Acta Biotechnol. 22 (2002) 1--2, 199--208
Phytoremediation of Polluted Waters Potentials and Prospects of Wetland Plants
WANG*, Q., CUI, Y., DONG, Y.
Chinese Academy of Sciences Research Center for Eco-Environmental Sciences P.O. Box: 2871 Beijing 100085, P. R. China
*
Corresponding author Present address University of Florida 18905 SW 280 ST Tropical Research and Education Center Homestead, FL, 33031-3314, USA Phone: + 1 305 246 7001 Fax: + 1 305 246 7003 E-mail:
[email protected]
Summary To investigate the possible use of plants to remediate polluted waters, a pot experiment was carried out in the laboratory with five wetland plant species, i.e., sharp dock (Polygonum amphibium L.), duckweed (Lemna minor L.), water hyacinth (Eichhornia crassipes), water dropwort [Oenathe javanica (BL.) DC.] and calamus [Lepironia articulata (RETZ.) DOMIN]. Nitrogen (N), phosphorus (P) and three heavy metals, cadmium (Cd), mercury (Hg) and lead (Pb), were the objects of remediation. Sharp dock was found to be a good accumulator of N and P. Indeed, on a dry weight basis the shoots of sharp dock accumulated up to 6.4% of N and 1.1% of P with BCF (bioconcentration factor) values of 2235 and 1568, respectively. Water hyacinth and duckweed strongly accumulated Cd with concentrations of 462 and 14200 mg/kg, respectively, and BCF values of 1225 and 2567, respectively. Water dropwort achieved the highest concentrations of Hg, i.e., 1.2 mg/kg with a BCF value of 807, whereas calamus achieved the highest concentrations of Pb, i.e., 512.4 mg/kg in its roots with a BCF value of 1217. Thus, it could be concluded that the above plant species are good candidates for phytoremediation of polluted waters, as follows: sharp dock through accumulation of N and P in its shoots, water hyacinth and duckweed as hyperaccumulators of Cd, water dropwort as an hyperaccumulator of Hg and calamus as an hyperaccumulator of Pb.
Introduction
The shortage and pollution of water are significant environmental problems in China. For instance, 300 cities in total suffer daily shortages of 10 million tons of water in total. Of these cities fifty are experiencing severe shortage. However, in 1989, up to 35.5 billion © WILEY-VCH Verlag Berlin GmbH, 13086 Berlin, 2002 0138-4988/02/01-205-0199 $ 17.50+.50/0
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tons of wastewater were discharged each day, industrial operations alone contributed 25.2 billion tons, and 80% of the latter was discharged directly into rivers, lakes and marine waters [1]. In addition, 92% of twenty five lakes into which domestic wastewater was released showed eutrophication. Moreover, 63.3% of such lakes in China have been found to be severely polluted [2]. Therefore, water treatment has become an important task. Conventional physical and chemical systems require major investments in equipment and facilities and have high running costs. More economical alternatives of wastewater remediation are being pursued, including phytoremediation with certain aquatic and wetland plants that have phytoextraction and phytofiltration capabilities. Phytoremediation of soils contaminated with heavy metals has proven to be costeffective and environmentally friendly. This approach has developed quickly throughout the world, because certain terrestrial plants possess the ability to accumulate large quantities of certain metals in their shoots [3–7]. Similarly, the use of aquatic and wetland plants, such as duckweed, water velvet, to extract pollutants in wastewater treatment is considered by researchers to be an effective measure for removing heavy metals [8–12]. However, the quantities of biomass produced by some species of aquatic plants, such as duckweed, are so small that such species are not readily utilizable in practical programmes. Furthermore, in some parts of China, surface waters have been polluted not only by heavy metals through point pollution but also by P and N from non-point sources. To clean up the contaminants in the waters (mostly lakes), the government has spent large sums of money on physical and chemical approaches but with only modest effect thus far [1, 2]. To investigate feasible approaches for reducing the eutrophication of surface waters and for the removal of heavy metals, an experiment was conducted using the biomass of different wetland plant species in a split-plot design. Specifically, this study focused on the ability of certain plant species to extract and bioaccumulate N, P, Cd, Pb and Hg. Materials and Methods Sand culture was used for Polygonum amphibium, Oenathe javanica and Lepironia articulata, while water culture was used for Lemna minor and Eichhornia crassipes. Each culture of the latter plant species was established in a container with quarter-strength HOAGLAND’s nutrient solution. Two grams (fresh weight) of duckweed was used per container, and three evenly developed water hyacinth plants were collected from a pond, weighed, and used per container. All the plants were maintained in a growth chamber at 20 to 25 °C with an artificial irradiance of 400 µmol/m2 s photon flux density over a 14-h photoperiod each day. The nutrient solutions for the water culture were replaced every other day. Within the split-plot design, five species of wetland plants, sharp dock (Polygonum amphibium L.), duckweed (Lemna minor L.), water hyacinth (Eichhornia crassipes), water dropwort [Oenathe javanica (BL.) DC.] and calamus [Lepironia articulata (RETZ.) DOMIN] were treated as the main plot. Nitrogen, P and the different metal concentrations constituted sub-plots. With full strength of modified HOAGLAND’s nutrient solution containing macro-nutrients [mmol/l]: 5.0 Ca (NO3)2 × H2O, 5.0 KNO3, 2.0 MgSO4 × 7 H2O, 1.0 KH2PO2 and micro-nutrients; the metals were supplied as follows: Cd as Cd (Cl)2 at 0, 0.1, 1, 2, 4, 8 mg/l; Hg as HgCl2 at 0, 0.1, 0.5, 1, 1.5 µg/l; and Pb as Pb (NO3)2 at 0, 0.1, 1, 10, 20, 40, 80 mg/l. However, for N and P treatments, 1.2, 2.4 and 4.8 mmol/l of N as NaNO3, and 0.25, 0.5 and 1.0 mmol/l of P as NaH2PO4 were supplied instead of using KNO3
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and KH2PO4 in the HOAGLAND’s recipe associated with other basal nutrients in the solution, corresponding amounts of KCl, CaCl2 and NaCl2 were supplied to balance the nutrients for various rates. Three replicates in each group were conducted. After being cultured as stated above the plants were harvested, washed, then rinsed thoroughly with distilled water, and the shoots were separated from roots, whenever necessary. These plant parts were dried at 105 °C for 2 h – to quickly remove the excess water – and then held at 70 °C for 48 h. After the dry weights of these samples had been recorded, sub-samples were ground in an agate mill for chemical determination. For the chemical analysis of N and P, the samples were digested with concentrated H2SO4 and HClO4. For the analysis of metals, the samples were digested with HNO3, HF and HClO4. Nitrogen was determined by means of the KJEDAHL method. Phosphorus was determined colorimetrically with the molybdate-ascorbic acid method described by KITSON and MELLON [14]. An atomic absorption spectrometer with graphite furnace atomization (GFAAS) and deuterium background correction was used for Cd determination. An atomic absorption spectrometer with flame furnace atomization (FFAAS) was used for Pb determination, and an atomic fluorescence spectrometry mercury analyzer (YYG-4B) was used for Hg determination. The data were statistically subjected to ANOVA and subsequently to FISHER’s protected least significant difference (LSD) for comparisons of means by application of SAS (SAS Inst. Inc., Cary, NC, USA). The bioconcentration factors (BCF) were calculated as described by ZAYED et al. [12] based on the initial concentration of the given element in the culture medium. BCF =
Trace element concentration in plant tissue [mg/kg] at harvest Initial concentration of the element in the external nutrient solution [mg/l]
Results Effect of Elements on Dry Matter Production
The amount of biomass produced by sharp dock, water dropwort and calamus was enhanced by increasing the supply of N and P. For duckweed and water hyacinth, dry matter production increased as the concentration of P increased from 0.25 to 0.5 mmol/l but higher concentrations depressed dry matter production. Dry matter production was also depressed by increasing concentrations of N (data not shown). Plant growth generally decreased as metal supply increased, however, some stimulatory effects were found at low levels of supply. For example, increasing Cd supply caused progressive increases in the biomass of sharp dock and of calamus up to 2 mg/l of Cd, in duckweed up to 1 mg/l of Cd, but in water hyacinth only up to 0.1 mg/l of Cd. The growth of sharp dock and of water dropwort were promoted up to 10 mg/l of Pb, and the growth of calamus increased up to 20 mg/l of Pb. By contrast, such a stimulating effect on duckweed growth occurred only up to 0.1 mg/l of Pb, and Pb depressed the growth of water hyacinth at all levels. The growth of sharp dock was also improved up to 0.5 µg/l of Hg supply. Uptake and Bioconcentration Factors Nitrogen and Phosphorus
The concentrations of N and P in all five-plant species were increased as the concentrations of these elements increased. By contrast the magnitudes of the bioconcentra-
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tion factors (BCF) for N and P in all five-plant species varied inversely with concentrations of these elements in the culture media (Fig. 1). Among all the plant species, sharp dock achieved the greatest accumulation of N in its shoots (Fig. 1). The highest N concentration in the sharp dock was 63.6 g/kg at 4.8 mmol/l N, but the highest BCF of 2235 was observed at the lowest concentration, 1.2 mmol/l N. Water hyacinth was the second greatest accumulator of N, 42.8 mg/kg and a maximum BCF of 1315. These two-plant species showed the same pattern for P uptake. Thus, the highest P concentrations in sharp dock and water hyacinth were 11.3 and 6.5 g/kg at 1.0 mmol/l P and with respective BCF values of 1568 and 331 at the lowest P concentration (Fig. 1).
Fig. 1. Uptake (left) of N (top), P (bottom) and bioconcentration factors (right) in different plants
Heavy Metals
As in the cases of N and P, the five plant species exhibited differences in accumulated concentrations and BCF values of the different heavy metals. For instance, water hyacinth and duckweed accumulated surprisingly high concentrations of Cd. Remarkably in duckweed, the greatest Cd concentration was 14200 mg/kg (Fig. 2). Water hyacinth accumulated 462 mg Cd/kg in the shoots, and up to 832 mg Cd/kg in the roots (Fig. 2).
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The trends of the BCFs for Cd were similar in duckweed and in the shoots of water hyacinth. The value of the BCF was the highest at the lowest Cd supply (0.1 mg/l), but as Cd supply increased, the BCF decreased sharply and then rose steeply. From its maximum value at the lowest Cd supply concentration, the BCF in the roots of water hyacinth decreased sharply to a low level plateau with increasing Cd supply (Fig. 2).
Fig. 2. Concentration and bioconcentration factors of Cd in water hyacinth (top), duckweed (bottom left) and Hg in water dropwort plants (bottom right)
In water dropwort, the relationships between plant uptake and BCF values for Hg were strikingly different from those for Cd in water hyacinth and duckweed. In water dropwort, both Hg concentration and BCF values increased as Hg supply increased, and the highest Hg concentration was 1.2 mg/kg with a BCF value of 807 in the shoots at 1.5 µg/l of Hg supply (Fig. 2). Among the five-plant species, calamus attained the greatest uptake of Pb. Furthermore, the same patterns of uptake and BCF occurred in both shoots and roots of calamus (Fig. 3). The highest accumulations in shoots and roots were 323.6 and 512.4 mg/kg at
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the highest Pb supply, but the corresponding BCF values of 621 and 1217 were the greatest at the lowest Pb supply.
Fig. 3. Pb accumulation and bioconcentration factors in shoots and roots of calamus plants
For the plant species other than calamus, the uptake and BCF values were quite similar with respect to Cd, Pb (Fig. 4) and Hg (data not shown), with the exception that the uptake of Cd attained by sharp dock reached the remarkable value of 62 mg Cd/kg in its shoots (Fig. 4). Discussion
The finding that sharp dock can accumulate large quantities of N and P indicates that this species may be used to extract these plant nutrients from polluted waters to avert the eutrophication of surface waters. Based on our field investigations, we calculated that at least 11.25 tons/ha of dry biomass in sharp dock could be produced annually with 6.4% of N and 1.13% of P in the shoots. Thus, each year more than 722 kg N and 127 kg P/ha could be removed from the wastewater through only a single growth season. This can be achieved by cutting and removing the shoots two or three times per year, since the shoots regenerate readily from roots or from short remnants of stems. Moreover, like sorrel, rumex spp., sharp dock is a high quality fodder for domestic animals with high protein, 33.3% crude protein, 4.36% crude fat, substantial levels of calcium, vitamin C, etc. Some variations in the accumulation and distribution of heavy metals in the different plant species were found. The greatest uptake of heavy metals occurred in the roots rather than in the shoots. This distribution caused some difficulties in utilizing these
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species for phytoremediation on a practical scale, because plant roots are not as easily harvested as shoots [3]. Nevertheless floating plants, such as water hyacinth and duckweed, are readily harvested as whole plants. Clearly, water hyacinth is a very promising candidate for the remediation of Cd-polluted waters. Although duckweed accumulated much higher concentrations of Cd than did water hyacinth, duckweed is less promising for practical use, because it produces substantially less biomass than water hyacinth. Both species meet the definition of hyperaccumulators (Cd ≥ 100 mg/kg) [4–6], and our results are consistent with other reports [8, 12, 15].
Fig. 4. Uptake (left) and bioconcentration factors (right) of Cd (top) and Pb (bottom) in different plants
The BCF values varied for different heavy metals and their supply concentrations. In most instances, the highest BCF value occurred at the lowest element supply. A BCF
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value ≥ 1000 has been suggested to indicate that a plant species is a hyperaccumulator [12, 15]. However, as indicated by our experimental results, this definition must be qualified, since in most instances the BCF value varies inversely with the supply concentration. This relationship must be considered when selecting a plant species for phytoremediation. For example, calamus growing in sand culture with 0.1 mg/l of Pb has a BCF value of 1217 and meets the definition of a hyperaccumulator, but when grown with higher Pb concentrations, the BCF value of this plant species fails to meet this definition. Nevertheless, calamus accumulates large quantities of Pb when grown in sand culture with moderate Pb concentrations. A second limitation of the practice of identifying candidate plant species for phytoremediation based merely on the hyperaccumulation of metals is that it fails to take into account the amount of biomass produced. For example, we found that duckweed with a concentration of 14200 mg Cd/kg is a hyperaccumulator. However, the amount of biomass produced by duckweed was only 0.5 to 1% of that produced by water hyacinth, which accumulates about 700 mg Cd/kg including the roots. Hence, the total removal of Cd by water hyacinth from the wastewater is at least 5 to 10 times higher than by duckweed. Clearly, plant species should be selected as candidates for phytoremediation based on both the tissue concentration of the pollutant and the total biomass produced. Thus, the total amount of a metal accumulated in the harvested parts is the critical factor in assessing the efficiency of a given plant species for the phytoremediation of contaminated soil or wastewater. In the selection of plant species for phytoremediation of contaminated soils the main consideration is the amount of the pollutant that accumulated in the shoots. However, for phytoremediation of wastewater, consideration of the amount of pollutant accumulated by roots may be an important factor as well in selecting a plant species candidate. Our results have identified some encouraging prospects for using wetland plants to remediate polluted waters on a practical basis.
Conclusions
Clearly sharp dock has a high potential to extract large quantities of N and P from soil or wastewater under the experimental conditions. Water hyacinth and duckweed have good potential for the practical remediation of Cd contaminated water. Water dropwort is a good candidate for remediation of wastewater contaminated with Hg, and calamus shows promise for coping with Pb contamination. The development of practical technologies for phytoremediation with these plant species will require some additional investigations, such as plant uptake efficiency, mechanism, optimal combinations of plant species for specific circumstances and proper post-harvest treatment processing.
Acknowledgements The programme was financially supported by the Chinese Academy of Sciences as a key project in the strategy of Knowledge Innovation (KZCX2-401-01-1). The authors would like to thank
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Dr. W. K LASSEN from the Tropical Research and Education Center, University of Florida, for his contribution in revising the manuscript.
Received 15 June 2001 Received in revised form 27 March 2002 Accepted 2 April 2002
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