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Definition

Phytoremediation is best applied at sites with shallow contamination by organic, nutrient, or metal pollutants. Phytoremediation is well-suited for use at very large field sites where other methods of remediation are not cost-effective or practicable; at sites with low concentrations of contaminants where only "polishing treatment" is required over long periods of time; and in conjunction with other technologies where vegetation is used as a final cap and closure of the site. There are limitations to the technology that need to be considered carefully before it is selected for site remediation. These include: limited regulatory acceptance, long duration of time sometimes required for clean-up to below action levels, potential contamination of the vegetation and food chain, and difficulty establishing and maintaining vegetation at some toxic waste sites. This detailed report discusses the current status of phytoremediation to treat soils and ground water. Several field demonstration summaries are presented, with such information as: participants, compounds treated, site characteristics, results, and contacts.

An emerging technology for cleaning contaminated soils and shallow ground water is phytoremediation, an environmentally friendly, low-cost, and low-tech process. Phytoremediation encompasses all plant-influenced biological, chemical, and physical processes that aid in the uptake, degradation, and metabolism of contaminants by either plants or free-living organisms in the plant's rhizosphere. Aphytoremediation system can be viewed as a biological, solar-driven, pump-and-treat system with an extensive, self-extending uptake network (the root system) that enhances the soil and below-ground ecosystem for subsequent productive use.

Mechanisms of Phytoremediation

Plants and bacteria can form specific associations in which the plant provides the bacteria with a specific carbon source that induces the bacteria to reduce the toxicity of the contaminated soil. Alternatively, plants and bacteria can form nonspecific associations in which normal plant processes stimulate the microbial community, which in the course of normal metabolic activity degrades contaminants in soil. Plants can provide carbon substrates and nutrients, as well as increase contaminant solubility. These biochemical mechanisms increase the degradative activity of bacteria associated with plant roots. In return, bacteria can augment the degradative capacity of plants or reduce the toxicity of the contaminated soil.

During phytoremediation, PAHs that are resistant to degradation may adsorb to the surfaces of plant roots, making the roots an important sink for specific PAHs. Tall fescue and alfalfa were grown in a greenhouse under controlled conditions, and roots were harvested at three growth stages: vegetative, flowering, and mature. Naphthalene adsorption to the various plant roots was then evaluated. Results show that the mass of naphthalene volatilized was the largest component of the mass balance (32-45%). The mass in solution was usually greater than that adsorbed to the roots. The affinity of naphthalene for alfalfa roots was greater thanthat for tall fescue roots, but fescue roots were present in much greater quantities in the soil compared with alfalfa. Naphthalene adsorption on the roots of both plant species increased with plant age.

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