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Journal of Environmental Quality 31:1893-1900 (2002)
© 2002 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America

TECHNICAL REPORTS
Heavy Metals in the Environment

Lead Phytoextraction from Contaminated Soil with High-Biomass Plant Species

Zhen-Guo Shena, Xiang-Dong Li*,b, Chun-Chun Wangc, Huai-Man Chena and Hong Chuad

a Inst. of Soil Science, Chinese Academy of Sciences, Nanjing, 210008, China
b Dep. of Civil and Structural Engineering, The Hong Kong Polytechnic University, Kowloon, Hong Kong
c Dep. of Agronomy, Nanjing Agricultural University, Nanjing, 210095, China
d Dep. of Civil & Structural Engineering, The Hong Kong Polytechnic University, Kowloon, Hong Kong

* Corresponding author (cexdli{at}polyu.edu.hk)

Received for publication October 3, 2001.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
In this study, cabbage [Brassica rapa L. subsp. chinensis (L.) Hanelt cv. Xinza No 1], mung bean [Vigna radiata (L.) R. Wilczek var. radiata cv. VC-3762], and wheat (Triticum aestivum L. cv. Altas 66) were grown in Pb-contaminated soils. Application of ethylenediaminetetraacetic acid (EDTA) (3.0 mmol of EDTA/kg soil) to the soil significantly increased the concentrations of Pb in the shoots and roots of all the plants. Lead concentrations in the cabbage shoots reached 5010 and 4620 mg/kg dry matter on Days 7 and 14 after EDTA application, respectively. EDTA was the best in solubilizing soil-bound Pb and enhancing Pb accumulation in the cabbage shoots among various chelates (EDTA, diethylenetriaminepentaacetic acid [DTPA], hydroxyethylenediaminetriacetic acid [HEDTA], nitrilotriacetic acid [NTA], and citric acid). Results of the sequential chemical extraction of soil samples showed that the Pb concentrations in the carbonate–specifically adsorbed and Fe–Mn oxide phases were significantly decreased after EDTA treatment. The results indicated that EDTA solubilized Pb mainly from these two phases in the soil. The relative efficiency of EDTA enhancing Pb accumulation in shoots (defined as the ratio of shoot Pb concentration to EDTA concentration applied) was highest when 1.5 or 3.0 mmol EDTA/kg soil was used. Application of EDTA in three separate doses was most effective in enhancing the accumulation of Pb in cabbage shoots and decreased mobility of Pb in soil compared with one- and two-dose application methods. This approach could help to minimize the amount of chelate applied in the field and to reduce the potential risk of soluble Pb movement into ground water.

Abbreviations: DTPA, diethylenetriaminepentaacetic acid • EDTA, ethylenediaminetetraacetic acid • HEDTA, hydroxyethylenediaminetriacetic acid • ICP–AES, inductively coupled plasma atomic emission spectrometry • NTA, nitrilotriacetic acid


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
SOIL LEAD contamination is a major environmental problem facing the modern world (Body et al., 1991). Sources of Pb contamination in soils can be classified into three broad categories: industrial activities such as mining and smelting processes, agricultural activities such as application of insecticide and municipal sewage sludges, and urban activities such as use of lead in gasoline, paints, and other materials. Much research has been conducted on remediation of Pb-contaminated soils by employing chemical, physical, or biological treatments, and significant progress has been made (Holden, 1989). Conventional cleanup technology is generally too costly, and often harmful to desirable soil properties (i.e., texture, organic matter) for the restoration of contaminated sites. More recently, increasing attention has been given to the development of a plant-based technology (phytoremediation) to remediate heavy metal–contaminated soils (McGrath et al., 1993; Raskin et al., 1994; Chaney et al., 1997). In the phytoremediation process, several sequential crops of selected plant species can be cultivated to reduce the concentrations of heavy metals in contaminated soils to environmentally acceptable levels (Raskin et al., 1994). Heavy metals can be translocated to aboveground plant parts. The metal-rich plant material may be safely harvested and removed from the site without extensive excavation, disposal costs, and loss of topsoil associated with traditional remediation practices (Blaylock et al., 1997).

The success of a phytoremediation process is dependent on adequate plant yield and high metal concentrations in plant shoots. Plants must produce sufficient biomass while accumulating high concentrations of heavy metals. Hyperaccumulator plants possess an ability to take up abnormally high amounts of heavy metals in their shoots (Chaney et al., 1997; Shen et al., 1997). However, most hyperaccumulator species are not suitable for phytoremediation application in the field due to their small biomass and slow growth. As an alternative, it has been suggested to use high biomass species, such as maize (Zea mays L.), pea (Pisum sativum L.), oat (Avena sativa L.), canola (Brassica napus L.), barley (Hordeum vulgare L.), and Indian mustard [Brassica juncea (L.) Czern.], with improved plant husbandry and soil management practices to enhance metal uptake by these high biomass species (Huang and Cunningham, 1996; Banuelos et al., 1997; Blaylock et al., 1997; Huang et al., 1997; Ajwa et al., 1998; Ebbs and Kochian, 1998).

Geochemical forms of heavy metals in contaminated soils affect their solubilities, which directly influence their availability to plants (Davis et al., 1993; Zhang et al., 1997). Although the total Pb concentration in many contaminated soils may be high, the phytoavailable Pb fraction (water soluble and exchangeable) is usually very low due to the strong association of Pb with organic matter, Fe–Mn oxides, and clays, and precipitation as carbonates, hydroxides, and phosphates (McBride, 1994). Thus, increasing and maintaining Pb concentrations in soil solution are key factors in the phytoremediation of Pb-contaminated soil. Complexing agents such as EDTA, HEDTA, and NTA have been used in pot and field experiments to enhance heavy metal uptake by plants (Blaylock et al., 1997; Huang and Cunningham, 1996; Huang et al., 1997; Ebbs and Kochian, 1998; Kayser et al., 2000). These synthetic chelates can desorb heavy metals from the soil matrix to form water-soluble metal complexes and to increase metal uptake by plants. However, in situ application of such chelates may pose the potential risk of water pollution by uncontrolled metal solubilization and leaching. The biodegradation and toxicity of the chelating agents and their metal complexes in soils need careful assessment and evaluation (Means et al., 1980; Borgmann and Norwood, 1995; Nortemann, 1999; Grcman et al., 2001). To avoid possible metal–chelate movement into ground water and the effects of remaining EDTA on soil microorganisms, the amount and process of chelate application are important in addition to novel irrigation techniques and time control of chelate application.

Although there have been a few examples of phytoremediation application in small-scale field tests, the technique is still in the early developing stage (Banuelos, 2000; Blaylock, 2000). The objectives of the present study were to (i) study the solubilization of heavy metals in soil by various chelate treatments, (ii) compare the relative effectiveness of selected chelates in enhancing Pb accumulation in selected high-biomass plants, particularly cabbage, and (iii) investigate different chelate application methods to minimize chelate usage and potential risk of the migration of solubilized metals into ground water.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Soil contaminated with heavy metals was collected from an old mining site in Lin Ma Hau, Hong Kong. The soil sample can be classified as Red Soil in China. The pH of the original soil was 3.9. To adjust soil pH for the pot experiment, lime was applied and mixed thoroughly with the soil. Soil moisture was raised to 85% of the water holding capacity and maintained by daily addition of water after weighing the pots. The soil was incubated in the greenhouse for 8 wk. Selected characterization data of the soil sample used in the study are presented in Table 1. All experiments were conducted in the greenhouse under natural light. Air temperature ranged from 15 to 27°C, the natural variation of the greenhouse.


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  		Table 1. Physical and chemical characteristics of the soil.{dagger}

 
Metal Solubilization and Chemical Forms in Soil
Samples of the air-dried soil (1000 g) were placed in plastic pots. Basal fertilizers applied to the soil were 100 mg N kg-1 dry soil as urea, 80 mg P kg-1, and 100 mg kg-1 K as KH2PO4. The fertilizers were applied to each pot and were mixed thoroughly with the soil. Soil moisture was raised to 60% of the water holding capacity and maintained by daily addition of water after weighing every pot. Each treatment was repeated three times. The soil solution of samples without chelate application was extracted by a centrifuge method (Elkhatib et al., 1987). The supernatant solution was filtered through a 0.45-µm filter paper. The concentrations of heavy metals in soil solution were determined by inductively coupled plasma atomic emission spectrometry (ICP–AES).

For soil Pb dissolution experiment by chelates, about 2.0 g soil was shaken in extraction solution (1:5 soil to water ratio) containing 0.5 mM chelates (EDTA, DTPA, HEDTA, NTA, and citric acid) for 6, 24, 48, and 120 h. After centrifugation, the supernatant was filtered through a 0.45-µm filter paper (Whatman [Maidstone, UK] 42) and analyzed for metal concentrations by ICP–AES.

For the soil samples with chelate application, about 1.0 g soil was extracted in 5 mL 0.5 mmol/L EDTA for 48 h. After centrifugation, the residual was washed in 10 mL of deionized water. The soil extracted with or without EDTA was used to analyze the chemical partitioning of heavy metals with the sequential extraction procedure suggested by Tessier et al. (1979) and Li et al. (1995). The scheme consisted of sequential chemical extractions in the following operationally defined chemical forms and associated reagents: (i) exchangeable fraction (1 M MgCl2, pH 7.0, for 20 min), (ii) carbonate–specifically adsorbed fraction (1 M NaOAc adjusted to pH 5.0 with acetic acid, for 6 h), (iii) Fe–Mn oxide bound fraction (reducible phase) (0.04 M NH2OH·HCl in 25% [v/v] HOAc at 96°C, for 6 h), (iv) organic–sulfide fraction (oxidizable phase) (5 mL of 30% H2O2 and 0.02 M HNO3 for 2 h, a second 3 mL of 30% H2O2 for 3 h, at 85°C), and (v) residual fractions (total digestion with a concentrated mixture of HNO3 and HClO4 [4:1 v/v]).

EDTA Application on Plant Growth and Lead Uptake
About 15 seeds of cabbage, mung bean, and wheat were sown in each pot. After germination, the seedlings were thinned to six plants per pot and grown for 31 d. A subset of pots for each species was treated with 30 mL of 50 mM EDTA (as trisodium salt) by application to soil surface and with another 30 mL EDTA (50 mM) after 2 d to a final concentration to 3.0 mmol/kg soil. Three plants were harvested by cutting the shoots at the soil surface, and removing the roots from the pots 7 and 14 d after the first application of EDTA. The shoots and roots were washed with tap water, rinsed with deionized water, and then dried at 80°C for 24 h. The dry weights of the plants were measured. The plant materials were ground and digested with a mixture of HNO3 and HClO4 (Zhao et al., 1994). Concentrations of major and trace elements in the solutions were determined with ICP–AES (PerkinElmer [Wellesley, MA] 3300 DV).

Effects of Different Chelates on Cabbage Growth and Lead Accumulation
In this experiment, cabbage was chosen for further study of Pb uptake in plant shoots. Five cabbage seedlings in each pot were grown for 32 d before selected chelates (EDTA, DTPA, HEDTA, NTA, and citric acid) were applied to the soil. For the chelate treatments, 30 mL of 50 mM chelate was added in a single application to the soil surface to make up the amount of chelate to 1.5 mmol/kg soil. Each treatment was replicated three times. Cabbage shoots were cut 1.5 cm above the root–shoot junction 10 d after chelate application, dried, weighed, and ground. Immediately following the cutting, xylem sap was collected every 30 min for a period of 8 h (Huang et al., 1997). The xylem sap collected from each pot (five plants per pot) during this period was combined to give a composite sample for each pot for the analysis of Pb concentration in the sap. Major and trace element analysis in plants and xylem sap was performed with ICP–AES as previously described.

Amount and Regime of EDTA Application
The seedlings of cabbage were grown for 32 d on the Pb-contaminated soil. The EDTA was applied as a solution to the soil surface at five different amounts (1, 1.5, 3, 5, and 10 mmol/kg soil) with a control group without EDTA treatment. Plants were harvested 8 d after EDTA application.

For the different regimes of the EDTA application, EDTA solution was added to the soil surface in three different ways: a single application (1.5 mmol/kg soil), a double application (0.75 mmol/kg soil each) at Days 1 and 3, and a triple application (0.5 mmol/kg soil each) at Days 1, 3, and 5. The plants were harvested 8 d after the first application of EDTA. The soil at 8.0 to 8.5 cm below the surface (3 cm above the pot bottom) was collected for soluble Pb analysis. Soluble Pb was extracted by deionized water with 1:5 soil to water ratio and determined by ICP–AES.

All the values reported in the following tables and figures are the mean values based on the three replicate experiment results. Statistical analysis of variance was performed on all data sets. Least significant difference (LSD) was used for multiple comparisons between different chelate treatment and control groups.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Metal Solubilization and Chemical Forms in Soil
The concentration of Pb in soil solution was examined to assess the relative efficiency of five chelates in enhancing Pb solubilization from the soil. When no chelate was added, the Pb concentration in soil solution was 15.2 mg/L. The application of selected chelates (EDTA, DTPA, HEDTA, NTA, and citric acid) at 1.5 mmol/kg soil for 3 d significantly increased the Pb concentration in soil solution (Fig. 1) . Among the selected chelates, EDTA was the most effective in solubilizing soil-bound Pb. The Pb concentration in soil solution of the EDTA-treated soil was 42-fold higher than that of the control soil. Citric acid application to the soil produced only a small increase in the Pb concentration of soil solution and was much less effective than other chelates used.



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  		Fig. 1. Effects of chelate application on the Pb concentrations in soil solution (3 d after chelate application). Values are means ± SEs (n = 3).

 
Figure 2 displays the variations of water-, EDTA-, and DTPA-extractable Pb concentrations over time. At 6 h after EDTA and DTPA application, the soluble Pb concentration in soil increased 70-fold and 63-fold compared with the control samples, respectively. After 6 h, the soluble Pb concentration in soil increased slightly or remained relatively constant over the experiment period up to 120 h. This result indicated that soil Pb could be solubilized by EDTA and DTPA chelates in a short time and maintained at a high level afterward.



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  		Fig. 2. Lead solubilization from the soil after ethylenediaminetetraacetic acid (EDTA) and diethylenetriaminepentaacetic acid (DTPA) application. Values are means ± SEs (n = 3).

 
Results of sequential chemical extraction showed that Pb was mostly concentrated in the residual fraction, although it was present in small amounts in other phases (Table 2). Among the nonresidual fractions, the Fe–Mn oxide fraction contained the largest amount of Pb (16.1%), followed by the carbonate–specifically adsorbed (9.0%) and organic–sulfide fractions (5.3%). Only a very small percentage of Pb (0.2%) was associated with the exchangeable phase.


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  		Table 2. Lead partitioning in soil before and after ethylenediaminetetraacetic acid (EDTA) extraction.{dagger}

 
In comparison with the soil before EDTA extraction, 0.5 mmol/L EDTA treatment for 48 h significantly decreased the concentrations of Pb associated with the carbonate–specifically adsorbed and Fe–Mn oxide fractions, while Pb in the residual fraction remained relatively unchanged (Table 2). Thus, the EDTA solution mainly solubilized Pb from the carbonate–specifically adsorbed and Fe–Mn oxide fractions in the soil samples.

Effects of EDTA on Plant Growth and Lead Uptake
The shoot dry yields and metal concentrations of the three plant species are shown in Table 3. When no EDTA was added to the soil, all plants showed normal development without signs of heavy metal stress. After 7 d of EDTA (3.0 mmol/kg) application, dry matter yields of all plant species decreased significantly compared with those without EDTA application. When plants were grown for an additional 7 d, the inhibition of plant growth by EDTA became more pronounced, particularly for cabbage.


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  		Table 3. Shoot dry matter yields and metal concentrations in cabbage, mung bean, and wheat grown in soil with and without ethylenediaminetetraacetic acid (EDTA) application.{dagger}

 
Application of EDTA to the soil significantly increased the Pb concentrations in the shoots of all plant species (Table 3). There were large differences of Pb accumulation among plant species. The differences were much greater among the plants grown on EDTA-treated soils than those of the control group. Of the species tested, Pb uptake in cabbage shoots was highest. The shoot Pb concentration in cabbage reached 5010 and 4620 mg/kg dry matter on Days 7 and 14 after EDTA application, respectively, which were 40-fold and 46-fold of the controls (without EDTA application).

No significant differences of Zn, Cu, and Cd concentrations in plant shoots were observed among the species tested (Table 3). Shoot Zn concentration in all species was not significantly affected by the EDTA treatment, although the addition of EDTA to the soil tended to slightly increase shoot Zn concentrations. There were no significant effects of the EDTA application on the concentrations of Cu and Cd in species studied.

Effects of Different Chelates on Cabbage Growth and Lead Accumulation
Cabbage was chosen in the study of the effects of chelates on Pb accumulation in shoots. Within 10 d after the application of chelates (EDTA, DTPA, HEDTA, NTA, and citric acid) to the soil at 1.5 mmol/kg soil, there was no significant difference in cabbage growth, although the chelate treatments tended to decrease shoot dry matter yield (Fig. 3a) . The Pb uptake in cabbage shoots was significantly increased by the application of chelates to the soil (Fig. 3b). Of the chelates tested, EDTA was the most effective in increasing Pb concentrations in shoots. The effectiveness of stimulating Pb accumulation in cabbage shoots was in the decreasing order of EDTA > HEDTA > DTPA > NTA > citric acid. Chelate ability to solubilize soil Pb decreased in the same order (Fig. 1). A close positive relationship was found between the Pb concentrations in shoots and the Pb concentrations in soil solution (R2 = 0.97) (Fig. 4) .



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  		Fig. 3. Effects of chelate application on (a) shoot dry yields and (b) Pb concentrations in cabbage shoots. Values are means ± SEs (n = 3).

 


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  		Fig. 4. Relationship between the Pb concentration in soil solution and cabbage shoots.

 
Distribution of Pb in the shoots and roots of cabbage was also significantly affected by the chelate application (Table 4). When no chelate was applied to the soil, the concentration of Pb in the roots of cabbage was significantly higher than that in the shoots. Most of the Pb absorbed by cabbage was concentrated in the roots, accounting for more than 65% of total absorbed Pb from the soil. Treatments with various chelates significantly increased the shoot to root ratios of Pb concentrations in cabbage. The percentage of the absorbed Pb translocated from roots to shoots increased from 35% in the control samples to 73 to 92% with 1.5 mol/L of HEDTA, EDTA, and DTPA application. The NTA and citric acid were less effective in increasing Pb translocation in cabbage compared with other chelates used (Table 4).


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  		Table 4. Effects of chelate treatments on Pb translocation from cabbage roots to shoots and Pb concentration in xylem sap.{dagger}

 
Similar to the effects of chelates on the concentration of Pb in cabbage shoots, the application of chelates also significantly increased the Pb concentration in shoot xylem sap within 24 h of the application (Table 4). The effect of different chelates on the Pb concentration in shoot xylem sap was similar to the effects of chelates on the Pb concentration in cabbage shoots.

Effects of Amount and Regime of EDTA Application
Low-level EDTA (1.0 mmol/kg soil) application to the soil had some effects on the shoot dry matter yield of cabbage compared with that of the control plants (Fig. 5a) . The application of EDTA at the level of 5.0 mmol/kg soil or higher significantly depressed the plant growth, and decreased dry matter yield of the shoots. The concentrations of soluble Pb in soil and Pb concentrations in the shoots significantly increased with the increasing level of EDTA applied to the soil (Fig. 5b and 5c). However, the relative efficiency of EDTA in enhancing Pb accumulation in the shoots (defined as the ratio of Pb concentration in shoot to the EDTA concentration applied) was highest when 1.5 or 3.0 mmol EDTA/kg soil was applied (Fig. 6) .



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  		Fig. 5. Effects of ethylenediaminetetraacetic acid (EDTA) application level on (a) dry matter yield, (b) soluble Pb concentration in soil, and (c) Pb concentration in cabbage shoots. Values are means ± SEs (n = 3).

 


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  		Fig. 6. Relative efficiency of ethylenediaminetetraacetic acid (EDTA) application in enhancing the Pb accumulation in cabbage shoots. Values are means ± SEs (n = 3).

 
To test Pb solubility and plant uptake, EDTA (total rate of 1.5 mmol/kg soil) was added to the soil in one-, two-, and three-dose application procedures. At the same total application amount, the procedure of the three-separate EDTA applications produced the highest shoot Pb concentration in cabbage, followed by the two-separate application method and the single-application method (Fig. 7a) . The effects of these EDTA application methods on the mobility of Pb in the soil were also examined. When the plants were harvested 8 d after initial EDTA application, the concentrations of water-soluble Pb in the bottom soil of the pots were measured. Results showed that the soluble Pb concentration in the bottom soil was in the order: one-dose application > two-dose application > three-dose application (Fig. 7b). The data indicated that the multidose chelate application method could limit the solubility and migration potential of heavy metals in soils.



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  		Fig. 7. Effects of ethylenediaminetetraacetic acid (EDTA) application methods on (a) the Pb concentrations in cabbage shoots and (b) the soluble Pb concentration in soil at 8.0 to 8.5 cm below the surface at the same total application amount (1.5 mmol/kg soil). Values are means ± SEs (n = 3).

 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
In soils, heavy metals are present in various chemical forms and generally exhibit different physical and chemical behaviors in terms of chemical interactions, mobility, biological availability, and potential toxicity. Lead in soils is identified as a soft Lewis acid, and it can form stable complexes with organic matter, Fe and Mn oxides, and clays, and precipitate as carbonates, hydroxides, and phosphates (Kinniburgh et al., 1976; McBride, 1994). In the present study, the residual fraction dominated the Pb distribution in the soil, accounting for more than 69% of total Pb concentration (Table 4). This may be related to the fact that the soil was obtained from mine wastes at the old mining site. Among the nonresidual fractions, the Fe–Mn oxide fraction was predominant (Table 4). This result is in agreement with observations of Ramos et al. (1994), Lopez-Sanchez et al. (1996), and Jones and Turki (1997). Results in Table 4 also indicated that the greatest contribution to EDTA-extractable Pb was from the carbonate–specifically adsorbed and Fe–Mn oxide fractions. Generally, the solubility of metal fractions was in the order: exchangeable > carbonate–specifically adsorbed > Fe–Mn oxide > organic–sulfide > residual (Tessier et al., 1979; Ma and Rao, 1997; Li and Thornton, 2001).

Results from this study demonstrate that some chelates can rapidly and significantly increase Pb concentrations in soil solution due to strong chelation of Pb. For the five chelates tested, their efficiencies in solubilizing soil-bound Pb varied significantly. The EDTA was the most effective chelate in solubilizing Pb from the soil (Fig. 1). This may be related to the high binding capacity of EDTA for Pb (Blaylock et al., 1997; Huang et al., 1997; Wu et al., 1999). The solubilization effect by DTPA was not as effective as EDTA, although both have a similar binding constant for Pb. In the experiment of Wu et al. (1999), the concentration of DTPA-extractable Pb in soil decreased with increasing extraction time from 6 to 120 h. It has been demonstrated that DTPA is more rapidly degraded than EDTA (Means et al., 1980). These could explain the different abilities of EDTA and DTPA to enhance Pb solubilization in the soil.

In situ application of chelates may pose potential risks of water pollution through uncontrolled metal solubilization and migration. Mobilization of heavy metals into ground water is not only dependent on soil properties and water status, but also on the balance of solubilized soil-bound metals and metal uptake by plant. To avoid possible chelate-metal movement into ground water, the amount, time, and method of chelate application should be carefully controlled. Since soil-bound Pb was solubilized by EDTA and DTPA in a short duration (6 h), and majority of Pb uptake by the plants may have occurred rapidly after EDTA application (Huang et al., 1997; Epstein et al., 1999). One approach for this situation is to reduce dissolution rate of soil-bound Pb by EDTA application and to maintain a certain soluble Pb concentration over the plant growth period. Results in this study indicated that EDTA applied in three separate applications was the most effective in enhancing Pb accumulation in cabbage shoots compared with other application methods at the same total level of EDTA application (Fig. 7a). The multidose application of EDTA at a given level also decreased the mobility of Pb in soil (Fig. 7b). This method could help to minimize the amount of chelate applied in the field, reduce the operation cost, and alleviate the potential risks of EDTA and heavy metal migration to ground water. Moreover, it has been proposed that transpiration is a major force that drives Pb accumulation in plant shoots (Blaylock et al., 1997). The EDTA treatment decreased the transpiration rate of Indian mustard (Vassil et al., 1998; Epstein et al., 1999). Therefore, applying EDTA in three or more separate additions during the treatment process may minimize its adverse effects on transpiration rate.

Chelation of heavy metal ions has long been recognized as an important factor in metal uptake by plants (Lindsay, 1978). Results from this study demonstrate that synthetic chelates can induce Pb accumulation in cabbage. Most of the increased uptake of Pb after the chelate treatments could be explained as an effect of enhancing Pb solubility (Fig. 4). It is well known that metal solubility and bioavailabilty can be increased by the application of synthetic chelates such as EDTA, HEDTA, NTA, and citric acid to soil (Blaylock et al., 1997; Huang et al., 1997; Huang et al., 1998; Ebbs and Kochian, 1998; Kayser et al., 2000; Wu et al., 1999). In contrast, no significant effect of 210Pb-EDTA and -DTPA on plant uptake was observed in solution culture and soil culture (Athalye et al., 1995). The addition of NTA, DTPA, and EDTA (0.5–4 g/kg soil) caused a significant decrease in the uptake of Ni by Berkheya coddii (Robinson et al., 1999). Treatments with EDTA or DTPA decreased uptake of Zn, Mn, and Cu by the hyperaccumulator plant Thlaspi caerulescens in solution culture (Shen et al., 1998). According to the common understanding, plants take up metals in the free ionic form, and the chelated forms of metals are less available for uptake than the free ionic form (Chaney, 1988; Parker et al., 1995). However, some evidence indicates that chelated metals may be absorbed by plant roots. The effects of chelates on metal uptake may also be related to the different uptake mechanisms of the metal concerned. For example, Zn transport into the root cytosol is via a protein-mediated transport system with a fairly high affinity for Zn (Lasat et al., 2000), which can be contrasted to the passive uptake of Pb by plants (Huang and Cunningham, 1996). The application of EDTA solubilizes large quantities of Pb from soil, which then diffuses down its concentration gradient into plant root, and can be taken up by mass flow. Recent studies have shown that Pb accumulation in plant shoots is correlated with the formation of Pb–EDTA complex, and Pb–EDTA is the major form of Pb absorbed and translocated by the plant (Vassil et al., 1998; Epstein et al., 1999). It has been suggested that metal chelate complexes may enter the root at breaks in the root endodermis and Casparian strip, and be rapidly transported to the shoots (Romheld and Marschner, 1981; Bell, et al., 1991). Therefore, plant uptake of metal–chelate complexes can be related to the total metal concentration in the nutrient solution rather than the activity of the free ionic metal.

It is reported that members of Brassicae have an ability to take up heavy metals from contaminated soils and transport these metals to the shoots. This ability may be inherited from some wild species in Brassicaceae (Kumar, et al., 1995). Furthermore, the ability of a plant to absorb and accumulate heavy metals depends also on metal concentration and chemical forms in soil. Metal solubility in soil and accumulation by plants can be enhanced by the chelate application to soils. However, the response of different plant species to EDTA application was also markedly different. The results of cabbage in the present study are compatible with those for other plants reported previously (Blaylock et al., 1997; Huang and Cunningham, 1996; Huang et al., 1997; Wu et al., 1999; Stanhope et al., 2000), although the Pb concentration in shoots and the magnitudes of the increase observed in the present experiment were somewhat smaller than those reported previously. It was noticed that the development of all cabbage plants appeared normal after EDTA application (Table 2), and cabbage shoot biomass was similar in both the control and chelate-treated soil (Fig. 3). It was reported that some plants grown on chelate-treated soils died or exhibited visible toxic symptoms (Huang and Cunningham, 1996; Epstein et al., 1999; Stanhope et al., 2000). Good growth of cabbage plants over the chelate-treatment period in the current study may also help the roots to absorb more Pb from soil and to reduce the risk of soluble Pb movement into ground water.


   ACKNOWLEDGMENTS
 
This work was supported by the National Natural Science Foundation of China (no. 29977009) and the Hong Kong Polytechnic University (Project no. A-PC60 and G-YB69). The authors thank the reviewers and editor, whose constructive and detailed comments greatly improved the manuscript.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 




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Enhancing Phytoextraction: The Effect of Chemical Soil Manipulation on Mobility, Plant Accumulation, and Leaching of Heavy Metals
J. Environ. Qual., November 1, 2003; 32(6): 1939 - 1954.
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The SCI Journals Agronomy Journal Crop Science
Vadose Zone Journal Journal of Plant Registrations
Journal of Natural Resources
and Life Sciences Education		 Soil Science Society of America Journal