ANNOUNCEMENTS
Fossil fuels such as petroleum, coal and natural gas are essential energy sources to support human life on this planet. Fossil fuels act as predominant energy reservoirs and it has been estimated that 85% of total energy generated is derived from fossil fuels, followed by nuclear power (8%) and other energy sources such as hydroelectric power and wood (7%). Oil leads the list of the total world consumption (40%), followed by coal (24%) and natural gas (22%). Combustion of fossil fuels with high levels of sulphur results in the release of toxic sulphur oxides into the atmosphere. Emission of sulphur - oxides to the atmosphere causes serious environmental problems such as acid rain, which destroy buildings, kill forests and poison lakes. Some sulphur and nitrogen heterocycles are suspected carcinogens and sulphur compounds in oil have been implicated in pipeline corrosion. Sulphur and nitrogen in heterocyclic compounds are capable of poisoning catalysts used in hydrodesulphurization process. Government regulations are demanding that the sulphur content of petroleum products for use in motor vehicles needs to be further reduced over the next decade, requiring refiners to increase desulphurization capacity.
The U.S specifications for diesel oil mandate that the sulphur concentration should be 50 ppm for the year 2005 and to 15 ppm sulphur by 2006. The Indian government has also imposed regulations on the level of sulfur in diesel and directed the refineries to reduce sulphur levels from the current level of 500 ppm to 50 ppm by 2010. Biocatalytic desulphurization or biodesulphurization (BDS) is a process based on the metabolism of naturally occurring aerobic bacteria that can remove organically bound sulphur in sulphur heterocycles or petroleum without degrading the fuel value of the hydrocarbon matrix. The present work entitled “Studies on C-S bond targeted biodesulphurization of middle-distillate range fuels by Mycobacterium phlei SM120-1” was undertaken with an objective to isolate an efficient desulphurizing bacterial strain for biodesulphurization of middle-distillate range fuels such as diesel and light gas oil.
The summary of the above study is as follows:
• A total of 18 bacterial strains were isolated on BSM-DBT agar plates following the enrichment culture technique using simulated microcosm.
• Analysis of desulphurization activity using Gibb’s assay showed that six mesophilic bacterial strains and two thermophilic bacterial strains designated, as SM120-1 and KT-1 were desulphurization competent.
• The bacterial strains RR and SM120-1 were found to utilise 4, 6 dimethyl DBT as sole source of sulphur, whereas the other bacterial strains could not grow in the presence of 4, 6 dimethyl DBT.
• The bacterial strain SM120-1 identified as Mycobacterium phlei was selected for further characterization and process development, since it showed wide temperature range for growth and desulphurization. In addition, the desulphurization efficiency of this bacterial strain was found to be higher than the other bacterial strains.
• M.phlei SM120-1 was found to desulphurize DBT to 0.02mM DBT from an initial concentration of 0.54 mM within 5 days to produce 2-HBP (0.24 mM) at 50â—¦C. Further, the specific desulphurization activity of this strain with respect to DBT was determined to be 0.17 ± 0.04 μmol 2-HBP min-1 (g dry cell)-1 using the resting cells of the bacterial strain SM120-1
• 0.54 mM 4,6 dimethyl DBT (100 ppm) was completely desulphurized by the growing cells of M.phlei SM120-1 within 104 hours. The resting cells of the bacterial strain SM120-1 desulphurized 0.8 mMDBT and 0.8 mM 4,6 dimethyl DBT completely within 8 hours.
• The 4 S pathway of desulphurization followed by M.phlei SM120-1 was confirmed by the presence of products of DBT desulphurization pathway in the culture broth as dibenothiophene sulfone and 2-hydroxybiphenyl.
• Sulphur specific desulphurization pathway followed by M.phlei SM120-1 for desulphurization of 4,6 dimethyl DBT was also confirmed by identification of 2-hydroxy 3-3’-dimethyl biphenyl an end product of 4,6 dimethyl DBT desulphurization.
• Cloning and sequencing analysis of a 3 Kb bdsA-C gene fragment of M.phlei SM120-1 shared a homology of 98% with bdsA and bdsC genes coding for dibenzothiophene desulphurization enzyme A and dibenzothiophene desulphurization enzyme C of Bacillus subtilis respectively
• The deduced amino acid residues of bdsA and bdsC genes of the bacterial strain Mycobacterium phlei SM120-1 revealed a homology level of 74.4% with that of dszA and dszC genes of Rhodococcus erythropolis IGTS8 respectively.
• The bacterial strain M.phlei SM120-1 showed differences in desulphurization performance with different diesel oils. For Japanese hydrodesulphurized light gas oil with an initial sulphur content of 508 ppm, the desulphurization efficiency was found to be 70%, with the decrease in initial sulphur levels from 508 ppm to 151 ppm.
• In case of deeply hydrodesulphurized Indian diesel oil with an initial sulphur content of 224 ppm, the desulphurization efficiency of 66%, with the decrease in sulphur content of diesel from 224 ppm to 76 ppm. Biodesulphurization of hydrodesulphurized Indian diesel oil with an initial sulphur content of 820 ppm resulted in 53% of total sulphur removal from the diesel oil
• This study also showed that the differences in the desulphurization abilities of M.phlei SM120-1 and M.phlei GTIS10 were due to the following reasons. The bacterial strain M.phlei GTIS10 was isolated on dibenzothiophene and cultured on this substrate for >4 years. This selection pressure may have rendered it incapable of growing in the presence of complex hydrocarbon fuels, whereas the bacterial strain M. phlei SM120-1 was continuously enriched on various diesel oils of Indian and Japanese origin for >3 years.
• The results of fatty acid methyl ester analysis of the cell membranes of the bacterial strain M.phlei SM120-1 and the canonical Mycobacterium phlei GTIS10 showed that there was a clear difference in membrane fatty acid profiles, when LGO was used as the sole sulphur source.
• Based on the data of difference in major fatty acid profiles between these two strains, it was presumed that the bacterial strain M.phlei SM120-1 changes its fatty acid profiles when exposed to LGO, which could be responsible for the survival of bacteria from the toxicity caused by LGO and the subsequent uptake of sulphur compounds from the oil phase by this bacterial strain.
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