Enhancement and stabilization of desulfurization activity of Rhodococcus erythropolis KA2-5-1 by feeding ethanol and sulfur components

Enhancing the biodesulphurization capacity of Rhodococcus sp. FUM94 in a biphasic system through optimization of operational factors

The biodesulfurization activity of bacteria through the 4S pathway in aqueous-oil emulsions is affected by various operational factors. These factors also demonstrate interacting effects that influence the potential for field applications of biodesulfurization technology and can solely be deciphered through multi-variable experiments. In this study, the effects of the influential factors and their interactions on the desulfurizing activity of a newly identified desulfurizing bacterium, Rhodococcus sp, FUM94 were quantitatively investigated. The capacity improvement achieved through optimized values obtained in this study is significant due to its simple implementation to large scale processes. This is the most simple and the most cost-effective way to scale-up a biodesulfurization process.Using response surface methodology (RSM). Optimum values of the factors were identified with the objective of maximizing biodesulfurization activity. Results revealed that the desulfurization activity of the biocatalyst increased from 0.323 ± 0.072 to 46.57 ± 4.556 mmol 2-Hydroxybiphenyl (kg dry cell weight)^-1 h^-1 at the optimized conditions of 6 h reaction time, 2 g.L^-1 biocatalyst concentration, 0.54 mM (100 ppm) dibenzothiophene (DBT) concentration (sulfur source), and 25% oil phase fraction. Desirability analysis proved that the selected conditions are the most desirable combination of factors (desirability value = 0.896) to achieve the highest biodesulfurization activity of the biocatalyst. A comparison between the biodesulfurization capacity achieved in this study and the capacities reported in similar studies published in the past two decades revealed that biodesulfurization under optimized operational conditions outperforms previously proposed techniques.

An overview of conventional and alternative technologies for the production of ultra-low-sulfur fuels

Environmental concerns have given a great deal of attention for the production of ultra-low-sulfur fuels. The conventional hydrodesulfurization (HDS) process has high operating cost and also encounters difficulty in removing sulfur compound with steric hindrance. Consequently, various research efforts have been made to overcome the limitation of conventional HDS process and exploring the alternative technologies for deep desulfurization. The alternative processes being explored for the production of ultra-low-sulfur content fuel are adsorptive desulfurization (ADS), biodesulfurization (BDS), oxidative desulfurization (ODS), and extractive desulfurization (EDS). The present article provided the comprehensive information on the basic principle, reaction mechanism, workability, advantages, and disadvantages of conventional and alternative technologies. This review article aims to provide valuable insight into the recent advances made in conventional HDS process and alternative techniques. For deep desulfurization of liquid fuels, integration of conventional HDS with an alternative technique is also proposed.

Advances in the Reduction of the Costs Inherent to Fossil Fuels' Biodesulfurization towards Its Potential Industrial Application

Biodesulfurization (BDS) process consists on the use of microorganisms for the removal of sulfur from fossil fuels. Through BDS it is possible to treat most of the organosulfur compounds recalcitrant to the conventional hydrodesulfurization (HDS), the petroleum industry's solution, at mild operating conditions, without the need for molecular hydrogen or metal catalysts. This technique results in lower emissions, smaller residue production and less energy consumption, which makes BDS an eco-friendly process that can complement HDS making it more efficient. BDS has been extensively studied and much is already known about the process. Clearly, BDS presents advantages as a complementary technique to HDS; however its commercial use has been delayed by several limitations both upstream and downstream the process. This study will comprehensively review and discuss key issues, like reduction of the BDS costs, advances and/or challenges for a competitive BDS towards its potential industrial application aiming ultra low sulfur fuels.

Reconstruction of a genome-scale metabolic network of Rhodococcus erythropolis for desulfurization studies

The remarkable catabolic diversity of Rhodococcus erythropolis makes it an interesting organism for bioremediation and fuel desulfurization. However, a model that can describe and explain the combined influence of various intracellular metabolic activities on its desulfurizing capabilities is missing from the literature. Such a model can greatly aid the development of R. erythropolis as an effective desulfurizing biocatalyst. This work reports the reconstruction of the first genome-scale metabolic model for R. erythropolis using the available genomic, experimental, and biochemical information. We have validated our in silico model by successfully predicting cell growth results and explaining several experimental observations in the literature on biodesulfurization using dibenzothiophene. We report several in silico experiments and flux balance analyses to propose minimal media, determine gene and reaction essentiality, and compare effectiveness of carbon, nitrogen, and sulfur sources. We demonstrate the usefulness of our model by studying a few in silico mutants of R. erythropolis for improved biodesulfurization, and comparing the desulfurization abilities of R. erythropolis with an in silico mutant of E. coli.

Desulfurization of dibenzothiophene (DBT) by a novel strain Lysinibacillus sphaericus DMT-7 isolated from diesel contaminated soil

A new bacterial strain DMT-7 capable of selectively desulfurizing dibenzothiophene (DBT) was isolated from diesel contaminated soil. The DMT-7 was characterized and identified as Lysinibacillus sphaericus DMT-7 (NCBI GenBank Accession No. GQ496620) using 16S rDNA gene sequence analysis. The desulfurized product of DBT, 2-hydroxybiphenyl (2HBP), was identified and confirmed by high performance liquid chromatography analysis and gas chromatography-mass spectroscopy analysis respectively. The desulfurization kinetics revealed that DMT-7 started desulfurization of DBT into 2HBP after the lag phase of 24 hr, exponentially increasing the accumulation of 2HBP up to 15 days leading to approximately 60% desulfurization of the DBT. However, further growth resulted into DBT degradation. The induced culture of DMT-7 showed shorter lag phase of 6 hr and early onset of stationary phase within 10 days for desulfurization as compared to that of non-induced culture clearly indicating the inducibility of the desulfurization pathway of DMT-7. In addition, Lysinibacillus sphaericus DMT-7 also possess the ability to utilize broad range of substrates as sole source of sulfur such as benzothiophene, 3,4-benzo DBT, 4,6-dimethyl DBT, and 4,6-dibutyl DBT. Therefore, Lysinibacillus sphaericus DMT-7 could serve as model system for efficient biodesulfurization of diesel and petrol.

Biodesulfurization of dibenzothiophene by recombinant Gordonia alkanivorans RIPI90A

The dszABC genes from newly reported dibenzothiophene biodesulfurizing bacterium, Gordonia alkanivorans RIPI90A were cloned and sequenced. The overall nucleotide sequence similarity between the dszABC genes of G. alkanivorans RIPI90A and those of Rhodococcus erythropolis IGTS8 and Gordonia nitida were 83.1% and 83.2%, respectively. A gene transfer system for G. alkanivorans RIPI90A was established employing the Escherichia coli-Rhodococcus shuttle vector pRSG43 as suitable cloning vector, resulting in transformation efficiencies up to 1.6 x 10(5)CFUs microg(-1) plasmid DNA. This stable vector was applied to cloning and efficient expression of the dsz genes under the control of lac promoter. The recombinant strain was able to desulfurize dibenzothiophene in the presence of inorganic sulfate and sulfur-containing amino acids. The maximum desulfurization activity by recombinant resting cells (131.8 microM2-hydroxybiphenylg(dry cell weight)(-1)h(-1)) was increased 2.67-fold in comparison to the highest desulfurization activity of native resting cells.

Biocatalytic desulfurization (BDS) of petrodiesel fuels

Oil refineries are facing many challenges, including heavier crude oils, increased fuel quality standards, and a need to reduce air pollution emissions. Global society is stepping on the road to zero-sulfur fuel, with only differences in the starting point of sulfur level and rate reduction of sulfur content between different countries. Hydrodesulfurization (HDS) is the most common technology used by refineries to remove sulfur from intermediate streams. However, HDS has several disadvantages, in that it is energy intensive, costly to install and to operate, and does not work well on refractory organosulfur compounds. Recent research has therefore focused on improving HDS catalysts and processes and also on the development of alternative technologies. Among the new technologies one possible approach is biocatalytic desulfurization (BDS). The advantage of BDS is that it can be operated in conditions that require less energy and hydrogen. BDS operates at ambient temperature and pressure with high selectivity, resulting in decreased energy costs, low emission, and no generation of undesirable side products. Over the last two decades several research groups have attempted to isolate bacteria capable of efficient desulfurization of oil fractions. This review examines the developments in our knowledge of the application of bacteria in BDS processes, assesses the technical viability of this technology and examines its future challenges.

Dimethyl sulfoxide (DMSO) as the sulfur source for the production of desulfurizing resting cells of Gordonia alkanivorans RIPI90A

The sulfate repression of desulfurization (Dsz) phenotype represents a major barrier to the mass production of desulfurizing resting cells. This repression can be avoided by replacing sulfate with dibenzothiophene (DBT) as the main substrate for the 4S pathway. However, mass production of biocatalyst using DBT is impractical because of its high price, low water solubility, and growth inhibition by 2-hydroxybiphenyl (2-HBP), which is the end product of the 4S pathway. In this work, the results showed that readily bioavailable sulfur compounds led to repression of the desulfurization activity of Gordonia alkanivorans RIPI90A. However, the Dsz phenotype was expressed through the 4S pathway in the presence of DMSO as the sulfur source for growth. Resting cells grown on DMSO were more active than the resting cells grown on DBT. The growth rate of strain RIPI90A on DMSO was higher than when DBT was used as the sole sulfur source. DMSO concentration significantly influenced the growth pattern of the strain, and the highest growth rate was observed at a concentration of 200 microg ml(-1). Above this concentration, the growth rate gradually decreased. DBT was found to induce the Dsz phenotype, with no observed lag period, in cells grown on DMSO as the sole sulfur source. Prior to induction, the specific activity was detected as 1.4 micromol 2-HBP (g dry cell weight)(-1) h(-1), and following incubation (5 h) the highest specific activity was observed as 5.11 micromol 2-HBP (g dry cell weight)(-1 )h(-1). This study identified that resting cells can be prepared in a two-step process. First, resting cells can be produced using DMSO as the sulfur source for growth; in the second step, improvements to their desulfurizing activity can be made using DBT as an inducer. DMSO is recommended as an appropriate sulfur source for the mass production of G. alkanivorans RIPI90A.

Methods for the preparation of a biodesulfurization biocatalyst using Rhodococcus sp

Several methods to prepare a biodesulfurization (BDS) biocatalyst were investigated in this study using a strain of Rhodococcus sp. 1awq. This bacterium could selectively remove sulfur from dibenzothiophene (DBT) via the "4S" pathway. DBT, dimethylsulfoxide (DMSO), sodium sulphate and mixed sulfur sources were used to study their influence on cell density, desulfurization activity, desulfurization ability, and the cost of biocatalyst production. In contrast to that observed from bacteria cultured in DBT, only partial desulfurization activity of strain 1awq was induced by DBT after cultivation in a medium containing inorganic sulfur as the sole sulfur source. The biocatalyst, prepared from culture with mixed sulfur sources, was found to possess desulfurization activity. With DMSO as the sole sulfur source, the desulfurization activity was shown to be similar to that of bacteria incubated in medium with DBT as the sole sulfur source. The biocatalyst prepared by this method with the least cost could remove sulfur from hydrodesulfurization (HDS)-treated diesel oil efficiently, providing a total desulfurization percent of 78% and suggesting its cost-effective advantage.

Effect of sulfur sources on specific desulfurization activity of Rhodococcus erythropolis KA2-5-1 in exponential fed-batch culture

The effects of sulfur sources on the desulfurization activity of Rhodococcus erythropolis KA2-5-1 were investigated by using an exponential fed-batch culture technique. The feed rate of a sulfur source was controlled independently of the feed rate of ethanol, which was used as a carbon and energy source. Among the sulfur sources examined were dibenzothiophene (DBT), ammonium sulfate, L-cysteine, L-methionine, and 2-amino-ethanesulfonic acid. When the fed-medium contained DBT as the sole sulfur source, KA2-5-1 cells showed a maximum desulfurization activity of approximately 130 mmol 2-HBP kg-cell(-1) h(-1). Similar levels of enzyme activity were also achieved with inexpensive ammonium sulfate by using the exponential fed-batch culture technique. In addition, higher levels of desulfurization activity were achieved by increasing the dosage of the DBT desulfurization (dsz) operon and dszD gene in R. erythropolis KA2-5-1. The recombinant strain showed a maximum desulfurization activity of approximately 250 mmol 2-HBP kg-cell(-1) h(-1) in the exponential fed-batch cultures.