in Metallurgical Industries. Editors: P. Mahant. Ontario Ministry of Environment and Energy. Toronto, Ontario. C. Pickles. Queen's University. Kingston, Ontario.
PROCEEDINGS OF THE INTERNATIONAL SYYIPOSIU~l ON RESOURCE CONSERVATION AND ENVIRONMENTAL TECHNOLOGIES IN METALLURGICAL INDUSTRIES TORONTO, ONTARIO, AUGUST 20-25, 1994
Resource Conservation and Environmental Technologies in Metallurgical Industries Editors: P. Mahant Ontario Ministry of Environment and Energy Toronto, Ontario C. Pickles Queen's University Kingston, Ontario W-K. Lu McMaster University Hamilton, ontario
Symposium organized by the Non-ferrous P;.TometalIurgy Section, the Iron and Steel Section and the Environmental Committee of The Metallurgical Society of CIM. 33rd M"1'.'UAL CONFERENCE OF METALLURGISTS OF CIM 33e CONrERENCE A.t~LLE DES META.lLURGISTES DE L'ICM
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Phytoremediation: A new technology for the environmental cleanup of toxic metals D.E. Salt, P.B.A. Nanda Kumar, S. Dushenkov and I. Raskin AgBiolech Center, Rutgers r..;niversity, Cook College, P.O, Box 231, Sew Brunswick, SJ 08903-0231, U.S.A.
Toxic metal-contaminated soils, aqueous waste streams and ground waters pose a major environmental and human health problem. The use of plants to remove these pollutants would provide an efficient, low cost, in sim cleanup technology able to remove toxic metals from the site leaving it intact for normal ecosystem redevelopment. We have demonstrated that the Indian mustard plant Brassica juneea can efficiently accumulate Pd, Zn, Cd, Cr(VI). Ni and Cu from soils or water into both roots and shoots. These uptake characteristics coupled with B. juneea high biomass and good agronomic practices make this an ideal plant for phytoremediation.
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RESOCRCE CONSERVATION AND ENVIRONMENTAL TECHNOLOGIES
Toxic heavy metals are present in soils as natural components or as a result of human activity. Metal-rich urine tailings, metal smelting, electroplating, gas exhausts, energy and fuel production, down wash from power lines, intensive agriculture and sludge dumping are the most important human activities which contaminate soils and aqueous streams with large quantities of toxic heavy metals (1). It should be noted. however. that in some areas natural background concentrations of heavy metals in soils and water exceed those which are considered safe by regulatory agencies (2). Toxic metal-contamination of soil, aqueous waste streams and ground water poses a major environmental and human health problem which is still in need of an effective technological soultion. Bioremediation, Le. the use of living organisms to degrade organic wastes, is increasingly favored by both the public and private sectors as a treatment method because of its low cost and minimal environmental impact Currently microbial-based bioremediation technologies cannot clean up toxic metals, thus making many metal contaminated sites suitable only for conventional and '.ery costly "removal and burial" methods. The ability of plants to accumulate toxic heavy metals in both their roots and shoots has been recognized for along time as being detrimental. Being at the bottom of many food chains, metal-accumulating plants are directly or indirectly responsible for a large proportion of the dietary intake of toxic heavy metals by humans and other animals. (3). Only recently has the value of metal-accumulating plants for environmental remediation be fully realized (4-7), this is an emerging technology which has been called Phytoremediation (7). Phytoremediation combines economic advantages with in-situ treatment, leaving cleaned soil in place. The high concentrations of metals in the plant residues may permit recycling, thus eliminating or reducing the need for land filling of hazardous wastes. Three subsets of this technology are being studied in our laboratory. (i) Phytoextraction - the use of metal-accumulating plants to extract, transpon and concentrate toxic metals from soils into the harvestable pans of roots and above ground shoots. (ii) Rhizofiltration the use of plant roots to absorb, concentrate and precipitate toxic metals from polluted aqueous streams. (iii) Phytostabilization - the use of plants to reduce the bioavalibility of toxic-metals in soils. PbytoextractjQQ - In the phytoextraction process several sequential crops of laboratory-improved metal· accumulating plants may be used to reduce soil concentrations of toxic metals to environmentally acceptable levels. Extensive screening procedures have identified the most promising metal-accumulating lines of the fast-growing crop plant, Brassica juncea, a variety of mustard plant which can effectively remove lead. chromium and other metals from soils. In developing phytoextraction technology we have avoided using non-crop metal accumulating species because of their low biomass production, handling difficulties. genetic variability and lack of established cultivation practices. Instead, we limited our search for phytoextracting species to crop or crop·related Brassica species because of their ability to accumulate metals and their good agronomic characteristics. Of 16 species srudied, Brassica jun.cea was the best accumulator of lead in shoots. It is also a high biomass producer (average yield of 18 tons!hectare) - meaning that it can extract and concentrate store more toxic metals from the soil. The ease of cultivation and predictable field perfonnance led to the choice of B. jUllcea as one of the best metal-accumulating species for phytoextraction. Identification of B. jUllcea was followed by an extensive screening of 120 B. jUllcea varieties in order to utilize existing genetic variability and to find the best phytoextracting line(s). Lead accumulation in shoots differed 30 fold between the best and worst B. juncea lines, the best cultivar accumulating almost 3.5% of lead by weight in the dried shoots. This corresponds to a phytoextTaction coefficient of 55 (ratio between lead concentration in dried shoots and in the soil) compared to a coefficient of 3-5 for non-crop species. B juncea is also able to efficiently accumulate other environmentally important contaminating toxic metals, including Zn, Cr(VI), Cd, Ni and Cu (Table 1).
RESOURCE CONSERVA nON AND ENVIRON!\.1ENTAL TECHNOLOGIES
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Table 1- Toxic metal accumulation in shoots of Brassicajuncea. Seedlings were grown for 17 days in an acid washed I: I (v/v) mixture of coarse sand and perlite. On the 17th day plants were treated with aqueous solutions of various metal salts. After 14 days plants were harvested and shoots analvsed for metal content. Concentration Phytoaccumulation Shoots coefficient Soil Metal (ugfg Drv Wt.) (mgfg) 1723 ± 44 17 Zn 100 Cr(ill) 6±2 o 50 202 ± 16 3.5 58 cnvD 104 ± 17 2 52 Cd 100 3086 ± 184 31 Ni
cum)
10
69 ± 3
7
Rbizofiltratjon - Rhizoflltration technology exploits the ability of plant roots to remove toxic metals from
solutions applying it to metal-contaminated aqueous streams. In addition to concentrating metals in root tissue, rhizofiltrating plants can precipitate large amounts of metal with compounds exuded from the roots. Metal-enriched roots are harvested after remediation cycles and are replaced by new plants to maintain the continuity of operation. The ability to absorb different metal ions is a common property of all plants but it varies between plant species. Turfgrasses, Brassicas and Sunflower were promising species for use in the rhizoflltration system. To demonstrate the rate and efficiency of lead removal during rhizoftltration 1.1 g dry weight of sunflower or B juncea roots immersed in 400 ml of water containing 300 J..I.gfml of lead brought the lead concentration to below I Ilg/ml in 8 hours. Removal of lead from the solution was accompanied by a dramatic concentration of lead in the root tissue, over 10% on a dry weight basis. B. juncea roots were also capable of removing other environmentally important toxic-metals from aqueous solution, including Zn, Cr(VD, Cd, Ni and Cu (Table 2).
Table II- Heavy metal accumulation in roots of Brassica juncea. Seedlings were grown hydroponically in aerated nutrient solution. After 28 days roots were washed in deionised water and placed into an aerated solution containing various metal salts. Roots were exposed to the metal solution for 24 hours then harvested and analvsed for metal content. Concentration Phvtoextraction Solution Roots Metal ~fficient OJ.gImll (j.tgfg Dry Wt.) Zn 95.9 13147 ± 1917 137 Cr(VI) 4.7 716±34 153 1.7 268 ± 23 Cd 160 Ni 11.9 2080 ±206 175 Cu(lI) 6.1 2943 ± 158 484 PhytQstabilization - The ability of plant roots to absorb, chemically reduce and precipitate large quantities of toxic metals can be utilized to reduce their bioavalibility in soils thus preventing their entry into ground water and the food chain. A technology which renders toxic metals not bioavalible will greatly reduce human exposure to them. Seedlings of B. juncea were grown in a sand - Perlite mixture. Pots containing the same sand - Perlite mixture but without plants were maintained as controls. 'When plants were 3 weeks old, a lead solution was administered to the pots to a [mal concentration of 625 I.l.gfml. Control pots and pots containing plants were watered with tap water evety other day. On the sixth day after lead treatment solution leached from each pot was collected and analyzed for lead to determine available lead in each pot. Leachate from pots containing plants had 97% less lead in it than leachate from control pots containing no plants. This experiment demonstrates that B. juncea roots can effectively immobilize lead in soil, thereby making it much more harmless to the environment. Chromium, a serious pollutant metal. can be present in soils in two major oxidation states, Cr(ill) and Cr(VD. Due to chemical differences between these tWO oxidation states Cr(Vl)
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RESOURCE CONSERVATION AND ENVIRON\1ENTAL TECHNOLOGIES
is much more readily accumulated by plants than Cr(llI) (Table I). Using X-ray absorption spectroscopy we
have demonstrated that B.juncea plants exposed to 4 j.J.glml Cr(VI) in solution are able to accumulate Cr(VI)
and reduce it to Cr(IIl) in pianra.
Phytoremediation is a cost·effective alternative for the cleanup of toxic metal contaminated soils. Since the
roots of crop-related Brassicas can reach a depth of 80 cm, soil can easily be treated to a depth of 50 em. The
estimated cost for phytoremediation of one acre of sandy loan soil to a depth of 50 em is $50,000 70,000
compared to at least $450,000 for excavation and storage. Furthermore, this method is ecologically preferred
since it reclaims soil at the site, recycling it to a biologically safe state rather than removing it to a permanent
storage site.
Toxic metals in industrial process and ground water are most commonly removed by precipitation or
flocculation, followed by sedimentation and disposal of the re"ulting sludge. For many of these streams
treatment is relatively simple and inexpensive. However, the special characteristics of some toxic metals in
water makes these streams much more difficult and expensive to treat, for example, chelated copper in
electroplating rinse water. Also, certain metals, because of their highly toxic nature, must be polished after
conventional treatment to meet stringent discharge standards. An important example of this class of
contaminants is radionucleotide waste. It is in these type of areas that rhizofiltration will have its greatest
impact.
Phytoremediation, although still in its infancy, may one day become an established environmental clean up technology. Further development of phytoremediation requires an integrated multidiSCiplinary research effort combining plant biology, soil chemistry, soil microbiology, and agricultural and environmental engineering. As a major renewable resource exploited by man plants already give us food energy, construction materials, natural flbres and various chemicals. The use of plants in environmental cleanup may guarantee a greener and cleaner planet for us all.
References 1. M.R.D. Seaward, D.I·LS. Richardson, "Atmospheric Sources of Metal Pollution and Effects on Vegetation". Heayy Metal Tolerance in Plants: Evolutionary Aspects. AI. Shaw, Ed., CRC Press, 1990,75-92. 2. D.D. Runnells, T.A. Shepherd, E.E. Angino, "Metals in Water. Determining Natural Background Concentrations in Mineralized Areas". Environ Sci-Techno!" 26, 1992, 2316·2323. 3. A.K. Kabata-Pendias, H. Pendias, Trace Elements in Soils and Plan!. , CRC Press, 1989. 4. A Baker, R. Brooks, R. Reeves, "Growing for Gold... and Copper... and Zinc". New Scientist, 1603, 1985 44-48. 5. R.L. Chaney, "Plant Uptake of Inorganic Waste". Land Treatment of Hazardous Wastes. lE. Parr, P.B. Marsh, I.M. KIa, Eds., Park Ridge, Noyes Data Corp; 1983: 50-76. 6. AI.M. Baker, S.P. McGrath, C.M.D. Sidoll. R.D. Reeves, "The possibility of in situ heavy metal decontamination of metal-polluted soils using crops of metal-accumulating plants". Resource. Conservation and Recycline. in press. 7. I. Raskin, P.B.A Nanda Kumar. S. Dushenkov and D. E. Salt, "Bioconcentration of heavy metals by plants", Current Opinions in Bjotechnolocy. in press.
