African Journal of Biotechnology Vol. 7 (17), pp. 3053-3060, 3 September, 2008 Available online at http://www.academicjournals.org/AJB ISSN 1684–5315 © 2008 Academic Journals Full Length Research Paper Heavy metal concentrations in soils and accumulation in plants growing in a deserted slag dumpsite in Nigeria Mary B. Ogundiran* and Oladele Osibanjo Department of Chemistry, University of Ibadan, Ibadan, Nigeria. Accepted 28 July, 2008 Early detection and remediation of heavy metals in soil and vegetation will ameliorate serious threats posed to human existence. An auto battery manufacturing company dumped slag containing proportion of heavy metals in an hectare of land at Lalupon, Ibadan, Nigeria. The extent of contamination of soil by heavy metals and their accumulation in plants around the abandoned slag was studied. Plants and the surrounding soils were sampled from different directions at increasing distance from the vicinity of the waste pile and their concentrations of heavy metals were determined. The levels of Pb, Zn and Ni in mg/kg ranged from 34.8 – 41500, 16.3 – 849 and ND – 48.2; 9.2 – 9700, 16.0 – 271 and 2.83 – 36.9; 4.5- 5670, 8.00 – 174 and ND – 322 in soil, plant root and plant shoot, respectively. The plant samples from the immediate environment of the waste were highly contaminated with Pb. Six plant species, particularly Sporobolus pyramidalis, met some of the conditions to be classified as hyperaccumulators for Pb, Ni and Zn, and three other plants fulfilled the criteria for heavy metal excluders. We conclude that the potential hyperaccumulators and excluders, under controlled conditions, can be used for phytoremediation of the site. Key words: Heavy metals, contamination, hyperaccumulator, excluder, phytoremediation. INTRODUCTION Soil contamination by heavy metals as a result of human currently researched soil decontamination methods for activities is a serious environmental issue all over the heavy metal polluted matter is phytoremediation which is world. Mining, smelting of metalliferous ores and metal the use of plants to accumulate heavy metal contami- scraps, electroplating, application of fertilizer and pesti- nants (phytoextraction) and also to restrict their cides, sludge dumping and generation of municipal waste dissemination from the polluting source (phytostabiliza- have been identified as the principal sources of soil tion) (Smith and Bradshaw, 1979; Kumar et al., 1995). contamination by heavy metals. Failure to mitigate high Low cost, little or no landscape disruption, preservation of heavy metal concentrations in soils may result in ecosystem, generation of a recyclable metal-rich plant mobilization of heavy metal contaminants into the flora residue, applicability to a range of toxic metals and public and fauna and subsequently into man with consequent acceptance, among others, have been identified as deleterious health effects. merits of phytoremediation. Once heavy metals get into the environment, whether Three important uses of plants in environmental studies in small or large quantities, they cannot be completely have been investigated which are as indicators of eliminated. However, their effects on the ecosystem can pollution (Gabriella and Attila, 2002; Fatoki and Ayodele, be mitigated through immobilization. Research continues 1991) as excluders and as accumulators. Excluders are on the effective, less expensive and environment friendly plants that limit the levels of heavy metal translocation methods of immobilizing heavy metals in contaminated within them and maintain relatively low concentrations in soil thereby making them less bioavailable. One of the their shoot over a wide range of soil concentrations. They are employed in regenerating heavy metal contaminated soils (Baker, 1981). Accumulators concentrate heavy metals in their shoots at both low and high soil metal concentrations and are utilized in extracting heavy metals *Corresponding author: E-mail: mbogundiran@yahoo.com. from contaminated soils (Rotkittikhun et al., 2006). UNIVERSITY OF IBADAN LIBRARY 3054 Afr. J. Biotechnol. Figure 1. Map of Ibadan showing Lalupon, the sampling site. Many field studies have investigated the accumulating turing company located in Lalupon, Ibadan, Nigeria. Lalupon lies o 1 o 1 capacity of plants that grew naturally on metalliferous between longitude 7 28 N and latitude 4 04 E as shown in Figure wastes and on contaminated soils. It has been observed 1. The area occupied by the waste is void of vegetation which may be a result of heavy metal toxicity while the adjacent land some from such surveys that some plant species accumulate distance away blossoms with different plant species. levels of heavy metals more than the normal levels encountered generally in plants (Boularbah et al., 2006; Kidd and Monterroso, 2005; Yanqun et al., 2004; Walter Plants and soil sampling et al., 2003; Bunzl et al., 2001; Escarre et al., 2000; Twenty six (26) plant samples consisting 15 different species were Smith and Bradshaw, 1979) thereby proposing that such collected at different distances and along three different directions, plants can be used as decontaminants of heavy metal west, north and north east from the centre of the waste pile and polluted soils. In Nigeria there is paucity of information identified. These were directions suspected to be contaminated by about such metal tolerant plants locally. Hence in this run-off and other erosion agents from the waste-pile. Soils were study, we looked at the levels of Pb, Zn and Ni in soils sampled at the same locations as the plant samples at 15 cm depth and plants from a site contaminated by an abandoned rooting zone and mixed to form composite samples at each location. The choice of plant species collected was based on Pb-acid battery waste in Lalupon, an outskirt of Ibadan, distance to the source of contamination and the availability at the Nigeria. point of collection as previously reported (Steinborn and Breen, 1999). For each plant species, depending on the biomass, two to six replicate samples were collected from each location within the MATERIALS AND METHODS area of 4 m2. The samples were mixed to form a composite of the particular species, stored and transported in plastic bags to the Site description laboratory for detailed analysis. Plant samples used for identifi- cation were collected separately and kept in a brown envelope. The study site was an abandoned auto battery waste dumpsite on Sample identification was done at Forestry Research Institute of an hectare of land owned by a defunct lead-acid battery manufac- Nigeria (FRIN), Ibadan. UNIVERSITY OF IBADAN LIBRARY Ogundiran and Osibanjo 3055 Plant and soil samples pretreatment that were made from previously analyzed samples. Replicate analyses were performed on the spiked soil and plant samples to Plant samples collected from the field were washed under running yield a mean which is used to determine trueness and also stan- tap water to remove adhered soils, and were then separated into dard deviation of the mean to measure precision (Stanton 1966; parts including roots and shoots. The samples were dried in an Valcarcel, 2000). oven for 48 h at 80oC. The dried samples were ground using agate 10% duplicate samples were included to evaluate measurement mortal and pestle, sieved to < 2 mm and transferred to polyethylene precision. Procedural blanks and standard solutions were also bags for storage until later analysis. The soil samples were air-dried included for analytical quality control to assure the accuracy and at room temperature for two weeks, mechanically ground and reproducibility of the results. sieved to < 2 mm diameter size. Soil analysis Statistical analysis The < 2 mm fraction soil samples were digested to determine the Pearson’s correlation coefficient was used to determine the maximal environmentally available heavy metals. This was done relationship between concentrations of soil Pb/Zn; soil Pb/Ni; soil using 2 M HNO3 in centrifuge tubes which were placed in boiling Pb/plant Pb and between soil Zn/plant Zn using Statistical Package water in a 1 L beaker on a hot plate for 2 h and shaken at 20 min for Social Science (SPSS) version 14.0. intervals. The digested samples were filtered into 25, 50 or 100 mL standard flasks for varying distances of sampling points to the waste pile. The filtrates were diluted to the marks on the flasks with RESULTS deionized water and stored in polyethylene tubes prior to instrumental analysis. Chemical properties of composite soil samples Composite sample The soil physico-chemical properties of the composite samples are presented in Table 1. The pH of the soils Seven composite soil sub-samples were obtained from the soil ranged between moderately acidic (5.82) and slightly samples collected from the different locations in addition to one composite from the waste pile. Bulk soils were tested for basic alkaline (7.58). The highest levels of Pb and Zn were physico-chemical properties including pH, potassium (K), calcium found in the composite waste pile sample. The levels of (Ca) and magnesium (Mg). Total K, Ca and Mg concentrations were Mg and K were much lower in the wastes when com- determined using flame emission after digestion of the composite pared with their high levels in adjacent soils. All the soils samples with boiling 2 M HNO3 for 2 h. The pH of the soil was were low in Ca content. The organic carbon content and measured in water at ratio 1:1 soil/water. the soil texture were reported elsewhere (Ogundiran, 2007). Plant analysis The roots and shoots of the different plants were analyzed Distribution of heavy metals in soils, roots and separately for heavy metal content. 1 g of < 2 mm fraction plant shoots of plant species samples was weighed into porcelain crucibles and was ignited in a muffle furnace for 6 h at a temperature between 450 - 500oC. Grey white ash was obtained at the completion of the ashing. The ash Chromolaena odorata and Imperata cylindrical were samples were allowed to cool and then 10 mL of 2 M HNO3 was predominant plant species on the site followed by added to each sample. The solution was evaporated to near Passiflora foetida, Sporobolus pyramidalis and Andro- dryness on a hot plate and the cooled residues were re-dissolved in pogon tectonum. The presence of other plant species 10 mL 2 M HNO3. The solutions were then filtered into 25 mL was sporadic. The concentration of heavy metals in volumetric flasks. Both the crucible and the filter paper were washed into the flasks, made up with deionized water and then plants and soils are as given in Table 2. Soil conta- stored in polyethylene tubes for instrumental analysis. Atomic mination with Pb and Zn was obvious in the west, north absorption spectrophotometer Buck Scientific 200 was used to and north east directions of the site. Generally, the analyse soil and plant digests for Pb, Zn and Ni. concentration of metals in soil and plants were found to decrease with distance from the waste pile to background Shoot/root quotient and extraction coefficient levels as reported (Kabata-Pendias and Pendias, 2001). Pb and Zn concentrations in soil and plants were greatest Shoot/root quotient was determined by dividing the concentration of at 40 m west and at 60 m north of the waste pile which the heavy metals in the shoot by that of the root while the extraction happened to be in the immediate vicinity of the waste. At coefficient was calculated by dividing the concentration of the heavy 40 m west, Pb and Zn levels in soils ranged from 1310 - metals in the shoot by that of the soil as previously described 19600 mg/kg and 96.0 - 849 mg/kg, respectively; Pb and (Rotkittikhun et al., 2006). Zn levels in plants ranged from 18.7 - 15410 mg/kg and 73 - 1270 mg/kg, respectively. Also, at 60 m north Pb and Quality control Zn levels in soils ranged from 797 - 41500 mg/kg and 232 974 mg/kg, respectively; Pb and Zn levels in plants The efficiency of digestion procedures for both soil and plant ranged from 317-693 mg/kg and 101 - 389 mg/kg, samples was tested by analyzing spiked samples (recovery studies) respectively. Heavy metals content in plants were ob- UNIVERSITY OF IBADAN LIBRARY 3056 Afr. J. Biotechnol. Table 1. Chemical properties of composite soil and waste samples in abandoned auto battery waste dumpsite in Lalupon, Ibadan, Nigeria. Concentration (mg/Kg) Location pH Pb Cd Ni Zn Ca Mg K 40 mW 5.82 12600 ND ND 131.2 10.0 1450 3620 60 mW 6.29 21.5 ND ND 20.75 3.25 1730 1410 100 mW 7.05 44.5 ND 1.91 37.3 74.5 4820 8740 60 mN 5.86 18900 ND 10.3 182 27.0 3060 4020 100 mN 6.53 1140 ND 9.90 52.0 32.0 2720 4020 140 mN 7.58 1450 ND ND 48.0 53.0 3640 3620 60 mNE 6.09 49.0 ND ND 31.0 10.0 1130 4420 Waste pile 6.87 95700 ND 31.1 1670 75.0 974 804 ND = Not detectable. Table 2. Concentration of Pb, Zn and Ni in soils and plants collected from auto battery waste dumpsite, Lalupon. Pb concentration (mg/kg) Zn concentration (mg/kg) Ni concentration (mg/kg) Location Plant species Soil Root Shoot Soil Root Shoot Soil Root Shoot 40 mW S. pyramidalis 19600 9740 5670 849 197 174 4.11 16.2 322 Py. polystachyos 23500 9640 94.8 817 1250 18.3 ND 18.6 35.5 C. odorata 5540 275 95.5 270 275 23.5 15.6 20.9 ND I. cylindrical 1310 9.20 9.50 96.0 65.0 8.00 25.9 11.8 ND 60 mW C. odorata 89.5 25.0 104 20.5 283 41.3 30.2 16.4 8.2 P. foetida 182 15.0 29.5 18.5 83.8 12.0 30.0 16.0 7.25 A. tectonum 44.8 42.5 22.0 19.8 27.5 41.3 31.4 9.71 3.83 H. suaveolens 34.8 20.0 24.5 90.8 33.3 12.3 30.3 6.80 1.59 I. cylindrical 149 22.5 19.5 1.70 88.3 25.0 31.3 9.77 1.82 100 mW C. mucunoids 88.8 74.8 17.0 86.3 96.5 70.8 31.9 17.2 ND P. foetida 44.8 22.5 27.5 88.5 84.0 93.0 33.0 9.39 ND 60 mN P. foetida 797 569 99.8 242 93 86 38.7 11.2 ND A. tectonum 5990 280 37.0 237 277 112 47.7 14.5 61.5 C. odorata 4200 489 190 232 76.8 24.5 43.1 16.3 ND C. digitalis 41500 457 226 974 272 83.8 68.6 17.3 ND 100 mN Se. sesban 3450 67.3 62.0 69.8 82.8 20.0 21.9 16.3 76.2 T. platycarpa 2220 24.5 17.0 261 62.5 15.5 21.7 7.31 ND Cal. mucunoides 120 23.5 22.0 62.8 78.0 23.5 25.3 12.7 84.0 I. cylindrical 3130 22.0 4.50 65.3 89.5 69.8 27.8 12.6 ND 140 mN T. platycarpa 612 26.2 69.0 79.3 76.4 16.8 29.1 12.6 ND Ca. cajan 2950 79.8 39.3 81.5 83.8 13.3 23.4 15.9 ND C. odorata 291 50.0 37.0 16.3 16.0 16.0 25.9 8.52 ND 60 mNE D. aegypticum 428 41.0 7.00 77.0 96.5 20.8 48.2 36.9 237 Sc. dulcis 1490 14.5 9.50 208 82.5 91.8 13.1 17.0 ND C. odorata 209 49.0 64.7 48.8 82.5 23.8 22.2 12.9 118 S. pyramidalis 431 399 17.0 71.0 82.5 16.8 15.2 2.83 137 ND = Not detectable. tained by summation of the leves in roots and shoots at other countries (Kabata-Pendias and Pendias, 2001) and the locations considered. These levels fall within the also met remediation criteria for Pb and Zn contaminated concentrations of these metals in contaminated soils of soils (Ma and Rao, 1999; Vanik et al., 2005). UNIVERSITY OF IBADAN LIBRARY Ogundiran and Osibanjo 3057 Variations in heavy metals concentration with plant Unlike what was observed with Pb accumulation, Py. species polystachyos {1250 mg/kg (root), 18.3 mg/kg (shoot)}, C. odorata {275 mg/kg (root), 23.5 mg/kg (shoot)} and Cy. By and large, S. pyramidalis, Pycreus polystachyos, C. digitals {272 mg/kg (root), 83.8 mg/kg (shoot)} accu- odorata, P. foetida, A. tectonum, Cajanus cajan and mulated high concentrations of Zn only in the root with Cyperus digitals accumulated Pb (Table 2). S. pyrami- very little transfer to the shoot. dalis contained the highest concentrations of Pb in its The site did not show a considerable pollution with Ni. roots (9740 mg/kg) and shoots (5670 mg/kg), followed by However, some of the plant species showed high levels Py. polystachyos which had a high concentration in its of Ni in the shoots compared to the levels found in soils roots (9640 mg/kg). Plants such as I. cylindrical, (Table 2). Ni extraction coefficients and shoots to roots Tephrosia platycarpa and Scoparia dulcis sampled from a quotients for S. pyramidalis, Py. polystachyos, A. tecto- highly Pb-contaminated soil indicated low Pb concentra- num, Se. sesban, Cal. mucunoide and D. aegypticum tions in their morphologies in spite of the corresponding were greater than 1, with S. pyramidalis indicating the high soil Pb concentrations (Table 2). Some of the plants highest (78.3, 19.9) which showed high accumulation towards Pb also showed accumulation towards Zn particularly S. pyrami- dalis, Py. polystachyos and C. odorata (Table 2). Ni on Extraction coefficient and shoot/root quotient the other hand indicated weak accumulation towards C. odorata but excessively towards S. pyramidalis, Py. None of the plants that were analyzed had extraction polystachyos and Calopogonum mucunoide (Table 2). coefficient and shoot/root quotient greater than 1 for Pb. However S. pyramidalis, Cal. mucunoide and Se. sesban showed appreciable shoot/root quotients of 0.58, 0.94 Variations in heavy metals concentration in plants and 0.92 respectively. Sc. dulcis (1.11), C. odorata (1.49) with levels in soil and S. pyramidalis (0.88) had their shoot/root quotients >1 for Zn (Table 3). Ni extraction coefficients and shoots Lead and Zn concentrations in plants varied with levels in to roots quotients for S. pyramidalis, Py. polystachyos, A. soil. This was evident by the concentrations of heavy tectonum, Se. sesban, Cal. mucunoide and D. metals noticed in the roots of C. odorata, P. foetida, S. aegypticum were greater than 1 with S. pyramidalis pyramidalis and A. tectonum collected at different loca- indicating the highest (78.3, 19.9). tions. C. odorata indicated 275, 489, 50.0 and 49 and 25.0 mg/kg Pb in roots that grew on soils with 5540, 4200, 291, 209 and 89 mg/kg at 40 m W, 60 m N, 140 m DISCUSSION N, 60 m NE and 60 m W respectively. Similar trends were also observed for S. pyramidalis at 40 m W and 60 m NE. Chemical properties of composite soil samples Pearson correlation analysis established that Pb concentrations in the plants were positively correlated The highest levels of Pb and Zn were found in the with Pb concentrations in soil (r = 0.93, p < 0.01). The composite soil from the waste indicating the waste as the relationship between concentration of Zn in the soil and source of soil pollution. The presence or absence of soil plant was weak. micronutrients and macronutrients determine the viability of plants on a particular soil. This necessitated the need to determine the levels of some of the important nutrients Heavy metals accumulation in plants in the study area. Mg and K, which are important macronutrients for plant growth, were at much lower S. pyramidalis showed noticeable levels of Pb in roots levels in the wastes when compared with their high levels (9740 mg/kg) and shoots (5670 mg/kg shoot) followed by in adjacent soils. This, in addition to the high Pb and Zn C. odorata (489 and 190 mg/kg), Cy. digitals (457 and levels may explain the absence of plants on the waste 225 mg/kg) and P. foetida (569 and 100 mg/kg), while Py. pile. This may imply the need for nutrient enrichment of polystachyos (9640 mg/kg) and A. tectonum (270 mg/kg) the waste prior to future experiments with the waste and indicated high extraction of Pb only in their roots. Plants regeneration of vegetations of the bared area with the such as I. cylindrical {1310 mg/kg (soil), 9.20 mg/kg discovered local metal tolerant plants. It is also obvious (root), 9.20 mg/kg (shoot)}, T. platycarpa {2220 mg/kg from the table as indicated by the pH 40 mW (5.82) and (soil), 24.5 mg/kg (root), 17.0 mg/kg (shoot)} and Sc. pH 140 m N (7.58) that S. pyramidalis and C. odorata can dulcis {1490 mg/kg (soil), 14.5 mg/kg (root), 9.50 mg/kg thrive both in slightly acidic and alkaline environments, (shoot )} did not indicate Pb accumulation although they suggesting that they can be potential materials for grew on highly Pb-contaminated soil (Table 2). phytoremediation regardless of the pH of the con- S. pyramidalis {197 mg/kg (root), 174 mg/kg (shoot)} taminated area. and A. tectonum {277 mg/kg (root), 112 mg/kg (shoot)} All the soil samples collected 40 m west and at 60 m accumulated substantial levels of Zn in roots and shoots. north of the waste pile had higher levels of Pb than the UNIVERSITY OF IBADAN LIBRARY 3058 Afr. J. Biotechnol. Table 3. Extraction coefficients and shoot/root quotient of plants that grew on highly lead-contaminated soils. Pb Zn Ni Extraction shoot/root Extraction shoot/root Extraction shoot/root coefficient = quotients = coefficient = quotients = coefficient = quotients = Plant species Shoot : soil Shoot : root Shoot : soil Shoot : root Shoot : soil Shoot : root S. pyramidalis 0.29 0.58 0.23 0.88 78.3 19.9 Py. polystachyos 0.004 0.01 0.02 0.01 35.5 1.91 C. odorata 0.02 0.35 0.45 1.49 ND ND P. foetida 0.13 0.18 0.36 0.92 ND ND A. tectonum 0.01 0.13 0.47 0.40 1.29 4.26 C. digitalis 0.01 0.49 0.09 0.31 ND ND Sc. dulcis 0.01 0.7 0.44 1.11 ND ND Se. sesban 0.02 0.92 0.29 0.24 3.48 3.51 D. aegypticum 0.02 0.48 0.27 0.22 4.92 6.42 Cal. mucunoides 0.18 0.94 0.82 0.73 3.32 6.60 normal cleanup level of 400 mg/kg of Pb in soils (Chen et contaminated soils (Baker, 1981). In plants that accumu- al., 2003). The levels of Zn were outside the range found late heavy metals, shoot/root quotients greater than 1 are in uncontaminated soils (20 - 300 mg/kg) (Steinborn and commonly reported while shoot/root quotients less than 1 Breen, 1999). The levels of Ni found in the soil samples characterize heavy metal excluders. Based on this were within the normal range of 1-110 mg/kg reported for criterion, I. cylindrical, T. platycarpa, and Sc. dulcis could uncontaminated soils (Kabata-Pendias and Pendias, be regarded as potential Pb excluders or hypertolerant as 2001). These levels impacted on the concentration of Ni described (Boularbah et al., 2006). Consequently these observed in the plants. plants if found so in laboratory trials may be proposed for The apparent diminishing of heavy metals concen- revegetation of the bare Pb-contaminated soils at tration away from the waste pile almost certainly confirms Lalupon the site under consideration. the waste as the potential source of soil contamination Lead and Zn concentrations in the plants varied with and their concentrations in plants. About 58% of the soil their levels in soil as demonstrated by the concentrations samples analyzed indicated levels higher than the normal of heavy metals in C. odorata, P. foetida, S. pyramidalis range (2 - 200 mg/kg) of Pb in soil (Kabata-Pendias and and A. tectonum that were collected from different Pendias, 2001). Samples of plant collected from the locations. Nevertheless, the degree at which a particular immediate environment of the waste were grossly conta- plant species accumulated Pb at the different locations in minated with Pb. The high levels of these metals present the present study differed, such that S. pyramidalis at 40 the site as potentially hazardous and highly inimical to m W transferred 50% (i.e Pb in root/Pb in soil X 100) of food chain and biological life in the environment. This the soil Pb to the root while the same plant transferred makes remediation of the site a matter of urgency for 92% of the soil Pb to its root at 60 m NE (Table 2). A safe biological life and for a clean environment. similar trend was observed for C. odorata which The results obtained showed that heavy metal concen- transferred 5.0% soil Pb to the root at 40 m W (pH = trations in the plants varied with plant species, levels of 5.82) and 11.6% to its root at 60 m N. The lower levels of heavy metals in the soils and heavy metal contaminants. Pb in S. pyramidalis and C. odorata at 40 m W when Plant species such as S. pyramidalis and Py. Polysta- compared to the level at 60 m N where the level in soil is chyos accumulated Pb from the soil while I. cylindrical, T. lower may suggest that a few other soil factors may be platycarpa and Sc. dulcis sampled from highly Pb- responsible for the concentration of heavy metals in plant contaminated soils contained very low levels of Pb (Table species apart from their levels in the soil. Some of such 2). This observation has been attributed to the diffe- factors may be the form in which the metal exists in total rences in metal tolerance mechanisms. In actual fact, soil, soil pH, soil organic matter and the degree of accumulation and exclusion have been viewed as the two moisture in soil (Steinborn and Breen, 999; Xian, 1989). fundamental mechanisms by which plants respond to The implication is that the characteristics of the high levels of heavy metals in soil (Steinborn and Breen, contaminated soil are also crucial to the effective field 1999; Yanqun et al., 2004). Excluders are plants that limit application of phytoremediation technique. the levels of heavy metal translocation within them and The extraction coefficient and shoot/root quotient have maintain relatively low concentrations in their shoot over been used to demonstrate the ability of plants to accumu- a wide range of soil concentrations. They are mostly late heavy metals (Rotkittikhun et al., 2006). The extent employed in revegetating and stabilization of heavy metal of accumulation of heavy metals by the plants studied dif- UNIVERSITY OF IBADAN LIBRARY Ogundiran and Osibanjo 3059 fered with the type of metal being considered. In the their shoots at both low and high soil metal concen- present study, S. pyramidalis, Py. polystachyos, A. trations and are utilized in extracting heavy metals from tectonum and Cal. mucunoide accumulated the metals in contaminated soils (Rotkittikhun et al., 2006). In this the order Ni > Zn > Pb; Se. sesban and D. aegypticum in study, S. pyramidalis, Py. polystachyos, Se. sesban, D. the order Ni > Pb > Zn as indicated by extraction aegypticum and Cal. mucunoides showed extraction coefficient and shoot/root quotient values (Table 3). Also, coefficients and shoot/root quotients > 1 for Ni, with S. C. odorata, P. foetida, Cy. digitals and Sc. dulcis accu- pyramidalis showing the highest (78.3 and 19.9 mulated Zn and Pb whereas the values for Ni were respectively). Taylor et al. (1992) also observed exces- undetectable. Criteria have been reported for a plant to sive accumulation of Ni by cowpea (Cal. mucunoide). be categorised as hyperaccumulator, that is, plant with This presupposes that any of these plants, especially S. exceptional ability to concentrate heavy metals in their pyramidalis, may likely hyperaccumulate Ni when grown shoots at both low and high soil metal concentrations. in Ni-contaminated soils. Se. sesban has been reported (Boularbah et al., 2006; Yanqun et al., 2004; Ginocchio to accumulate Pb and Zn both in its root and shoot (Yang and Baker, 2004) established that plant species which et al., 2003). In this work, the same plant species accu- contained more than 0.1% (1,000 mg/kg) of copper, lead, mulated a reasonable level of Pb, had Zn plant levels nickel chromium or cobalt, cadmium >100 mg/kg, with an higher than what was found in the soil and exception for zinc and Mn which have a threshold of 1% hyperaccumulated Ni. (10,000 mg/kg) in their dried tissues are hyperaccu- The relative percent differences (RPD) for duplicate mulators. Hyperaccumulator was also defined (Shen and analyses for Pb, Zn and Ni included as quality control Liu, 1998) as concentrations of heavy metals in shoots by were less than 20% which is the control limit between 10 – 500 times more than concentration in normal plants, duplicate as set by United States Environmental protect- if shoot/root quotient is greater than 1, and if extraction tion Agency (USEPA, 2002) indicating high precision. In coefficient is greater than 1 (Rotkittikhun et al., 2006). the soils and plants used for recovery studies, Pb, Zn and In this study, S. pyramidalis (5670 mg Pb/kg shoot) met Ni percent recoveries were within 100 ± 20% suggesting the first two conditions as Pb hyperaccumulator since the that errors attributable to total metal analysis were concentrations of Pb in the shoot was > 1000 mg/kg and negligible. The coefficient of variations among replicate 10 - 500 times more than 0.5 - 10 mg/kg Pb levels in determinations was  5.0. normal plants (Boularbah et al., 2006), C. odorata (190 mg Pb/kg shoot) and Cy. digitals (225 mg Pb/kg shoot) met the second condition where the levels of Pb in the Conclusion shoot were more than 10 times what was obtainable in uncontaminated plants, suggesting that they are potential The results showed that some of the soils and plants Pb hyperaccumulators. Similar observations were repor- studied were highly contaminated with Pb and Zn sug- ted for Sporobolus indicus, C. odorata and C. difformis gesting that the site poses potential hazards to grazing collected from a lead mine in Thailand (Rotkittikhun et al., animals and the food chain. These findings suggest that 2006). Considering the last two conditions, none of the there is need for urgent attention to proffer far reaching plants can be regarded as Pb hyperaccumulator Se. solutions to the problems of the exposed contaminated sesban (shoot/root quotients = 0.92) and Dactyloctenium site. aegypticum (shoot/root quotients = 0.94) are close to On the other hand, some of the plants analyzed, parti- fulfilling the third and fourth conditions. cularly S. pyramidalis, showed high levels of metal C. odorata and Sc. dulcis indicated shoot to root tolerance to lead, zinc and nickel. 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