Microbial pretreatment of coals: A tool for solubilization of lignite in organic solvent – quinoline

A method was developed for the conversion of low rank coals to products soluble in an organic solvent (quinoline). A selected group of polynuclear aromatic-compound-degrading and lignin-degrading facultative pure cultures and enriched anaerobic mixed microbial cultures developed for this purpose were used separately as well as together under co-culture conditions for stepwise treatment of Neyveli lignite (NL). This aerobic-anaerobic co-metabolic (co-culture) biodegradation (AACB) process resulted in the enhancement of quinoline extractability of the lignite, thereby yielding clean coal substance (the extract). The residual lignite obtained after quinoline extraction was subjected to a second step of AACB fermentation treatment. This resulted in further extraction of lignite in quinoline. The conditions were optimised for AACB fermentation treatment. The two-step AACB fermentation process under optimum conditions, resulted in an overall enhancement of yield of extract from 18% for the original lignite sample to 56% for the treated sample. The changes in the filtrate were evaluated using UV spectra, those in the residue were evaluated using FTIR spectroscopy and UV-reflectance and those in the extract using proton NMR spectra of the chloroform soluble fraction. The results indicated a decreased absorption in the carbonyl region in the AACB-treated residue and also a decrease in the overall mineral matter in the lignite samples. The mechanism of the AACB fermentation process is discussed. The process affords biosolubilization of lignite in organic solvent (quinoline) under milder conditions along with a simultaneous removal of a part of the mineral matter present in the coal. Uses for the clean coal extract obtained are suggested.