Microbial Desulfurization of Organosulfur Containing Coal

Abstract

Biodesulfurization (BDS) can be an efficient way of reducing the impact of toxic gases owing to its inbuilt operational feasibility under ambient environmental conditions. In present research, bacteria were isolated from hydron carbon contaminated soil samples for their metabolic potential of desulfurization of dibenzothiophene (DBT). Enrichment culture technique was applied for screening of bacteria containing “4S” pathway. Based on the screening, a bacterial consortium was developed and used for desulfurization of DBT. The results from Gibb’s assay and high-performance liquid chromatography (HPLC) analysis showed that six of the bacterial isolates, transform DBT to 2-hydroxybiphenyl (2HBP) following 4S pathway. Among these isolates, I-5 (Named IQMJ-5) showed a maximum growth rate of 0.0064 g dry cell weight L-1h-1 and desulfurization activity of about 77% as indicated by HPLC analysis. The 16S rRNA based identification revealed that, our consortium contained bacteria belongs to the phyla such as Proteobacteria, Bacteroidetes, Firmicutes, Patescibacteria, and Actinobacteria (in order of abundance). The consortium showed significant tolerance to the inhibitory effect of both 2-HBP and sulfate and even maintained growth in the presence of about 1.0 mM initial concentration of both the end products. The effect of different process parameters (such as temperature, pH, and DBT initial concentrations) on the growth and desulfurization capability of the bacterial consortium IQMJ-5 was examined. In addition, the effect of several carbon and sulfur compounds on the growth of consortia was also analyzed. The results showed that 25 ºC temperature, 7.6 pH, and 0.3 mM DBT were the optimum conditions for the highest growth and desulfurization of the DBT. In addition, glycerol, and sodium sulfate (Na2SO4) were the bioavailable carbon and sulfur sources respectively, at which the consortium IQMJ-5 showed maximum growth. Moreover, 2 gL-1 glycerol appeared as the carbon concentration at which the consortium IQMJ-5 showed the highest activity (activity). An enhanced rate of desulfurization was achieved when a medium with optimized conditions was employed, compared to non-optimized conditions. In next phase, two strategies for BDS of a standard organosulfur compound such as DBT were investigated under laboratory conditions. In the first treatment, the role of different surfactants such as Tween-20, Tween-80, SDS, and EDTA on the desulfurization of DBT was investigated by the application of bacterial consortium IQMJ-5. In the second treatment, viii iron oxide nanoparticles were synthesized and immobilized on the surface of bacteria cells. Shake flask experiments were conducted with immobilized cells, surfactant amended immobilized cells, and control or noncoated cells. Among different surfactant treatments, Tween-80 was found to be the most effective surfactant, showing maximum desulfurization activity at a concentration of 5g/L. The transmission electron microscopy and X-ray diffraction analysis indicated that produced nanoparticles were spherical in shape with a size of about 46 nm and had a stoichiometric ratio of 55.85 % and 44.15 % between O and Fe, respectively. The nanoparticle treatment enhanced the DBT desulfurization process up to 11.37 % as compared to the control, specifically when immobilized cells were used. Therefore, it was concluded that nanoparticles treatments with immobilization of the bacterial cells enhanced the desulfurization rate of DBT under ambient reaction conditions and provide a sustainable alternative for commercial coal BDS. Finally, the iron oxide (Fe3O4) coated consortium IQMJ-5 cells were applied for the desulfurization of three types of coals namely B1, B2, and K (names given according to their place of collection), collected from different coal mines in Pakistan. The total sulfur content identified was 5.25%, 1.78%, and 4.50% in B1, B2, and K coal, respectively. The desulfurization activity was characterized by different techniques such as proximate analysis, ultimate analysis, heating value analysis, FTIR, XRD, and EDS. The results of the analysis showed that after treatment with the coated IQMJ-5 consortia cells, an increase in the carbon content and heating value of all the coal samples was detected. However, the maximum increase in carbon content (about 14%) and heating value (about 2068 Btu/lb) was recorded for that of the K coal sample. The coated consortium remained successful in the removal of about 61%, 88.76%, and 54.46% of organic sulfur from B1, B2, and K coal samples, respectively. The desulfurization capabilities of the consortium were additionally confirmed by FTIR and XRD analysis. Therefore, the extent of desulfurization obtained with the Fe3O4 coated consortium suggest that the consortium can be a potential candidate for reducing sulfur-related environmental pollution of fossil fuels at more advanced commercial scales.