Contamination of Lead and Mercury in Coal Basin of India

The coal is contaminated with toxic metals at the trace levels. They are released into the environment during mining, handling and burning of coal. The Korba basin has one of the largest coal exploitation areas in the country. In this work, contamination and sources of toxic metals i.e. Hg and Pb in the air, soil and sediment of the Korba basin, India are described. The concentration of Hg and Pb in the ambient air was ranged from 7.4 29 and 7.0 585 ng/m3 with mean value of 18 ± 4 and 129 ± 104 ng/m3 in the winter season. The mean concentration of Hg in the soil and sediment was 0.22 ± 0.03 and 0.44 ± 0.08 mg/kg, respectively. The higher concentration of Pb in the environmental samples was observed.


Introduction
Heavy metals i.e.Pb and Hg are highly toxic to the developing brain and nervous system [1]- [2].Coal is a naturally occurring combustible fuel containing the toxic metals at trace levels [3]- [5].They are emitted from the thermal power plant stacks in both gaseous and particulate forms by retaining some fractions in the ash residues [6]- [8].They are persistent in the environment, could enter bodies through the oral route and prove a great threat to people, especially those living in the vicinity of these thermal power plants.The contamination of environment with the Pb and Hg in various parts of the World was reported [9]- [45].In the present work, contamination, enrichment and sources of Pb and Hg in the environment of the Korba basin, India are described.

Study Area
The Korba basin (22˚21'N, 82˚40'E) has largest coal deposit in the country extending over ≈530 km 2 .Several open and underground coal mines are in operation with production of >10,000 MT/Yr coal.They are consumed by the thermal power plants running in the basin for production of electricity (40,000 MW) by pouring effluents into the environment.The population (≈0.5 million) of the basin is exposed severely with particulate and fly ash pollution.The increased prevalence rate of air and water borne diseases in human and domestic animals was observed.

Sample Collection and Meteorological Analysis
The sampling network for collection of particulate matters (PM), soil and sediment is shown in Figure 1.The Lata Envirotech air sampler was used for the collection of the coarse particulate matters (PM 10 ).The PM 10 was collected over the quartz filer paper (47-mm, Whatmann) at flow rate of 16 lit/min.The sampler was run for duration of 24 hr at the 1 st floor of the building (≥3 m above the ground level).Ten samples were collected from 10 different locations of the Korba basin during period: December, 2012-February, 2013.The Anderson sampler (1531-107B-G289X) with eight stages: PM 10.0-9.0 , PM 9.0-5.8, PM 5.8-4.7 , PM 4.7-3.3, PM 3.3-2.1 , PM 2.1-1.1 , PM 1.1-0.7 and PM 0.7 was used for the collection of respirable particulate matters in the segregation forms.The sampler was run for 72 hr at four locations in January, 2013.The mass of dried loaded and blank filters were weighted out.
The surface soil (0 -10 cm) samples from 30 locations of the basin were collected in January, 2013 [46].The pond sediment was sampled from 26 locations of the basin in January, 2013 [47].They were dried, crushed and sieved out particles of ≤0.1 mm sizes.The meteorological parameters i.e. ambient temperature (T), humidity (H), wind speed (WS) and wind direction (WD) were measured during the sampling period.

Analysis
The weighed amount of the sample was digested with acids by the microwave probe by using prescribed procedure.The Pb and Hg contents were analyzed by the Varian GF-AAS and CV-AAS techniques.The elemental or black carbon (BC or EC) and organic carbon (OC) were analyzed by the thermal and titration methods [48].The pH value of soil and sediment extracts was determined, by shaking the soil or sediment sample in a 1:2 volume ratio with the deionized water.

Results and Discussion
The atmosphere of Korba basin is dusty due to huge emission of coal and ash particulates.The meteorology of the studied area during the winter period (2012-13) is summarized in Table 1.The value of T, H and WS was ranged from 8˚C -30˚C, 24% -93% and 3 -8 k/hr, respectively during the studied period.The air mass flow was varied from E to S with domination in the SE direction.

Distribution of PM in Ambient Air
The distribution of the PM in the ambient air is summarized in Table 2. Extremely high concentration of PM 10 in ambient air during the winter period was observed, ranging from 196 -775 µg/m 3 with mean value of 390 ± 137 µg/m 3 .The highest concentration of the PM 10 was observed at location No. 2 due to input of the thermal power plant (TPP) effluents.The segregation chromatogram of the PM 10 at four locations is summarized in Figure 2. The mean (n = 4) relative concentration of PM 10.0-9.0 , PM 9.0-5.8, PM 5.8-4.7 , PM 4.7-3.3, PM 3.3-2.1 , PM 2.1-1.1 , PM 1.1-0.7 and PM 0.7 in the air was 17, 14, 9, 9, 15, 19, 12 and 6%, respectively.At least, 52% ± 14% PM lie in the fine and ultrafine modes (≤3.3 µm).The concentration of respirable PM in the air was found to be too much higher than recommended limit of 50 µg/m 3 [49].

Distribution of Hg in Air, Soil and Sediment
Mercury is a highly mobile metal, being liquid at room temperature, and behaves like organic compounds.The concentration of the Hg (n = 10) in the air (associated to PM 10 ) was ranged from 7.4 -29 ng/m 3 with mean value of 18 ± 4 ng/m 3 , Table 2. Its concentration in the PM 10 (n = 10) was ranged from 23 -86 mg/kg with mean value of 52 ± 13 mg/kg, Table 2.The higher concentration of Hg was observed in the studied area as compared to other regions of the World (i.e.Canada, China, Korea, Mexico, Taiwan, Thailand and Vietnam), may be due to huge coal burnings [11]- [15].All the soil and sediment were composed of micrometer-scale grains with nano-scale structure.They were colored, ranging from whitish (W) to black (B) due to deposition of the fly ash and black carbon, Table 3 and Table 4.The mean pH value of the soil and sediment extracts were found to be slightly acidic, ≈6.6 due to presence of high content of chloride and sulfate ions.The concentration of Hg in the soils (n = 30) was ranged from 0.11 -0.39 mg/kg with the mean value of 0.22 ± 0.03 mg/kg.The concentration of Hg in the soil of studied area was found to be comparable to the contents reported in other regions of the World (i.e.China, Greece, Iran, Poland, USA) [16]- [20].At least 2-folds higher concentration of Hg in the sediment was observed and may be due to its highly mobile nature.The concentration of Hg in the sediments (n = 26) was ranged from 0.12 -0.82 mg/kg with the mean value of 0.44 ± 0.08 mg/kg.Extremely high concentration of Hg in the sediment of Hong Kong and Mumbai, maximum up to in the 0.855 -2.66 mg/kg was reported [21]- [22].The Hg loading in the sediments of Amazonian basin, Brazil, China, and Poland was ranged from 55 -386 µg/kg [23]- [28].

Distribution of Pb in Air, Soil and Sediment
Lead is least mobile and lethargic metal, generally deposited in surface soil.The concentration of Pb (n = 10) in the ambient air and PM 10 was ranged from 7 -585 ng/m 3 and 22 -755 mg/kg with mean value of 129 ± 104 ng/m 3 and 311 ± 136 mg/kg, respectively.Unlikely to Hg, lead concentration in the soil was found to be remarkably higher than the sediment, may be due to its hydrophobic nature.Lead concentration in the soil and sediment of the studied area was ranged from 72 -194 and 26 -127 mg/kg with mean value of 130 ± 10 and 75 ± 13 mg/kg, respectively.The concentration of Pb reported in the ambient air of various parts of the World i.e.China, India, Nigeria and USA was ranged from 3.9 -1123 ng/m 3 [29]- [33].The Pb levels in soil and sediment of other regions of the World reported were ranged maximum up to 926 mg/kg [34] [45].

Spatial and Vertical Distribution of Metals
Similar spatial distribution patterns of Hg and Pb in the soil and sediment of the studied area was observed.The higher concentration of the metals was observed in the locations near point sources i.e. mining and thermal power plant, Figure 3 and Figure 4.The concentration of the Hg was found to increase as the depth profile was increased from 0 -30 cm, may be due to its hydrophilic nature and less binding with the soil organic compounds, Figure 5.However, the reverse trend was observed with Pb, may be due to its hydrophobic nature and higher binding with the soil components, Figure 5.

Enrichment of Metals
The background concentration of Hg and Pb in the soil reported is 0.05 and 17 mg/kg, respectively [50].The Hg and Pb in the PM 10 was found to be extremely (1049 ± 261) and significantly (18 ± 8) enriched, respectively.However, they were found to be moderately enriched in the soil and sediment of the studied area, Figure 6.

Sources of Metals
The PM and EC in the air are mainly generated during the burning processes.A good correlation (r = 0.98) of the EC with the PM was achieved, showing origin from the burning processes.Similarly, the EC with Pb and Hg contents in the air, soil and sediment was found to be fairly correlated (r = 0.63 -0.81), indicating origin mainly from the coal burning processes.

Conclusion
The enrichment of Hg and Pb in the environment depends strongly on the particle size of the sorbent, and their remarkable higher concentrations are observed in the ambient PM 10 .Mercury was enriched > 100-folds higher in the ambient PM 10 with respect to the soil and sediment media.The increased vertical mobility of Hg via the soil was marked unlikely to Pb.The coal burning and mining are expected to be the major sources for contaminations of the metals in the environment.

Figure 2 .
Figure 2. Segregation of PM 10 in ambient air of Korba.

Figure 6 .
Figure 6.Enrichment of Pb and Hg in soil and sediment.

Table 1 .
Meteorology of the studied area.

Table 2 .
Concentration of EC, Pb and Hg in ambient air and PM.

Table 3 .
Distribution of Pb and Hg in soil.

Table 4 .
Distribution of Pb and Hg in sediment.