytoremediation is an active approach to site cleanup. Although it utilizes natural processes, a successful phytoremediation system must be similar in certain ways to conventional methods. Arborists must:

• fully understand the geology of the site and contaminants within the area.

• clearly define remedial objectives. Are they quantitative, to reduce to a specific level of contamination such as can be defined by parts per million, or are they more qualitative, such as establishing a vegetative cover to reduce windblown dust?

• establish a time frame for remediation. Since the technology is new, specific cleanup time frames have not been fully established, so flexibility must be built into the system.

• know the fate of contaminants within the plant system. A well-designed laboratory potting study or greenhouse experiment with soil from the site is needed to document this.

• know the species of vegetation and planting techniques. Appropriate choices will largely define whether a phytoremediation project is a success or failure. Site constraints often result in selection of sub-optimal species.

• expect operation and maintenance. A common pitfall is the misconception that phytoremediation is maintenance-free. Such assumptions have led to the failure of numerous projects. Maintenance requirements include watering, insecticide applications, and dealing with natural predation by wildlife.

It is important to choose plants suitable to the site conditions. Although some plants grow quickly, most take time to be effective. Phytoremediation is not an overnight solution to pollution.

Future of Phytoremediation

Because of the trend toward more passive remedial technologies and the recognition of the importance of natural processes, phytoremediation holds promise in the overall remediation marketplace. There will be growth in three areas:

1. the types of vegetation used for phytoremediation

2. expanded applications of phytoremediation and integration of phytoremediation with natural processes

3. engineered systems, architectural design, and site planning

No one thinks plants will replace mechanical digging and pumping to make polluted sites safe. Phytoremediation is not the complete answer. More research is required to determine long-term effects, how toxins are absorbed from the soil, and whether they evaporate into the atmosphere, causing potential harm. Is phytoremediation the next frontier in forestry?

References

• Briggs, G.G., R.H. Bromilow, A.A. Bromilow, ”Relationships between Lipophilicity and Root Uptake and Translocation of Non-ionized Chemicals”, Pesticide Science (13):495–503, 1982.

• Carman, Eric P. and Tom L. Crossman, “Phytoremediation”, In Situ Treatment Technology, 2001.

• Cross, John, “Phytoremediation”, Missouri Botanical Garden, 2007.

• Phillips, Len, Editor , “Phytoremediation”, City Trees, The Journal of The Society of Municipal Arborists, Vol. 39, No. 2, March/April 2003.

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