Phytoextraction
Background
The use of plants to remove contaminants from the environment and concentrate them in above-ground plant tissue is known as phytoextraction. Research and development efforts focus on two areas of study: (1) remediation of contaminants such as Pb, As, Cr, Hg, and radionuclides and (2) mining, or recovery, of inorganic compounds, mainly Ni and Cu, having intrinsic economic value. Phytoextraction can be used in both water and soil environments.
Applicability
Phytoextraction was primarily employed to recover heavy metals from soils, however, this technology is now applicable to other materials in different media. Greenhouse-based hydroponic systems using plants with high contaminant root uptake and poor translocation to the shoots are currently being researched for removal of heavy metals and radionuclides from water [1]. These plants also are referred to as hyperaccumulators. Plants with high growth rates (>3 tons dry matter/hectare-year) and the ability to tolerate high metal concentrations in harvestable parts of the plants (>1,000 mg/kg) are needed for practicable treatment [2].It has been reported that Cd, Ni, Zn, As, Se, and Cu are readily bioavailable metals. Co, Mn, and Fe are considered moderately bioavailable metals. Pb, Cr, and U are not very bioavailable, although the addition of ethylenediaminetetraacetic acid, or EDTA, to soil (0.5 to 10 µg EDTA/kg soil) can improve the bioavailability of Pb [2].
Limitations
Effective extraction of toxic metals by hyperaccumulators is limited to shallow soil depths of up to 24 inches. If contamination is at substantially greater depths (e.g., 6 to 10 feet), deep-rooted poplar trees can be used, however, there is concern about leaf litter and associated toxic residues [3].Despite having amiable metal-accumulating characteristics, currently available hyperaccumulators lack suitable biomass production, physiological adaptability to varying climatic conditions, and adaptability to current agronomic techniques [3].
Performance
Early research revealed that phytoextraction via constructed wetlands (used to purify water) was ineffective because it was difficult to remove inorganic elements that precipitated from the water into the sediments. In addition, floating plant systems, with subsequent biomass harvesting, was determined to be inefficient and uneconomical [1].
Research on specific plant species determined that some plants concentrated toxic heavy metals of up to several percent of their dried shoot biomass. These plants (designated as hyperaccumulators) had toxic element levels in the leaf and stalk biomass of about 100 times those of non-accumulator plants in the same soil, and in some cases more than a thousand times [4]. Lead concentrations in plant shoots (dry weight basis) of several plants growing on contaminated sites were reported to range from 130 to 8,200 mg per kg [3].Another investigation of Pb phytoextraction revealed that Brassica juncea and Brassica nigra have high metal-accumulating ability [5]. The Pb contents of roots and shoots of various species areincluded in Table 1. Other salient findings include: (1) cultivar 426308 of Brassica juncea was the most efficient shoot accumulator (3.5% lead on a dry weight basis); (2) tight binding of Pb to soils and plant material partially explains relatively low mobility in soils and plants; (3) the rate of Pb uptake to roots decreased and the rate of translocation to the shoots increased as a function of exposure time; and (4) insoluble inorganic complexes in soil and the plant significantly reduces phytoextraction efficiency of Brassica juncea [5].
Table 1. Lead content of roots and shoots of crop Brassica and other plants [5].
Plant species*
mg of Pb per g dry weight ± SE
Shoot
Root
Brassica juncea (L.) Czern. 10.3 ± 2.9
103.5 ± 12.3
Brassica nigra (L.) Koch 9.4 ± 2.5
106.6 ± 10.7
Brassica campestris L. 7.2 ± 2.2
103.4 ± 7.7
Brassica carinata A. Br. 4.6 ± 2.6
108.9 ± 13.9
Brassica napus L. 3.4 ± 1.0
61.2 ± 11.9
Brassica oleracea L. 0.6 ± 0.2
52.7 ± 3.8
Helianthus annuus L. 5.6 ± 1.3
61.6 ± 3.3
Nicotiana tabacum L. 0.8 ± 0.3
24.9 ± 7.8
Sorghum bicolor L. 0.3 ± 0.0
8.2 ± 0.6
Amaranthus hybridus L. 0.3 ± 0.04
8.7 ± 0.7
Amaranthus paniculata L. 0.4 ± 0.04
8.9 ± 0.3
Zea mays L. 0.2 ± 0.1
14.7 ± 0.9
* plants grown for 14-20 days in sand/Perlite mixture containing 625 µg Pb(2-) per g
dry weight supplied as Pb(NO3)2; SE (n=4)
Performance data for two phytoextraction field studies are provided in Table 2.Table 2. Performance data of phytoextraction field investigations [6].
Site/Media
Contaminants
Plants
Performance
Location
60,000 sq. ft. plot brownfield (demonstration)
Pb
Brassica juncea
Pb removed to below action level in
one season SITE programTrenton, NJ mine wastes
(pilot)Zn, Cd
Thlaspi caerulescens
Rapid uptake; soil decontamination difficult
Pennsylvania
SITE: Superfund Innovative Technology Evaluations (by EPA)
Data Requirements
Successful implementation of phytoextraction depends on the following: (1) bioavailability of the contaminant in the environmental matrix; (2) root uptake; (3) internal translocation of the plant; and (4) plant tolerance [1]. Plant productivity (i.e., amount of dry matter that is harvestable each season) and the accumulation factor (ratio of metal in plant tissue to that in the soil) are important design parameters [2].
Cost
The total cost for phytoextraction is estimated to be between $60,000 to $100,000 per acre. This includes maintenance, monitoring, verification testing, and $10,000 per acre for planting. Other estimates place the total cost at approximately $80 per cubic yard of contaminated soil [6] and $15-$40 per cubic meter [2].
There are many non-plant based technologies that meet current regulatory guidelines in the wastewater purification industry. It is viewed that outlay of additional capital for technologies other than the incumbent systems is unlikely unless water quality standards change [1].
Status of Technology
Applications to recover inorganic elements using plants is still in its infancy and phytoextraction of Pb is in the developmental stage. Further research is needed to understand cellular mechanisms of contaminant transport in plants and the physiology of contaminant uptake, translocation, and accumulation. Field testing of phytoextraction of Pb is being conducted at several sites in the U.S. [3].
References
1. Cunningham, S.D., J.R. Shann, D.E. Crowley, and T.A. Anderson, 1997, Phytoremediation of Contaminated Soil and Water, in Phytoremediation of Soil and Water Contaminants, E.L. Kruger, T.A. Anderson, and J.R. Coats, Eds., ACS Symposium Series 664, American Chemical Society, Washington, DC.2. Schnoor, J.L., 1997, Phytoremediation, Technology Overview Report, Ground-Water Remediation Technologies Analysis Center, Series E, Vol. 1, October.
3. Huang, J.W, J. Chen, and S.D. Cunningham, 1997, Phytoextraction of Lead from Contaminated Soils, in Phytoremediation of Soil and Water Contaminants, E.L. Kruger, T.A. Anderson, and J.R. Coats, Eds., ACS Symposium Series 664, American Chemical Society, Washington, DC.
4. Suthersan, S., 1997, Remediation Engineering Design Concepts, CRC Press, Boca Raton, FL.
5. Kumar, P.B.A.N., V. Dushenkov, H. Motto, and I. Raskin, 1995, Phytoextraction: The Use of Plants to Remove Heavy Metals from Soils, Environmental Science and Technology, 29 (5), pp. 1232-1238.
6. Miller, R., 1996, Phytoremediation, Technology Overview Report, Ground-Water Remediation Technologies Analysis Center, Series O, Vol. 3, October.