Separation and analysis of lignite bioconversion products

International Journal of Mining Science and Technology 22 (2012) 529–532

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International Journal of Mining Science and Technology journal homepage: www.elsevier.com/locate/ijmst

Separation and analysis of lignite bioconversion products Yao Jinghua a,b,⇑, Xiao Lei a,b, Wang Liqiang a a b

School of Chemical Engineering and Technology, China University of Mining & Technology, Xuzhou 221116, China Key Laboratory of Coal Processing & Efficient Utilization, Ministry of Education, Xuzhou 221116, China

a r t i c l e

i n f o

Article history: Received 24 November 2011 Received in revised form 26 December 2011 Accepted 25 January 2012 Available online 12 July 2012 Keywords: Lignite Lignite bioconversion Fractional extraction GC/MS

a b s t r a c t The bioconversion of coal at ambient conditions is a promising technology for coal processing, although the mechanisms of coal degradation are still not understood fully. In this work, the bioconversion of lignite was studied using a fungus isolated from decaying wood. The lignite samples were oxidized with nitric acid under moderate conditions and then the oxidized samples were placed on a potato medium with isolated fungus for lignite bioconversion. Lignite, oxidized lignite and residual products after bioconversion of lignite were sequentially extracted with petroleum ether, CS2, methanol, acetone and tetrahydrofuran (THF), and then each extract was characterized by gas chromatography–mass spectrometry (GC/MS). The differences in composition and structure among the samples were inferred by comparing the differences between the extracts. The results show that aromatics with one or several benzene rings and their derivatives; and some long-chain alkanes containing oxygen decreased in the methanol-, acetone-, and THF-soluble fraction from residual lignite, whereas long chain or a few branched alkanes and small quantities of aromatic compounds increased in petroleum ether and CS2 soluble fractions. Ó 2012 Published by Elsevier B.V. on behalf of China University of Mining & Technology.

1. Introduction Lignite is a kind of low-rank coal and serves as a poor fuel. There is an abundance of lignite in China and the current reserves are 130 billion tons, accounting for about 13% of total coal reserves [1–3]. However, its usage poses a serious potential threat to the environment. It is true that lignite can be used as fuel, but because of the problems associated with its low heating value and the possibility of producing spontaneous combustion byproducts make lignite difficulty for industrial use, especially for thermoelectricity generation [4,5]. Since the chemical structure of lignite is similar to lignin, i.e., the bridge bonds and the more active functional groups in the molecule are similar and are both coupled with more side chains, therefore, lignin-degrading fungi also show abilities to convert lignite into smaller molecular compounds and other value-added products [6–10]. Due to a few advantages over thermo-chemical conversion, microbial treatment of coal has been considered an economically effective and environmentally safe way of transforming macromolecules into simpler, low molecular weight products [11]. The bioconversion of coal was earlier broadly carried out, but the systematic studies on biosolubilization of coal were not conducted until the 1980s. Presently there are many researches on ⇑ Corresponding author. Tel.: +86 516 83591073. E-mail address: [email protected] (Y. Jinghua).

the bioconversion of lignite, of which most are concerned with the microbial species and conditions of bioconversion and less on the analysis of the products [12–22]. In this study, we firstly degraded the oxidized lignite with the isolated fungus, and then extracted the raw lignite, oxidized lignite and residual lignite after biodegradation with five organic solvents. Each extract was then analyzed with gas chromatography–mass spectrometry to provide a theoretical basis for the mechanism of lignite bioconversion by comparing the differences in composition and structure among the extracts.

2. Materials and methods 2.1. Lignite samples The lignite samples, collected from Xiaolongtan, Yunnan of China, were ground and sieved with a 90 mesh screen, and then dried in an oven at 70 °C for 4 h. The pretreated lignite samples were prepared by putting lignite (0.5 g per milliliter of nitric acid) into 5 mol/L nitric acid for 48 h at room temperature. Then the solutions were separated with a vacuum filtration system, and the residues repeatedly were washed with distilled water until the filtrates became neutral (pH 7.0). The oxidized lignite was dried in an oven at 70 °C for 4 h and stored for further use. The ultimate analyses of the lignite and the oxidized lignite were carried by an element analyzer, and results are shown in Table 1.

2095-2686/$ - see front matter Ó 2012 Published by Elsevier B.V. on behalf of China University of Mining & Technology. http://dx.doi.org/10.1016/j.ijmst.2012.01.015

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software. The compounds were identified by comparing mass spectra with NIST library data.

Table 1 Ultimate analysis of the three samples (%). Sample

C

H

N

O

S

Lignite Oxidized lignite Residual lignite

57.61 50.36 39.98

4.23 3.67 3.32

1.09 5.34 3.25

25.56 37.15 40.08

3.05 2.30 1.09

2.2. Bioconversion of lignite The isolated white-rot fungi were inoculated onto slants with the potato medium (200 mL potato extraction, 20 g of glucose, 20 g of agar and 1 L of distilled water) and incubated in a biological incubator at 28 °C for further use. After the development of mycelia on the medium surface, the fungi were then inoculated into 500 mL flasks containing 150 mL liquid medium (K2HPO4 0.1 g, (NH4)2SO4 1.0 g, Na2HPO4 0.1 g, MgSO4
7H2O 0.2 g, NaCl 0.1 g, CaCl2 0.01 g in 1 L of distilled water) supplied with 1.8 g oxidized lignite. The cultures were incubated by shaking at 150 rpm at 33 °C for 7 days. The percentage of bioconversion, determined by the net weight loss of the lignite samples, was 28.56%. The residue lignite was obtained as a precipitate by centrifugation at 10,000 rpm for 5 min, and was dried in an oven at 70 °C for 4 h for the further use. 2.3. Fractional extraction of samples According to the theory of ‘‘similarity and intermiscibility’’, each sample (5 g) including lignite, the oxidized lignite and the residue lignite after bioconversion was sequentially extracted with 150 mL of petroleum ether, CS2, methanol, acetone, and THF in a Soxhlet extractor with a water bath. These organic solvents have different polarity and their polarity increase gradually. Each extraction lasted for 4 days [23–25]. In this experiment, the solvent-extractable fraction was distilled with a rotary evaporator to remove the organic solvent for GC/MS analysis, and the solvent-inextractable was further extracted with the next solvent. All the solvents used in the experiment are analytical reagents and were distilled with a rotary evaporator prior to use. Fig. 1 shows the yields of the extracts from the three samples.

3. Results and discussion 3.1. Element analysis As shown in Table 1: the carbon content of the residual lignite after bioconversion decreased obviously, because to a certain extent the carbon–carbon bonds were broken by the fungus due to the occurrence of the water-soluble compounds containing carbon, the sulfur content decreased as a result of desulfurization of oxidized lignite by the microorganism; the hydrogen content slightly decreased as a result of microbial oxidization. 3.2. Gas chromatography–mass spectrometry (GC/MS) of the samples Each organic solvent extract from the lignite, oxidized lignite and residual lignite was analyzed with GC–MS, and the GC–MS analyses of residual lignite were compared with those of lignite and oxidized lignite as illustrated in Figs. 2–6. Samples A, B, and C in Fig. 2 refer to the raw lignite, the nitric acid pretreated lignite and the residual lignite after bioconversion, respectively. From Fig. 2, it is seen the GC/MS chromatogram of the oxidized lignite is similar to that of lignite, but different from that of the residual lignite. The Nos. 1, 2, and 3 materials occurred, however the Nos. 4 and 5 materials disappeared in the oxidized lignite as a resulting of the oxidization. The petroleum ether extract from lignite mostly contained aliphatic hydrocarbons with straight chains and a few branched chains, but the extract from nitric acid oxidation caused incorporation of nitrogen and oxygen in the alkanes, at the same time the number of the side chains in the aliphatic hydrocarbons also increased. In graph a 12 kinds of compounds and in graph b more than 20 kinds of compounds disappeared after the bioconversion. Microbial effects bring about the formation of long-chain aliphatic hydrocarbon, circular hydrocarbons and a small amount of compounds containing benzene rings.

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2.4. GC/MS analysis of the extracts

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28

Law lignite Oxidized lignite Risidual lignite

Extract yield (%)

24

4

60

5

Sample B

3

40

Relative abundance (%)

Each extract was analyzed with a Hewlett Packard 6890/5973 GC/MS equipped with a capillary column coated with HP-5MS (crosslink 5% PH ME siloxane, 30 m  0.25 mm i.d., 0.25 lm film thickness) and a quadrupole analyzer and operated in electron impact (70 eV) mode. The mass range scanned was from 30 to 500 amu. Data were acquired and processed using Chemstation

Sample A

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Sample B Sample A 1 2

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8

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Retention time (min)

0 E1

E2

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E4

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Fig. 1. Extract yields of three samples.

Total Fig. 2. Total ion chromatograms of petroleum ether-extractable fractions from A, B, C.

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100

100 80

SampleA

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Relative abundance (%)

20 0 100

Sample B O O N

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O OH

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Re lative abunda nce (%)

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20 0 100 80

Sample A

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0 100

Sample B

80 60 40 20 0 100

Sample C

80 60 O N OO O O O

O O

O

OHO Sample C O O

40 20

60 0

40

10

15

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30

Retention time (min)

20 0

10

Fig. 5. Total ion chromatograms of acetone-soluble fractions from A, B, C.

20

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Retention time (min) Fig. 3. Total ion chromatograms of CS2-soluble fractions from A, B, C.

100

Sample A

80

100

0 100

0 100 Sample B O

O OO O

60 40 20

Relative abundance (%)

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40 20

R elative abundanc e (%)

O O

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80 60 40 20 0 100

Sample C

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0 100 OO

80 60

Sample C

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60 40 20

40 20 0

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Sample A

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NH

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Retention time (min) Fig. 6. Total ion chromatograms of THF-soluble fractions from A, B, C.

Fig. 4. Total ion chromatograms of methanol-soluble fractions from A, B, C.

From Fig. 3, it can be seen that the CS2-soluble fraction from lignite mostly contained long-chain aliphatic hydrocarbons and a small amounts of short side-chain compounds with benzene rings. The peak at 6–8 min (Fig. 3, sample A) which refers to short chain compounds with benzene rings disappeared in the CS2-soluble fraction from oxidized lignite. After bioconversion, the CS2-soluble compounds decreased, the aliphatic hydrocarbons with branched chains disappeared and branched chains became less. Therefore, the microbial bioconversion significantly changed the composition and caused the occurrence of some organic matters with benzene rings. In Fig. 4, the peaks at 6–15 min significantly increased and the peak posterior to 16 min which shows the presence of straightchain or side-chain hydrocarbons containing oxygen decreased significantly in the oxidized lignite. It indicates that the complex macromolecules in the raw lignite were degraded into small molecules during the oxidization with nitric acid. In the methanol-soluble fraction of the residual lignite, only several peaks were

detected. It can be partly explained that the microbial enzymes broke down some of the oxidized lignite structures by oxidization and then produced methanol-insoluble compounds. The GC/MS analyses of three samples from acetone extract are presented in Fig. 5. The peaks posterior to 18 min show that the presence of long-chain aliphatic hydrocarbons disappeared in the nitric acid pretreated lignite. While the peaks at 6–14 min which show hydrocarbons containing oxygen and double bonds; and a few compounds containing sulfur disappeared. The peaks posterior to 14 min show the presence of chain hydrocarbons and a small amount of aromatics with benzene rings. In Fig. 6, the peaks after 10 min showing the presence of the compounds containing two benzene rings and small amounts of long chain aliphatic containing oxygen in the oxidized lignite disappeared. The peaks at 6–14 min which refer to aromatics and long chain hydrocarbons containing oxygen increased, and the peaks posterior to 14 min represent presence of long-chain aliphatic hydrocarbons and aromatics in residual lignite. This indicates that microbial bioconversion changed the composition of oxidized

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lignite, since no GC–MS detectable compounds were detected in the THF-soluble fraction. 4. Conclusions (1) The oxidative pretreatment of lignite with nitric acid introduced nitrogen and oxygen in the lignite structure, thus leading to increase of carbonyl and nitro groups in the oxidized lignite. (2) Nitric acid pretreated lignites were degraded by the fungi isolated from decaying wood. The bioconversion percentage of pretreated lignites determined by the net weight loss of the lignite samples was 28.56%. (3) Each sample, including lignite, the lignite oxidized by nitric acid and the residual lignite after bioconversion were extracted with petroleum ether, CS2, methanol, acetone and THF sequentially. The extract yields of petroleum ether and CS2 from the residue lignite were higher than from those of oxidized lignite, and the extract yields of methanol, acetone and THF were low. It indicated that methanol-soluble, acetone-soluble, and THF-soluble organic matters were degraded by the fungi. (4) The GC/MS analyses showed: the relative abundances aromatics containing one or more benzene rings and their derivatives; and some long-chain aliphatic hydrocarbons with oxygen decreased in methanol-, acetone- and THF-soluble fractions of the residue lignite; whereas relative abundances of alkanes or hydrocarbons with a few side-chains and some aromatics containing benzene rings increased in petroleum ether-soluble and CS2-soluble fractions of the residual lignite.

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