Phytoextraction

In the past, removing metal and metalloid contaminants from soil has been impossible, and site clean-up has meant excavation and disposal in a secure landfill. An exciting new approach to this problem is phytoextraction, where plants are used to extract contaminants from the soil and harvested. Immobilization and Toxicity-Minimization. 

Bacterial remediation of selenium oxyanions in San Joaquin, California, drainage water is under active investigation (96,97), but has not yet been commercialized. Agricultural drainage rich in selenium is also typically rich in nitrates, so bioremediation must also include conditions that stimulate denitrification (98). Phytoextraction of selenium is also being tested, but is not yet being used on a large scale. 

Extraction of contaminants from the soil and accumulation in the plant tissue for removal (phytoextraction)... 

Phytoextraction is mainly carried out by certain plants called hyperaccumulators, which absorb unusually large amounts of metals compared to other plants. A hyperaccumulator is a plant species capable of accumulating 100 times more metal than a common nonaccumulating plant. Therefore, a hyperaccumulator will concentrate more than 1000 mg/kg or 0.1% (dry weight) of Co, Cu, Cr, or Pb, or 10,000 mg/kg (1%) of Zn and Ni (dry matter).43-44 Similarly, halophytes are plants that can tolerate and, in many cases, accumulate large amounts of salt (typically sodium chloride but also Ca and Mg chlorides). Hyperaccumulators and halophytes may be selected and planted at a site based on the type of metals or salts present, the concentrations of these constituents, and other site conditions. 

Phytoextraction Soils, sediments Metals (Ag, Au, Cd, Co, Cr, Cu, Hg, Mn, Mo, Ni, Pb, Zn) Radionuclides (90Sr, 137Cs, 239Pu, 234JJ 238JJ) Sunflowers Indian mustard Rape seed plants Barley, hops Crucifers Serpentine plants Nettles, dandelions... 

Higher phytoextraction coefficients indicate higher metal uptake. The effectiveness of phytoextraction can be limited by the sorption of metals to soil particles and the low solubility of the metals however, metals can be solubilized through the addition of acids or chelating agents and so allow uptake of the contaminant by the plant. Ethylene diamine tetra-acetic acid (EDTA), citric acid, and ammonium nitrate have been reported to help in the solubilization of lead, uranium, and cesium... 

Source Kumar, P.B.A.N., Dushenkov, V., Motto, H. and Raskin, I., Phytoextraction The use of plants to remove heavy metals from soils, Environ. Sci. Technol., 29, 1232-1238, 1995. With permission. 

Phytoextraction has several advantages. The contaminants are permanently removed from the soil and the quantity of the waste material produced is substantially decreased. In some cases, the contaminant can be recycled from the contaminated biomass. However, the use of hyperaccumul-ating plants is limited by their slow growth, shallow root systems, and small biomass production. In order for this remediation scheme to be feasible, plants must tolerate high metal concentrations, extract large concentrations of heavy metals into their roots, translocate them into the surface biomass, and produce a large quantity of plant biomass. 

These maximum depths are not likely to occur in most cases. The effective depth for phytoremediation using most nonwoody plant species is likely to be only 30 or 61 cm (1 or 2 ft). Most accumulators have root zones limited to the top foot of soil, which restricts the use of phytoextraction to shallow soils. The effective depth of tree roots is likely to be in the few tens of feet or less, with one optimistic estimate that trees will be useful for extraction of groundwater up to 9 m (30 ft) deep.41-58... 

Equations 14.24 to 14.27 can be applied to most sites where soil cleanup regulations are known for metals or organic contaminants. Two examples follow, one for TCE treatment by phytotransformation and another for lead removal by phytoextraction, which demonstrate the use of the design equations. 

Lead at a lightly contaminated brownfield site has a concentration in soil of 600 mg/kg to a depth of 1 ft. The cleanup standard has been set at 400 mg/kg. Indian mustard, Bmssica juncea, will be planted, fertilized, and harvested three times each year for phytoextraction. Using small doses of EDTA, it is possible to achieve concentrations in the plant of 5000 mg/kg (dry weight basis), and harvestable densities of 2.721 (3 short tons) dry matter per crop. Estimate the time required for cleanup ... 

Blaylock M.J., Huang J.W. Phytoextraction of metals. In Phytoremediation of Toxic Metals Using Plants to Clean Up the Environment, Raskin I., Ensley B.D., eds. New York, NY John Wiley Sons, Inc., 2000. 

Ebbs S.D., Kochian L.V. Phytoextraction of zinc by oat (aAvena sativa), barley (Hordeum vulgare), and Indian mustard (Brassica juncea). Environ Sc Technol 1998 32 802-806. 

Huang J.W., Cunningham S.D. Lead phytoextraction Species variation in lead uptake and translocation. NewPhytol 1996 134 75-84. 

Chaney R. L., Angle J. S., Wang A. S., McIntosh M.S., Broadhurst L., and Reeves R. D., 2005, Phytoextraction of soil Cd, Ni and Zn using hyperaccumulator plants to alleviate risks of metal contaminated soils requiring remediation. International Workshop Current developments in remediation of contaminated lands p. 39, 27-29 October 2005. Pulawy, Poland. 

Watt, N.R., Willey, N.J., Hall, S.C., and Cobb, A., 2002,. Phytoextraction of 137Cs the effect of soil Cs concentration on Cs uptake by Beta vulgaris. Acta Biotechnol. 22 183-188. 

The U.S. EPA claims that the 30-year cost of treating lead at a 12-acre site using a phytoextraction technology would cost approximately 200,000. In contrast, the EPA estimated that excavation and disposal would cost 12,000,000, soil washing would cost 6,300,000, and soil capping would cost 600,000. For a 1-acre site with thick sandy loam, phytoextraction technologies are estimated to cost between 60,000 and 100,000. This estimate assumes treatment to a depth of 20 inches (D21292A, p. 17). 

Cost estimates for phytoremediation vary widely. One estimate for phytoextraction included 10,000 per acre for planting, with total remediation costs estimated at 60,000 to 100,000 per acre. Total costs included expenses associated with maintenance, monitoring, and verification testing. Another estimate placed phytoremediation costs at 80/yd of contaminated soil (D131431). Cleanup costs for an acre of metal-contaminated soil were estimated to range from 60,000 to 100,000. This estimate assumes remediation to a depth of 50 cm. In contrast, excavation and disposal storage without treatment for a comparable site would cost at least 400,000 (D16482T). 

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