1. Introduction
The quality of environment (i.e. air, water and soil), microorganism activities, plant growth, etc. are affected by heavy metal (HMs) contaminations [1] -[3] . The woody plants are being excellent tool for biomonitoring of the metals due to rapid growth, high biomass, profuse root apparatus and low impact on the food chain and human health [4] -[9] . The accumulation of HMs in the tree leaves was reported to assess air quality and bioaccumulation fluxes [10] - [18] . The coal is widely used as source of energy in India and its huge exploitation causes the contamination of the environment [19] -[23] . In the present work, concentration, enrichment, and sources of the HMs (i.e. As, Fe, Cr, Mn, Cu, Zn, Cd, Pb and Hg) in the dried leaves of common woody trees (i.e. Eucalyptus, Butea monosperma, Ficus religiosa, Tectona grandis and Azadirachta indica) in the largest coal burning basin, Korba, India are described as a bioindicator to assess the HMs contamination of the environment.
2. Methods and Materials
2.1. Study Area
The Korba coal basin, Chhattisgarh, India (22˚21'N, 82˚40'E, above >250 m from seas levels) was selected for the proposed investigation due to extreme environmental pollution. The area is covered by the dense tropical moist and deciduous forest dominated by trees i.e. Tectona grandis, Shorea Robusta, Pterocarpus marsupium, Anogeissus latifolia, Madhuca indica, Dendrocalamus strictus, Azadirachta indica, Butea monosperma, Eucalyptus, Ficus religiosa, Mangifera indica, etc. Several open and underground coal mines are in operation with production of >10,000 MT/Yr coal. The most of electricity (40,000 MW) of the state is produced by the thermal power plants emitting effluents in the city. The Asia biggest aluminium plant with captive power plant of 1200 MW is also in operation. The population of city is ≈ 1 million, being exposed with severs particulate and fly ash pollution.
2.2. Collection of Soil and Plant Leaf Samples
The sampling networks of soil and plant samples are shown in Figure 1. Leaves of six trees (i.e. Azadirachta indica, Butea monosperma, Eucalyptus, Ficus religiosa, Mangifera indica and Tectona grandis) of great social economic values were selected for this work. Azadirachta indica is a fast-growing broad-leaved evergreen large tree of India. The leaves have antibacterial and antiviral properties, and are often used in cosmetic and skin treatment preparations. Butea monosperma is a medium sized dry season-deciduous ≈ 15 m tall tree. The leaves are used as fodder for animals. Eucalyptus is quick growing tall tree with leaf of leathery in texture, hang obliquely and containing a fragrant volatile oil. Ficus religiosa is a large dry season―deciduous or semi-ever green tree up to 30 m tall with a large trunk with long and broad leaves. The Mangifera indica is a fruity tree with leafy green foliage of height ranging from 0.5 - 15 m. Tectona grandis is a large deciduous tree having height up to 35 m with simple large leaves.
A 0.5 × 5 kg of the surface soil samples (0 - 10 cm) from rhizospheric zone of five trees of each species was collected in January, 2013 from Manikpur, Korba as prescribed in the literature [24] . They were stored in polyethylene bottles and dried in an oven for overnight at 60˚C.
The leaves from five trees of each species (≥10 Yrs old) were collected manually in January, 2013, washed thoroughly with deionized water and dried in a shed [25] . The samples were compressed into a powder with the help of a manual grinder and sieved out the particles of ≤1 mm.
2.3. Analysis
The soil sample was mixed with deionised water into 1:2 (w/v) ratio in a 100-ml conical flask. The suspension was allowed to stand an overnight, and pH value of the extract was measured by Hanna pH meter type-HI991300.
A 0.25 g each of soil sample was digested with 8 ml aqua regia in the closed microwave vessel for 15 min. Similarly, 0.25 g dried leave sample was digested with 5 ml HNO3 + 2 ml H2O2. An aliquot of the extract was diluted with deionized distilled water for further analysis. Similar procedure was applied for the digestion of the soil (NCS DC73382, Spex Certiprep, UK) and leaf (NIST-1515, USA) reference samples.
The Varian Liberty AX Sequential ICP-AES (plasma flow: 15 L/min, auxiliary flow: 1.5 L/min, power: 1 KW, PMT voltage: 650 V) was used for analysis of the metals i.e. Cr, Mn, Fe, Cu and Zn. A VARIAN “SpectrAA 220Z” model graphite furnace atomic absorption spectrometer (GF-AAS) equipped with a longitudinal Zeeman effect background corrector and THGA tube, auto sampler and automatic data processor was employed for analysis of metals i.e. Cd and Pb. The VARIAN “SpectraAA 55B equipped with hydride/cold vapour regenerator accessories was selected for analysis of elements i.e. As and Hg. The accuracy of the method for analysis of the metals (n = 3) in the reference materials was found to be ≤2%. The precision of the analysis for the metals in soil and leaf samples was ≤ 5%.
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Figure 1. Representation of sampling location in Korba.
2.4. Pollution Indices
The enrichment factor (Ef) is used to assess pollution of heavy metals in soil with respect to crustal contributions [26] .
where, symbols Cm and CAl denote the concentration of the metal and aluminium in the soil and crust. The background concentration of Al, Fe, As, Cr, Mn, Cu, Zn, Cd, Pb and Hg in the earth crust reported was 81,500, 39,200, 4.8, 92, 775, 28, 67, 0.09, 17 and 0.05 mg/kg, respectively [27] . Five contamination categories are generally recognized on the basis of the enrichment factor: Ef < 2, depletion to mineral enrichment; 2 ≥ Ef < 5, moderate enrichment; 5 ≥ Ef < 20, significant enrichment; 20 ≥ Ef < 40, very high enrichment; and Ef > 40, extremely high enrichment [28] .
3. Results and Discussion
3.1. Concentration of Metals in Soils
The physico-chemical characteristics of the rhizospheric soils are summarized in Table 1. The soils were colored, ranging from brown to blackish. The pH value of the soil extract (n = 6) was ranged from 6.2 - 7.2 with mean value of 6.6 ± 0.5 at 95% probability. The concentration of As, Cr, Mn, Fe, Cu, Zn, Cd, Pb and Hg in the soil (n = 6) was ranged from 10.8 - 20.0, 34 - 72, 314 - 1760, 17,718 - 45,426, 49 - 62, 26 - 62, 1.1 - 1.5, 19 - 48 and 1.2 - 2.0 mg/kg with mean value of 15.0 ± 2.5, 57 ± 10, 809 ± 395, 34,730 ± 7462, 55 ± 4, 43 ± 10, 1.3 ± 0.1, 33 ± 8 and 1.5 ± 0.3 mg/kg at 95% probability, respectively. The mean Ef value (n = 6) for Cr, Zn, Fe, Mn, Cu, Pb, As, Cd and Hg was found to be 7, 7, 10, 12, 22, 22, 35, 161 and 337, respectively. The metals i.e. Cr, Zn, Fe and Mn; Cu, Pb and As; and Cd and Hg were significantly, highly and extremely enriched in the soil, respectively. The concentration of the HMs in the soil of Korba city was seen to be higher than reported in the soil of other region of the country, may be due to higher exploitation (i.e. mining and burning) of coal [21] -[23] .
3.2. Concentration of Metals in Leaves
The concentration of HMs in the dried leaves is summarized in Table 2. High concentration of As, Fe, Cr, Mn, Cu, Zn, Cd, Pb and Hg in the tree leaves was observed, ranging from 2.8 - 43, 728 - 5182, 8.6 - 49, 48 - 1196, 43 - 406, 79 - 360, 1.12 - 1.65, 1.6 - 16.4 and 0.13 - 0.76 mg/kg, respectively. The highest content of all metals (except Mn, Cd and Hg) was marked with the Azadirachta indica leaves, may be due to bio-adsorption of the metals with the inherent alkaloids [29] . Similarly, the highest accumulation of metals i.e. Mn, Cd and Hg was observed in the Eucalyptus, Mangifera indica and Butea monosperma leaves, respectively, Table 2. The recommended values of As, Fe, Cr, Mn, Cu, Zn, Cd, Pb and Hg in the vegetables were reported to be 0.1, 425, 2.3, 500, 40, 100, 0.1, 0.2 and 0.03 mg/kg, respectively [30] . Several folds higher concentration of metals i.e. As, Fe, Cr, Cu, Cd, Pb and Cd than the recommended values was marked in all tree leaves. Extremely high Mn content in the Eucaliptus leaves was observed, being higher than recommended value of 500 mg/kg.
The As content in the lichens of Mandav Monuments, India was reported maximum up to 51.95 mg/kg [10] . The content of Cu, Zn, Cd and Pb in leaves of natural and exotic plants of Samsun City, Turkey was ranged 40 - 60, 70 - 75, 0.1 - 0.3 and 12 - 35 mg/kg, respectively [12] . The Mn content in leaf of holoptelia, cassia and neem trees of Gumgaon, India was reported in the range of 168.59 - 437.56 mg/kg [17] . The content of Cu, Pb and Cd in the horse chestnut leaves at Studentski Park site was amounted to 110.2, 20.3 and 4.9 mg/kg, respectively [18] . The HMs contents in the tree leaves of Korba city was found to be comparable with the values reported in other location of the country and World [10] -[12] [17] [18] .
3.3. Bioaccumulation of Metals in Leaves
The bioaccumulation factor (Bf) of the metals (ratio of leaf to soil content) in the leaves is presented in Table 3. Significant Bf value (>1) of metals i.e. As, Fe, Cu, Zn and Cd in the Azadirachta indica leaves was observed. Similarly, high Bf value (>1) of Zn was marked in all tested leaves. The highest BF value for metals i.e. As, Fe, Cr, Cu, Zn and Pb was observed with the Azadirachta indica leaves, may be due to interaction of bioactive compounds (i.e. azadirachtin, nimbin, nimbidin and nimbolides). Significant Bf value for Mn, Cd and Hg was seen with Eucaliptus, M. Indica and B. Monosperma leaves, respectively. The results showed that the Azadirachta indica leaves are seemed to be a hyperaccumulator for five heavy metals i.e. As, Fe, Cu, Zn and Cd in a huge coal burning area. Leaves of other trees i.e. M. Indica, Eucaliptus, B. Monosperma, F. Religiosa and T. Grandis were observed to be hyperaccumulator for heavy metals i.e. Cu, Zn and Cd, respectively. Among them, only Eucaliptus leaves was marked as good hyperaccumulator for Mn.
3.4. Factor Analysis
The result of factor analysis in tree leaves is given in Table 4. Three factors were extracted, and a 65.60% of the total variance was accounted by Factor-I. Six HMs i.e. Cr, Cu, Fe, Pb, Zn and As were positively correlated (r = 0.69 - 1.00) to each other in the leaves, Table 5. A 13.90% of the total variance was included in the Factor-II, and positively and negatively correlated with Cd and Hg, suggesting aerial accumulation of Hg in the leaves rather than soil [31] . A 12.74% of the total variance was contributed by Factor-III, and is negatively correlated with Mn.
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Table 1. Physico-chemical characteristics of rhizospheric soil.
S = Soil, Br = Brown, Bk = Blackish.
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Table 2. Mean concentration (n = 5) of heavy metals in tree dried leaves, mg/kg.
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Table 3. Mean bioaccumulation factor of metals in leaves.
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Table 4. Results of factor analysis of heavy metals in the tree leaves.
Loading value > 0.70, significant at p < 0.05.
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Table 5. Correlation matrix of HMs in the tree leaves.
4. Conclusion
The soil of the Korba city, India is highly polluted with As, Cu, Pb and Hg due to coal burning and mining. The highest phytoextraction of the HMs was observed with Azadirachta indica leaves; it may be due to interaction of the alkaloids with the metals. Significant Bf values (>1) for metals i.e. As, Cu and Zn with the Azadirachta indica leaves were marked. The Azadirachta indica leaves could be considered as bioindicator for soil pollution of the HMs in the coal burning area of the country.
Acknowledgements
We are thankful to the Alexander von Humboldt Foundation, Bonn, Germany for granting financial support to one of the author: KSP.