- Rhizofiltration
Rhizofiltration is a form of
bioremediation that involves filteringwater through a mass ofroot s to removetoxic substances or excessnutrient s.Overview
Rhizofiltration is a type of
phytoremediation , which refers to the approach of using hydroponically cultivated plant roots to remediate contaminated water through absorption, concentration, and precipitation ofpollutant s. This contaminated water is either collected from a waste site and brought to the plants, or the plants are planted in the contaminated area, where the roots then take up the water and the contaminants dissolved in it. Many plant species naturally uptakeheavy metals and excessnutrient s for a variety of reasons:sequestration ,drought resistance, disposal by leafabscission , interference with other plants, and defense againstpathogen s andherbivore s. [Boyd, R. S. Plants That Hyperaccumulate Heavy Metals. Ed. R. R. Brooks. Willingford, U.K.: CAB International, 1998. 181-201.] Some of these species are better than others and can accumulate extraordinary amounts of these contaminants. Identification of such plant species has led environmental researchers to realize the potential for using these plants for remediation of contaminated soil andwastewater .Process
This process is very similar to phytoextraction in that it removes contaminants by trapping them into harvestable plant
biomass . Both phytoextraction and rhizofiltration follow the same basic path to remediation. First, plants are put in contact with the contamination. They absorb contaminants through their root systems and store them in root biomass and/or transport them up into the stems and/or leaves. The plants continue to absorb contaminants until they are harvested. The plants are then replaced to continue the growth/harvest cycle until satisfactory levels of contaminant are achieved. Both processes are also aimed more toward concentrating and precipitating heavy metals than organic contaminants. The major difference between rhizofiltration and phytoextraction is that rhizofiltration is used for treatment in aquatic environments, while phytoextraction deals with soil remediation.Applications
Rhizofiltration may be applicable to the treatment of surface water and groundwater, industrial and residential effluents, downwashes from power lines, storm waters,
acid mine drainage , agricultural runoffs, diluted sludges, andradionuclide -contaminated solutions.Plants suitable for rhizofiltration applications can efficiently remove toxic metals from a solution using rapid-growth root systems. Variousterrestrial plant species have been found to effectively remove toxic metals such as Cu2+, Cd2+, Cr6+, Ni2+, Pb2+, and Zn2+ from aqueous solutions. [Dushenkov, Viatcheslav, Harry Motto, Ilya Raskin, and P.b.a. Nanda Kumar. "Rhizofiltration: the Use of Plants to Remove Heavy Metals From Aqueous Streams." Environmental Science Technology 30 (1995): 1239-1245.] It was also found that low levelradioactive contaminants can successfully be removed from liquid streams. [EPA, 1998, A Citizen's Guide to Phytoremediation, U.S. Environmental Protection Agency, Office of Solid Waste and Emergency Response, EPA 542-F-98-011, August.]Tree s have also been applied to remediation. Trees are the lowest cost plant type. They can grow on land of marginal quality and have long life-spans. This results in little or no maintenance costs. The most commonly used arewillow s andpoplar s, which can grow 6 - 8’ per year and have a high flood tolerance. For deep contamination, hybrid poplars with roots extending 30 feet deep have been used. Their roots penetrate microscopic scale pores in the soil matrix and can cycle 100 L of water per day per tree. These trees act almost like a pump and treat remediation system.Suthersan, Susan S. Remediation Engineering: Design Concepts. CRC P, 1997.]Cost
Rhizofiltration is cost-effective for large volumes of water having low concentrations of contaminants that are subjected to stringent standards. [Miller, R., 1996, Phytoremediation, Technology Overview Report, Ground-Water Remediatoin Technologies Analysis Center, Series O, Vol. 3, October.] It is relatively inexpensive, yet potentially more effective than comparable technologies. The removal of radionuclides from water using
sunflower s was estimated to cost between $2 and $6 per thousand gallons of water treated, including waste disposal and capital costs. [Cooney, C M. "Sunflowers Remove Radionuclides From Water in Ongoing Phytoremediation Field Tests." Environmental Science and Technology 30 (1996): 194.]Advantages
Rhizofiltration is a treatment method that may be conducted
in situ , with plants being grown directly in the contaminated water body. This allows for a relatively inexpensive procedure with low capital costs. Operation costs are also low but depend on the type of contaminant. This treatment method is also aesthetically pleasing and results in a decrease of water infiltration andleaching of contaminants.After harvesting, the crop may be converted to biofuel briquette, a substitute for fossil fuel.Modeling rhizofiltration: heavy-metal uptake by plant roots P. Verma*, K. V. George, H. V. Singh, S. K. Singh, A. Juwarkar and R. N. Singh National Environmental Engineering Research Institute (NEERI), Nehru Marg, Nagpur 440 020, India]Disadvantages
This treatment method has its limits. Any contaminant that is below the rooting depth will not be extracted. The plants used may not be able to grow in highly contaminated areas. Most importantly, it can take years to reach regulatory levels. This results in long-term maintenance. Also, most contaminated sites are polluted with many different kinds of contaminants. There can be a combination of metals and organics, in which treatment through rhizofiltration will not suffice.Plants grown on polluted water and soils become a potential threat to human and animal health, and therefore, careful attention must be paid to the harvesting process and only non-fodder crop should be chosen for the rhizofiltration remediation method.
ee also
*
Phytoremediation
*Hyperaccumulating Plants
*Biodegradation
*Bioremediation External links
* [http://www.hort.purdue.edu/hort/research/murphy/pdfs/metals11.pdf Phytoremediation and Hyperaccumulator Plants] Comprehensive overview.
* [http://www.ars.usda.gov/is/AR/archive/jun00/soil0600.htm Using Plants To Clean Up Soils] - from [http://www.ars.usda.gov/is/AR/ Agricultural Research magazine]
* [http://www.springerlink.com/content/l6lw05ru51884214/ Modeling rhizofiltration: heavy-metal uptake by plant roots] Lots of data.References
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