Potential Use of Nonionic Surfactants in the Biodesulfurization of

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Potential Use of Nonionic Surfactants in the Biodesulfurization of Bunker-C Oil Ji-Won Han,† Hyung-Soo Park,‡ Byung-Hong Kim,‡ Pyung-Gyun Shin,‡ Sang-Kwon Park,† and Jong-Choo Lim*,† Department of Chemical Engineering, Dongguk University, Seoul 100-715, Korea, and Korea Institute of Science and Technology, Seoul 136-791, Korea Received August 14, 2000. Revised Manuscript Received November 7, 2000

For an efficient operation in biodesulfurization of petroleum and related fuels, the aqueous solubility of insoluble or very slightly soluble sulfur compounds contained in the petroleum products has to be increased. In this study, polyoxyethylene nonionic surfactants were used in order to enhance the aqueous solubility of insoluble or very slightly soluble sulfur compounds contained in the bunker-C oil and the solubilized sulfur contents in the aqueous surfactant solutions were measured by X-ray sulfur spectrophotometer. The most hydrophobic surfactant used during this study showed the maximum solubilization capacity for sulfur compounds contained in the bunker-C oil and the solubilization of sulfur compounds was found to increase with temperature and to be abruptly increased at above 5 wt % surfactant concentrations. It was found that Tergitol series surfactants showed higher solubilizing capacity than Neodol series surfactants presumably due to the disruption of the regular packing in the hydrocarbon region of the surfactant micellar aggregates and that the addition of a cosurfactant and/or an electrolyte increased the solubilization of sulfur compounds in the bunker-C oil. It was also shown that partitioning phenomena were shown to be significant with a hydrophobic surfactant especially at high temperature and pH of the Tergitol surfactant solution did not affect the solubilization of sulfur compounds. The growth of M6 sulfur-reducing bacteria was not greatly affected by the addition of both nonionic surfactant and cosurfactant. Desulfurization experiments with M6 sulfurreducing bacteria showed that the biodesulfurization rate of bunker-C oil was enhanced with addition of nonionic surfactant and these data suggested the potential applicability of surfactant to the actual biodesulfurization system.

Introduction Sulfur oxides (SOx) emission through fossil fuel combustion possesses a major environmental problem because these oxides are a major cause of acid rain.1 As environmental regulations become even more stringent and availability of low-sulfur fuels decreases, efficient techniques for desulfurization of the petroleum products are required. To reduce the sulfur content in the petroleum, the hydrodesulfurization (HDS) process has been routinely used. However, this process is carried out under severe conditions such as extremely high temperature and pressure and with catalysts which are vulnerable to catalyst poisoning. Also the HDS process is found to have some limitations in complete treatment of many types of organosulfur compounds and to require relatively high cost.2-5 Recently, biodesulfurization (BDS) process using microorganisms is known to be promising in terms of * Author to whom correspondence should be addressed. † Dongguk University. ‡ Korea Institute of Science and Technology. (1) National Air Pollution Control Administration. Air Quality Criteria for Sulfur Dioxide; Public AP-50, 1969. (2) Grange, P. Catal. Rev. Sci. Eng. 1980, 21, 135. (3) Angelici, R. J. Acc. Chem. Res. 1988, 21, 387. (4) Massoth, F. E. Adv. Cat. 1978, 27, 265. (5) Gates, B. C.; Katzer, J. R.; Schuit, G. C. A. Chemistry of Catalytic Processes; McGraw-Hill: New York, 1979.

safety since being carried out under mild conditions and reasonably low treatment cost.6-8 The BDS has been reported since 1935 and much work has been done in order to understand the biochemistry of microbial attack on organic sulfur compounds and thus the mechanisms of BDS process.9 For the BDS process to be effective, the solubilization of insoluble or slightly soluble organosulfur compounds into aqueous solution is a prerequisite. Solvents, frequently used for this purpose, have been known to rupture the microbial cell during the BDS process and to cause secondary pollution problems after treatment. Also the separation of solvents from aqueous solution requires conventional energy-intensive processes such as distillation or extraction. Surface active agents or surfactants have a characteristic molecular structure consisting of a structural group that has very little attraction for the solvent, known as lyophobic group, and a group that has strong attraction with solvent, known as lyophilic group.10 (6) Sublette, K. L.; Gwozdz, K. J. Biochemistry Biotechnology 1991, 28, 635. (7) Yamada, K.; Minoda, Y.; Kodama, K.; Nakatani, S.; Akasaki, T. Agr. Biol. Chem. 1968, 23, 7, 840. (8) Shennan J. L. J. Chem. Technol. Biotechnol. 1996, 67, 109. (9) Malik, K. A. Process Biochemistry 1978, 13, 10. (10) Rosen, M. J. Surfactants and Interfacial Phenomena; Wiley: New York, 1989.

10.1021/ef000181q CCC: $20.00 © 2001 American Chemical Society Published on Web 12/29/2000

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Table 1. Physical Properties of Neodol Series Nonionic Surfactants physical property

Neodol 91a-6b

Neodol 91-8

Neodol 23-6.5

Neodol 25-7

Neodol 25-9

Neodol 25-12

Neodol 45-13

HLB number CMC (wt %) cloud pointc (°C) molecular weight densityd (g/cm3)

12.5 0.005 52 425 0.991

14.0 0.027 80 529 1.002

12.0 0.0017 45 484 0.981

12.2 0.0009 50 519 0.967

13.3 0.0018 74 610 0.982

14.4 0.0018 97 729 0.998

14.4 0.006 78 790 1.008

a The hydrocarbon chains contain carbon atoms from 9 to 11. b The hydrophilic ethylene oxide chains contain, on the average, 6 ethoxy groups. c Measured with 1 wt % surfactant solution. d At 25 °C.

Surfactant molecules can accumulate along the airliquid and liquid-liquid interfaces and thus reduce both surface tensions and interfacial tensions at the same time. In addition, surfactant molecules can form aggregates, called micelles, in aqueous solution and the surfactant concentration at which monomers begin to assemble in ordered, colloidal aggregates is termed the critical micelle concentration (CMC). Substantial amounts of insoluble or very slightly soluble compounds or components can be solubilized into surfactant micelles directly if surfactant concentrations are high enoughs a few times larger than the CMC.10 When micelles solubilize very large amounts of organic, the micelles are often said to be swollen and are referred to as microemulsions. This phenomenon is known as solubilization. Solubilization, first proposed by McBain11 in 1955, is defined as the spontaneous formation of a thermodynamically stable solution of a substance (the solubilizate) normally insoluble or very slightly soluble in a given solvent, by addition of a surfactant. Solubilization is believed to occur at a number of different sites in the micelles depending on the compound solubilized: (1) on the surface of the micelle (2) between the hydrophilic headgroups (3) in the so-called palisade layer of the micelle between the hydrophilic headgroups and the hydrophobic groups and (4) in the inner core of the micelle interior. It has been observed that nonpolar compounds tend to solubilize in the hydrophobic micellar core and polar and amphiphilic compounds are found in the more hydrophilic palisade layer of the micelles.11 The amount solubilized in the surfactant micellar solution depends on structure of surfactant, structure of solubilizate, concentration of surfactant, temperature, and additives. The effective solubility of the solute in the surfactant solution can be increased by orders of magnitude in comparison to its aqueous solubility. Solubilization is an interfacial mass transfer process which takes place relatively slowly and at a rate proportional to the oil/water interfacial area and thus it is necessary to make the latter as large as possible to achieve high rates of solubilization.12,13 Solubilization phenomenon turns toward a variety of industrial applications in addition to enhanced oil recovery,14 detergency,15 cosmetic,16 cleanup of contaminated soils and aquifer,17,18 and drug delivery systems (11) McBain, M. E.; Hutchison, E. Solubilization and Related Phenomena; Academic Press: New York, 1955. (12) Carroll, B. J. J. Colloid Interface Sci. 1976, 57, 488. (13) Carroll, B. J.; O’Rourke, B. G. C.; Ward, A. J. I. J. Pharm. Pharmacol. 1982, 34, 287. (14) Miller, C. A.; Qutubuddin, S. Interfacial Phenomena in NonAqueous Media; Marcel Dekker: New York, 1986. (15) Shaeiwitz, J. A.; Chan, A. F. C.; Cussler, E. L.; Evans, D. F. J. Colloid Interface Sci. 1981, 84, 47. (16) Tokiwa, F. J. Phys. Chem. 1968, 72, 4331. (17) Bury, S. J.; Miller, C. A. Environ. Sci. Technol. 1993, 27, 104.

for pharmaceutical actives.19,20 Pertaining to systems of biological reactions, Rouse et al.21 point out that the efforts to enhance bioremediation have renewed interest in the microbial transport and oxidation of organic compounds especially in the presence of surfactants. The unique ability of surfactants to act as emulsifiers or solubilizers has appeared to enhance the availability of insoluble materials to some microbes in some cases, but to other microbes and in other cases it has appeared to be inhibitory. It is clear that surfactants can affect the biological reaction process in different ways that are dependent on the solubilizate, the microorganisms, and the surfactant. Even though much work has been done on the BDS process in order to understand the biochemistry of microbial attack on organic sulfur compounds and thus the mechanisms of BDS process, there has been no systematic study on the potential use of surfactant during BDS process. In this study, two different types of commercial polyoxyethylene nonionic surfactants were used in order to enhance the aqueous solubility of sulfur compounds contained in the bunker-C oil and the solubilized sulfur contents in the aqueous surfactant solutions were measured by X-ray sulfur spectrophotometer. Effects of surfactant concentration, temperature, cosurfactant, electrolyte, and pH on solubilization of insoluble or very little soluble sulfur compounds in the bunker-C oil were studied. Also both the growth of M6 sulfur-reducing bacteria and biodesulfurization rate in the presence of nonionic surfactant were investigated in order to test the potential applicability of surfactant to the actual BDS process. Experimental Section Two different types of commercial polyoxyethylene nonionic surfactants were used during this study. Neodol series surfactant, supplied by Shell Development Co. (Houston, TX), is a primary alcohol ethoxylate where Neodol 23-6.5 is made up of various individual compounds whose hydrocarbon chains contain 12 or 13 carbon atoms and whose ethylene oxide chains contain, on the average, 6.5 ethoxy groups. Tergitol 15-S is a secondary alcohol ethoxylate which is a mixture of species with the alcohol group located at various positions along a chain of 11 to 15 carbon atoms and was obtained from Union Carbide Co. (Charleston, WV) and used as received. The products having average ethylene oxide (EO) numbers of 7, 9, 12, and 15 were used in the present work. The important physical properties of the two commercial surfactants used are shown (18) Rosenberg, E.; Legmann, R.; Kushmaro, A.; Taube, R.; Alder, E.; Ron, E. Z. Biodegradation 1992, 3, 337. (19) Skodvin, T.; Sjoblom, J.; Saeten, J. O. J. Colloid Interface Sci. 1993, 155, 392. (20) Shah, D. O.; Jhonson, K. A. J. Colloid Interface Sci. 1985, 107, 269. (21) Rouse, J. D.; Sabatini, D. A.; Suflita, J. M.; Harwell, J. H. Environ. Sci. and Eng. 1994, 21, 325.

Nonionic Surfactants in the Biodesulfurization of Oil

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Table 2. Physical Properties of Tergitol Series Nonionic Surfactants physical property

Tergitol 15a-S-7b

Tergitol 15-S-9

Tergitol 15-S-12

Tergitol 15-S-15

HLB number CMC (wt %) cloud pointc (°C) molecular weight densityd (g/cm3)

12.4 0.0039 37 515 0.992

13.3 0.0056 60 584 1.006

14.7 0.011 88 738 1.020

15.6 0.018 >100 877 1.009

a The hydrocarbon chains contain carbon atoms from 11 to 15. b The hydrophilic ethylene oxide chains contain, on the average, 7 ethoxy groups. c Measured with 1 wt % surfactant solution. d At 25 °C.

in Tables1 and 2, respectively. Hydrophilic-lipophilic balance (HLB) number as shown in Tables 1 and 2 characterizes the potential relative solubility of surfactant in water and in oil. The smaller the HLB value, the more oil soluble (hydrophobic) the surfactant is. The value of 7 corresponds to approximately equal solubility of a nonionic surfactant in oil and in water. Cloud point, unique characteristic of a nonionic surfactant, is the temperature where a micellar solution of a given concentration turns turbid and separates into two phases of a liquid surfactant-rich phase in equilibrium with a dilute surfactant solution. The cloud point is strongly influenced by the length of the ethylene oxide chain and higher cloud point values are observed for those with longer ethylene oxide chains. Aside from effectiveness in solubilizing sulfur compounds, three criteria have been considered for screening surfactants to be used: (1) their melting points should be below 30 °C, (2) their cloud points should be higher than 35 °C, and (3) they must not be toxic to sulfur-reducing microorganisms. The first two criteria are based on the fact that biodesulfurization reaction with sulfur-reducing bacteria proceeds in the temperature range of 30 to 40 °C, and the last criterion will be discussed later. Bunker-C oil, supplied by Hyundai Oil Co. (DaeSan, Korea), contains sulfur content of 8.08 wt % and was used without further purification. Cosurfactants such as n-butanol, n-pentanol, n-hexanol, and n-octanol were purchased from Aldrich Inc. and had reported purities of 99%. The inorganic salts of reagent grade such as CaCl2‚2H2O (99.0% purity), MgCl2(98% purity), and NaCl(99.5% purity) were obtained from Sigma Inc. and used as received. Water used for solution preparation was ultrapure having been double distilled and deionized. The pH of the solution was adjusted by using 0.01 N HCl and 0.01 N NaOH. Sample solutions were prepared in Teflon-capped, 13 mm i.d., flat-bottomed test tubes and mixed for 20 s by vortex mixing prior to being put into the constant-temperature bath. Equilibrium was considered to be reached when there was no further change in phase appearances and volumes. Typically, 0.5 g of bunker-C oil was added to the 10 g of surfactant solution, unless otherwise specified and equilibration time was about 6 h. After reaching equilibrium, the sample was passed through a filter paper in order to remove undissolved solid particles and the sulfur content solubilized in the aqueous surfactant solution was measured using X-ray spectrometer (Model 2000T(SS), Asoma Instruments Inc.). The solubilization capacity of each surfactant was characterized by using two parameters So and Sw where the former was defined as the weight percent of the sulfur solubilized relative to the initial sulfur content in the bunker-C oil and the latter as the weight percent of that relative to the initial surfactant content. Each experiment was conducted with three samples made under the same conditions and the averaged value was presented. The error percent calculated based on the average value of three measurements was found to be less than 5% in most cases. Direct chemical analysis of the solubilized sulfur compounds in the aqueous surfactant solution was performed by gas chromatography (GC) with a packed column (OV-101) using a flame photometric detector (FPD) (HP 5890II). After being equilibrated at the desired temperature, the samples were rotated under centrifugation of 14000 rpm for 15 min in order to remove the remaining solid particles from the solution.

Table 3. Composition of Postgate Medium C in 1 L Aqueous Solutiona

a

component

amount (g)

KH2PO4 NH4Cl Na2SO4 CaCl2 ‚ 6H2O MgSO4 ‚ 7H2O sodium lactate yeast extract sodium citrate ‚ 2H2O

0.5 1 4.5 0.06 0.06 6 1 0.3

pH at 7.

Typically, the sample volume of 1 × 10-9 m3 was injected and the N2 carrier gas flow rate was maintained at 5 × 10-6 m3/ min. The injection and detector temperatures were maintained at 200 °C and 300 °C, respectively, and the oven temperature was set at from 200 to 300 °C. The growth of M6 sulfur-reducing bacteria (SRB) and biodesulfurization rate of bunker-C oil in the presence of surfactant were investigated where the isolate M6 SRB, characterized as a mesophilic obligatory anaerobe, was identified as Desulfovibrio desulfuricans.22 Samples were anaerobically inoculated into anaerobic tubes containing Postgate medium C under a stream of oxygen-free nitrogen gas and the tube was incubated for 5 days at 30 °C. Postgate medium C, whose composition is shown in Table 3, was made anaerobically in anaerobic pressure tubes (Bellco Glass Inc., Millville, NJ), and the pH was adjusted to 7. The growth was initiated by 5% inoculation into 10 g of Postgate medium C, and the culture turbidity was monitored by measuring optical density (OD) by UV/VIS Spectrophotometer (Model Optizen, Hanson Technology, Seoul, Korea). Biodesulfurization experiments of bunker-C oil with M6 SRB were performed and the decrease of sulfur concentration was monitored by the gas chromatographic method after extraction with n-butanol. The details of experimental procedures employed are described elsewhere.22

Results and Discussion Two different types of commercial polyoxyethylene nonionic surfactants were used in order to enhance the aqueous solubility of insoluble or very slightly soluble sulfur compounds contained in the bunker-C oil and the solubilized sulfur contents in the aqueous surfactant solutions were measured by X-ray sulfur spectrophotometer. Experimental results for the solubilizing capacity of 5 wt % Neodol series surfactant at three different temperatures of 30, 35, and 40 °C are shown in Figure 1. As shown in Figure 1, the most hydrophobic surfactant Neodol 23-6.5 among Neodol series surfactants used showed the maximum solubilizing capacity for the sulfur compounds and the solubilizing power is found to increase with temperature. In general, the extent of solubilization can be expected to increase at a (22) Kim, H. Y.; Kim, T. S.; Kim, B. H. J. Microbiology Bioeng. 1991, 1, 1.

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Figure 1. Solubilization of sulfur compounds in the bunker-C oil by 10 g of 5 wt % Neodol series nonionic surfactant solution at three different temperatures of 30, 35, and 40 °C: A ) water; B ) 25-7; C ) 25-9; D ) 25-12; E ) 91-6; F ) 91-8; G ) 45-13; H ) 23-6.5.

given temperature with increasing length of the hydrophobic hydrocarbon chain and decreasing length of the polyoxyethylene chain due to a decrease in CMC. Both changes also increase an aggregation number, which is defined by the number of surfactant molecules contained in a micelle, and thus increase a solubilization power of a given surfactant. It is known that the length of hydrophilic polyoxyethylene chain is more important than the length of hydrocarbon chain.23 However, a minimum hydrophilic group size is required to achieve surfactant solubility and formation of a micellar solution. Within this limit, a surfactant whose hydrophilic portion is smaller seems relatively more efficient for solubilizing sulfur compounds. The dependence of micellar solubilization on temperature depends on the structure of the solubilizate and surfactant, and in most cases increases with temperature. The effects can generally be ascribed to either (1) an increase in the aqueous solubility of the solubilizate or (2) a change in the properties of the micelles such as increases in micelle size and aggregation number. The solubilization capacity of the Tergitol series surfactant, as shown in Figure 2, exhibits trends similar to those found with the Neodol series surfactant. That is, the most hydrophobic surfactant Tergitol 15-S-7 showed the maximum solubilizing power for the sulfur compounds in bunker-C oil. One feature of interest is that Tergitol 15-S-7 showed higher solubilizing capacity than Neodol 23-6.5 of almost the same HLB number. Neodol series surfactants are primary alcohol ethoxylates where the hydrocarbon chains are long, straight, and of uniform length. On the other hand, Tergitol series surfactants are secondary alcohol ethoxylates which are double-chain surfactants with chains of various lengths. The total number of carbon atoms in the two chains is nearly the same for all the molecules present, but the position of the ethoxylated alcohol group varies, yielding different species with chains of different lengths. Therefore, Tergitol series surfactant forms less ordered surfactant films due to the disruption of the regular packing in the hydrocarbon region of the surfactant micellar aggregates and thus provides better (23) Salib, N. N.; Isamil, A. A.; Geneidi, A. S. Pharm. Ind. 1974, 36, 108.

Han et al.

Figure 2. Solubilization of sulfur compounds in the bunker-C oil by 10 g of 5 wt % Tergitol series nonionic surfactant solution at three different temperatures of 30, 35, and 40 °C.

Figure 3. Effect of surfactant concentration on solubilization of sulfur compounds by 10 g of Tergitol series surfactant solution at 35 °C.

solubilization capacity than Neodol series surfactant of the same HLB number. This result is consistent with detergency experiments performed with the same surfactant systems for the solubilization of triglycerides.24 Another approach to forming less ordered surfactant films is to add a cosurfactant and that effect will be discussed later. Effect of surfactant concentration on solubilization of sulfur compounds was investigated with Tergitol series surfactants at 35 and 40 °C. In both systems, effective solubilization was shown to occur at concentrations considerably in excess of CMC, typically at higher than 5 wt % surfactant concentrations. Therefore the solubilization experimental results with higher than 5 wt % surfactant concentrations were presented in Figures 3 and 4, respectively. For all of the Tergitol series surfactants examined, the solubilization capacity for sulfur compounds was found to be linearly proportional to surfactant concentration. The results in the present system reveal that selective solubilization of sulfur compounds of bunker-C oil does not occur. That is, the preferential solubilization of short-chain hydrocarbons, which are relatively easy to be incorporated into micellar aggregates, occurred first. And the preferential solubilization of hydrocarbons permits easy incorpora(24) Tungsubutra, T.; Miller, C. A. Organized Solutions; Marcel Dekker: New York, 1992.


Figure 4. Effect of surfactant concentration on solubilization of sulfur compounds by 10 g of Tergitol series surfactant solution at 40 °C.

Figure 5. Effect of amount of bunker-C oil on solubilization of sulfur compounds by 10 g of 5 wt % Tergitol series surfactant solution at 40 °C.

Figure 6. Effect of cosurfactant on solubilization of sulfur compounds in 10 g of 5 wt % Tergitol series surfactant system at 40 °C where bunker-C oil content was fixed at 0.5 g.

tion of relatively large molecules, such as insoluble or very little soluble sulfur compounds in our case, into surfactant micellar aggregates by making the hydrocarbon chain region of the surfactant films sufficiently disordered. It is well-known that in mixed solubilizate systems, the presence of one solubilized component in the micellar aggregate affects the uptake of other (25) Mori, F.; Lim, J. C.; Raney, O. G.; Elsik, C. M.; Miller, C. A. Colloids Surf. 1989, 40, 323.


Figure 7. Effect of amount of bunker-C oil on solubilization of sulfur compounds in systems containing 10 g of 5 wt % Tergitol 15-S-7 surfactant solution where the ratio of n-hexanol to Tergitol 15-S-7 is 0.04 by weight.

Figure 8. Effect of electrolyte concentration on solubilization of sulfur compounds in the bunker-C oil by 10 g of 5 wt % Tergitol 15-S-7 at 40 °C.

components from the bulk oil phase and similar results have been reported for other surfactant systems.25 Solubilization of sulfur compounds by 5 wt % Tergitol series surfactant solution was measured as a function of the amount of bunker-C oil added at 40 °C. As indicated in Figure 5, the three other surfactant systems except Tergitol 15-S-7 showed constant Sw values, where Sw was defined as the weight percent of the sulfur solubilized relative to the initial surfactant content, with an increase in oil contents. That is, the amount of sulfur compounds solubilized was not greatly affected by the amount of bunker-C oil added. On the other hand, Tergitol 15-S-7, which showed the best solubilizing capacity among surfactants used during this study, showed a different behavior. Solubilization of sulfur compounds was found to increase until the addition of 0.5 g of bunker-C oil and to decrease with a further increase in oil contents. This behavior confirms that some surfactant is incorporated into the oil phase with an increase in the amount of bunker-C oil. This partitioning phenomenon is important with a hydrophobic surfactant, such as Tergitol 15-S-7 in our case, and also with increasing temperature since both effects substantially increase surfactant solubility into oil phase. From the result of Figure 5, it may be concluded that the maximum amount of bunker-C oil that can be used with 10 g of 5 wt % Tergitol surfactant solution is 0.5 g.

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Figure 9. GC-FPD analysis for the solubilization of sulfur compounds in the bunker-C oil by 10 g of Tergitol 15-S-7 surfactant solution at 40 °C: (a) ) 1 wt % surfactant solution; (b) ) 5 wt % surfactant solution; (c) ) 10 wt % surfactant solution.

If the surfactant films of micellar aggregates were less ordered, e.g., if the hydrocarbon chains were nonuniform length and/or branched as found from the results in Figures 1 and 2, incorporation of the large and bulky sulfur-containing molecules into the surfactant aggregates would be facilitated. One possible way is to add a cosurfactant which would introduce greater disorder into the surfactant films and thereby facilitate the solubilization of the large and bulky sulfur-containing molecules into the surfactant aggregates.26 Effect of addition of various kinds of cosurfactants to 5 wt % solutions of Tergitol 15-S-7 was investigated and the

results are presented in Figure 6. Addition of shortchain alcohols such as n-butanol and n-pentanol was found to have no effect on the solubilization of sulfur compounds since most of the water-soluble alcohol remained in the water and so was not present in the surfactant films to facilitate solubilization. On the other hand, addition of longer chain alcohols such as nhexanol and n-octanol to aqueous surfactant solutions increases the solubilization of sulfur compounds by (26) Lim, J. C.; Miller, C. A.; Yang, J. H. Colloids Surf. 1992, 66, 45.


increasing disorder in the hydrocarbon chain region of the surfactant aggregates. For example, the addition of 0.03 g of n-octanol to 10 g of surfactant solution increases the solubilization of sulfur compounds by almost 61%. However, excess addition of a long-chain alcohol to aqueous surfactant solution depresses the cloud point of a surfactant system and thus induces phase separation of the micellar solution into two phases of a liquid surfactant-rich phase in equilibrium with a dilute surfactant solution. As shown in Figure 6, 0.06 g of n-octanol represents maximum amount that can be added to 10 g of 5 wt % Tergitol 15-S-7 surfactant solution in order to promote solubilization of sulfur compounds without undergoing phase separation. On the other hand, other cosurfactant systems did not exhibit phase separation until 0.08 g of alcohol addition to 10 g of surfactant solution but the effect of cosurfactant addition became less pronounced with a short-chain alcohol mainly due to an increase in water solubility. Effect of amount of bunker-C oil on solubilization of sulfur compounds in systems containing 5 wt % Tergitol 15-S-7 was investigated both at 35 and 40 °C. In these experiments, the amount of surfactant solution and the ratio of n-hexanol to Tergitol 15-S-7 were kept constant at 10 g and 0.04 by weight, respectively, while the amount of bunker-C oil was varied. The results presented in Figure 7 indicated that an increase in oil contents decreased the amount of sulfur solubilized in the aqueous surfactant solution mainly due to significant partitioning of surfactant into oil phase. This partitioning phenomenon was observed with an increase in oil contents at both temperatures and was more pronounced at 40 °C. The former behavior is expected since both Tergitol 15-S-7 and n-hexanol are hydrophobic in nature at these temperatures as seen from Figure 6 and the latter can be explained as a substantial increase in surfactant solubility into oil phase with increasing temperature. Addition of electrolytes generally depresses the cloud point of a nonionic surfactant.27,28 Thus it is expected that this phenomenon will be reflected in the solubilization and that a solubilization at a definite temperature will increase. Also the location of the solubilizate molecule within the micelle of a nonionic surfactant may be an important factor in determining the effect of electrolyte addition on solubilization.29 In cases where the solubilizate is located within the core or deep within the palisade layer, such as the case of insoluble or very slightly soluble sulfur compounds in the bunker-C oil, it is readily appreciated that solubilization may be increased due to the increase in micellar volume following electrolyte addition. The effect of added electrolytes on the solubilization of sulfur compounds by 5 wt % Tergitol 15-S-7 at 40 °C is shown in Figure 8. Evidently the addition of electrolytes enhanced the solubilization capacity for sulfur compounds and the effect was found to be more pronounced with CaCl2 and MgCl2 than with NaCl. These results agree with the findings that the micellar weight and the solubilizing power increases with the amount of added salt and the (27) Schick, M. J. J. Colloid Sci. 1962, 17, 801. (28) Maclay, W. N. J. Colloid Sci. 1956, 11, 272. (29) Saito, H.; Shinoda, K. J. Colloid Interface Sci. 1967, 24, 10.


Figure 10. Solubilization of sulfur compounds in the bunker-C oil by 10 g of 5 wt % Tergitol surfactant in the Postgate medium C at three different temperatures of 30, 35, and 40 °C where the content of Postgate medium was fixed at 10 g.

order in which electrolytes increase the solubilization is the same as that in which they depress the cloud point.30 Effect of pH on the solubilization of sulfur compounds in the bunker-C oil by 5 wt % Tergitol 15-S-7 was studied at 40 °C where the pH of the surfactant solution was adjusted by using 0.01 N HCl and 0.01 N NaOH. Effect of amount of aqueous surfactant solution on solubilization of sulfur compounds was also investigated at 40 °C. In these experiments, the amount of bunker-C oil and the concentration of Tergitol 15-S-7 were kept constant at 0.5 g and 5 wt %, respectively, while the amount of 5 wt % Tergitol 15-S-7 surfactant solution was varied. Both the amount of aqueous surfactant solution and pH did not affect the solubilization of sulfur compounds in Tergitol 15-S-7 surfactant systems. In order to confirm the solubilization of insoluble or very little sulfur compounds by surfactant solution, gas chromatography (GC) system equipped with an flame photometric detector (FPD) was employed. As shown in Figure 9, it is evident that surfactant is effective in increasing solubility of insoluble or very little soluble sulfur compounds contained in the bunker-C oil. Especially it is noteworthy that the increase in surfactant concentration enhances the solubilization of large and bulky sulfur-containing molecules into the surfactant micellar aggregates present. Figure 10 shows the solubilization capacity of 5 wt % Tergitol surfactant for sulfur compounds into the Postgate medium C at there different temperatures of 30, 35, and 40 °C. It is evident from Figure 10 that the addition of surfactant into the Postgate medium C greatly increases the solubility of sulfur compounds. As in the case of aqueous surfactant solution, the most hydrophobic surfactant Tergitol 15-S-7 shows the maximum solubilization capacity, which also slightly increases with temperature. The growth of M6 sulfur-reducing bacteria in the presence of both Tergitol series surfactant and cosurfactant was investigated at 30 °C in order to test surfactant toxicity or inhibition of microbial processes. Each of three different cosurfactants such as n-butanol, n-hexanol and n-octanol was added into 5 wt % Tergitol (30) Mankowich, A. M. Ind. Eng. Chem. 1953, 44, 1151.

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Conclusion

Figure 11. Optical density measurement of culture turbidity as a function of time where 0.01 g of each cosurfactant was added to 10 g of 5 wt % Tergitol 15-S-7 solution at 30 °C.

Figure 12. Results of biodesulfurization of 2 g of bunker-C oil with M6 sulfur-reducing bacteria at 40 °C for 5 days where total volume of 20 mL and 10 g of 5 wt % of Tergitol surfactant solution were used in each experiment.

15-S-7 surfactant solution and the culture turbidity was monitored by measuring optical density. As shown in Figure 11, the addition of both surfactant and cosurfactant was found to have little effect on the growth of M6 sulfur-reducing bacteria. Desulfurization experiments with M6 sulfur-reducing bacteria were carried out at 40 °C. As shown in Figure 12, the biodesulfurization of bunker-C oil in the absence of surfactant showed about 13% sulfur removal. On the other hand, the addition of nonionic surfactant enhanced the biodesulfurization of bunker-C oil. For example, the addition of 5 wt % Tergitol 15-S-7 in the same system increased the biodesulfurization rate of bunker-C oil by 10%.

In this study, polyoxyethylene nonionic surfactants were used in order to enhance the aqueous solubility of insoluble or very slightly soluble sulfur compounds contained in the bunker-C oil and the solubilized sulfur contents in the aqueous surfactant solutions were measured by X-ray sulfur spectrophotometer. Two commercial nonionic surfactants used during this study basically showed the same trends. That is, the most hydrophobic surfactant of each series showed the maximum solubilization capacity for sulfur compounds contained in the bunker-C oil and the solubilization of sulfur compounds was found to increase with temperature and to be abruptly increased at above 5 wt % surfactant concentrations. One feature of interest is that Tergitol 15-S-7 showed higher solubilizing capacity than Neodol 23-6.5 of almost the same HLB number, presumably due to the disruption of the regular packing in the hydrocarbon region of the surfactant micellar aggregates. Another approach to forming less ordered surfactant films is to add a cosurfactant and that effect was observed with addition of longer chain alcohols such as n-hexanol and n-octanol to aqueous surfactant solutions. The partitioning phenomenon was found to be important with a hydrophobic surfactant, such as Tergitol 15S-7, and also with increasing temperature since both effects substantially increase surfactant solubility into oil phase. Addition of electrolytes enhanced the solubilization capacity for sulfur compounds and the effect was found to be more pronounced with CaCl2 and MgCl2 than with NaCl. These results agree with the findings that the micellar weight and the solubilizing power increases with the amount of added salt and the order in which electrolytes increase the solubilization is the same as that in which they depress the cloud point. On the other hand, both the amount of aqueous surfactant solution and pH did not affect the solubilization of sulfur compounds. The growth of M6 sulfur-reducing bacteria was not greatly affected by the addition of both nonionic surfactant and cosurfactant and desulfurization experiments with M6 sulfur-reducing bacteria showed that the addition of nonionic surfactant enhanced the biodesulfurization rate of bunker-C oil by about 10%. These data suggested the potential applicability of surfactant to the actual biodesulfurization system. EF000181Q

Potential Use of Nonionic Surfactants in the Biodesulfurization of

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