Environ. Sci. Technol. 1998, 32, 802-806
Phytoextraction of Zinc by Oat (Avena sativa), Barley (Hordeum vulgare), and Indian Mustard (Brassica juncea) STEPHEN D. EBBS AND LEON V. KOCHIAN* U.S. Plant, Soil, and Nutrition Laboratory, USDA-ARS, Cornell University, Ithaca, New York 14853
The success of phytoremediation hinges on the selection of plant species and soil amendments that maximize contaminant removal. Indian mustard (Brassica juncea) has been shown to be effective in phytoextracting Zn, particularly after the synthetic chelate EDTA has been applied to the soil. However, the effectiveness of grass species for phytoremediation has not been addressed in great detail. A hydroponic screening of 22 grass species indicated that oat (Avena sativa) and barley (Hordeum vulgare) tolerated the high Cu, Cd, and Zn concentrations present in the solution and also accumulated elevated concentrations of these metals in the plant shoots. A hydroponic experiment comparing these two grasses to Indian mustard indicated that, although shoot Zn concentrations were greater for Indian mustard, the grasses were considerably more tolerant. A pot experiment conducted using a Zncontaminated soil showed that the addition of EDTA to the soil significantly increased Zn accumulation by B. juncea but not oat or barley. Nevertheless, barley accumulated >2 mg of Zn plant-1, 2-4 times more Zn than what was observed in Indian mustard in the presence of EDTA. The results of this experiment suggest that barley has a phytoremediation potential equal to, if not greater than, that for B. juncea.
Introduction The primary objective of phytoextraction is to maximize the transfer of contaminant to the plant shoots so that the greatest total mass of contaminant is removed by each cropping. Initial phytoremediation research suggested that this could be achieved with ‘hyperaccumulator’ plant species such as Thlaspi caerulescens (1-4). While the results from these studies were promising, some researchers have suggested that the small size and slow growth of this species may limit its utility for phytoremediation (5, 6). Recent evidence suggests that moderate accumulating, high biomass species, such as Indian mustard (Brassica juncea), may be more effective than T. caerulescens in phytoextracting Zn (7-9). For example, when B. juncea and T. caerulescens were grown for 6 weeks in a contaminated soil [(>11 000 mg of Zn (kg of soil)-1], it was found that B. juncea removed 4-fold more Zn than T. caerulescens (9). This was due primarily to the fact that B. juncea produced 10 times more biomass than T. caerulescens. These results * Corresponding author phone: (607)255-2454; fax: (607)255-2459; e-mail:
[email protected]. 802
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 32, NO. 6, 1998
suggested that a greater shoot biomass can more than compensate for a lower shoot metal concentration and that the plant species suitable for phytoremediation may not be limited to hyperaccumulators. There is evidence that grass species such as corn, barley, and ryegrass have varieties that display significant heavy metal tolerance (10, 11). Several studies (12-17) have indicated that certain varieties from the genera Agrostis, Deschampsia, Festuca, and Holcus possess a nonspecific heavy metal ‘co-tolerance’ similar to that of the hyperaccumulating Thlaspi species (18, 19). That is, certain varieties display a tolerance to metals whose concentrations were not elevated in the parent soil from which the particular variety originated. Some tolerant grass species are also capable of accumulating moderate to high levels of heavy metals in the plant shoots (20-25). Because several of these grass species also produce a high biomass, the possibility exists that such species may be as effective as B. juncea in phytoextracting heavy metals. The first part of this study utilized a hydroponic screening experiment to identify grass species with the ability to tolerate and accumulate heavy metals (Cd, Cu, Zn) in plant shoots. A subsequent hydroponic experiment and a pot study using Zn-contaminated soil were then conducted to compare the phytoextraction potential of two candidate species from the screening experiment, oat (Avena sativa) and barley (Hordeum vulgare), to B. juncea. In addition, the effect of EDTA on Zn phytoextraction was examined in the pot experiment. Recent studies have shown that the removal of heavy metals can be greatly enhanced through the addition of synthetic chelates to the soil (25-27). In the study in which the effect of synthetic chelates on Zn phytoextraction was examined, however, the soil Zn concentration was achieved by spiking clean soil with zinc carbonate. Thus, there are no studies in the literature describing the effect of synthetic chelates on Zn phytoextraction from ‘aged’ contaminated soil.
Materials and Methods In the initial screening experiment, the grasses tested were obtained commercially: Agropyron elongatum, Agrostis tenuis, Elymus angustos, Elymus cinereus, Elymus junceus, Elymus triticoides, Eragrostis curvula, Festuca megalura, Leptochloa dubia, Pannicum virgatum, Phalaris arundinacea, Poa compressa, Poa sandbergii, Puccinellia distans (Granite Seed, Lehi, UT), Paspallum notatum, and Sorghum bicolor (Valley Seed, Fresno, CA). Two varieties each of Agrostis alba, Festuca longifolia, and Festuca rubra were also tested (Granite Seed, Lehi, UT; Big Sky Wholesale Seeds, Shelby, MT). Seed of Agrostis capillaris and a third variety of Festuca rubra were provided by Dr. David Morrey (Golder Associates, Boulder, CO). Oat (Avena sativa) and barley (Hordeum vulgare) were included for comparative purposes. Seed of Indian mustard (Brassica juncea, accessions 426308 and 184290), was obtained from the USDA-ARS Plant Introduction Station, Ames, IA. Grass seeds were germinated for 3-4 days on filter paper before being transferred to two 200-L polyethylene tanks containing a nutrient solution comparable to the soil solution extracted from a Zn-contaminated soil (28). A 1.25-cm-thick piece of Styrofoam, into which 100 wells (2.5 cm diameter) had been cut, was floated on the solution in each tank. Disks of polyethylene mesh (1 mm) attached to the bottom of each well supported the seeds. Within each tank, each species was replicated three times. The composition of the nutrient solution in each tank was as follows: 6.0 mM KNO3; 4.0 mM Ca(NO3)2; 0.1 mM NH4H2PO4; 1.0 mM MgSO4; 25.0 µM CaCl2; S0013-936X(97)00698-6 CCC: $15.00
1998 American Chemical Society Published on Web 01/27/1998
TABLE 1. Shoot Biomass and Removal of Heavy Metals from the Control (-) and Heavy Metal-Supplemented (+) Hypoponic Solutions during the Hydroponic Screening Experimenta biomass (g plant-1)
Cd (µg plant-1)
Cu (µg plant-1)
Zn (µg plant-1)
species
-
+
-
+
-
+
-
+
Agropyron elongatum Agrostis alba Agrostis capillaris Agrostis tenuis Avena sativa Elymus cinereus Elymus curvula Elymus junceus Elymus triticoides Festuca megalura Festuca rubra Hordeum vulgare Leptochloa dubia Phalaris arundinacea Poa compressa Puccinellia distans
0.34 0.09
