phytoextraction and accumulation of lead from contaminated soil by vetiver grass: laboratory and...

PHYTOEXTRACTION AND ACCUMULATION OF LEAD FROMCONTAMINATED SOIL BY VETIVER GRASS: LABORATORY AND

SIMULATED FIELD STUDY

S. CHANTACHON1, M. KRUATRACHUE1,2∗, P. POKETHITIYOOK1, S. UPATHAM3,S. TANTANASARIT4 and V. SOONTHORNSARATHOOL1

1 Department of Biology, Faculty of Science, Mahidol University, Rama 6 Road, Bangkok 10400,Thailand; 2 Department of Biology, Faculty of Science, Bangkok 10400, and Mahidol UniversityInternational College, Salaya, Nakhonpathom, 73170, Thailand; 3 Burapha University, Chonburi

20130, Thailand; 4 Department of Conservation, Faculty of Forestry, Kasetsart University, Bangkok10900, Thailand

(∗ author for correspondence, e-mail: [email protected], Fax: 662 247 7058)

(Received 8 July 2003; accepted 31 October 2003)

Abstract. A soil-culture study was conducted to investigate the phytoextraction of lead (Pb) in twospecies of vetiver grass (Vetiveria zizanioides and V. nemoralis) irrigated with an increasing level ofPb(NO3)2 (5, 7, 9 and 11 g L−1) for 12 weeks. In a laboratory study, the removal of lead from soilwas correlated with lead accumulation by roots and shoots of both species of vetiver grass. Highconcentration of lead (9–11 g L−1) resulted in decrease of growth, total chlorophyll content andbiomass of V. zizanioides, while V. nemoralis died after one week of application. Toxicity symptoms(e.g., burning leaf margins, shoot die back) occurred in vetiver grass at a high concentration of lead.Based on the data V. zizanioides tolerated and accumulated the greatest amount of lead the best. Asimulated field experiment was conducted to examine the efficiency of vetiver grass in removinglead from contaminated soil. The vetiver grasses, V. zizanioides and V. nemoralis, were grown in soilcontaminated with Pb(NO3)2 (5, 7, 9, and 11 g L−1) for 3 months. The removal of lead from soilwas correlated with lead accumulation by roots and shoots of both grass species. The grass rootstook up more lead than the shoots. V. zizanioides could uptake more lead from soil than V. nemoralis.The effects of lead on the biomass of V. zizanioides and V. nemoralis showed that in both species,the biomass was decreased when the lead concentration was increased. In comparison, V. zizanioidesshowed greater biomass than V. nemoralis.

Keywords: accumulation, lead, phytoextraction, vetiver grass

1. Introduction

Lead (Pb) is a common contaminant of surface soils, giving high toxicity to plants,animals, and man. There is a great need for techniques to reduce lead at contam-inated sites (Gregorio et al., 2000). Concern about the spreading of these contam-inants has resulted in strict guidelines being set to prevent the increasing concen-tration of heavy metal pollutants. In some cases, industrial and mining projectshave been ceased until appropriate methods of decontamination or rehabilitationhave been implemented at the source. The method used in these situations, is to

Water, Air, and Soil Pollution 00: 1–20, 2004.© 2004 Kluwer Academic Publishers. Printed in the Netherlands.

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treat the contaminants chemically to remove them from the site. These methodsare expensive and at times impossible to carry out as the volume of contaminatedmaterial is very large (Truong and Baker, 1998). If these wastes cannot be treated orremoved economically, off-site contamination must be prevented. Wind and watererosion and leaching are often the causes of off-site contamination. An effectiveerosion and sediment control program can be used to rehabilitate such sites.

Phytoremediation or vegetation method offers a lower cost for soil remediationand extraction of metals. Phytoextraction is an integrated multidisciplinary ap-proach to the clean up of contaminated soils using accumulator plants. Phytoextrac-tion requires that the target metal must be (1) available for plant roots, (2) absorbedby the roots, (3) translocated from the root to the shoot, and (4) that the biomassproduction is substantial (Vandenhove et al., 2001). The metal is removed from thesite by harvesting. After harvesting, the biomass is processed to either recover themetals or further concentration of that metal (thermal, microbial, chemical treat-ment) to facilitate disposal (Vandenhove et al., 2001). Phytoextraction of metal byplants offers a great promise for commercial development (Chaney et al., 1997).

Vetiver grass (Vetiveria sp.) has been widely known for its effectiveness inerosion and sediment control (Truong et al., 1995). It is a versatile, hardy plantthat is fast growing and can survive in a harsh environment. It also has been foundto be highly tolerant to extreme soil condition including high metal concentration(Randloff et al., 1995; Knoll, 1997; Truong and Baker, 1998; Chen, 2000). Thevetiver grass has been successfully used to stabilize mining overburden and highlysaline, sodic, and alkaline tailing of gold mines (Randloff et al., 1995). In SouthAfrica, it was used effectively to stabilize waste and slime dams from platinum andgold mines (Knoll, 1997). In Australia, vetiver grass was used to stabilize landfilland industrial waste site contaminated with heavy metals such as As, Cd, Cr, Ni,Cu, Pb and Hg (Truong and Baker, 1998). In China, vetiver grass was planted on alarge scale for pollution control and mine tail stabilization (Chen, 2000).

In Thailand, vetiver grass is found widely distributed naturally in all parts ofthe country. It has been used for erosion control and slope stabilization. It wasindicated that vetiver hedges have an important role in the process of captivity anddecontamination of pesticides, preventing them from contaminating and accumu-lating in crops (Pinthong et al., 1998). Moreover, it is believed that, compared toother plants, vetiver should be more efficient in absorbing certain heavy metalsand chemicals due to the capacity of its root system to reach greater depth andwidths (FAO-RAP, 1998). However, to the best of our knowledge, the study onphytoextraction and accumulation of heavy metals by vetiver grass is still lacking.Hence, the purpose of this study was to investigate the efficiency of lead extrac-tion and accumulation by two species of vetiver grass, Vetiveria zizanioides and V.nemoralis, under laboratory (soil culture) and simulated field condition.

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PHYTOEXTRACTION OF LEAD BY VETIVER GRASS 3

TABLE I

Soil properties and methods used for soil analysis prior to the experiment

Property Constituent Method Reference

Organic matter 0.76% Walkley-Black titration Walkley and Black, 1934

N 0.038% Kjedhal method Black, 1965

P <1 mg L−1 Bray II method Bray and Kurtz, 1945

K 51% Conc. HClO4 digestion Pratt, 1965

Atomic absorption

spectrophotometer

pH 5.34–6.55 pH meter National Soil Survey

Center, 1996

ECe 0.44 ms cm−1 Electrical conductivity Richards, 1954

Pb 22.0 mg kg−1 Conc. HNO3 atomic APHA, 1981

absorption spectrophotometer

Soil texture Sandy loams USDA (United States National Soil Survey

Department of Agriculture) Center, 1996

2. Materials and Methods

2.1. EXPERIMENT 1: PHYTOEXTRACTION OF LEAD FROM CONTAMINATED

SOIL BY VETIVER GRASS: LABORATORY STUDY

The experiment was performed at the laboratory of the Department of Biology,Faculty of Science and Technology, Rajabhat Institute, Mahasarakham, Thailand.

2.1.1. Experimental MaterialsTwo species of vetiver grass were used, i.e., Vetiveria zizanioides and V. nemor-alis. Vetiver sprouts were obtained from the Land Development Station, Mahas-arakham Province, Northeast Thailand. Before planting in separate pots, the topsand the roots were pruned to 10 and 5 cm, respectively. Plants were cultured in potscontaining sandy loam soil for one month prior to the lead treatment.

Sandy loam soil was used in the experiment. Soil samples were analyzed forlead concentration prior to the experiment. In addition, the pH, organic matter,total N, P, K and electrical conductivity were measured (Table I).

2.1.2. Experimental ProceduresLead solution was prepared by dissolving Pb(NO3)2 in distilled water as a stocksolution at the concentration of 20 g L−1. Then, the required serial dilution wasmade up: 5, 7, 9 and 11 g Pb L−1. One-month-old vetiver grass sprouts were ex-posed to these concentrations of Pb; the volume of each concentration was 500 mL

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for each pot, which contained 3 sprouts. Fifteen sprouts were used for each leadconcentration. Three replicates of each treatment were performed. The controlswere not exposed to lead solution. The soil in each pot was watered to field ca-pacity. After watering, the pots were placed in the laboratory under controlledtemperature (28–30 ◦C), 100 watt cool white fluorescence lamp and a photoperiodof 8 hr d−1. Water was replenished every week (200 mL) to maintain a constant os-motic potential. At 4, 8 and 12 weeks, plants were sampled randomly and measuredfor biomass, total chlorophyll content and lead concentration.

2.2. EXPERIMENT 2: PHYTOEXTRACTION OF LEAD FROM CONTAMINATED

SOIL BY VETIVER GRASS: SIMULATED FIELD STUDY

The experiment was performed in the simulated field at the Department of Biology,Faculty of Science and Technology, Rajabhat Institute, Mahasarakham.

2.2.1. Experimental MaterialsV. zizanioides and V. nemoralis were collected from the Land Development Station,Mahasarakham Province, Northeast Thailand. Before planting on the experimentalplots, the roots of vetiver grass were cut about 5 cm below the surface and theleaves were cut about 10 cm above the roots. The clump was broken into about 3tillers.

Before planting, soil core at 30 cm in depth and 3 cm in diameter was randomlycollected from six points of each plot. Samples were air dried, sieved to 2 mm andanalyzed for pH, organic matter, N, P, K, ECe, soil texture, and soil lead (Table I).

2.2.2. Experimental ProceduresExperimental plots of V. zizanioides and V. nemoralis were established at the Ra-jabhat Institute, Mahasarakham, in January 2000 to June 2000. The plots were2 × 3 m in size. Five planting plots were used for each species of vetiver grass. Ineach plot, 24 clumps of vetiver were planted (4 row and 6 columns). The distancebetween row and column was 50 cm. The grass was planted at a depth of 30 cm.At the bottom, a thin plastic sheet was lined to prevent the leachate of lead.

A lead solution was prepared by dissolving Pb(NO3)2 in distilled water as astock solution at the concentration of 20 g L−1. Then, the required serial dilutionwas made up: 5, 7, 9 and 11 g Pb L−1.

One-month-old vetiver grass sprouts in the plots were exposed to each concen-tration of lead (5, 7, 9, 11 g Pb L−1); the volume of each concentration was 10 L.The controls were not exposed to lead solution. The soil in each plot was wateredto field capacity. Water was replenished every week (10 L per plot) to supplementwater reduced by transpiration and evaporation.

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2.3. PLANT AND SOIL ANALYSIS

The plants were harvested after 12 weeks or three months in both experiments.The plant tops were harvested by cutting the stem just above the soil surface. Theplant roots were harvested after soaking the pots and their contents in a water bathand gently washing the soil from the roots. The roots and shoots were rinsed withdeionized water before drying at 85 ◦C in a hot air oven. Dry matter yield of rootsand shoots was determined by weighing. Lead concentration in the soil, shoots, androots was determined by an atomic absorption spectrophotometer (AAS) followingdigestion with concentrated nitric acid and 30% H2O2 (APHA, 1981). Samples ofeach treatment were measured for the total chlorophyll content according to themethods of Whittingham (1974). In addition, toxicity symptoms were observed.

2.4. DATA ANALYSIS

Two-way analysis of variance (ANOVA) was used to determine the variance of thedata and means. The protected least square difference (LSD) was used to comparethe means for each time of measurement or sampling.

2.5. PHYTOEXTRACTION COEFFICIENT

Metal was reported as phytoextraction coefficient, i.e., the ratio of metal concen-tration in the plant roots and/or above ground tissue (g metal/g dry weight oftissue) to the initial soil concentration of the metal (g metal/g dry weight soil) (U.S.EPA, 2000). The phytoextraction coefficients of vetiver, both from laboratory andsimulated field study, were determined.

3. Results

3.1. EXPERIMENT 1. PHYTOEXTRACTION OF LEAD FROM CONTAMINATED

SOIL BY VETIVER GRASS: LABORATORY STUDY

3.1.1. Toxicity SymptomsThe comparison of two species of vetiver grass showed that V. zizanioides wasmore tolerant to lead than V. nemoralis. At lead concentrations of 5–7 g L−1, V.zizanioides could survive and grow moderately well, while V. nemoralis started toshow toxicity symptoms e.g., chlorosis, burning of leaf magins, leaf abscission andshoot die black. These occurred in both species of vetiver grass, grown in highconcentration (9–11 g L−1) of lead. V. nemoralis began to die one week after theexposure, while V. zizanioides started to show toxicity symptoms.

3.1.2. BiomassLead had a significant effect on the biomass (dry weight yield) of the two speciesof vetiver grass. The results showed that there was a significant increase in biomass

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Figure 1. Effect of lead concentration on biomass of V. zizanioides (A) and V. nemoralis (B) in thelaboratory study.

(P≤0.05) in V. zizanioides, as the lead concentration was increased from 5 to 9 gL−1 (Figure 1A). In contrast, there was a significant reduction (P≤0.05) in thebiomass of V. nemoralis (Figure 1B). All V. nemoralis died after one week of expos-ure to a lead concentration of 9–11 g L−1. However, at a lead concentration of 11 gL−1, there was a significant reduction (P≤0.05) in the biomass of V. zizanioides(Figure 1A).

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Figure 2. Effect of lead concentration on the total chlorophyll content of V. zizanioides (A) and V.nemoralis (B) in the laboratory study.

3.1.3. Total chlorophyll contentLead has a significant effect on the total chlorophyll content (P≤0.05) of bothspecies of vetiver grass (Figure 2). In V. zizanioides, the total chlorophyll contentsof control, 5 and 7 g L−1 treatments were subsequently increased (P≤0.05) (Fig-ure 2A), while in V. nemoralis, only those of control and 5 g L−1 treatment weresignificantly increased (P≤0.05) (Figure 2B). At higher lead concentrations (9 and

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Figure 3. Lead accumulation in shoots of V. zizanioides (A) and V. nemoralis (B) in the laboratorystudy.

11 g L−1 for V. zizaniodies; 7, 9 and 11 g L−1 for V. nemoralis), the total chlorophyllcontents significantly decreased (P≤0.05) (Figure 2). The results showed that V.zizanioides was more tolerant to lead than V. nemoralis.

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Figure 4. Lead accumulation in roots of V. zizanioides (A) and V. nemoralis (B) in the laboratorystudy.

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TABLE II

Phytoextraction coefficients of V. zizanioides (Vz) and V. nemoralis(Vn) in the laboratory study

Concentration Phytoextraction coefficient

of Pb Week 4 Week 8 Week 12

(g L−1) Vz Vn Vz Vn Vz Vn

5 1.13 1.52 1.77 2.37 2.51 3.00

7 1.58 0.91 2.00 1.14 2.70 1.32

9 1.35 – 1.52 – 1.65 –

11 1.82 – 2.02 – 2.11 –

3.1.4. Lead accumulation in plantsLead was found to accumulate more in roots than in shoots of both vetiver species(P≤0.05) (Figures 3 and 4). The accumulation increased significantly (P≤0.05)when the exposure time and lead concentration were increased (Figures 3 and 4).The comparison between the two species of vetiver grass shows that V. zizanioidescould tolerate and accumulate higher concentration of lead (7, 9 and 11 g L−1

treatments) than V. nemoralis (P≤0.05) (Figures 3 and 4). However, at a lower con-centration of lead (5 g L−1), V. nemoralis could accumulate more lead (817.4 mgkg−1) in roots than V. zizanioides (618.8 mg kg−1) (Figure 4A). The maximumaccumulation (4545.9 mg kg−1) was found in roots of V. zizanioides at 11 g L−1

lead exposure for 12 weeks (Figure 4A).

3.1.5. Lead content in soilThe results showed that lead content in soil significantly decreased (P≤0.05) atlower levels of lead concentration (5, 7 g L−1) in both species of vetiver (Figure 5).In V. zizanioides, at higher lead treatments (9, 11 g L−1), the lead content in soilsignificantly decreased at 1 and 4 weeks (P≤0.05); however, it did not decreasesignificantly at 8 and 12 weeks (P > 0.05) (Figure 5A). V. nemoralis died afterone week of exposure to 9 and 11 g L−1 of lead (Figure 5B).

3.2. PHYTOEXTRACTION COEFFICIENT

The data on phytoextraction coefficient showed that V. zizanioides was more ef-ficient (P≤0.05) in extraction of lead from soil than V. nemoralis at 7 g L−1 oflead concentration (Table II). However, at 5 g L−1 of lead, the efficiency was rathersimilar (P > 0.05). The highest phytoextraction coefficient (2.7) was obtained inV. zizanioides at 7 g L−1 lead after 12 weeks (Table II).

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Figure 5. Lead content in soil from various treatments of V. zizanioides (A) and V. nemoralis (B) inthe laboratory study.

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3.3. EXPERIMENT 2. PHYTOEXTRACTION OF LEAD FROM CONTAMINATED

SOIL BY VETIVER GRASS: SIMULATED FIELD STUDY

3.3.1. Toxicity symptomsThe comparison of two species of vetiver grass, grown in simulated field condi-tion for three months, showed similar results to those of the laboratory study. V.zizanioides and V. nemoralis could survive and grow moderately well at a lowlead concentration (5–7 g L−1). However, at higher lead concentration (9–11 gL−1), they showed toxicity symptoms, such as the burning of leaf margins and thechlorosis of leaves.

3.3.2. BiomassLead had a significant effect (P≤0.05) on the biomass of the two species of vet-iver grass under the simulated field conditions (Figure 6). In V. zizanioides and V.nemoralis, the biomass was reduced when the lead concentration was increased.However, the exposure time did not seem to have any significant effect (P > 0.05)on the biomass of both species. The highest biomass was found in V. zizanioidesexposed to 5 g L−1 lead (404.4 g; Figure 6A) and V. nemoralis (295.5 g; Figure 6B)after 12 weeks. In comparison, V. zizanioides showed a greater biomass than V.nemoralis at every lead concentration (Figure 6).

3.3.3. Lead accumulation in plantsLead was found to accumulate more in roots than in shoots of both vetiver species(P≤0.05) (Figures 7 and 8). The accumulation increased significantly (P≤0.05)when the exposure time and lead concentration were increased. The comparisonbetween the two species of vetiver grass shows that V. zizanioides could accumulatemuch higher lead content than V. nemoralis (P≤0.05). The highest accumulationwas found in the roots of V. zizanioides (2842.5 mg kg−1) exposed to lead at 11 gL−1 after 12 weeks (Figure 8A).

3.3.4. Lead content in soilThe results showed that lead content in soil significantly decreased (P≤0.05) whenlead concentration and exposure time were increased in both species of vetiver(Figure 9). The comparison between the two species of vetiver grass shows thatV. zizanioides could extract more lead (P≤0.05) than V. nemoralis at 5 g L−1

concentration (Figure 9).

3.3.5. Phytoextraction coefficientThe data on phytoextraction coefficient showed that V. zizanioides was more effi-cient (P≤0.05) in the extraction of lead from soil than V. nemoralis at every leadtreatment (Table III). Furthermore, at 7 g Pb L−1, V. zizanioides performed the bestin phytoextraction of lead with the highest phytoextraction coefficient (32.0) after12 weeks (Table III).

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Figure 6. Effect of lead concentration on the biomass of V. zizanioides (A) and V. nemoralis (B) inthe simulated field study.

4. Discussion

The purpose of this study was to demonstrate the application of phytoremedi-ation as a cleanup technology for metal contaminated soils, specifically lead. Tomake this type of remediation strategy successful, it is necessary to utilize metal-accumulator plants to extract environmentally important toxic metals. In addition,

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Figure 7. Lead accumulation in shoots of V. zizanioides (A) and V. nemoralis (B) in the simulatedfield study.

the establishment of benchmark tolerance level of plants to adverse soil conditionmeasures, which has a direct application in environment protection, has to be done.

The results from this study showed that V. zizanioides had a higher toleranceto lead than V. nemoralis. In the soil culture study, the results showed the highestbiomass in V. zizanioides at 7 g L−1 lead after 12 weeks and this started to decrease

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Figure 8. Lead accumulation in roots of V. zizanioides (A) and V. nemoralis (B) in the simulated fieldstudy.

in 9 and 11 g L−1 treatments. V. nemoralis could not tolerate lead at a high con-centration (5–11 g L−1); they attained the highest biomass at 5 g L−1 lead after 12weeks and they died when they were treated with 9 and 11 g L−1 lead. The resultsfrom the simulated field study correlated very well with those from the laboratorystudy. However, in the field, V. zizanioides grew best at 5 g L−1 lead after 12 weeksand the biomass started to decline in 7–11 g L−1 lead treatments. In V. nemoralis,

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Figure 9. Lead content in soil from various treatments of V. zizanioides (A) and V. nemoralis (B) inthe simulated field study.

the highest growth was found in the control. However, they did not die at a higherconcentration of lead (9–11 g L−1), but the growth was decreased. Significantlyhigher biomass was found in V. zizanioides (404.3 compared to 269.6).

The results of this experiment confirmed the ability of vetiver to establish andgrow well in shallow contaminated soil. Important is that the results also clearly

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TABLE III

Phytoextraction coefficients of V. zizanioides (Vz) and V. nemoralis (Vn)in the simulated field study

Concentration Phytoextraction coefficient

of Pb Week 4 Week 8 Week 12

(g L−1) Vz Vn Vz Vn Vz Vn

5 6.33 2.04 11.50 2.88 22.00 8.71

7 4.70 1.40 9.61 2.10 32.00 2.73

9 5.55 1.39 8.00 1.75 10.38 2.41

11 5.10 1.22 7.25 1.74 9.1 2.05

demonstrated that the growth and performance of the two species of vetiver werenot affected by the exposure to the lead solution. Indeed, V. zizanioides and V.nemoralis were little affected by lead at a concentration as high as 11 g L−1 level,which is encountered in the environment only in situations of accidental spillage,or direct application to soil. Vetiver’s tolerance of lead was demonstrated on termof growth. However, the good growth of vetiver at a high lead nitrate concentrationis probably partly due to the presence of soluble nitrate, which enhances all plantgrowth.

Chaney et al. (1997) reported that hypertolerance is the key plant characteristicrequired for hyperaccumulation. V. zizanioides could tolerate the lead concentrationup to 7 g L−1. This result is similar to the finding of Troung and Baker (1998), whoreported that the levels of lead up to 800 mg kg−1 did not affect the growth ofvetiver. In addition, they also reported the tolerance of vetiver grass to other heavymetals such as Cd (toxic level 20–60 mg kg−1), Cu (50–100 mg kg−1), Cr (50 mgkg−1) and Hg (1500 mg kg−1). Chen (2000) conducted a similar experiment on theeffect of heavy metals (Cu, Pb, Zn, Cd and As) on vetiver growth. He found thatthe high contents of metals (Cu 100 mg kg−1, Zn 200 mg kg−1, Pb 300 mg kg−1,Cd 1.5 mg kg−1, As 30 mg kg−1) limited the growth of vetiver grass during the firstyear, but the effect was reduced in the second year (Chen, 2000). Cull et al. (2000)found that vetiver was highly tolerant to these heavy metals. The toxic of cadmiumfor vetiver was 45 mg kg−1 and for other plants between 5 and 20 mg kg−1. Animpressive finding was that, while the toxic threshold of vetiver was between 5 and18 mg kg−1 and that for nickel was 347 mg kg−1, the growth of most plants wasaffected at a content between 10 and 30 mg kg−1, for nickel. Vetiver had similartolerance to copper as other plants at 15 mg kg−1.

Experimental results from glasshouse studies showed that, when adequatelysupplied with nitrogen and phosphorus fertilizers, vetiver can grow in soil withextremely high acidity and high manganese. Vetiver growth was not affected and

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no obvious symptoms were observed when the extractable manganese in the soilreached 578 mg kg−1, soil pH as low as 3.3 and plant manganese was 890 mg kg−1

(Truong and Baker, 1996). Bermuda grass (Cynodon dactylon), which has beenrecommended as a suitable species for acid mine rehabilitation, has 314 mg kg−1

of manganese in plant top when growing in mine soil containing 106 mg kg−1 ofmanganese (Taylor et al., 1989).

Landfill sites and industrial wastes are usually contaminated with heavy metalsthat are highly toxic. The results from the work by Truong and Baker (1998)conducted in Queensland, Australia, have conclusively shown that vetiver could re-habilitate in reducing leachate from an old landfill near Brisbane. Similarly, Sripen(1996) also found that vetiver could absorb a substantial amount of cadmium,mercury and lead in the wastewater in Thailand.

The results from this study showed that V. zizanioides was a better accumu-lator of lead than V. nemoralis. The removal of lead from contaminated soil wascorrelated with more lead accumulation by roots and leaves of both species. Inaddition, higher accumulation of lead was found in roots more than in shoots ofboth species. The maximum lead accumulation in roots (4545.9 mg kg−1 from thelaboratory study; 2842 mg kg−1 from the simulated field study) was found in V.zizanioides treated with 11 mg L−1 lead after 12 weeks, while V. nemoralis couldaccumulate much less lead in roots (1229.6 mg kg−1 at 7 mg L−1 lead from thelaboratory study; 35.3 mg kg−1 at 11 mg L−1 lead from the simulated field study).In comparison, the lead accumulation in shoots of both species of vetiver was muchlower, i.e., 773.7 and 573.4 mg kg−1 in V. zizanioides, from the laboratory andsimulated field study, respectively; and 197.5 and 191.5 mg kg−1 in V. nemoralis,from the laboratory and simulated field study, respectively. Tight binding of leadto soils and plant material explains at least partially the low mobilization of themetal in soil and plants. While plants are known to concentrate lead in the roots,the lead translocation to the shoots is normally very low (Malone et al., 1974;Reeves and Brooks, 1983; Kabata-Pendias and Pendias, 2001). Kabata-Pendias andPendias (2001) found that lead accumulation by the above-ground parts of barley,Hordeum vulgare, was below 5% relative to the lead contents in the roots, thusconfirming that the lead was not translocated from the roots to the tops. Lan et al.(1992) used Typha latifolia to treat wastewater from a lead/zinc mine and foundthat it assimilated significant amounts of lead and zinc, especially in its roots. Leadprecipitate may have formed in or on plant roots with little translocation into aerialportions of the plant (Malone et al., 1974). Harrison and Laxen (1981) studied theabsorption of lead by many plants and found that translocation of lead into the rootslimited the quantities of lead that move into the above-ground portions. Likewise,the present study indicated that vetiver grass has the ability to accumulate lead athigh concentrations in roots and translocate it to shoot in limited quantities.

This is the first report in Thailand to show that vetiver grass can tolerate andaccumulate very high concentration of lead. Vetiver grass has a world-wide dis-tribution, especially in tropic regions such as Thailand. It is fast growing and has

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greater biomass when compared to other grass species. The present study indicatedthat V. zizanioides possessed many beneficial characteristics to uptake lead fromcontaminated soil, was the most tolerant and could be growing in soil contamin-ated with high lead concentration. The important implication of these findings arewhen vetiver is used for phytoextraction on sites contaminated with high levels ofheavy metals. It can be used to remove the heavy metals and disposed off safelyelsewhere, thus gradually reducing the contaminant levels.

5. Conclusion

The vetiver system is a proven technology; its effectiveness as an environmentalprotection tool has been demonstrated around the world. In addition, it offers apotential avenue for soil phytoremediation. The two species of vetiver grass, testedin this study, showed high Pb accumulation in roots. Among the two tested species,V. zizanioides was more tolerant to high concentrations of Pb than V. nemoralis.The vetiver grass is a very cost effective, environmental friendly and practicalphytoremedial tool for the control and attenuation of heavy metal pollution whenappropriately applied.

Acknowledgement

This work was partially supported by the Secondary Education Quality Improve-ment Project Office of Rajabhat Institutes Council (CRIC), Ministry of Education,Thailand.

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phytoextraction and accumulation of lead from contaminated soil by vetiver grass: laboratory and...

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