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Applied and Environmental Microbiology, January 1999, p. 181-188, Vol. 65, No. 1
0099-2240/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.
Microbial Desulfurization of a Crude Oil Middle-Distillate
Fraction: Analysis of the Extent of Sulfur Removal and the Effect
of Removal on Remaining Sulfur
M. J.
Grossman,1,*
M. K.
Lee,1
R. C.
Prince,1
K. K.
Garrett,1
G. N.
George,2 and
I.
J.
Pickering2
Exxon Research and Engineering Co.,
Annandale, New Jersey 08801,1 and
Stanford Synchrotron Radiation Laboratory, Stanford,
California 943092
Received 10 July 1998/Accepted 27 October 1998
 |
ABSTRACT |
Rhodococcus sp. strain ECRD-1 was evaluated for its
ability to desulfurize a 232 to 343°C middle-distillate (diesel
range) fraction of Oregon basin (OB) crude oil. OB oil was provided as the sole source of sulfur in batch cultures, and the extent of desulfurization and the chemical fate of the residual sulfur in the oil
after treatment were determined. Gas chromatography (GC), flame
ionization detection, and GC sulfur chemiluminesce detection analysis
were used to qualitatively evaluate the effect of
Rhodococcus sp. strain ECRD-1 treatment on the hydrocarbon
and sulfur content of the oil, respectively. Total sulfur was
determined by combustion of samples and measurement of released sulfur
dioxide by infrared absorption. Up to 30% of the total sulfur in the
middle distillate cut was removed, and compounds across the entire
boiling range of the oil were affected. Sulfur K-edge X-ray
absorption-edge spectroscopy was used to examine the chemical state of
the sulfur remaining in the treated OB oil. Approximately
equal amounts of thiophenic and sulfidic sulfur compounds were
removed by ECRD-1 treatment, and over 50% of the sulfur remaining
after treatment was in an oxidized form. The presence of partially
oxidized sulfur compounds indicates that these compounds were en route
to desulfurization. Overall, more than two-thirds of the sulfur
had been removed or oxidized by the microbial treatment.
| |
INTRODUCTION |
The concentration of sulfur in crude
oil is typically between 0.05 and 5.0% (by weight), although values as
high as 13.95% have been reported (24). In general, the
distribution of sulfur in crude oil is such that the proportion of
sulfur increases along with the boiling point of the distillate
fraction (24). As a result, the higher the boiling range of
the fuel the higher the sulfur content will tend to be. For example, a
middle-distillate-range fraction, e.g., diesel fuel, will typically
have a higher sulfur content than the lower-boiling-range gasoline
fraction. Upon combustion, the sulfur in fuels can contribute to air
pollution in the form of particulate material and acidic gases, such as
sulfur dioxide (26). To reduce sulfur-related air pollution,
the level of sulfur in fuels is regulated, and to meet these
regulations sulfur must be removed from fuels during the refining process.
Refineries remove organic sulfur from crude oil-derived fuels by
hydrodesulfurization (HDS). HDS is a catalytic process that converts
organic sulfur to hydrogen sulfide gas by reacting crude oil fractions
with hydrogen at pressures between 150 and 3,000 lb/in2 and
temperatures between 290 and 455°C, depending upon the feed and level
of desulfurization required (26). Organic sulfur compounds in the lower-boiling fractions of petroleum, e.g., the gasoline range,
are mainly thiols, sulfides, and thiophenes, which are readily removed
by HDS. However, middle-distillate fractions, e.g., the diesel and fuel
oil range, contain significant amounts of benzothiophenes and
dibenzothiophenes (DBTs), which are considerably more difficult to
remove by HDS. Among the most refractory of these compounds are DBTs
with substitutions adjacent to the sulfur moiety (12).
Compounds of this type are referred to as sterically hindered compounds
because the substitutions are believed to sterically hinder access of
the sulfur atom to the catalyst surface. Due to their resistance to
HDS, sterically hindered compounds represent a significant barrier to
reaching very low sulfur levels in middle- and heavy-distillate-range
fuels. The high cost and inherent chemical limitations associated with
HDS make alternatives to this technology of interest to the petroleum
industry. Moreover, current trends toward stricter regulations on the
content of sulfur in fuels provide incentive for the continued search
for improved desulfurization processes.
Biodesulfurization has been studied as an alternative to HDS for the
removal of organic sulfur from fuels. The use of hydrocarbon degradation pathways that attacked DBT were unsuccessful because these
systems relied on the oxidation and mineralization of the carbon
skeleton instead of on sulfur removal and therefore significantly reduced the fuel value of the desulfurized end product (9, 10, 14,
15, 16, 17, 18, 19, 20, 30). More recently, bacteria that
desulfurize DBT and a variety of other organic sulfur compounds
typically found in petroleum oils via a sulfur selective oxidative
pathway that does not remove carbon (1, 7, 11, 13, 22, 27,
28) have been isolated. This pathway involves the sequential
oxidation of the sulfur moiety followed by cleavage of the carbon
sulfur bonds.
To be commercially useful, biodesulfurization must be able to remove
the sulfur from fuels. Although considerable research on the
desulfurization of model compounds via the sulfur selective oxidative
pathway has been reported, little information on the desulfurization of
fuel oils has been published. Rhodococcus sp. strain ECRD-1
was previously shown to be able to desulfurize sterically hindered DBTs
by using the sulfur selective oxidative pathway (17). In
this study, we evaluated the ability of ECRD-1 to desulfurize a
middle-distillate fraction of crude and characterized the chemical fate
of the remaining sulfur.
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MATERIALS AND METHODS |
Bacterial strain.
Biodesulfurization experiments using
Rhodococcus sp. strain ECRD-1 (ATCC 55309), previously
designated Arthrobacter sp. strain D-1 (ATCC 55309),
were performed. This organism was isolated by enrichment culture from
marine sediments based on its ability to selectively remove sulfur from
the sterically hindered organic sulfur compound 4,6-DEDBT
(17). ECRD-1 uses a sulfur selective oxidative pathway for
sulfur removal, resulting in the formation of a hydroxylated
sulfur-free end product (17). Frozen stocks were maintained
at 
80°C prior to use in desulfurization experiments.
Oil.
Oregon Basin (OB) crude oil, containing 2.1% sulfur,
was obtained from the Exxon Company, Baytown, Tex. A 450-to-650°F
(232-to-343°C) middle-distillate cut was prepared and used for
desulfurization experiments. The oil was artificially weathered by
evaporation under a stream of nitrogen to a constant weight to
eliminate inconsistencies caused by evaporative loss of oil during
culturing or extraction. Weathering of the OB oil resulted in a weight
loss of less than 10%.
Biodesulfurization of oil.
Biodesulfurization of OB oil was
performed by growing Rhodococcus sp. strain ECRD-1 in
mineral salts sulfur-free medium (MSSF) using the OB oil as sulfur
source. MSSF contained the following components per liter: 0.4 g
of KH2PO4, 1.6 g of
K2HPO4, 1.5 g of NH4Cl,
0.17 g of MgCl2 · 6H2O, 0.09 g
of CaCl2 · 2H2O, 50 mg of
Na2WO4 · 2H2O, 1 ml of
vitamin solution, and 5 ml of mineral solution. The vitamin solution
contained the following components per liter: 100 mg of thiamine, 50 mg
of p-aminobenzoic acid, 50 mg of vitamin B12,
and 10 mg of biotin. The mineral solution contained (per liter of
deionized water) 1.5 g of nitrilotriacetic acid (dissolved in 500 ml of H2O and adjusted to pH 6.5 with 10 M KOH), 5.1 g
of MgCl2 · 6H2O, 0.66 g of
MnCl2 · 2H2O, 1.0 g of NaCl, 1.0 g of FeCl3 · 6H2O, 0.1 g
of CaCl2 · 6H2O, 0.01 g of
CuCl2 · 6H2O, 0.08 g of
ZnCl2, 0.05 g of AlCl3, 0.01 g of
H3BO3, and 0.04 g of
Na2MoO4 · 2H2O. MSSF plates
containing 130 mg of 4,6-DEDBT/liter (added as a 100× solution
dissolved in ethanol) as the sole sulfur source were prepared from MSSF
containing 0.8% SeaKem Gold agarose (FMC Corp.), a low-free-sulfate agarose.
Desulfurization of undiluted OB oil was performed with 1 ml of OB oil
per liter of culture, which provided approximately 20 mg of organic
sulfur (final concentration, 20 ppm). To minimize the degradation by
ECRD-1 of alkanes present in the OB oil, desulfurization experiments
were also performed with OB oil diluted in decane. Decane served as an
abundant source of a readily assimilated alkane substrate for ECRD-1,
minimizing the degradation of alkanes present in the OB oil. Two
dilutions of OB oil were used to evaluate the effect of different
sulfur concentrations on the extent of desulfurization. Ten milliliters
of a 10-fold dilution of oil in decane per liter of culture was added
to provide 20 mg of organic sulfur (final concentration, 20 ppm), and
10 ml of a 20-fold dilution of oil in decane per liter of culture was
added to provide 10 mg of organic sulfur (final concentration, 10 ppm).
Oil was autoclaved in sealed jars for 15 min at 121°C and 15 lb/in2, cooled, and then added at the indicated amounts
prior to inoculation.
Inocula for the biodesulfurization experiments were prepared by
inoculating 50 ml of sterile Luria-Bertani medium (
25) with
Rhodococcus sp. strain ECRD-1 grown on MSSF agar plates
containing
4,6-DEDBT as the sole sulfur source. Cells were grown for
20 h
until mid-log phase, and the entire culture was harvested by
centrifugation
at 3,000 ×
g at 4°C. The resulting
cell pellet was washed twice
with 50 ml of sterile 12 mM phosphate
buffer (pH 7.0). Washed
cell pellets were resuspended in 1/10 the
original culture volume
of chilled phosphate buffer and used
immediately for inoculation.
Two milliliters of a concentrated inoculum
suspension was used
to inoculate 1 liter of culture medium contained in
2-liter Erlenmeyer
flasks. Biodesulfurization cultures were incubated
for 7 days
at 25°C with shaking at 200 rpm on a rotary shaker. The pH
of
the cultures was monitored at 1- to 2-day intervals with pH paper
(J. T. Baker Inc.) by using 1-ml aliquots of the aqueous phase
withdrawn with a sterile pipette. Care was taken to avoid removal
of
oil. If the pH deviated by more than 1.0 pH unit, it was adjusted
to pH
7.0 with 1 M phosphoric acid. Sterile controls, which were
not
inoculated with ECRD-1, were prepared and treated in a manner
identical
to that for inoculated
cultures.
Extraction of oil cultures.
Before extraction, cultures were
brought to a pH of 2.0 with 1 N HCl. The entire content of each flask
was extracted three times with 100 ml of methylene chloride, and the
combined extracts were filtered through anhydrous sodium sulfate to
remove water. The samples were evaporated to approximately 10 ml under
a stream of nitrogen gas. Samples were subsequently filtered through a 0.22-µm-pore-size Teflon hydrophobic membrane syringe filter (13-mm diameter; Gelman Sciences) to remove turbidity (attributed to water
condensate) appearing after volume reduction. The solutions were then
concentrated by evaporation at room temperature under a stream of
nitrogen gas to approximately 3.0 ml and used for analysis by gas
chromatography (GC) and sulfur K-edge X-ray absorption-edge spectroscopy. For total sulfur analysis, a portion of the concentrated sample was evaporated under a stream of nitrogen gas to constant weight
to remove residual methylene chloride and, if present, decane.
GC, FID, and SCD analyses.
GC analysis was performed in
duplicate on 1 µl of sample extract by using a Perkin-Elmer GC
Autosystem (split/splitless injector) and a Supelco SPB-1 column (30 m
by 0.32 mm, 0.25-µm film thickness). The temperature zones for the GC
were as follows: injector and detector temperature, 300°C; initial
oven temperature, 40°C for 1 min, followed by a 4°C/min temperature
ramp to 300°C for a final 10-min hold. The temperature zones for
the GC were as follows: injector temperature, 275°C; detector
temperature, 325°C; initial oven temperature, 50°C for 1 min
followed by a 5°C/min temperature ramp to 300°C with a final 20-min
hold. Hydrocarbon- and sulfur-containing compounds were detected by
using a Perkin-Elmer flame ionization detection (FID) instrument and a
Sievers Instruments model 355 sulfur chemiluminescence detection (SCD)
instrument in tandem.
Total sulfur analysis.
Sulfur removal from OB oil was
determined by the difference in sulfur content in sterile control oil
and that in oil treated with ECRD-1. The total percentage of sulfur (by
weight) was determined in triplicate for each sample by combustion of
samples, and measurement of released sulfur dioxide was performed by
infrared absorption using a model SC-432DR sulfur analyzer (LECO
Corporation, St. Joseph, Mich.). Analysis was carried out in accordance
with the procedures described in American Society for Testing Materials Method D-4239. The variation on the procedure included the use of
Com-Aid (501-426) combustion aid (LECO).
Sulfur K-edge X-ray absorption-edge spectroscopy.
Sulfur
K-edge X-ray absorption-edge spectroscopy was used to determine the
effect of biodesulfurization on the residual sulfur content of the
treated oils. This technique allows for the evaluation of the chemical
state of sulfur compounds in a sample, e.g., sulfidic versus
thiophenic, and the oxidation state, e.g., sulfoxide versus sulfone.
Sulfur K-edge X-ray absorption-edge spectra were obtained on beamline
6-2 at the Stanford Synchrotron Radiation Laboratory. Where
appropriate, model compounds were run as powder films, using electron
yield detection, and liquid model compounds and the oils were run as
dilute solutions in toluene, using fluorescence detection (4). In general, there is a trend toward higher absorbance energies in sulfidic, thiophenic, and oxidized species (in that order)
(Fig. 1).
Crude oil and its distillate fractions contain a mixture of sulfur
compounds. The sulfur K-edge X-ray absorption-edge spectra
of samples
with a mixture of sulfur compounds is equal to the
sum of the spectra
of the individual components. The contribution
of each component sulfur
compound to the total sulfur spectra
is proportional to the amount of
the total sulfur in the sample
it represents. The relative amount of
different sulfur types in
biodesulfurized and control oils was
determined from the combination
and proportion of model compound
spectra which gave the best fit
to the spectra of the oil. Fits to the
spectra were performed
by using least-squares nonlinear optimization as
previously described
(
5).
Although middle-distillate crude oil fractions contain a complex
mixture of sulfur compounds, they can be represented by two
general
chemical types, aliphatic sulfides and thiophenes. These
compounds,
representing the substrates for biodesulfurization,
were modeled by
using DBT as a model thiophene and benzylsulfide
(BS) as a model
aliphatic sulfide. DBT sulfoxide and sulfone have
been identified as
intermediates of DBT desulfurization by
Rhodococcus sp.
strain ECRD-1 (
17). The compounds
2-hydroxybiphenyl-2'-sulfinic
acid and 2-hydroxybiphenyl-2'-sulfonic
acid, cyclized during acid
extraction to
dibenz[
c,
e][1,2]-oxanthiin 6-oxide (HBP-sultine)
and dibenz[
c,
e][1,2]-oxanthiin 6,6-dioxide
(HBP-sultone), respectively,
have been identified as intermediates of
DBT desulfurization by
Rhodococcus sp. strain IGTS8, which
also uses a sulfur selective
oxidative desulfurization pathway (
6,
21). Potential intermediates
of biodesulfurization were
modeled by using dimethyl sulfoxide
(DMSO), DBT sulfone, HBP-sultine,
and HBP-sultone. Figure
1 shows
the sulfur K-edge X-ray absorption-edge
spectra of these compounds.
DBT, BS, and DMSO were purchased from
Aldrich. DBT sulfone was
synthesized as previously described
(
17), and HBP-sultine and
HBP-sultone were synthesized from
2-hydroxybiphenyl by the method
of Hanson and Kemp (
8). All
compounds were greater than 98%
pure as determined by GC
analysis.
| |
RESULTS |
GC, SCD, FID, and total sulfur analyses.
Desulfurization of OB
oil by Rhodococcus sp. strain ECRD-1 was performed by using
neat OB oil and OB oil diluted in decane. GC-SCD analysis of cultures
grown on undiluted OB oil providing approximately 20 ppm of organic
sulfur revealed extensive depletion of sulfur compounds across the
entire boiling range of the oil (Fig.
2A). Comparison of the sulfur content of
the treated versus sterile control oil showed that the treated oil had
8.1% less sulfur than the sterile control oil (Table
1). In addition to sulfur removal, the
GC-FID chromatogram showed marked reduction of the resolvable peaks,
largely linear alkanes (Fig. 2B). ECRD-1 is able to degrade alkanes but
is unable to attack aromatic hydrocarbons (17). The loss of
resolvable peaks in the treated OB oil chromatogram was attributed to
alkane degradation.
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FIG. 2.
GC-SCD (A) and GC-FID (B) chromatograms of sterile
control and Rhodococcus sp. strain ECRD-1-desulfurized OB
oil treated without decane.
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|
The loss of oil due to alkane degradation by ECRD-1 resulted in an
underestimation of the extent of sulfur removal. To reduce
the
degradation of hydrocarbons in the OB oil, experiments using
the oil
diluted in decane were performed. The added decane provided
a preferred
alkane substrate that competed for alkane substrates
present in the OB
oil and was completely resolved from the hydrocarbons
present in the OB
oil by GC. This allowed for the evaluation of
sulfur removal without
significant hydrocarbon loss from the OB
oil
itself.
Equal volumes of OB oil diluted 10-fold and 20-fold in decane,
providing a final sulfur concentration of approximately 20
and 10 ppm,
respectively, were evaluated to assess the maximum
amount of sulfur
removal possible by ECRD-1. The GC-FID chromatogram
of the OB oil
sample from the culture receiving 10-fold-diluted
OB oil (Fig.
3B) and
the culture receiving 20-fold-diluted OB
oil (data not shown) showed
little change in the resolvable peaks,
demonstrating that the decane
effectively competed for other alkane
substrates in the OB oil. The
GC-SCD chromatogram of the OB oil
samples from the culture receiving
10-fold-diluted OB oil (Fig.
3A) and the
culture receiving 20-fold-diluted OB oil (data not
shown) revealed
effectively the same pattern of sulfur compound
removal as observed
without decane. Sulfur removal was calculated
to be 30% for both
cultures (Table
1). If sulfur removal in the
culture receiving
10-fold-diluted OB oil was limited by the sulfur
needs of ECRD-1, a
greater sulfur reduction should have occurred
in the culture receiving
20-fold-diluted OB oil. The fact that
this did not occur indicates that
30% sulfur removal is the maximum
obtainable by this organism for this
oil.
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FIG. 3.
GC-SCD (A) and GC-FID (B) chromatograms of sterile
control and Rhodococcus sp. strain ECRD-1-desulfurized OB
oil diluted in decane.
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|
Sulfur K-edge X-ray absorption-edge spectroscopy.
The effect
of ECRD-1 biodesulfurization on the chemical state of the sulfur
remaining in the OB oil after treatment was determined by analysis of
the sulfur K-edge X-ray absorption-edge spectra of the recovered oils.
The sulfur spectra of the sterile control and desulfurized OB oil
without decane samples are shown in Fig. 4. The spectra of the original and the
sterile control were virtually identical, indicating that there were no
abiological effects on the sulfur composition due to the culture
conditions used (data not shown). In contrast, the spectrum of the
treated oil is markedly different from that of the sterile control,
showing an increase in absorbance at approximately 2,473 and 2,477 eV
that is characteristic of more highly oxidized sulfur species (Fig. 1).
Table 2 shows the best-fit composition of
sulfur compounds in the sterile control and in the treated oil. In the
sterile control, the sulfur compounds are almost equally split between
thiophenic and sulfidic forms with a small amount of oxidized sulfur
present, modeled in this case by a 4% contribution of HBP-sultine. In
marked contrast, the best fit to the treated OB oil sample spectra is
obtained by including significant amounts of HBP-sultine (22%) and DBT sulfone (38%). Interestingly, thiophenic and sulfidic sulfur appeared to be affected to an equal extent by the microbial desulfurization. In
this analysis and subsequent analyses of OB oil in decane cultures, the
sultone could not be meaningfully fit to the data and was not
considered to contribute to their spectra.
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FIG. 4.
Sulfur K-edge X-ray absorption spectra of sterile
control and Rhodococcus sp. strain ECRD-1-desulfurized OB
oil treated without decane. Spectra are scaled relative to the levels
of sulfur in the oils.
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|
The sulfur K-edge X-ray absorption-edge spectrum of the biodesulfurized
OB oil diluted 10-fold in decane also showed a dramatic
increase in
oxidized sulfur species (Fig.
5). The
fits to the
spectra and the model spectra contributing to the fits are
shown
in Fig.
6A and B. The composition
of oxidized species was similar
to that of the OB oil desulfurized in
the absence of decane, but
less of the remaining sulfur species were
present as HBP-sultine
(14% versus 22%), and the total percentage of
oxidized species
was lower (Table
3).
Overall, about 30% of the sulfur was removed,
and 50% of the
remaining sulfur was oxidized to a chemical state
most similar to that
of DBT sulfone and HBP-sultine, possibly
representing compounds that
had proceeded only partially along
the desulfurization pathway. The
sulfur remaining in the biodesulfurized
sample from the culture
receiving OB oil diluted 20-fold in decane
was somewhat more oxidized
than that from the culture receiving
10-fold-diluted OB oil, most
noticeably with regard to species
with spectra similar to HBP-sultine
(data not shown).
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FIG. 5.
Sulfur K-edge X-ray absorption spectra of sterile
control and Rhodococcus sp. strain ECRD-1-desulfurized OB
oil diluted in decane. Spectra are scaled relative to the levels of
sulfur in the oils.
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FIG. 6.
Linear fits (solid lines) of the sulfur K-edge X-ray
absorption spectra of a combination of model compounds to the spectra
of OB oil diluted in decane (dotted lines) from sterile control (A) and
Rhodococcus sp. strain ECRD-1-desulfurized (B) cultures. The
spectra of the individual model compounds contributing to the fit are
shown below the oil spectra and fits. The difference between the fit
and the experimental data is shown beneath each panel.
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|
| |
DISCUSSION |
The use of excess decane significantly reduced the extent of OB
oil hydrocarbon degradation by Rhodococcus sp. strain ECRD-1 and allowed for an accurate measure of sulfur removal. ECRD-1 was able
to remove a maximum of 30% of the total sulfur present in the
middle-distillate OB oil. Reducing the amount of sulfur added for
biodesulfurization from 20 ppm (OB oil diluted 10-fold in decane) to 10 ppm (OB oil diluted 20-fold in decane) did not increase sulfur removal
but appeared to increase HBP-sultine formation. The fact that the final
sulfur content of the OB oil was the same in both cases suggests that
the substrate range of the desulfurization enzymes and/or
bioavailability, not the sulfur requirements of the bacteria, limited
further sulfur removal. Comparison of the GC-SCD chromatograms of the
oils treated in the presence and absence of decane indicates that the
pattern of sulfur compound removal is the same. This suggests that
sulfur-containing compounds are not substrates for hydrocarbon
degradation enzymes and that sulfur removal is strictly due to the
activity of the desulfurization pathway.
Sulfur K-edge X-ray absorption-edge analysis revealed that sulfidic and
thiophenic sulfur compounds were equivalently reduced, indicating that
this pathway is active against a wide variety of sulfur compounds
relevant to the fuel industry. Good fits to the biodesulfurized OB oil
sample spectra were obtained with DBT sulfone and HBP-sultine as model
oxidized sulfur compounds, consistent with the oxidative
desulfurization pathway of Rhodococcus sp. strain IGTS8
described by Gray et al. (6). Gallagher et al. (3) proposed two desulfurization pathways for IGTS8 that are dependent upon the physiological state of the organism. During growth,
the pathway followed the sequence DBT sulfoxide, DBT sulfone, HBP-sultone, and 2,2'-dihydroxybiphenyl. In contrast, when DBT was
metabolized during stationary phase, 2-hydroxybiphenyl was identified
as the end product, and HBP-sultine was observed as an intermediate,
which is analogous to the results of Gray et al. (6). In our
analysis, HBP-sultone could not be meaningfully fitted to the
experimental data, suggesting that it is not a pathway intermediate or
that under the conditions used here it is rapidly converted to other products.
In addition to removing 30% of the sulfur in the OB oil,
biodesulfurization by ECRD-1 resulted in the conversion of greater than
50% of the remaining sulfur into oxidized forms. This suggests that
the substrate range of the sulfur oxidation enzymes is broader than
that of the enzymes involved in carbon-sulfur bond cleavage. If true,
broadening the substrate range of the carbon-sulfur cleavage enzymes to
include those oxidized compounds present in the treated oils
should allow for considerably more desulfurization of
middle-distillate crude oil fractions. The genes required for DBT
desulfurization via the sulfur oxidative pathway have been
cloned from strain IGTS8 (2, 23), and the enzymes and
cofactors involved have recently been characterized (6). Not
all of the oxidized species formed in the treated oils are necessarily
due to the desulfurization activity. It is known that alkane
degradation enzymes will oxidize sulfur compounds to the sulfoxides,
although the production of sulfones and sultines has not been reported
(29).
To be commercially useful, microbial desulfurization must be able to
greatly reduce the sulfur content of fuel oils which contain a broad
range of organic sulfur compounds. The results shown here demonstrate
that significant biological sulfur removal can be achieved for a
middle-distillate fraction of a crude oil. Further, hydrocarbon
degradation of the desulfurized oil does not appear to be involved in
desulfurization, indicating that hydrocarbon degradation and
desulfurization are distinct and separable activities.
Biodesulfurization using a strain lacking the ability to degrade
hydrocarbons will clarify any ambiguities regarding the affect of
hydrocarbon degradation on desulfurization.
Although significant, the degree of desulfurization shown here is not
sufficient to meet the required sulfur levels for all fuels.
Nonetheless, the ability to remove sterically hindered compounds not
affected by HDS could prove valuable in and of itself.
| |
ACKNOWLEDGMENTS |
Stanford Synchrotron Radiation Laboratory is funded by the U.S.
Department of Energy, Office of Basic Energy Sciences. The Biotechnology Program is supported by the National Institutes of
Health, Biomedical Research Technology Program, Division of Research
Resources. Further support is provided by the Department of Energy,
Office of Health and Environmental Research.
| |
FOOTNOTES |
*
Corresponding author. Mailing address: Room LA270,
Exxon Research and Engineering Co., Route 22 East, Annandale, NJ 08801. Phone: (908) 730-2205. Fax: (908) 730-3301. E-mail:
mjgross{at}erenj.com.
| |
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Applied and Environmental Microbiology, January 1999, p. 181-188, Vol. 65, No. 1
0099-2240/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.
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Abstract
Introduction
