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EPRI Sows Seeds of Plant-Based Waste-Cleaning Systems

Some plants naturally absorb and accumulate trace metals such as chromium, selenium, nickel, mercury, lead, copper, and arsenic. In a process known as phytoremediation, scientists are harnessing the plants' ability to remove these substances to help cleanse contaminated soils and water. A recent project directed by the Electric Power Research Institute (EPRI) has measured the capacity of various plants for removing toxic substances from polluted water.

Because plants differ in the extent to which they can accumulate the different elements, judicious selection of appropriate plants can greatly enhance the removal of the desired substance. Selection is based on the type of substance to be removed, the geographic location, environmental conditions, and the known capabilities of the species. Therefore it is important to understand the capabilities of the specific plants under varying conditions.

In the EPRI-directed study, researchers collected plant species from wetlands and nurseries, grew them hydroponically (in liquid nutrients but no soil), and identified the plant species that removed the most toxic substances without harming themselves.

Some of the findings were that:
  • Water hyacinth, duckweed, brass buttons, cattail, saltmarsh bulrush, parrot's feather, iris-leaved rush, and smartweed were excellent candidates for remediating wastewater contaminated with trace elements such as manganese, cadmium, copper, nickel, chromium, lead, mercury, boron, arsenic, and selenium. Of the 12 plants tested, smartweed proved the most effective.
  • Water hyacinth, an aquatic floating plant that is easily harvested, most efficiently accumulated cadmium and chromium. Brass buttons, a wetland plant, also proved an excellent choice for the remediation of chromium-contaminated water.
  • Of the 20 aquatic plant species screened, saltmarsh bulrush, parrot's feather, and iris-leaved rush were identified as the best candidates for removing selenium.

    John Goodrich-Mahoney, manager in EPRI's water and ecosystems research program, explains, "The use of hydroponics enabled the investigators to directly compare different plant species under fixed conditions. Plant species that were highly efficient in removing specific trace elements under controlled environmental conditions will likely prove effective for trace element removal under field conditions."

    Conventional chemical treatment techniques for metal-bearing industrial discharges are resource-intensive, requiring chemical additives, human supervision, and regular maintenance. Hazardous by-products, like chemical sludges, can be generated, resulting in potentially high disposal costs. And these systems may not even be able to reduce contaminant concentrations to the levels called for by water quality criteria. "Passive treatment systems containing specially selected plants, by contrast, require significantly less management, says Goodrich-Mahoney. "They eliminate the transport of treatment chemicals on the highways, there are no undesirable by-products, and the fuel and manpower requirements are absolutely minimal."

    EPRI is directing a comprehensive program to develop design and engineering guidelines for remediating metal-containing industrial waste streams using plants. The program includes demonstration projects, such as the Allegheny Power's award-winning Springdale constructed-wetland installation, field and laboratory research to increase understanding of treatment processes, and experimental work to accelerate or otherwise enhance removal of individual chemicals. Though EPRI's tests to-date have focused on power plant effluents, the technology could potentially have applicability for metal-bearing streams from other sources.

    Says Goodrich-Mahoney, "EPRI's program is introducing new dimensions to passive treatment technology. Efforts to optimize these processes are on the leading edge of microbiology, plant physiology, genetic engineering, and other disciplines."

    The future of this work will involve genetic engineering to develop enhanced plants for phytoremediation. Genetically-engineered plants will have greater capacity to absorb metals and can also survive in waters or soil that are highly contaminated.

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