Phytoremediation - A Green Solution to Pollution
Plants may be the raw material of the future for everything from fibers to fuels, but in the meantime they're busy mending the mistakes of the past.
Indian mustard, poplar trees and sunflowers are the active agents in a new technology being used to clean up land and water contaminated with everything from lead to petroleum hydrocarbons and pesticides.
It's the flower power of the '90s: technically known as phytoremediation. For quite some time scientists have been aware of plants' ability to absorb, store or degrade contaminants found in their environments. Advances in research during the last decade caught the attention of regulators and owners of contaminated sites and a handful of phytoremediation companies have sprung up in response.
Results from field studies are impressive and the economics seem favorable, but it remains to be seen whether or not phytoremediation companies will take a chunk of the $12 billion and growing annual market for cleaning up polluted sites. Attempts to commercialize the technology began only in the last few years. To date, much of the phytoremediation work has consisted of demonstration projects, though each company says it is ready for or has already begun full-scale commercial applications.
How It Works
The term phytoremediation actually encompasses a number of processes. Phytoextraction refers to the ability of some plants to absorb soil contaminants and accumulate them in their tissue. This process is most commonly used to remove heavy metals. Rhizofiltration is fundamentally the same process applied to contaminated water rather than soil. It may involve the precipitation of compounds and often works in a matter of days or hours. Through phytodegradation, organic contaminants may be taken up and metabolized into less toxic or even harmless substances. Plants also stimulate microbial degradation in the soil surrounding their roots. Finally, plants may stabilize or sequester hazardous substances or be used as natural "pumps" to remove large volumes of contaminated groundwater.
Contaminants tackled by phytoremediation in field studies include heavy metals, radionuclides, chlorinated solvents, petroleum hydrocarbons, polychlorinated biphenyls (PCBs), pesticides and explosives.
How It Measures Up
Phytoremediation is significantly cheaper than the conventional clean-up method of soil excavation followed by incineration, chemical treatment or landfilling. Estimates vary widely depending on the type and concentration of contaminants, as well as the soil or water conditions, but phytoremediation is generally one-half to one-fifth as expensive as traditional techniques. Excavation and incineration of metal-contaminated soil, for instance, costs between $200 and $1,500 per ton (depending on the location), while phytoremediation ranges from $25 to $100 per ton.
Other advantages include greatly reduced environmental disturbance and applicability to very large sites. Phytoremediation can also be a permanent solution; many organic contaminants are degraded into inert substances rather than simply being transferred in their toxic state to a landfill.
Phytoremediation may require several growing seasons as well as several different types of plants. Soil remediation is limited to the depth of the plant's roots (about three feet). Other potential problems include the possibility that animals may feed on the contaminated plants, affecting the entire food chain. Although the plants selected for phytoremediation are typically not wildlife food sources, not enough evidence exists to lay this concern to rest.
Other research questions remain as well. In the case of a small number of contaminants, such as trichloroethylene (TCE), a portion of the pollutant is released directly into the atmosphere through transpiration. How much exits the plant in this way, as opposed to being metabolized or degraded, has not been quantified.
Phytoremediation companies have two things in common: they were all founded by university scientists and they are all very young. Each company has specialized, developing expertise in particular contaminants or plant processes. To market their services most rely on partnerships with environmental engineering and consulting firms.
Founded in 1993, Phytotech Inc. (Monmouth Junction, NJ), expects 1998 to be its first year of full-scale commercial operations. Phytotech specializes in the removal of metals and radionuclides. The company's field projects made headlines twice in 1996. At a pond near Chernobyl, sunflowers removed 95% of the pond's radioactive cesium and strontium within ten days. At a Department of Energy (DOE) facility in Ohio sunflowers reduced uranium concentrations in water from 350 parts per billion (ppb) to well below the federal safety limit of 20 ppb.
Phytotech has also developed expertise in remediation of toxic metals, with an emphasis on lead. After extensive screening the company selected Indian mustard plants (Brassica juncea), which can remove as much as 2000 kg of lead per hectare in a few months. While all plants absorb nutrients, including metals, through their roots, phytoextraction relies on naturally occurring or artificially improved varieties of hyperaccumulators, which have the ability to absorb large amounts of metals, even those not required by the plant.
Other criteria for metal accumulators include rapid growth and sufficient biomass for metal storage, as well as the ability to withstand high levels of toxicity. Absorption of some metals, including lead, require chelating agents, iron oxides or other soil amendments to facilitate solubility and plant uptake.
The harvested plant matter must be disposed of as hazardous waste, though it weighs only a small fraction of the hazardous material generated by excavation. The metal-rich plant tissue, however, offers an intriguing alternative. If combusted it produces ash containing as much as 25-65% metal. This raises the possibility of "mining" and recycling the metal, an option Phytotech is currently considering at a battery manufacturing site.
Phytotech's current and potential clients include railroad and oil companies, metal-platers, battery manufacturers, smelters, mining operations and the Department of Defense (DOD).
Another company, Phytokinetics Inc. (Logan, Utah), has focused on organic chemical contaminants in soils and groundwater. The company is using perennial ryegrass at a Portland, Oregon, demonstration site contaminated with pentachlorophenol, a wood preservative. Another grass, tall fescue, is tackling total petroleum hydrocarbon at a former petroleum terminal in Ogden, Utah.
A significant aspect of phytoremediation of organic contaminants occurs in the rhizosphere, or root zone, of the plant, which contains 10-100 times more microorganisms than unplanted soil. Certain microbes metabolize contaminants, a process that is significantly aided by enzymes secreted by particular plants. Compounds degraded in this way include petroleum hydrocarbons, chlorinated pesticides, chlorinated solvents and explosives.
Poplar trees and grasses are being used by Ecolotree, Inc. (Iowa City, Iowa) to filter water and sequester contaminants in municipal or industrial wastewater streams. The company has also constructed a number of landfill caps, which rely on poplar trees' ability to consume large quantities of water, thereby preventing contaminants from seeping offsite.
Other phytoremediation companies include Applied Natural Sciences (Hamilton, Ohio), which uses poplar and willow trees to address pesticides, nitrate fertilizers and TCE in groundwater. The most recent entrant into the field, Phytoworks, Inc. (Gladwyne, PA), is engaged in identifying and isolating plant enzymes capable of degrading contaminants such as PCBs, TNT and TCE or detoxifying metals such as mercury.
According to Charles Plummer of the Department of Agriculture, the U.S. is facing a $400-750 billion bill to clean up existing hazardous waste sites with conventional technologies, generating intense interest in developing cheaper alternatives. To this end, much of the funding for phytoremediation to date has come from field studies commissioned by the DOE and DOD, which have both publicly committed to cleaning up their polluted lands, and the EPA, through its Superfund Innovative Technology Evaluation (SITE) program.
These field tests provide mounting evidence of phytoremediation's effectiveness, a necessary step toward building commercial credibility and acceptance among regulators and the public.
It is too early to determine if this young industry will develop into a viable solution to polluted land and water. The next couple of years should reveal a great deal, and perhaps one day brownfields and other contaminated land will beckon to us as fields of opportunity rather than as scars of the past and impediments to the future.
from The Carbohydrate Economy
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