Variation in Endogenous Trace Elements during Methane

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Variation in Endogenous Trace Elements during Methane Generation from Different Coal Ranks Daping Xia,†,‡,§ Huaiwen Zhang,† Jiangtao Hong,† Xianbo Su,*,†,‡,§ and Xile Liu† †

School of Energy Science and Engineering and ‡College of Resource and Environment, Henan Polytechnic University, Jiaozuo, Henan 454000, People’s Republic of China § Collaborative Innovation Center of Coalbed Methane and Shale Gas for Central Plains Economic Region, Jiaozuo, Henan 454000, People’s Republic of China ABSTRACT: To investigate the variation in endogenous trace elements in different coal ranks during methane production, a series of biological gas production experiments were carried out using low- and high-rank coals. Changes in the content and status of occurrence of eight representative beneficial trace elements (i.e., Fe, Co, Ni, Mn, Zn, Cu, and Sn) in the gas production process were analyzed in this study. Under the same conditions, endogenous trace elements were 580.22 ppm in high-rank coal, a higher proportion than in low-rank coal (466.72 ppm). This indicates that the effect of the rank of coal is much more significant than the impact of endogenous trace elements. The average post-reaction rates of trace element content change in low- and highrank coals are 43.6 and 28.7%, respectively. This result shows that the proportion of trace elements in low-rank coal is more important than that in high-rank coal because of their contribution to response magnitude. Trace elements in different coal ranks exhibit a similar trend during different stages of biogas production. In other words, free and humic states conform to a “increase− decrease−increase” trend, while both the inorganic and macromolecular states exhibit tendencies to decrease. Results show that variation in the occurrence time of trace elements in high-rank coal is backward. Thus, this study addresses trace element variation during the process of biological gas production in different ranks of coal and augments our understanding of the process of coal-derived biogenic methane production. This study also provides a fundamental research basis for a more detailed understanding of the effect of trace elements as degradation substrates in the biogas production process.

1. INTRODUCTION Trace elements are among the five essential nutrients for microbial growth and are also important enzyme components in growth and metabolism.1−4 Most studies to date on the effects of trace elements have evaluated exogenous heavy-metalsoluble salts,5−10 and just a handful have addressed the effects of trace elements as degradation substrates during the process of methane generation. Biogeneration in this medium is an anaerobic process, in which coal acts as the fermentation substrate; however, trace element content changes in occurrence, migration, and transformation are less studied.4,11−17 The elements in coal are distributed into different states, including humic acid organic (humic acid state), macromolecule-bound organic (macromolecule), inorganic ion exchange (free state), and residual (other states). The occurrence of these states determines trace element bioavailability and, thus, the metabolic activity of microorganisms.18−23 Indeed, a significant relationship is known to exist between the trace element form and bioavailability; not all of the states of each element are involved in reactions, but the free state is most easily absorbed and used for biological functions, while the chelating state (humic acid in coal) enhances bioavailability and promotes methane fermentation when trace elements are insufficiently bioavailable.24 The rank of coal is another important factor that determines the output of biogenic methane.25 Previous research has shown that significantly more methane is generated from low-rank coal than from higher rank because of differences in the structure © XXXX American Chemical Society

and degree of coal utilization. Although the trace elements in coal play indispensable roles in biogenic methane output, to study endogenous trace element variation during different ranks of methane generation, our research is based on two ranks: lowrank coal A and high-rank coal B. Biogeneration experiments were carried out by adding exogenous enrichment culture strains to study changes in endogenous trace elements during biogenic methane production.

2. MATERIALS AND METHODS 2.1. Experimental Samples. Samples of low-rank coal from mine A and high-rank coal from mine B were selected for this experiment. Samples were sieved to a ca. 60 mesh size and stored in a refrigerator until processing. All samples were enriched with strains from water from the Daliuta mine in Inner Mongolia to ensure analytical consistency. 2.2. Detecting Trace Elements. Coal samples before and after reactions were tested using a sequential chemical extraction method that enables the quantitative detection of the occurrences of trace elements. Chemical reagents with specific solvents, which together with appropriate pH and temperature to control environmental conditions, were used to selectively extract trace elements from solid samples. The occurrences of trace elements were determined from solvents following a previously proposed approach,26 showed in Table 1. 2.3. Experimental Protocols. We used a microwave dissolver, an inductively coupled plasma mass spectrometer (ICP−MS), and a Received: June 29, 2017 Revised: September 14, 2017 Published: October 23, 2017 A

DOI: 10.1021/acs.energyfuels.7b01841 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels

methane reactions were 466.72 and 332.56 ppm, respectively, corresponding to an average rate of reduction of 28.74%. Trace element totals in high-rank coal before and after reactions were 580.22 and 452.52 ppm, respectively, corresponding to an average rate of reduction of 22%. These results reflect the level of coal trace element participation in the biogeneration process. In low-rank samples, because of the presence of more active groups and, thus, more material for microorganisms (i.e., hydrolytic bacteria, fermentation bacteria, etc.) to degrade, coal utilization and corresponding trace element utilization will be higher and, thus, reaction rates will also be greater. In contrast, high-rank coal will have a higher degree of aromatization, synusia packing, and closer structures and, therefore, will be less easily used by microorganisms. Trace elements in these samples cannot be easily used and are less likely to participate in reactions, and therefore, the rate of change will be lower. 3.2. Methane Generation Results and Analyses of Different Ranks of Coal Samples. The results presented in Figure 3 show that the total methane generated from low- and high-rank coals was 289 and 175 mL, respectively, and there were obvious differences between the two samples. Combining these results with the changes in coal trace elements from the generation process (Figure 1) provides a further explanation as to why the process of methane generation from high-rank coal is below that of low-rank samples, even though the trace element content of the former is much higher than that of the latter. The rank of coal is more determined to the extent to which it is used as a microbial substrate, which is a key carbon source according to total methane generation from the different ranks. 3.3. General Trends in the Occurrence of Trace Elements in Different Ranks of Coals. The data presented in Figures 4 and 5 show that, in an original coal state, the free and humic acid states of trace elements in a low-rank sample are 0.7 and 2.57%, respectively. These values are higher than the 0.14 and 1.26% results seen in the case of high-rank coal samples because there are less alkyl side chains and briskness functional groups in low-rank samples, and thus, the number of free and humic acid states in low-rank coal is relatively larger. Results show that, over the course of the 60 day generation period, both sets of samples conform to an “increase−decrease−increase” trend. Overall, the

Table 1. Chemical Reagent and Process for Extracting Different Forms of Metal Elements in Coal occurrence of trace elements

reagent

free state

1 mol of CH3COONa

inorganic

25% CH3COOH

humic acid organic macromolecule

1% NaOH (including 0.1 mol of sodium pyrophosphate) low-temperature ashing 0.2 mol of HNO3 and 10 mol of H2O2

environmental condition centrifugation for 30 min water bath at 90 °C 550 °C (temperature) 7.82 (pH)

super pure water machine to perform the analyses reported in this paper. Our experimental protocol involved running two parallel samples from different rank coals, fresh water from the Daliuta mine used as anaerobic bacterial sources after acclimatization and culture medium, were mixed and sparged with N2 and were sealed to ferment at 35 °C to perform biogeneration experiments and draw comparisons based on the cumulative total of methane generation. We detected the trace elements produced before, during, and after coal generation and analyzed and compared compositional differences. We then initiated methane generation on six coal samples in parallel and detected their trace element compositions using ICP−MS after 12, 24, 36, 48, and 60 days of gas generation. Finally, we performed a series of analytical tests to determine changes in the occurrences of trace elements.

3. RESULTS AND DISCUSSION 3.1. Variation in the Trace Element Content before and after Methane Generation. The experimental results presented in Figures 1 and 2 reveal that the proportion of trace elements tends to decrease both the pre- and post-reaction when the two different rank coals were used as reaction substrates for methane generation. Results show that the rates of Fe, Co, Ni, Mn, Zn, Cu, and Mo reduction in low-rank coal A were 27.6, 49.94, 52.68, 53.88, 63.03, 45.38, and 72.25%, respectively, before the methane reaction, while the proportion of Sn increased by 89.42% as a result of an experimental instrument error. In contrast, the rates of Fe, Co, Ni, Mn, Zn, Cu, and Mo reduction from high-rank coal B were reduced by 21.9, 11.89, 13.15, 38.12, 39.71, 27.50, and 25.51%, respectively, both before and after the reaction. Thus, the trace element totals in low-rank coal before and after

Figure 1. The trace element content of low rank coal from mine A mine pre- and post-reaction. B

DOI: 10.1021/acs.energyfuels.7b01841 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 2. The trace element content of high rank coal from mine B mine pre- and post-reaction.

Figure 3. Total methane generation from high and low rank coal.

free state content of low-rank coal increased from 0.7 to 4.77% before decreasing to 0.85% and then increased again to 4.24%, while the humic acid state content rose from an initial 2.57 to 19.38% before decreasing to 4.86% and then increased again to 10.5%. Similarly, the free state content in high-rank coal increased from 0.14 to 1.98%, decreased to 1.36%, and then increased again to 1.47%, while the humic acid state content increased from 1.26 to 12.38%, decreased to 4.23%, and then increased again to 8.76%. The average contents of free and humic acid trace element states in the two different rank coals occur at different proportions during the generation process, and these trends correspond to different times during generation, although less range is seen within the high-rank sample. These differences are inseparable from the generation process; at the beginning of gas production, the most active process within coal is degradation, up until the first peak of generation when available forms increase and methanogens begin to generate gas.25 When this reaction has continued for a while, the macromolecular part of coal begins to decompose, the occurrence form of some trace elements is changed, and microorganisms being to use chelating state trace elements to free key synthetic enzymes. At this point, exchangeable speciation and humic acid states are consumed and result in the tendency to initially increase and then decrease. At the end

Figure 4. Pre- and post-reaction occurrences of trace elements in low rank coal A.

of gas production, the growth and metabolism of microorganisms enter a decline phase, trace elements cannot be used, and, therefore, exchangeable speciation and humic acid states from late degradation are preserved. In addition, because of the structure and composition of high-rank coal, the microelement occurrence variation time tends to lag behind that of low-rank coal because microorganisms need more time to degrade highrank coal, which is difficult to decompose, and thus, a certain extended period is inevitable. There is an increasing volume of macromolecular states, while inorganic forms tend to decrease, from an original 27.82 and 40.26% in high-rank coal, decreasing to 14.22 and 22.99%, respectively, while that of low-rank coal decreases from the original levels of 14.81 and 35.12%, decreasing to 8.13 and 14.12%, respectively. The trends in residual states between high- and low-rank coals are similar, following an initial increase, a decrease, and then a further increase. Results also confirm the fact that macromolecule C

DOI: 10.1021/acs.energyfuels.7b01841 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 7. Trace element variation rates in Mine A low rank coal before, and after reaction. Figure 5. Pre- and post-reaction occurrences of trace elements in high rank coal from mine B.

phase, 17.5%, while the increase in Mn was the second highest, and Cu had the lowest increase in amplitude, 0.23%. Macromolecular states and inorganic forms exhibited different degrees of reduction; for example, Zn exhibited the maximum amplitude decrease of 12.98% as a macromolecule, while Co was the second most, and Cu had the least amplitude of decrease, 2.8%. Mo had the maximum amplitude of decrease, 23.1% in the inorganic state, while Fe was the second highest, and Zn exhibited the smallest amplitude decrease, 3.4%. Most elements increased in their residual states, but Ni decreased by 6.6%. The results presented in Figures 8 and 9 show that variation in the six elements between high-rank coal B and low-rank coal A was nearly the same. Most elements increased in their free states before gas production, while Mo decreased by 0.27%; the amplitude of increase of each element in its free state is

binding states conform to a decreasing trend as a result of the decomposition of the macromolecular structure, leading to state transformations. 3.4. Trace Element Variation in Different Rank Coals during the Reaction Process. The graphs presented in Figures 6 and 7 reveal that the ionic and humic acid states of six

Figure 6. Trace element variation rates in Mine A low rank coal before, and after reaction.

representative elements (i.e., Fe, Co, Ni, Mn, Zn, Cu, Mo, and Sn) in low-rank coal increase to different degrees following the cessation of gas production. At the same time, Mn increased to a maximum at different degrees after the end of gas production. The maximum increase in this element was seen in its free state, 4.37% following gas production, while the increase in Ni was the second highest, and Mo increased the least, 0.5%. The maximum amplitude of Ni increase was seen in the humic acid

Figure 8. Trace element variation rates in Mine B high rank coal before, and after reaction. D

DOI: 10.1021/acs.energyfuels.7b01841 Energy Fuels XXXX, XXX, XXX−XXX

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4. CONCLUSION Understanding the trace elements in coal is an important aspect of geochemistry; indeed, the influence of trace elements on biogenic coalbed methane outputs are being given more impetus. The trace elements in different ranks of coal vary significantly depending upon their migration and transformation as a result of pre- and post-biogenic gas production. Our results reveal that the effect of coal rank is significant and the total participation of trace elements in low-rank coal is higher than that in high-rank samples. The occurrence of trace elements in different coal ranks, however, leads to similar changes at different stages of gas production; results show that free and humic states conform to a “increase−decrease− increase” trend, but the inorganic and macromolecular states tend to decrease. Identifying these trends in the variation of trace element content and occurrence states in coal during biogenic gas production helps in the understanding of migration trends and the transformation of trace elements. It also helps to understand variations in structural and compositional processes during coal degradation. Our research therefore lays the foundations for understanding biological gas production from coal, a key future research direction. The coal in different regions and other rank coals need further investigation.

Figure 9. Trace element variation rates in Mine B high rank before, and after reaction.

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significantly lower than that in the low-rank coal A, where Ni exhibited a maximum amplitude increase of 3.78%, Zn was the second highest, and Cu exhibited a minimum amplitude increase of 0.53%. Increasing humic acid amplitudes in this case are also unlike those seen in the low-rank coal A; with the exception of Cu, the proportions of all other elements increased before gas production, with Mn exhibiting a maximum increase amplitude of 10%, Ni exhibiting the second highest, and Mn exhibiting the lowest amplitude increase at 0.35%. In terms of macromolecule states and inorganic form, all elements, with the exception of Sn, show trends in variation similar to those in low-rank coal A, decreasing prior to gas production. Ni exhibits the maximum decrease of 14.9% in the macromolecule state, followed by Fe, while Cu exhibits the minimum decrease, 6.3%. Cu shows a maximum decrease of 10.8% in the inorganic form, while Zn is second, and Co reveals a minimum decrease of 3.45%. Similarly, Sn increases by 6.97% in the macromolecule state and by 5.94% in its inorganic form; with the exception of Mn, the proportions of all other elements increase in their residual state. The free and humic acid states of each element are used by microorganisms. Results show that the two states of occurrences of elements actually increase late in reaction stages; this trend is also characteristic of macromolecule states, while inorganic forms are transformed to other states because of the sustained decrease in macromolecule and inorganic forms and the concomitant increase in free and humic acid states during late-stage gas production. On the one hand, variation in the states of occurrence and proportions of trace elements reflects the effects of biogenic methane and confirms the origin and participation of trace elements in coal during the biogenic production of this gas. This process also indicates that some trace elements are sensitive to participation in anaerobic reactions. Elements, such as Mn, Ni, and Zn, vary significantly in the occurrence state depending upon their utilization by microorganisms both before and after the process of generation; this indirectly reveals that this process is influenced by absorption and utilization by microorganisms as well as their degree of influence.

AUTHOR INFORMATION

Corresponding Author

*Telephone: 0391-3987981. E-mail: [email protected]. ORCID

Jiangtao Hong: 0000-0001-7159-9143 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science of Foundation of China (Grants 41472127, 41472129, and 41502158).

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REFERENCES

(1) Seravalli, J.; Gu, W.; Tam, A.; Strauss, E.; Begley, T. P.; Cramer, S. P.; Ragsdale, S. W. Proc. Natl. Acad. Sci. U. S. A. 2003, 100 (7), 3689−3694. (2) Ferry, J. G. FEMS Microbiol. Rev. 1999, 23 (1), 13−38. (3) Shima, S.; Warkentin, E.; Thauer, R. K.; Ermler, U. J. Biosci. Bioeng. 2002, 93 (6), 519−530. (4) Singh, P. K.; Singh, A. L.; Kumar, A.; Singh, M. P. Fuel 2012, 102 (14), 227−230. (5) Evranos, B.; Demirel, B. Environ. Technol. 2015, 36, 1556−1562. (6) Moestedt, J.; Nordell, E.; Shakeri Yekta, S.; Lundgren, J.; Martí, M.; Sundberg, C.; Ejlertsson, J.; Svensson, B. H.; Bjorn, A. Waste Manage. 2016, 47, 11−20. (7) Qiang, H.; Niu, Q. G.; Chi, Y. Z.; Li, Y. Y. Chem. Eng. J. 2013, 222, 330−336. (8) Schmidt, T.; Nelles, M.; Scholwin, F.; Pröter, J. Bioresour. Technol. 2014, 168, 80−85. (9) Zhang, L.; Jahng, D. Waste Manage. 2012, 32 (8), 1509−1515. (10) Banks, C. J.; Zhang, Y.; Jiang, Y.; Heaven, S. Bioresour. Technol. 2012, 104 (1), 127−135. (11) Singh, P. K.; Singh, A. L.; Kumar, A.; Singh, M. P. Fuel 2013, 106 (2), 876−879. (12) Singh, A. L.; Singh, P. K.; Kumar, A.; Yadav, A.; Singh, M. P. Energy Explor. Exploit. 2014, 32 (5), 831−846. (13) Singh, P. K.; Singh, A. L.; Kumar, A.; Singh, M. P. Energy Explor. Exploit. 2014, 32 (4), 709−718. E

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Energy & Fuels (14) Singh, A. L.; Singh, P. K.; Kumar, A.; Singh, M. P. Energy Explor. Exploit. 2015, 33 (5), 755−767. (15) Singh, A. L.; Singh, P. K.; Kumar, A.; Singh, M. P. Energy Sources, Part A 2015, 37 (13), 1432−1439. (16) Singh, P. K.; Singh, A. L.; Singh, M. P.; Naik, A. S.; Singh, D.; Rai, S.; Kumar, A. Int. J. Coal Sci. Technol. 2016, 3 (2), 235−245. (17) Singh, A. L.; Singh, P. K.; Kumar, A.; Singh, M. P. J. Geol. Soc. India 2017, 89 (6), 643−652. (18) Rizzi, F. R.; Stoll, S.; Senesi, N.; Buffle, J. Soil Sci. 2004, 169 (11), 765−775. (19) Hower, J. C.; Campbell, J. L.; Teesdale, W. J.; Nejedly, Z.; Robertson, J. D. Int. J. Coal Geol. 2008, 75 (2), 88−92. (20) Tao, S.; Tang, D. Z.; Xu, H.; Liang, J. L.; Shi, X. F. Int. J. Coal Geol. 2013, 115, 41−51. (21) Sutcu, E. C.; Karayigit, A. L. Int. J. Coal Geol. 2015, 144−145, 111−129. (22) Tang, Y. G.; Yin, Z. R.; Chang, C. X.; Hao, L. J. China Coal Soc. 2005, 30 (1), 80−84. (23) Swaine, D. J. Trace Elements in Coal; Butterworth: London, U.K., 1990; pp 278. (24) Prachiti, P. K.; Manikyamba, C.; Singh, P. K.; Balaram, V.; Lakshminarayana, G.; Raju, K.; Singh, M. P.; Kalpana, M. S.; Arora, M. Int. J. Coal Geol. 2011, 88 (2−3), 83−100. (25) Singh, A. L.; Singh, P. K.; Singh, M. P. Energy Explor. Exploit. 2012, 30 (5), 837−852. (26) Zhao, F. H.; Ren, D. Y.; Yi, J. S.; Li, Y. N.; Wang, X. Q. Environ. Sci. 1999, 79−81.

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DOI: 10.1021/acs.energyfuels.7b01841 Energy Fuels XXXX, XXX, XXX−XXX