Enhancement of bunker oil biodesulfurization by adding surfactant

Biodesulfurization of Fossil Fuels: Analysis and Prospective

Biodesulfurization (BDS) of fossil fuels is a promising method for treating the high content of sulfur in crude oils and their derivatives in the future, attributed to its environmental-friendly nature and the technical efficient ability to desulfurize the organosulfur compounds recalcitrant on other techniques. It was found that the bioreaction rate depends on the treated fluid, targeting sulfur compounds, and the microorganism applied. Also, many studies investigated the operation conditions, specificity, and biocatalysts modification to develop BDS efficiency. Furthermore, mathematical kinetics models were formulated to represent the process. In this review, the previous studies are analyzed and discussed. This review article is characterized by a clear picture of all BDS's experimental, industrial, procedural, theoretical, and hypothetical points.

Biodesulfurization of refractory sulfur compounds in petro-diesel by a novel hydrocarbon tolerable strain Paenibacillus glucanolyticus HN4

One of the main precursors of air pollution and acid rains is the presence of the recalcitrant thiophenic compounds, for example dibenzothiophene (DBT) and its derivatives in transportation fuels. In an attempt to achieve the worldwide regulations of ultra-low sulfur transportation fuels without affecting its hydrocarbon skeleton, a biphasic medium containing 100 mg/L DBT dissolved in n-hexadecane (1/4 oil/water v/v) used for enrichment and isolation of selective biodesulfurizing bacterium from an oil-polluted sediment sample collected from Egyptian Red Sea shoreline. The isolated bacterium is facultative anaerobe, motile, spore-former, and mesophile. It is genetically identified as Paenibacillus glucanolyticus strain HN4 (NCBI Gene Bank Accession No. MT645230). HN4 desulfurized DBT as a model of the recalcitrant thiophenic compounds without affecting its hydrocarbon skeleton via the 4S-pathway producing 2-hydroxybiphenyl (2-HBP) as a dead end product. HN4 substantiated to be a hydrocarbon tolerant, biosurfactants(s) producer, and endorsed unique enzymatic system capable of desulfurizing broad range of thiophenic compounds and expressed an efficient desulfurization activity against the recalcitrant alkylated DBTs. As far our knowledge, it is the first reported BDS study using P. glucanolyticus. Statistical optimization based on One-Factor-At-A-Time (OFAT) technique and response surface methodology (RSM) applied for elucidation of mathematical model correlations describing and optimizing the effect of different physicochemical parameters on batch biphasic BDS process. That illustrated an approximate increase in BDS efficiency by 1.34 fold and recorded 94% sulfur removal in biphasic batch process at optimum operation conditions of 120 h, 0.14 wt% S-content model oil (DBT dissolved in n-hexadecane), 33.5 °C, pH7 and 1/1 oil/water phase ratio, and 147 rpm. Resting cells of HN4 in a biphasic reactor (1/1 v/v) decreased the sulfur content of a refractory thiophenic model oil (thiophene, benzothiophene, DBT, and alkylated DBT dissolved in n-hexadecane) from 0.14 to 0.027 wt%, and petro-diesel from 0.2 to 0.04 wt%, within 120 h, keeping the calorific value of the treated fuel intact. Consequently, that novel strain could be recommended as a promising candidate for BDS as complementary to hydrodesulfurization process in oil refinery.

A lipopeptide biosurfactant from Bacillus sp. Lv13 and their combined effects on biodesulfurization of dibenzothiophene

The process of using biodesulfurization (BDS) to remove sulfur compounds in petroleum has limitations such as low efficiency and low mass transfer. Therefore, it is important to study the combined effects of biosurfactant and the strain on BDS. A thermophilic desulfurization strain, Bacillus sp. Lv13, was isolated from the oilfield and used to produce biosurfactant (BS). The strain was identified as Bacillus licheniformis, a moderate thermophilic bacterium. Its BS was identified as lipopeptide using thin-layer chromatography (TLC), gas chromatography-mass spectrometry (GC-MS) and Fourier transform infrared spectroscopy (FT-IR). The emulsification efficiency after 24 h (E₂₄) and critical micelle concentration (CMC) were determined to be 46.93% and 30 mg L⁻¹, respectively. The combined effects of biosurfactant and the strain on BDS was confirmed using the Gibbs assay, GC-MS and BaCl₂ test. Results showed that the yield of 2-hydroxybiphenyl (2-HBP) from dibenzothiophene significantly increased after the addition of lipopeptide into the reaction system. This could be illustrated by the stabilization of emulsion, lower CMC value, higher mass transfer rate with the addition of lipopeptide, and the enhancement in the capacity of BDS as well as the catalytic ability of the microbial cell.

Complex interactions among DBT, bacteria and biomolecules play a major role in the absence of lipopeptides. After adding lipopeptides, DBT degrades rapidly to HBP through BDS.

Metabolic and process engineering for biodesulfurization in Gram-negative bacteria

Microbial desulfurization or biodesulfurization (BDS) is an attractive low-cost and environmentally friendly complementary technology to the hydrotreating chemical process based on the potential of certain bacteria to specifically remove sulfur from S-heterocyclic compounds of crude fuels that are recalcitrant to the chemical treatments. The 4S or Dsz sulfur specific pathway for dibenzothiophene (DBT) and alkyl-substituted DBTs, widely used as model S-heterocyclic compounds, has been extensively studied at the physiological, biochemical and genetic levels mainly in Gram-positive bacteria. Nevertheless, several Gram-negative bacteria have been also used in BDS because they are endowed with some properties, e.g., broad metabolic versatility and easy genetic and genomic manipulation, that make them suitable chassis for systems metabolic engineering strategies. A high number of recombinant bacteria, many of which are Pseudomonas strains, have been constructed to overcome the major bottlenecks of the desulfurization process, i.e., expression of the dsz operon, activity of the Dsz enzymes, retro-inhibition of the Dsz pathway, availability of reducing power, uptake-secretion of substrate and intermediates, tolerance to organic solvents and metals, and other host-specific limitations. However, to attain a BDS process with industrial applicability, it is necessary to apply all the knowledge and advances achieved at the genetic and metabolic levels to the process engineering level, i.e., kinetic modelling, scale-up of biphasic systems, enhancing mass transfer rates, biocatalyst separation, etc. The production of high-added value products derived from the organosulfur material present in oil can be regarded also as an economically viable process that has barely begun to be explored.

Biocatalytic Desulfurization Capabilities of a Mixed Culture during Non-Destructive Utilization of Recalcitrant Organosulfur Compounds

We investigated the biodesulfurization potential of a mixed culture AK6 enriched from petroleum hydrocarbons-polluted soil with dibenzothiophene (DBT) as a sulfur source. In addition to DBT, AK6 utilized the following compounds as sulfur sources: 4-methyldibenzothiophene (4-MDBT), benzothiophene (BT), and 4,6- dimethyldibenzothiophene (4,6-DM-DBT). None of these compounds supported the growth of AK6 as the sole carbon and sulfur source. AK6 could not grow on dibenzylsulfide (DBS) as a sulfur source. The AK6 community structure changed according to the provided sulfur source. The major DGGE bands represented members of the genera Sphingobacterium, Klebsiella, Pseudomonas, Stenotrophomonas, Arthrobacter, Mycobacterium, and Rhodococcus. Sphingobacterium sp. and Pseudomonas sp. were abundant across all cultures utilizing any of the tested thiophenic S-compounds. Mycobacterium/Rhodococcus spp. were restricted to the 4-MDBT culture. The 4-MDBT culture had the highest species richness and diversity. Biodesulfurization of DBT by resting cells of AK6 produced 2-hydroxybiphenyl (2-HBP) in addition to trace amounts of phenylacetate. AK6 transformed DBT to 2-hydroxybiphenyl with a specific activity of 9 ± 0.6 μM 2-HBP g dry cell weight⁻¹ h⁻¹. PCR confirmed the presence in the AK6 community of the sulfur-specific (4S) pathway genes dszB and dszC. Mixed cultures hold a better potential than axenic ones for the development of a biodesulfurization technology.

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.

A novel metabolite (1,3-benzenediol, 5-hexyl) production by Exophiala spinifera strain FM through dibenzothiophene desulfurization

Sulfur dioxide which is released from petroleum oil combustion causes pollution over the atmosphere and the soil. Biodesulfurization can be used as a complementary method of hydrodesulfurization, the common method of petroleum desulfurization in refineries. Many studies have been carried out to develop biological desulfurization of dibenzothiophene (DBT) with bacterial biocatalysts. However, fungi are capable to metabolize a wide range of aromatic hydrocarbons through cytochrome P450 and their extracellular enzymes. The aim of the present work was isolation and identification of fungi biocatalysts capable for DBT utilization as sulfur source and production of novel metabolites. DBT consumption and the related produced metabolites were analyzed by HPLC and GC-MS respectively. One of the isolated fungi that could utilize DBT as sole sulfur source was identified by both traditional and molecular experiments and registered in NCBI as Exophiala spinifera FM strain (accession no. KC952672). This strain could desulfurize 99 % of DBT (0.3 mM) as sulfur source by co-metabolism reaction with other carbon sources through the same pathway as 4S and produced 2-hydroxy biphenyl (2-HBP) during 7 days of incubation at 30 °C and 180 rpm shaking. However, the isolate was able to transform 2-HBP to 1,3-benzenediol, 5-hexyl. While biphenyl compounds are toxic to leaving cells, biotransformation of them can reduce their toxicity and the fungi will be more tolerant to the final product. These data are the first report about the desulfurization of DBT comparable to 4S-pathway and production of innovative metabolite by E. spinifera FM strain.

Isolation and classification of a soil actinomycete capable of sulphur-specific biotransformation of dibenzothiophene, benzothiophene and thianthrene

AIM

To isolate actinomycete spp with the ability to desulphurize sulphur-containing heterocyclic compounds present in petroleum.

METHODS AND RESULTS

Enrichment cultures were set up to select and isolate sulphur heterocycle metabolizing soil micro-organisms. Screening of the microbial isolates for the desulphurization property led to isolation of R3. The isolate was characterized by PCR screening of 16S rRNA genes and classical taxonomic investigations. HPLC analysis of the desulphurization assays with R3 showed ~85% transformation of dibenzothiophene (270 μmol l(-1)), present as the sole sulphur source in basal salt medium, in 4 days. Production of the desulphurized dibenzothiophene metabolite, 2-hydroxybiphenyl, was confirmed by GC/MS analyses. GC/MS analyses also established the ability of R3 to transform benzothiophene to benzothiophene-1-oxide and benzothiophene-1, 1-dioxide, and thianthrene to thianthrene-5-oxide. PCR primers computed based on the desulphurization operon (dszABC) of Rhodococcus erythropolis IGTS8 yielded the predicted amplification products with R3 genomic DNA as template. Southern hybridization and restriction endonuclease digestion profiles indicated that R3 amplicons were homologous to dsz AB.

CONCLUSIONS

The enrichment method used in this study yielded an environmental isolate with the ability to transform multiple sulphur heterocycles. The isolate R3 has taxonomic proximity to the Oerskovia sp, order Actinomycetales. The isolate R3 selectively removes sulphur from dibenzothiophene yielding 2-hydroxybiphenyl and sulphate. R3 also transforms benzothiophene and thianthrene in a sulphur-targeted manner. The desulphurization genes in R3 bear similarity to those in R. erythropolis IGTS8.

SIGNIFICANCE AND IMPACT OF THE STUDY

The actinomycetes present in soil can remove sulphur from different sulphur heterocycle substrates and have potential as biodesulphurization catalysts.

Biodesulfurization of model compounds and de-asphalted bunker oil by mixed culture

In this study, complicated model sulfur compounds in bunker oil and de-asphalted bunker oil were biodesulfurized in a batch process by microbial consortium enriched from oil sludge. Dibenzothiophene (DBT) and benzo[b]naphtho[1,2-d]thiophene (BNT1) were selected as model sulfur compounds. The results show that the mixed culture was able to grow by utilizing DBT and BNT1 as the sole sulfur source, while the cell density was higher using DBT than BNT1 as the sulfur source. GC-MS analysis of their desulfurized metabolites indicates that both DBT and BNT1 could be desulfurized through the sulfur-specific degradation pathway with the selective cleavage of carbon-sulfur bonds. When DBT and BNT1 coexisted, the biodesulfurization efficiency of BNT1 decreased significantly as the DBT concentrations increased (>0.1 mmol/L). BNT1 desulfurization efficiency also decreased along with the increase of 2-hydroxybiphenyl as the end product of DBT desulfurization. For real bunker oil, only 2.8 % of sulfur was removed without de-asphalting after 7 days of biotreatment. After de-asphalting, the biodesulfurization efficiency was significantly improved (26.2-36.5 %), which is mainly attributed to fully mixing of the oil and water due to the decreased viscosity of bunker oil.

Fructophilic behaviour of Gordonia alkanivorans strain 1B during dibenzothiophene desulfurization process

Biodesulfurization (BDS) aims at the removal of recalcitrant sulfur from fossil fuels at mild operating conditions with the aid of microorganisms. These microorganisms can remove sulfur from dibenzothiphene (DBT), a model compound, or other polycyclic aromatic used as sulfur source, making BDS an easy and environmental friendly process. Gordonia alkanivorans strain 1B has been described as a desulfurizing bacterium, able to desulfurize DBT to 2-hydroxybiphenyl (2-HBP), the final product of the 4S pathway, using d-glucose as carbon source. However, both cell growth and desulfurization can be largely affected by the nutrient composition of the growth medium, due to cofactor requirements of many enzymes involved in the BDS biochemical pathway. In this study, the main goal was to investigate the influence of several sugars, as carbon source, on the growth and DBT desulfurization ability of G. alkanivorans strain 1B. The results of desulfurization tests showed that the lowest values for the growth rate (0.025 hour(-1)) and for the overall 2-HBP production rate (1.80 μm/hour) by the strain 1B were obtained in glucose grown cultures. When using sucrose, the growth rate increase exhibited by strain 1B led to a higher biomass productivity, which induced a slightly increase in the 2-HBP production rate (1.91 μm/hour), conversely in terms of 2-HBP specific production rate (q2-HBP) the value obtained was markedly lower (0.718 μmol/g/hour in sucrose versus 1.22 μmol/g/hour in glucose). When a mixture of glucose and fructose was used as carbon source, strain 1B reached a value of q2-HBP=1.90 μmol/g/hour, close to that in fructose (q2-HBP=2.12 μmol/g/hour). The highest values for both cell growth (μ=0.091 hour(-1)) and 2-HPB production (9.29μm/hour) were obtained when strain 1B was desulfurizing DBT in the presence of fructose as the only carbon source, indicating a fructophilic behaviour by this bacterium. This fact is in agreement with the highest value of biomass productivity by strain 1B be in fructose, which resulted in a higher amount cells fulfilling the DBT-desulfurization. The greater number of functional cells conducted to a more effectiveness BDS process by strain 1B, as they attained a q2-HBP about 74% higher than in glucose grown cultures. Moreover, this significant BDS enhancement can better be observed in terms of the overall 2-HBP production rate, which increased over 5-fold, from 1.80 μm/hour (in glucose) to 9.29 μm/hour (in fructose).