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Mixed bacterial consortium as an emerging tool to remove hazardous trace metals from coal
Fuel 102 (2012) 227–230
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Mixed bacterial consortium as an emerging tool to remove hazardous trace metals from coal Prakash K. Singh a, Asha Lata Singh b,⇑, Aniruddha Kumar b, M.P. Singh a a b
Coal & Organic Petrology Lab, Centre of Advanced Study in Geology, Banaras Hindu University, Varanasi 221 005, India Environmental Science, Department of Botany, Banaras Hindu University, Varanasi 221 005, India
h i g h l i g h t s " These coals contain 1.6–15.7% ash, low carbon content and high S content. " Cd, Cu, Cr, Ni, Pb and Zn are in order of 1.96, 59.34, 26.98, 102.7, 14.4 and 172.5 ppm. " A mixed bacterial consortium was used to remove the hazardous trace elements. " It could remove >80% of Ni, Zn, Cd, Cu and Cr and 45% of Pb.
a r t i c l e
i n f o
Article history: Received 25 January 2011 Received in revised form 22 December 2011 Accepted 7 June 2012 Available online 30 June 2012 Keywords: Coal Hazardous trace metals Removal Mixed bacterial consortium
a b s t r a c t This paper presents the results of experimental work on the possibility of removal of environmentally sensitive trace elements from coal through treatment with mixed bacterial consortium. In the coals of Kalimantan area, Indonesia the metals like Cd, Cu, Cr, Ni, Pb and Zn have been found to occur in concentrations of 1.96, 59.34, 26.98, 102.68, 14.4 and 172.54 ppm respectively. The concentrations of these elements are higher when compared with Clarke values in bituminous coals. Mixed bacterial consortium has been used to explore the possibility of removal of these toxic trace elements. The result reveals that the bacterial consortium is efficient to remove more than 80% of metals like Ni, Zn, Cd, Cu and Cr while the removal of Pb is nearly 45%. The removal is seen in the order: Zn > Ni > Cd > Cu > Cr > Pb. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction The heterogeneity of coal is not limited to its visible organic and inorganic constituents but a large number of elements are also associated with it [1–4]. These elements occur, in association of organic and inorganic constituents, as major (>1 wt%), minor (1–0.1 wt%) or trace (<0.1 wt%) elements [3]. The trace elements get associated as elements-organic compounds, as impurities in the mineral matter and inorganic amorphous matter, as major components in the mineral matter, and as elements in fluid components [3–8]. The concern for the trace elements as potential health hazards has been growing for the last few decades [9–13]. Enormous amount of data have been generated from various parts of the world, particularly from the coal producing countries where coal is the dominant fuel for power generation. It has been shown that considerable quantities of hazardous trace elements are released along with volatile matter during combustion and also ⇑ Corresponding author. Tel.: +91 9935658465. E-mail address: [email protected] (A.L. Singh). 0016-2361/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.fuel.2012.06.039
as parts of the fly and bottom ash [9,10]. These elements eventually get into atmosphere and biosphere through various natural mechanisms and become effective. Based on the study on major mineral groups like sulphides (epigenetic and syngenetic pyrite), carbonates (epigenetic calcite and syngenetic siderite), clay minerals (kaolinite and illite), and tonsteins of various origin, Pickhardt [14] has established environmental relevance of trace elements like As, Be, Cd, Cr, Co, Cu, Pb, Mn, Hg, Mo, Ni, Sr, U, V, and Zn. Turiel et al. [11] have demonstrated that these metals get enriched up to 10 times during combustion of coal. The bulk ash (88%) produced by coal combustion in Thermal Power Plants is in the form of bottom ash [12] and contain large number of elements including Cd, Cr, Pb, Zn in variable concentrations. Burning of fossil fuels releases Cd into atmosphere with gases and fly ash and may enter our body through respiratory tract and cause serious health hazards. Besides, it may contaminate soil through leaching and affect human health through food chain and food web. Cadmium is a persistent environmental toxin and cannot be rendered harmless [13,10,15]. After its absorption in body, it damages the kidney and causes
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P.K. Singh et al. / Fuel 102 (2012) 227–230
osteomalacia. Lead affects our entire body system, the most sensitive being the central nervous system, especially of children. Lead damages kidney and the reproductive system. Hexavalent Cr is toxic at low concentration [16] and damages kidney and liver. It also causes allergic skin reactions and is regarded carcinogenic [17]. Elevated concentration of Cr and Ni is reported to have caused serious lung disease in several coal miners of South China [18]. High concentration of Zn also causes harmful effects. Coal combustion also generates finer particulates (<2.5 lm) known a respirable particles [19]. The ultrafine particles (<0.1 lm) are enriched in Cd, Cr, Co, Pb, Zn, etc. and cause adverse effects [20]. These particles may travel hundreds of km along with fly ash [21]. The leached out toxic elements get mixed with soil and are absorbed in the plants grown in such soil [22]. Elements like Co, Cd, Mn, Ni and Zn have been noticed in high concentration in respirable coal dust [23]. 2. Materials and methods 2.1. Coal sample collection Coal samples have been collected from surface exposures from five locations of Tarakan basin of Kalimantan, Indonesia. The area is yet to be fully explored for its coal resources and the outcrops of coal were located in dense tropical rain forest region. These coal samples have been crushed and reduced in quantity, to prepare five composite samples [24].
The treatment has been carried out for six days and the metal concentration has been determined for every day’s treatment. 3. Result and discussion 3.1. Trace elements in Kalimantan coals These coals are ‘Sub-Bituminous coals’ which have evolved in a telmatic environment under alternate oxic to anoxic moor conditions [24]. These coals contain 1.6–15.7 wt%; mean 5.7 wt% ash. These coals are low in carbon content (64.2–73.9 wt%; mean 70.24 wt%) but are high in sulphur (0.4–5.4 wt%; mean 2.52 wt%) [24]. Selected trace elements (Cd, Cu, Cr, Ni, Pb and Zn) which are environmentally hazardous were determined in these coal samples. These trace elements occur in variable concentration (Tables 1a–c) as Cd (0.2–2.9 ppm), Cu (41.2–75.8 ppm), Cr (8.3– 58.7 ppm), Ni (54.6–155.9 ppm), Pb (1.0–33.0 ppm) and Zn (87.0–296.8 ppm). The concentration of trace elements in each sample has been summarised in Table 1a. As compared to the Clarke values in bituminous coals (Table 1b), they are high in concentration [26,27]. While elements like Cr (r2 = 0.82) and total S (r2 = 0.75) are seen to have a strong affinity with inorganic matter, Ni (r2 = 0.75) is associated with organic matter. The metals like Cd (r2 = 0.04), Cu (r2 = 0.29), Pb (r2 = 0.29) and Zn (r2 = 0.02) show low affinity with organic matter. 3.2. Removal of hazardous trace elements
2.2. Digestion of coal samples The coal samples have been digested with 2.5 ml HNO3 and HClO4 in 10:1 ratio on hot water plate following the method of Eaton et al. [25]. It is filtered with Whatman No. 42 filter paper and then analysed for heavy trace metals in AAS of Perkin Elmer (Model 2380) at different wavelengths. 2.3. Isolation of bacteria from coal and its mass cultivation One gram of coal powder is added to 10 ml double distilled water and after thorough stirring the water sample is filtered. One ml filtered water is diluted hundred times with double distilled water and inoculated on basal salt containing solid agar plates (glucose 0.2%, KH2PO4 0.244%, Na2HPO4 0.577%, NH4Cl 0.2% and MgCl2 0.02%) at ambient temperature. Colonies developed on solid agar plates within 24 h, which are picked up and inoculated in liquid basal salt media. Mass cultivation of bacteria is done for the removal of hazardous metals [16]. 2.4. Immobilisation of mixed bacteria The exponential phase bacterial cells (1000 mg), procured through centrifugation and repeated washing, are mixed to 50 ml of 5% (w/v) solution of alginic acid prepared in the basal salt growth medium. The mixture is pumped; drop wise, into 0.2 M CaCl2 solution aseptically in a laminar flow cabinet. The beads, thus formed, are harvested and re-suspended in a 100 ml growth medium contained in 250 ml cotton stoppard culture flask. 2.5. Treatment of coal with bacterial beads of mixed consortium One gram sterilized coal sample is taken in 25 ml distilled water and treated with 100 beads equivalent to 1000 mg dry wt of bacteria. To know the metal absorption by bacterial consortium, the treated coal samples are analysed for their metal concentration.
The available physico-chemical methods of metal removal from coal are not only expensive but also produce secondary sludge and are, therefore, not eco-friendly. The removal of hazardous trace metals from coal with eco-friendly method is thus imperative. Solubilisation and absorption of metals from solids may be achieved using different acidophilic and chemoautolithotrophic bacteria such as Thiobacillus thiooxidans and Thiobacillus ferrooxidans. Some of the bacteria are very effective for specific metal removal like zinc bioleaching through Leptospirillum ferrooxidans [28], Zn absorption by Staphylococcus aureus [29], Cr uptake by Bacillus mycoides [16]. The binding affinity of each metal to the bacterial
Table 1a The contents of trace elements (in ppm) in East Kalimantan coal. Contents
Sample S-8
Sample T-0
Sample A-4
Sample A-7
Sample T-2
Cd Cu Cr Ni Pb Zn
2.0 49.2 58.7 54.6 7.0 149.8
2.9 72.8 41.6 90.7 33.0 296.8
0.2 75.8 15.7 126.1 17.0 221.2
1.8 57.7 10.6 155.9 14.0 87.0
2.9 41.2 8.3 86.1 1.0 107.9
Table 1b Concentration of trace elements in coals, ash and the upper part of the crust (UCC). Elements
Average content (ppm) UCC [26]
Cr Ni Cu Zn Cd Pb
35 20 25 71 0.1 20
World coal [27] Brown coal
Hard coal
All
15 9.0 15 18 0.24 6.6
17 17 16 28 0.20 9.0
16 13 16 23 0.22 7.8
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P.K. Singh et al. / Fuel 102 (2012) 227–230 Table 1c The contents of trace elements (in ppm) and their removal (in %) after treatment with bacterial consortium in Tarakan basin coal of Indonesia. Elements
Cd Cu Cr Ni Pb Zn
Initial concentration of elements (ppm)
Removal of elements (%)
Minimum
Maximum
Mean
Minimum
Maximum
Mean
0.2 41.2 8.3 54.6 1.0 87.0
2.9 75.8 58.7 155.9 33.0 296.8
1.96 59.34 26.98 102.68 14.4 172.54
75.52 80.10 65.93 79.67 42.00 82.67
91.72 86.68 75.47 90.30 47.88 91.61
85.226 83.176 71.618 85.744 44.748 87.656
3.5 2.5
A-4
2
A-7 T-2
1.5 1 0.5 0
S-8
70
T-0
Cu (ppm)
Cd (ppm)
80
S-8
3
T-0
60
A-4
50
A-7
40
T-2
30 20 10
0
1
2
3
4
5
0
6
0
1
2
Days 70
30
50
A-4
25
40
A-7 T-2
30 20
Pb (ppm)
Cr (ppm)
T-0
10
6
20 15 10 5
0
1
2
3
4
5
0
6
0
1
2
S-8
300
T-0
250
A-4
200
Ni (ppm)
A-7 T-2
150 100 50 0
1
2
3
3
4
5
6
Days
350
Zn (ppm)
5
S-8 T-0 A-4 A-7 T-2
Days
0
4
35
S-8
60
0
3
Days
4
5
6
Days
180 160 140 120 100 80 60 40 20 0
S-8 T-0 A-4 A-7 T-2
0
1
2
3
4
5
6
Days
Fig. 1. Removal of trace elements from Tarakan coal samples by bacterial consortium during six days.
biomass is highly variable. To understand the efficiency of trace element removal through bacteria, the coal from Tarakan basin (Kalimantan), Indonesia has been studied. The method is less expensive and eco-friendly in nature. When coal samples were treated with inoculums of mixed bacterial consortium in water solution, a sharp decline in pH was measured from 6.0 to 4.0, in a time span of six days, indicating the presence of sulphur oxidising bacteria. This has caused the conversion of sulphide minerals into sulphate form and consequently lowering of pH. This process appears to have control over trace element removal (bioleaching) from the coal samples. Similar results were also reported by Sharma and Wadhawa [30] while working on bioleaching of Neyveli lignites of India. However, it has been shown that Cr and Pb require more acidic condition for their
leaching from coal [31]. Different heavy metals have a varying energy state, which affects the efficiency of bioleaching process [32]. Metals, particularly Zn, Cr and Cu, in the form of exchangeable state are considered highly mobile and bio-available. The maximum removal of metals like Zn, Cu, Cr, Cd and Pb were noticed on fourth and fifth day of incubation probably due to more acidification during that period. The removal of Ni was maximum on fifth day in all the coal samples. The quantitative removal of trace elements has been shown in Table 1c and their removal pattern with time is shown in Fig. 1. It evolves further that the metal concentrations in coal samples, except Pb, are significantly decreased using mixed bacterial consortium. The removal of metals like Cd, Cu, Cr, Ni, Pb and Zn using bacterial strains are 85.226%, 83.176%, 71.618%, 85.744%, 44.748%
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P.K. Singh et al. / Fuel 102 (2012) 227–230
and 87.656% respectively. The order of removal is: Zn > Ni > Cd > Cu > Cr > Pb. 4. Conclusions In an attempt to probe the feasibility of removal of environmentally sensitive trace elements, the coal samples from Kalimantan, Indonesia, were subjected to bacterial treatment. These coals are ‘Sub-Bituminous coals’ in rank and have evolved in a telmatic environment under alternate oxic to anoxic moor conditions. These coals contain 1.6–15.7 wt%; mean 5.7 wt% ash, and have low carbon content (64.2–73.9 wt%; mean 70.24 wt%) and high sulphur (0.4–5.4 wt%; mean 2.52 wt%). These elements (Cd – 1.96 ppm, Cu – 59.34 ppm, Cr – 26.98 ppm, Ni – 102.68 ppm, Pb – 14.4 ppm and Zn – 172.54 ppm) are seen to occur in higher values than the Clarke values for coal. When these samples were subjected to a mix of bacterial consortium, under controlled conditions, significant reduction in their concentration has been observed. The consortium could efficiently remove over 80% of metals like Ni, Zn, Cd, Cu and Cr while the removal of Pb was nearly –45%. The removal is seen in the order: Zn > Ni > Cd > Cu > Cr > Pb.
[10] [11] [12] [13] [14] [15]
[16]
[17]
[18]
[19]
[20]
Acknowledgements The authors (P.K.S. and M.P.S.) thankfully acknowledge the Department of Geology, while A.L.S. and A.K. extend their thankfulness to the Department of Botany, Banaras Hindu University for extending the facilities. The authors thankfully acknowledge the two anonymous reviewers whose critical comments could improve the standard of paper.
[21] [22]
[23]
[24]
References [1] Mukherjee KN, Dutta NR, Chandra D, Singh MP. Geochemistry of trace elements of tertiary coals of India. Int J Coal Geol 1992;20:99–113. [2] Prachiti PK, Manikyamba C, Singh PK, Balaram V, Lakshminarayana G, Raju K, et al. Geochemical systematics and precious metal content of the sedimentary horizons of lower Gondwanas from the Sattupalli coal field, Godavari Valley, India. Int J Coal Geol 2011;88:83–100. [3] Vassilev S, Vassileva C. Geochemistry of coals, coal ashes and combustion wastes from coal-fired power stations. Fuel Process Technol 1997;51:19–45. [4] Seredin VV, Finkelman RB. Metalliferous coals: a review of the main genetic and geochemical types. Int J Coal Geol 2008;76:253–89. [5] Van der Flier-Keller E, Fyfe W. Relationships between inorganic constituents and organic matter in a Northern Ontario lignite. Fuel 1988;67:1048–52. [6] Vassilev S, Yossifova M, Vassilev C. Mineralogy and geochemistry of Bobov Dol coals, Bulgaria. Int J Coal Geol 1994;26:185–213. [7] Vassilev S, Eskenazy G, Tarassov M, Dimov V. Geol. Bale. Mineralogy and geochemistry of a vitrain lens with unique trace element content from the Vulche Pole coal deposit, Bulgaria. Geol Balcanica 1995;25:111–23. [8] Vassilev S, Vassileva C. Occurrence, abundance and origin of minerals in coals and coal ashes. Fuel Process Technol 1996;48:85–106. [9] Sabbioni E, Goetz L. Mobilization of heavy metals from fossil-fuelled power plants, potential ecological and biochemical implications. IV. Assessment
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studies of the European situation. Environment and quality of life series. IX. Luxembourg: Commission of the European Communities; 1983. Bernard A. Cadmium and its adverse effects on human health – a review article. Indian J Med Res 2008;128:557–64. Turiel JLF, Carvalho WD, Cabanas M, Querol X, Soler AL. Mobility of heavy metals from coal fly ash. Environ Geol 1994;23:264–70. Swaine DJ. Why trace elements are important. Fuel Process Technol 2000;65– 66:21–33. Bernard A. Renal dysfunction induced by cadmium: biomarkers of critical effects. Biometals 2004;17:519–23. Pickhardt W. Trace elements in minerals of German bituminous coals. Int J Coal Geol 1989;14:137–53. Nordberg G, Nogawa K, Nordberg M, Friberg L. Cadmium. In: Nordberg G, Fowler B, Nordberg M, Friberg L, editors. Handbook on toxicology of metals. New York: Academic Press; 2007. p. 65–78. Singh AL, Krishna PM, Rao TN, Sarma PN. Uptake of Cr (VI) in the presence of sulphate by Bacillus mycoides in aerobic culture. Indian J Biotechnol New Delhi 2006;5:506–9. IARC Monographs on the evaluation of the carcinogenic risk of chemicals to humans. Chemicals, industrial processes and industries associated with cancer in humans. IARC monographs. vol. 1–29; 1998. Ren D, Xu D, Zhao F. A preliminary study on the enrichment mechanism and occurrence of hazardous trace elements in the tertiary lignite from the Shenbei coalfield, China. Int J Coal Geol 2004;57:187–96. Querol X, Alastuey A, Lopz-Soler A, Mantilla E, Plana F. Mineral composition of atmospheric particulates around a large coal-fired power station. Atmos Environ 1996;30:3557–72. Allsopp M, Costner P, Johnston P. Incineration and human health: sta te of knowledge of the impacts of waste incinerators on human health. Amsterdam: Publ. Greenpeace International; 2001. p. 81. ISBN 9073361-69-9. Mason BJ. Acid Rain its causes and its effects on inland waters. Science technology and society series. Clarendon Press; 1992. p. 126. El-Mogazi D, Lisk DJ, Weinstein LH. A review of physical, chemical, and biological properties of fly ash and effects on agricultural ecosystems. Sci Total Environ 1988;74:1–37. Sahoo BN. Occurrence of trace elements in respirable coal dust. In: Sahu KC, editor. Environmental impacts of coal utilization from raw material to waste resources. In: Proceeding volume international conference. IIT Bombay; 1991. p. 85–95. Singh PK, Singh MP, Singh AK, Arora M. Petrographic characteristics of coal from the lati formation, Tarakan basin, East Kalimantan, Indonesia. Int J Coal Geol 2010;81:109–16. Eaton A D, Clesceri L S, Greenberg A E. Standard methods for the examination of water and waste water APHA. AWWA and WEF; 1995. Taylor SR, Mclennan SM. The continental crust; its composition and evolution. London: Blackwell; 1985. p. 312. Yudovich YaE, Ketris MP. Valuable trace elements in coal. Ekaterinburg: UrB RAS; 2006. p. 538 [in Russian]. Shi SY, Fang ZH. Bioleaching of marmatite flotation concentrate by adapted mixed mesoacidophilic cultures in an air-lift reactor. Int J Miner Process 2005;76:3–12. Singh AL, Krishna PM, Rao TN, Sarma PN. Biosorption of zinc by Staphylococcus aureus from synthetic waste water: effect of modifying factors. Pollut Res 2008;27:1–5. Sharma DK, Wadhava G. Determination of coal by stepwise bioleaching: a comparative study of three Indian coals by Fourier transform infra red and Xray diffraction techniques. World J Microbiol Biotechnol 1997:1329–36. Chan LC, Gu XY, Wong JWC. Comparison of bioleaching of heavy metals from sewage sludge using iron and sulfur oxidizing bacteria. Adv Environ Res 2003;7:603–7. Liu YG, Zhou M, Zeng GM, Li X, Xu WH, Fan T. Effects of solids concentration on removal of heavy metals from mine tailings via bioleaching. Elsevier; 2006.
Contents lists available at SciVerse ScienceDirect
Fuel journal homepage: www.elsevier.com/locate/fuel
Mixed bacterial consortium as an emerging tool to remove hazardous trace metals from coal Prakash K. Singh a, Asha Lata Singh b,⇑, Aniruddha Kumar b, M.P. Singh a a b
Coal & Organic Petrology Lab, Centre of Advanced Study in Geology, Banaras Hindu University, Varanasi 221 005, India Environmental Science, Department of Botany, Banaras Hindu University, Varanasi 221 005, India
h i g h l i g h t s " These coals contain 1.6–15.7% ash, low carbon content and high S content. " Cd, Cu, Cr, Ni, Pb and Zn are in order of 1.96, 59.34, 26.98, 102.7, 14.4 and 172.5 ppm. " A mixed bacterial consortium was used to remove the hazardous trace elements. " It could remove >80% of Ni, Zn, Cd, Cu and Cr and 45% of Pb.
a r t i c l e
i n f o
Article history: Received 25 January 2011 Received in revised form 22 December 2011 Accepted 7 June 2012 Available online 30 June 2012 Keywords: Coal Hazardous trace metals Removal Mixed bacterial consortium
a b s t r a c t This paper presents the results of experimental work on the possibility of removal of environmentally sensitive trace elements from coal through treatment with mixed bacterial consortium. In the coals of Kalimantan area, Indonesia the metals like Cd, Cu, Cr, Ni, Pb and Zn have been found to occur in concentrations of 1.96, 59.34, 26.98, 102.68, 14.4 and 172.54 ppm respectively. The concentrations of these elements are higher when compared with Clarke values in bituminous coals. Mixed bacterial consortium has been used to explore the possibility of removal of these toxic trace elements. The result reveals that the bacterial consortium is efficient to remove more than 80% of metals like Ni, Zn, Cd, Cu and Cr while the removal of Pb is nearly 45%. The removal is seen in the order: Zn > Ni > Cd > Cu > Cr > Pb. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction The heterogeneity of coal is not limited to its visible organic and inorganic constituents but a large number of elements are also associated with it [1–4]. These elements occur, in association of organic and inorganic constituents, as major (>1 wt%), minor (1–0.1 wt%) or trace (<0.1 wt%) elements [3]. The trace elements get associated as elements-organic compounds, as impurities in the mineral matter and inorganic amorphous matter, as major components in the mineral matter, and as elements in fluid components [3–8]. The concern for the trace elements as potential health hazards has been growing for the last few decades [9–13]. Enormous amount of data have been generated from various parts of the world, particularly from the coal producing countries where coal is the dominant fuel for power generation. It has been shown that considerable quantities of hazardous trace elements are released along with volatile matter during combustion and also ⇑ Corresponding author. Tel.: +91 9935658465. E-mail address: [email protected] (A.L. Singh). 0016-2361/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.fuel.2012.06.039
as parts of the fly and bottom ash [9,10]. These elements eventually get into atmosphere and biosphere through various natural mechanisms and become effective. Based on the study on major mineral groups like sulphides (epigenetic and syngenetic pyrite), carbonates (epigenetic calcite and syngenetic siderite), clay minerals (kaolinite and illite), and tonsteins of various origin, Pickhardt [14] has established environmental relevance of trace elements like As, Be, Cd, Cr, Co, Cu, Pb, Mn, Hg, Mo, Ni, Sr, U, V, and Zn. Turiel et al. [11] have demonstrated that these metals get enriched up to 10 times during combustion of coal. The bulk ash (88%) produced by coal combustion in Thermal Power Plants is in the form of bottom ash [12] and contain large number of elements including Cd, Cr, Pb, Zn in variable concentrations. Burning of fossil fuels releases Cd into atmosphere with gases and fly ash and may enter our body through respiratory tract and cause serious health hazards. Besides, it may contaminate soil through leaching and affect human health through food chain and food web. Cadmium is a persistent environmental toxin and cannot be rendered harmless [13,10,15]. After its absorption in body, it damages the kidney and causes
228
P.K. Singh et al. / Fuel 102 (2012) 227–230
osteomalacia. Lead affects our entire body system, the most sensitive being the central nervous system, especially of children. Lead damages kidney and the reproductive system. Hexavalent Cr is toxic at low concentration [16] and damages kidney and liver. It also causes allergic skin reactions and is regarded carcinogenic [17]. Elevated concentration of Cr and Ni is reported to have caused serious lung disease in several coal miners of South China [18]. High concentration of Zn also causes harmful effects. Coal combustion also generates finer particulates (<2.5 lm) known a respirable particles [19]. The ultrafine particles (<0.1 lm) are enriched in Cd, Cr, Co, Pb, Zn, etc. and cause adverse effects [20]. These particles may travel hundreds of km along with fly ash [21]. The leached out toxic elements get mixed with soil and are absorbed in the plants grown in such soil [22]. Elements like Co, Cd, Mn, Ni and Zn have been noticed in high concentration in respirable coal dust [23]. 2. Materials and methods 2.1. Coal sample collection Coal samples have been collected from surface exposures from five locations of Tarakan basin of Kalimantan, Indonesia. The area is yet to be fully explored for its coal resources and the outcrops of coal were located in dense tropical rain forest region. These coal samples have been crushed and reduced in quantity, to prepare five composite samples [24].
The treatment has been carried out for six days and the metal concentration has been determined for every day’s treatment. 3. Result and discussion 3.1. Trace elements in Kalimantan coals These coals are ‘Sub-Bituminous coals’ which have evolved in a telmatic environment under alternate oxic to anoxic moor conditions [24]. These coals contain 1.6–15.7 wt%; mean 5.7 wt% ash. These coals are low in carbon content (64.2–73.9 wt%; mean 70.24 wt%) but are high in sulphur (0.4–5.4 wt%; mean 2.52 wt%) [24]. Selected trace elements (Cd, Cu, Cr, Ni, Pb and Zn) which are environmentally hazardous were determined in these coal samples. These trace elements occur in variable concentration (Tables 1a–c) as Cd (0.2–2.9 ppm), Cu (41.2–75.8 ppm), Cr (8.3– 58.7 ppm), Ni (54.6–155.9 ppm), Pb (1.0–33.0 ppm) and Zn (87.0–296.8 ppm). The concentration of trace elements in each sample has been summarised in Table 1a. As compared to the Clarke values in bituminous coals (Table 1b), they are high in concentration [26,27]. While elements like Cr (r2 = 0.82) and total S (r2 = 0.75) are seen to have a strong affinity with inorganic matter, Ni (r2 = 0.75) is associated with organic matter. The metals like Cd (r2 = 0.04), Cu (r2 = 0.29), Pb (r2 = 0.29) and Zn (r2 = 0.02) show low affinity with organic matter. 3.2. Removal of hazardous trace elements
2.2. Digestion of coal samples The coal samples have been digested with 2.5 ml HNO3 and HClO4 in 10:1 ratio on hot water plate following the method of Eaton et al. [25]. It is filtered with Whatman No. 42 filter paper and then analysed for heavy trace metals in AAS of Perkin Elmer (Model 2380) at different wavelengths. 2.3. Isolation of bacteria from coal and its mass cultivation One gram of coal powder is added to 10 ml double distilled water and after thorough stirring the water sample is filtered. One ml filtered water is diluted hundred times with double distilled water and inoculated on basal salt containing solid agar plates (glucose 0.2%, KH2PO4 0.244%, Na2HPO4 0.577%, NH4Cl 0.2% and MgCl2 0.02%) at ambient temperature. Colonies developed on solid agar plates within 24 h, which are picked up and inoculated in liquid basal salt media. Mass cultivation of bacteria is done for the removal of hazardous metals [16]. 2.4. Immobilisation of mixed bacteria The exponential phase bacterial cells (1000 mg), procured through centrifugation and repeated washing, are mixed to 50 ml of 5% (w/v) solution of alginic acid prepared in the basal salt growth medium. The mixture is pumped; drop wise, into 0.2 M CaCl2 solution aseptically in a laminar flow cabinet. The beads, thus formed, are harvested and re-suspended in a 100 ml growth medium contained in 250 ml cotton stoppard culture flask. 2.5. Treatment of coal with bacterial beads of mixed consortium One gram sterilized coal sample is taken in 25 ml distilled water and treated with 100 beads equivalent to 1000 mg dry wt of bacteria. To know the metal absorption by bacterial consortium, the treated coal samples are analysed for their metal concentration.
The available physico-chemical methods of metal removal from coal are not only expensive but also produce secondary sludge and are, therefore, not eco-friendly. The removal of hazardous trace metals from coal with eco-friendly method is thus imperative. Solubilisation and absorption of metals from solids may be achieved using different acidophilic and chemoautolithotrophic bacteria such as Thiobacillus thiooxidans and Thiobacillus ferrooxidans. Some of the bacteria are very effective for specific metal removal like zinc bioleaching through Leptospirillum ferrooxidans [28], Zn absorption by Staphylococcus aureus [29], Cr uptake by Bacillus mycoides [16]. The binding affinity of each metal to the bacterial
Table 1a The contents of trace elements (in ppm) in East Kalimantan coal. Contents
Sample S-8
Sample T-0
Sample A-4
Sample A-7
Sample T-2
Cd Cu Cr Ni Pb Zn
2.0 49.2 58.7 54.6 7.0 149.8
2.9 72.8 41.6 90.7 33.0 296.8
0.2 75.8 15.7 126.1 17.0 221.2
1.8 57.7 10.6 155.9 14.0 87.0
2.9 41.2 8.3 86.1 1.0 107.9
Table 1b Concentration of trace elements in coals, ash and the upper part of the crust (UCC). Elements
Average content (ppm) UCC [26]
Cr Ni Cu Zn Cd Pb
35 20 25 71 0.1 20
World coal [27] Brown coal
Hard coal
All
15 9.0 15 18 0.24 6.6
17 17 16 28 0.20 9.0
16 13 16 23 0.22 7.8
229
P.K. Singh et al. / Fuel 102 (2012) 227–230 Table 1c The contents of trace elements (in ppm) and their removal (in %) after treatment with bacterial consortium in Tarakan basin coal of Indonesia. Elements
Cd Cu Cr Ni Pb Zn
Initial concentration of elements (ppm)
Removal of elements (%)
Minimum
Maximum
Mean
Minimum
Maximum
Mean
0.2 41.2 8.3 54.6 1.0 87.0
2.9 75.8 58.7 155.9 33.0 296.8
1.96 59.34 26.98 102.68 14.4 172.54
75.52 80.10 65.93 79.67 42.00 82.67
91.72 86.68 75.47 90.30 47.88 91.61
85.226 83.176 71.618 85.744 44.748 87.656
3.5 2.5
A-4
2
A-7 T-2
1.5 1 0.5 0
S-8
70
T-0
Cu (ppm)
Cd (ppm)
80
S-8
3
T-0
60
A-4
50
A-7
40
T-2
30 20 10
0
1
2
3
4
5
0
6
0
1
2
Days 70
30
50
A-4
25
40
A-7 T-2
30 20
Pb (ppm)
Cr (ppm)
T-0
10
6
20 15 10 5
0
1
2
3
4
5
0
6
0
1
2
S-8
300
T-0
250
A-4
200
Ni (ppm)
A-7 T-2
150 100 50 0
1
2
3
3
4
5
6
Days
350
Zn (ppm)
5
S-8 T-0 A-4 A-7 T-2
Days
0
4
35
S-8
60
0
3
Days
4
5
6
Days
180 160 140 120 100 80 60 40 20 0
S-8 T-0 A-4 A-7 T-2
0
1
2
3
4
5
6
Days
Fig. 1. Removal of trace elements from Tarakan coal samples by bacterial consortium during six days.
biomass is highly variable. To understand the efficiency of trace element removal through bacteria, the coal from Tarakan basin (Kalimantan), Indonesia has been studied. The method is less expensive and eco-friendly in nature. When coal samples were treated with inoculums of mixed bacterial consortium in water solution, a sharp decline in pH was measured from 6.0 to 4.0, in a time span of six days, indicating the presence of sulphur oxidising bacteria. This has caused the conversion of sulphide minerals into sulphate form and consequently lowering of pH. This process appears to have control over trace element removal (bioleaching) from the coal samples. Similar results were also reported by Sharma and Wadhawa [30] while working on bioleaching of Neyveli lignites of India. However, it has been shown that Cr and Pb require more acidic condition for their
leaching from coal [31]. Different heavy metals have a varying energy state, which affects the efficiency of bioleaching process [32]. Metals, particularly Zn, Cr and Cu, in the form of exchangeable state are considered highly mobile and bio-available. The maximum removal of metals like Zn, Cu, Cr, Cd and Pb were noticed on fourth and fifth day of incubation probably due to more acidification during that period. The removal of Ni was maximum on fifth day in all the coal samples. The quantitative removal of trace elements has been shown in Table 1c and their removal pattern with time is shown in Fig. 1. It evolves further that the metal concentrations in coal samples, except Pb, are significantly decreased using mixed bacterial consortium. The removal of metals like Cd, Cu, Cr, Ni, Pb and Zn using bacterial strains are 85.226%, 83.176%, 71.618%, 85.744%, 44.748%
230
P.K. Singh et al. / Fuel 102 (2012) 227–230
and 87.656% respectively. The order of removal is: Zn > Ni > Cd > Cu > Cr > Pb. 4. Conclusions In an attempt to probe the feasibility of removal of environmentally sensitive trace elements, the coal samples from Kalimantan, Indonesia, were subjected to bacterial treatment. These coals are ‘Sub-Bituminous coals’ in rank and have evolved in a telmatic environment under alternate oxic to anoxic moor conditions. These coals contain 1.6–15.7 wt%; mean 5.7 wt% ash, and have low carbon content (64.2–73.9 wt%; mean 70.24 wt%) and high sulphur (0.4–5.4 wt%; mean 2.52 wt%). These elements (Cd – 1.96 ppm, Cu – 59.34 ppm, Cr – 26.98 ppm, Ni – 102.68 ppm, Pb – 14.4 ppm and Zn – 172.54 ppm) are seen to occur in higher values than the Clarke values for coal. When these samples were subjected to a mix of bacterial consortium, under controlled conditions, significant reduction in their concentration has been observed. The consortium could efficiently remove over 80% of metals like Ni, Zn, Cd, Cu and Cr while the removal of Pb was nearly –45%. The removal is seen in the order: Zn > Ni > Cd > Cu > Cr > Pb.
[10] [11] [12] [13] [14] [15]
[16]
[17]
[18]
[19]
[20]
Acknowledgements The authors (P.K.S. and M.P.S.) thankfully acknowledge the Department of Geology, while A.L.S. and A.K. extend their thankfulness to the Department of Botany, Banaras Hindu University for extending the facilities. The authors thankfully acknowledge the two anonymous reviewers whose critical comments could improve the standard of paper.
[21] [22]
[23]
[24]
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