Rhizofiltration
Background
Rhizofiltration refers to the use of plant roots to absorb, concentrate, and precipitate toxic metals from contaminated groundwater. Initially, suitable plants with stable root systems are supplied with contaminated water to acclimate the plants. These plants are then transferred to the contaminated site to collect the contaminants, and once the roots are saturated, they are harvested. Rhizofiltration allows in-situ treatment, minimizing disturbance to the environment.
Applicability
A suitable plant for rhizofiltration applications can remove toxic metals from solution over an extended period of time with its rapid-growth root system. Various plant species have been found to effectively remove toxic metals such as Cu (2+), Cd (2+), Cr (6+), Ni (2+), Pb (2+), and Zn (2+) from aqueous solutions [1]. Low level radioactive contaminants also can be removed from liquid streams [2].
Limitations
Rhizofiltration is particularly effective in applications where low concentrations and large volumes of water are involved. Plants that are efficient at translocating metals to the shoots should not be used for rhizofiltration because more contaminated plant residue is produced [3].
Performance
One study involving pilot-scale research of rhizofiltration found that roots of sunflower (helianthus annuus L.) reduced levels of Pb, Cu, Zn, Ni, Sr, Cd, U(VI), Mn, and Cr(VI) to concentrations near or below regulated discharge limits within 24 hours [3].Results of field investigations are summarized in Table 1.
Table 1. Performance data of rhizofiltration field investigations [4].
Site/Media
Contaminants
Plants
Performance
Location
pond near nuclear disaster (demonstration)
137Cs, 90Sr
Helianthus annuus
90% reduction in 2 weeks; roots concentrated 8,000 fold
Chernobyl, Ukraine
USDOE energy wastes (demonstration)
U
Helianthus annuus
95% removal in 24 hours; from 350 ppb to <5 ppb
Ashtabula, OH
landfill leachate
U, nitrate
Brassica juncea
NA (SITE program underway)
Rocky Flats, CO
USDOE: U.S. Department of Energy
NA: not available
SITE: Superfund Innovative Technology Evaluations (by EPA)
It has been reported that certain plants used in rhizofiltration have the ability to remove up to 60% of their dry weight as toxic metals [4].
Data Requirements
Depth of contamination, types of heavy metal present, and level of contamination must be determined and monitored. Vegetation should be aquatic, emergent, or submergent plants. Hydraulic detention time and sorption by the plant roots must be considered for a successful design [4].
Cost
Rhizofiltration is cost-effective for large volumes of water having low concentrations of contaminants and subjected to low (stringent) standards [5]. It is relatively inexpensive, yet potentially more effective than comparable technologies.The removal of radionuclides from water using sunflowers was estimated to cost between $2 and $6 per thousand gallons of water treated, including waste disposal and capital costs [6].
Status of Technology
Experiments concerning rhizofiltration are ongoing at a DOE facility in Ohio. Rhizofiltration using sunflowers are used in the remediation of radionuclides (strontium and cesium) from surface water near Chernobyl [5]. Rhizofiltration also has been demonstrated at a DOE facility in Ohio.
References
1. Dushenkov, V, P.B.A.N. Kumar, H. Motto, and I. Raskin, 1995, Rhizofiltration: The Use of Plants to Remove Heavy Metals from Aqueous Streams, Environmental Science and Technology, 29 (5), pp. 1239-1245.2. 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.
3. Salt, D.E., M. Blaylock, N.P.B.A. Kumar, V. Dushenkov, B.D. Ensley, I.Chet, and I. Raskin, 1995, Phytoremediation: A novel Strategy for the Removal of Toxic Metals from the Environment Using Plants, Biotechnology, 13 (5), pp. 468-474.
4. Schnoor, J.L., 1997, Phytoremediation, Technology Overview Report, Ground-Water Remediation Technologies Analysis Center, Series E, Vol. 1, October.
5. Miller, R., 1996, Phytoremediation, Technology Overview Report, Ground-Water Remediatoin Technologies Analysis Center, Series O, Vol. 3, October.
6. Cooney, C.M., 1996, Sunflowers Remove Radionuclides from Water in Ongoing
Phytoremediation Field Tests, Environmental Science and Technology, 30 (5), pp.194A.