"Isolation and Characterization of Coal Solubilizing Microorganisms"

Abstract

Microbial solubilization of coal has been considered as a promising technology to convert raw coal into valuable products. The biological conversion of coal to alternative fuel products can be an efficient and environmentally friendly mean of utilizing the current coal reserves, including those that are difficult to utilize with conventional methods. Understanding the details of microbial coal solubilization leading up to methanogenesis is essential in order to establish new energy production techniques and industrial processes that are cost and energy efficient. The present study was aimed at investigating and exploring the prospects of possible intervention of biotechnological approaches into conventional fuel sciences for the extraction of alternative fuel options like methane. In this regard, seven coal samples, originating from different coal areas of Pakistan, were subjected to detailed chemical analyses including maceral analysis and these indicated that samples belonging to different coal ranks with vitrinite value ranging from 0.27% to 2.13% pointing towards their diverse geological history. To study coal solubilizing potential of aerobic microorganisms, initially a total of 50 different aerobic bacterial and fungal isolates have been isolated from soil, coal and water samples of Salt Range Coal Mines, Chakwal, Pakistan, but on the basis of solubilization potential, only four isolates selected for further study. The intensity of biosolubilization was measured by determining the weight loss of the coal pieces and was observed to be about 25.93% by Pseudomonas sp. AY2, 36.36% by Bacillus sp. AY3 and 50% by Trichoderma sp. AY6 while Phanerochaete sp. AY5 showed maximum coal solubilization potential, i.e. 66.67% in 30 days. Ultraviolet Visible spectrum revealed an increase in the pattern of absorbance of all treated samples as compared to control referring to solubilization. Fourier transform infrared spectroscopy indicated alterations in structure of treated coal in comparison to control coal suggesting breakdown in complex structure of coal. The major absorbance bands in infrared spectroscopy for solubilization product were attributed to carbonyl (1600cm-1), hydroxyl (3450cm-1), cyclane (2925cm-1), ether linkage (1000-1300cm-1), carboxyl (3300-2500cm-1) and side chains of aromatic ring (1000-500cm-1). The presence of microorganisms and surface erosion of coal residues as compared to control samples were observed by scanning electron microscopy, suggested that isolated microorganisms were able to survive in coal for a longer period of time. Therefore, these microorganisms isolated from coal mines have excellent potential for coal solubilization which is considered as a crucial step in coal methanogenesis allowing them to be used successfully for in-situ methane production to meet future energy demands. Coal samples were investigated further for their ability to support microbial methane production in laboratory incubations. For this purpose, bioassay with two different exogenous microorganisms WBC2 (collected from wetlands), and IF (from PRB) were employed. Among all samples, CH sample which is low volatile bituminous coal produced maximum methane 34.9 µmol CH4/g coal with WBC2 consortium, followed by SR (subbituminous coal) with 30.18 µmol CH4/g. Relatively lower methane level was observed with IF consortium, however, maximum concentration observed in case of SR coal was 25.1 µmol CH4/g coal. Acetate accumulated in control incubations where methanogenesis was inhibited, pointing towards acetoclastic pathway and indicated acetate utilization and production during the course of methanogenesis. Methanogenesis inhibited control and bioassay incubations showed nearly same levels of hydrogen, proposed that acetoclastic might be the dominant pathway for methanogenesis. Carbon dioxide and carbon monoxide was produced and consumed during the course of methane production, suggesting their role in complex methanogenic pathway chemistry. Liquid extracts were analyzed through Excitation-Emission Matrix Spectroscopy (EEMS) to obtain qualitative estimates of solubilized coal; these analyses exhibited the release of complex organic moieties. Quantative Polymerase chain reaction analysis for mcrA functional genes suggested microbial quality as well as quantity have significant influence on methane production levels. Therefore, bioassay, suggests an attractive tool for assessing the potential of coal for biogenic methane generation, and provides a platform for studying the mechanisms involved in this economically important activity. Conclusively, the current evidence of Pakistan’s coal potential to be used for cost effective and energy efficient processes particularly the low volatile bituminous coal, would open numerous advantages to the current coal energy infrastructure.