Removal of Sulfur Compounds from Coal by the Thermophilic Organism Sulfolobus acidocaldarius

Transcription

1 APPLIED AND ENVIRONMENTAL MICROBIOLOGY, OCt. 1982, p /82/ $02.OO/O Copyright 1982, American Society for Microbiology Vol. 44, No. 4 Removal of Sulfur Compounds from Coal by the Thermophilic Organism Sulfolobus acidocaldarius FIKRET KARGI* AND JAMES M. ROBINSON Department of Chemical Engineering, Biotechnology Research Center, Whitaker Laboratory 5, Lehigh University, Bethlehem, Pennsylvania Received 1 June 1982/Accepted 25 June 1982 The thermophilic, reduced-sulfur, iron-oxidizing bacterium Sulfolobus acidocaldarius was used for the removal of sulfur compounds from coal. The inclusion of complex nutrients such as yeast extract and peptone, and chemical oxidizing agents, 0.01 M FeCl3 into leaching medium, reduced the rate and the extent of sulfur removal from coal. The rate of sulfur removal by S. acidocaldarius was strongly dependent on the sulfur content of the coal and on the total external surface area of coal particles. Approximately 96% of inorganic sulfur was removed from a 5% slurry of coal which had an initial total sulfur content of 4% and an inorganic (pyritic S and sulfate) sulfur content of 2.1%. This resulted in removal of 50% of initial total sulfur present in coal. One of the problems associated with the direct utilization of coal is the emission of S02 into the atmosphere. Sulfur-containing gases (mainly SO2) emitted into the atmosphere upon direct combustion of coal have deleterious effects upon animal and plant life (14). Among various methods of sulfur removal from coal, chemical desulfurization processes operate at high temperatures (100 to 400 C) and pressures (100 to 800 lb/ in2) and, therefore, are energy intensive (22, 27). Physical methods (such as flotation) are more cost effective than chemical methods but have the disadvantage of resulting in energy loss by removing coal particles containing finely disseminated pyrite (27). Microbial coal desulfurization before combustion has low capital and operating costs and is more energy efficient compared with high-temperature chemical processes (13). Finely distributed sulfur compounds can be removed from coal by microbial catalysis without any loss of coal (i.e., no energy loss). The sulfur in coal exists as inorganic and organic sulfur compounds (20, 22, 27, 28). The major inorganic sulfur compound in coal is pyrite; organic sulfur compounds are diverse and contain mainly thiol, sulfide, disulfide, and thiophene groups (14, 27). The range of total sulfur in U.S. coals is from less than 0.5% to greater than 6%. The chemoautotrophic and acidophilic microorganism Thiobacillus ferrooxidans has been the most widely used organism for the removal of pyritic sulfur from coal (11, 13, 21, 25). Silverman et al. (25) have reported that a pure culture of F. ferrooxidans (now T. ferrooxidans) accelerated the oxidation of pyritic sulfur present in coal. The rate of pyrite oxidation was improved by reducing the coal particle size and by removing the alkaline gangue material from coal with acid treatment (26). The microbial removal of inorganic sulfur from a pulverized coal blend has been studied by Dugan and Apel (12). By using a coal enrichment culture (a mixed culture from acid mine waters adapted to sulfur-rich coal), higher sulfur removal rates were obtained compared with the study of Silverman et al. (26). The use of a mixed culture of T. ferrooxidans and T. thiooxidans was also reported to yield sulfur removal rates higher than that of the pure culture of T. ferrooxidans (13). A recent study on pyritic sulfur removal from coal by T. ferrooxidans was reported by Hoffmann et al. (17). The rate of pyritic sulfur removal was reported to vary linearly with the total external surface area of coal particles. About 90% of pyritic sulfur was removed from coal within 8 to 12 days. However, the major drawback in all reported studies is the fact that the rates of pyritic sulfur removal were not high enough to reduce the reactor size to a reasonable level (24). Using a concentrated cell suspension of T. ferrooxidans and an external supply of CO2 and air in a well-agitated vessel, Kargi (19) obtained significantly higher rates of pyritic sulfur removal from coal compared with those in other reported studies. T. ferrooxidans has limited use in sulfur removal from coal since it removes only pyritic sulfur with a relatively low rate (9, 18, 23). In an 878

2 VOL. 44, 1982 attempt to develop a more effective microbial process for coal desulfurization, the thermophilic, reduced-sulfur, iron-oxidizing microorganism Sulfolobus acidocaldarius was used in this study. The facultative autotrophic microorganism S. acidocaldarius was originally isolated by Brierley and Brierley (4) from acidic hot springs of Yellowstone National Park. The maximum and minimum growth temperatures of the isolate were reported to be 70 and 45 C, respectively, and the optimal ph of elemental sulfur oxidation was 2. Several high-temperature strains of Sulfolobus were isolated and further characterized by Brock et al. (7, 8). It was proven by later studies that the organism is a facultative autotroph, i.e., capable of using both inorganic substrates (Fe2+ and reduced sulfur) and simple organic compounds as energy sources (7). The organism was used in microbial leaching of several mineral sulfides (e.g., molybdenite and chalcopyrite) and was found to be more effective than other mesophilic thiobacilli for the oxidation of certain mineral sulfides (2, 3, 5, 6). The organisms similar to Sulfolobus were also isolated from hot sulfatara areas in Italy and Japan (10, 16). Detz and Barvinchak (11) have briefly reported the use of S. acidocaldarius for the removal of pyritic sulfur from coal. However, no extensive study has been reported on the utilization of S. acidocaldarius for sulfur removal from coal. The purpose of this study was to determine the effectiveness of S. acidocaldarius (strain 98-3, isolated by Brock et al. [8]) for the removal of sulfur from coal and to elucidate the influence of certain process variables on the performance of the organisms. MATERIALS AND METHODS Coal samples. Coal samples were obtained from the Pennsylvania Power and Light Company and were ground to desired particle sizes. Various size fractions were separated by using U.S. standard sieve plates. Experiments were conducted with 100 to 150 mesh size (104,um < Dp < 147 SLm) coal particles. Two different coal samples with different total sulfur contents were used throughout the study: (i) coal refuse with approximately 11% total sulfur (about 10.5% inorganic sulfur and 0.5% organic sulfur), and (ii) untreated plant feed coal with -4% total sulfur (-2.1% inorganic sulfur and 1.9%o organic sulfur). Microbiological methods. A pure culture of S. acidocaldarius originally isolated by Brock et al. (8) (strain 98-3) was used. The experiments were performed by using the 2 x concentrated mineral salts medium developed by Brock et al. (8). The composition of the mineral salts medium used was as follows (g/liter): (NH4)2SO4, 2.6; KH2PO4, 0.56; MgSO4 * 7H20, 0.5; CaCl2 * 2H20, 0.14; FeCl3 * 6H20, The mineral salts were dissolved in deionized tap water, and the initial ph was adjusted to 2.5 with 0.1 N HCI. The growth medium consisted of slurried coal samples in mineral salts medium. Other nutrients, such as yeast REMOVAL OF SULFUR COMPOUNDS FROM COAL 879 extract and peptone, were added to the medium in desired concentrations. The organisms were adapted to pyrite before inoculation of the reaction medium. The stock culture medium contained 5 g of pyrite mineral particles (35 to 100 mesh size) and 100 ml of mineral salts medium in a 500-ml baffled shake flask. The initial ph of the medium was adjusted to 2.5 with 0.1 N HCI. After sterilization of pyrite-containing flasks, the medium was inoculated with 5 ml of active culture of S. acidocaldarius and incubated in a controlled-environment incubator shaker (New Brunswick model G26; New Brunswick Scientific Co.) at 75 C and 200 rpm. After 8 days of incubation, the pyrite particles and cells were separated from the liquid medium by aseptic centrifugation at 12,000 rpm. The solids (pyrite and cells) were suspended in mineral salts medium and incubated as previously described. The same procedure was repeated, and cells were concentrated two times. The 2x concentrated active culture grown on pyrite was used for the inoculation of coal slurries. The coal desulfurization experiments were performed in 500-ml baffled shake flasks. The flasks were charged with 100 ml of mineral salts medium and certain amounts of coal particles of known particle size. The initial ph of the medium was adjusted to 2.5 with 0.1 N HCI, and the flasks were inoculated with 5 ml of active and concentrated cell suspension of S. acidocaldarius and incubated in a controlled-environment shaker (New Brunswick model G26) at 75 C and 200 rpm. The samples were withdrawn from the flasks every 24 h for the analysis of sulfate ion and total iron (ferrous and ferric) in the liquid medium. A control flask was used to determine non-biological sulfur removal from coal. The uninoculated control flask included 10o coal particles of 100 to 150 mesh size in mineral salts medium. The initial ph of the control flask was adjusted to ph 2.5 with 0.1 N HCI, and it was incubated with the other flasks. Evaporation losses were determined by measuring the liquid volume in the control flask every 24 h. Three milliliters of presterilized tap water was added into the flasks every day to compensate for volume changes due to evaporation. Analytical methods. The samples were filtered through Whatman no. 2 filter paper to remove coal particles from the liquid medium. The filtrated solids were washed with 5 ml of 0.1 N HCI to extract adsorbed sulfate and iron into the filtrate (26). The filtrate was analyzed for sulfate and total iron. Sulfate concentration was measured turbidimetrically (20). Two milliliters of 10% BaCl2 solution was added to 2 ml of diluted liquid filtrate, and the turbidity of the final mixture was measured by a spectrophotometer (Bausch & Lomb, Inc., spec. 700) at 450 nm. Total iron (ferrous and ferric) was measured colorimetrically. One milliliter of 1% hydroquinone was added to 1 ml of diluted filtrate to reduce ferric iron into ferrous, and total ferrous iron concentration was determined by the o-phenanthroline method (15). Total sulfur content of coal samples was determined by the Eschka method (20). Sulfate sulfur content of coal samples was determined by extraction of 1-g samples of coal with dilute hydrochloric acid (15% HCI) followed by turbidimetric determination of sulfate (1).

3 880 KARGI AND ROBINSON Pyritic sulfur content was determined by extraction of the coal residues from sulfate analysis with 2 N nitric acid followed by colorimetric determination of iron by the o-phenanthroline method (1). Organic sulfur was determined indirectly by subtracting the sum of pyritic and sulfate sulfur from the total sulfur content of the coal as determined by the Eschka method. RESULTS AND DISCUSSION The experiments were designed to determine the effects of (i) yeast extract and peptone, (ii) inclusion of FeCl3 into the reaction medium as a chemical oxidizing agent, (iii) sulfur content of coal particles, and (iv) surface area of coal particles on the rate and the extent of sulfur removal from coal by S. acidocaldarius. The influence of organic nutrients (yeast extract and peptone) on sulfur removal from coal is shown in Fig. 1. The rate and the extent of sulfur removal in the presence of 0.02% yeast extract were less than those obtained with the mineral salts medium free of yeast extract. The similar effect has been observed in the presence of both 0.02% yeast extract and 0.1% peptone (Fig. 1). In this case, the adverse effects of the presence of organic nutrients were more pronounced. The experiments indicated that the organisms could remove sulfur from coal in mineral salts medium, and that inclusion of yeast extract or peptone into the medium did not improve sulfur removal from coal. The inclusion of 0.01 M FeCl3 into reaction medium as a chemical oxidizing agent resulted in lower sulfur removal than that obtained by 7 z 5 - ~~~~~~~~~~~A microorganisms in FeCl3-free medium (Fig. 1). This may be due to the inhibitory effects offerric iron (Fe ") or chloride (Cl-) on microorganisms. High Fe3+ and Cl- concentrations should be avoided in the leaching medium to achieve high sulfur removal rates from coal by S. acidocaldarius Ṫhe extent of total sulfur removal was found to be dependent upon the coal concentration in the reaction medium. Maximum total sulfur removed from a 10% slurry of coal refuse (-11% total S, 10.5% inorganic S) was 46%, which resulted in 48% inorganic sulfur removal (Fig. 2) in a single-batch run. Maximum total sulfur removal was obtained with 5% untreated coal slurry containing -4% total sulfur and 2.1% inorganic (pyritic and sulfate) sulfur. This resulted in about 96% inorganic sulfur and 50% total sulfur removal (Fig. 3). The extent of total sulfur removal from untreated coal (-4% total S) at higher pulp densities (Pd > 5%) was lower than that obtained with 5% coal slurry; e.g., 40% of initial total sulfur (about 76% inorganic sulfur) was removed from a 10% untreated coal slurry (Fig. 2). To determine the influence of sulfur content of coal on the rate of desulfurization, experiments were performed with two different coal samples: ? 13. z 0 < 11. z w z 9. 0 w 1 7. U..0~ *- Az APPL. ENVIRON. MICROBIOL. -A S TIME IDAYS) FIG. 1. The influence of organic nutrients and FeCl3 on microbial removal of sulfur from coal by S. acidocaldarius. Symbols: 0, mineral salts medium; 0, mineral salts % yeast extract; A, mineral salts % yeast extract + 0.1% peptone; E, mineral salts M FeCl3; A, control flask (uninoculated, 10% coal slurry in mineral salts medium). The 10% coal slurry was at 750C, initial ph 2.5, and -4% total S. Particle size was 104 to 147,m. 1. I TIME (DAYS) FIG. 2. Variation of rate and extent of sulfur removal with sulfur content of coal. Symbols: 0, coal refuse, -11% total sulfur; 0, untreated plant feed coal, -4% total sulfur; A, control flask. The 10% coal slurry was at 75 C, initial ph 2.5. Particle size was 104 to 147,um.

4 VOL. 44, z 0 w a g TIME (DAYS) FIG. 3. Sulfur removal from coal by S. acidocaldarius at various pulp densities of coal slurry. Symbols: 0, control flask; other symbols represent coal slurry samples with the indicated pulp densities. The temperature was 75 C; initial ph was 2.5; initial total S was 4%; particle size was 104 to 147,um. (i) coal refuse with -11% total sulfur (approximately 10.5% inorganic S and 0.5% organic S) and (ii) untreated plant feed coal with -4% total sulfur (approximately 2.1% inorganic S and L.9o organic S). The rate of microbial sulfur removal was dependent upon the sulfur content of coal (Fig. 2). The maximum volumetric sulfur removal rate with coal refuse (-11% total sulfur) was approximately 13 mg of S per liter per h, whereas the rate obtained with untreated coal (-4% total sulfur) was -4.5 mg of S per liter per h. The corresponding surface reaction rates were 4.1 x 10 mg of S per cm2 per h and 1.4 x 10-4 mg of S per cm2 per h for 11% and 4% sulfur-containing coal samples, respectively. The results indicated that the sulfur removal rate based upon external surface area of coal particles increased with the sulfur content of coal particles. An increase in the sulfur content of coal by a factor of three resulted in an increase in the surface reaction rate by approximately the same factor. The sulfur removal rate obtained in this study (1.4 x 10-4 mg of S per cm2 per h) was slightly higher than that reported by Du an and Apel (12) (-1.1 x 10- mg of S per cm per h) using T. ferrooxidans. The experiments were performed with different pulp densities (percent coal concentration [wt/vol]) of untreated coal slurries (-4% total S) to determine the effect of external surface area of coal particles on the rate and the extent of REMOVAL OF SULFUR COMPOUNDS FROM COAL 881 sulfur removal from coal. Figure 3 shows the results of experiments performed with different pulp densities. The data shown in Fig. 3 were used to calculate the maximum (exponential phase) volumetric rate of sulfur removal. To compare the relative effectiveness of biological sulfur removal at different pulp densities, the maximum sulfur removal rates based upon external surface area of coal particles (i.e., surface reaction rate) were calculated from volumetric rates of sulfur removal. The relationship between volumetric and surface rates of sulfur removal is (17) rv = r. a Pd (1) where r, is volumetric rate of sulfur removal (milligrams of S removed per milliliter of liquid per hour); r, is the rate of sulfur removal based upon external surface area of coal particles (milligrams of S removed per square centimeter of coal surface area per hour); a is specific surface area of coal particles (square centimeters per gram of coal); and Pd is pulp density of coal slurry (grams of coal per milliliter of liquid). The specific surface area (a), in terms of particle size, is 6 Pc Dp (2) Substituting equation 2 into equation 1 yields 6 rv = rs Pd Pc Dp (3) where Pc is the specific gravity of coal (grams per cubic centimeter of coal) and Dp is the average diameter of coal particles (in centimeters). Since the microbial removal of sulfur from coal involves heterogeneous biocatalytic reactions on the surfaces of coal particles, the sulfur removal rate was based upon the surface area of coal particles instead of on reaction volume. Surface reaction rates were calculated from volumetric rates of sulfur removal by using equation 3 (Pc 1.5 g/cm3; Dp = 125 p.m). Surface and volumetric reaction rates were plotted against pulp density of coal slurry (which is a measure of total external surface area of coal particles for a given particle size) in Fig. 4. When the average particle size (Dp) and the surface reaction rate (r5) were constant, the volumetric rate of sulfur removal increased with the pulp density (i.e., the external surface area) of coal slurry (equation 3). At low pulp densities, the r, was approximately constant (Fig. 4A) and, therefore, r, increased almost linearly with the pulp density when Pd ' 15% (Fig. 4B). However, at higher pulp densities (15% < Pd < 30%), the surface reaction rate decreased with the pulp

5 882 KARGI AND ROBINSON E x.10 -Z.- E IA) I(B) or PULP DENSITY (9 coal/100ml) PULP DENSITY (gcoal/100ml) APPL. ENVIRON. MICROBIOL. In [(S) - (S)o] =,ut + K' (6) - A plot of In [(S) - (S)0] versus time yields u 0.27 day-1 and a doubling time for sulfate formation as td(so42-) 2.6 days. - Among the possible reasons for low sulfur * removal rate are low CO2 content of air; suboptimal reaction conditions such as temperature, 25 X ph, and the ratio of biocatalyst to coal surface area; and insufficient aeration and agitation. The ph of the reaction medium dropped from an initial value of 2.5 to a final ph of 2 with low -- sulfur content coal, and from 2.5 to 1.5 with high sulfur content coal (i.e., coal refuse). Since the ph was not controlled at its optimal level, the change in ph of reaction medium may be partially responsible for the low sulfur removal rates obtained in this study. is30 The thermophilic microorganism S. acidocaldarius has shown promise for the removal of FIG. 4. Influence of pulp density (i.e external sulfur from coal. About 96% of inorganic sulfur surface area) of coal slurry on volumetric and surface has been removed within 10 days from coal rates of sulfur removal. Initial total S was--4%.(a)srversus pulp density; (B) r, versus pulp de nsity. inorganic sulfur, which resulted in 50% total samples containing 4% total sulfur and 2.1% sulfur removal. The optimal pulp density result- volumet- ing in maximum surface rate of sulfur removal density, resulting in a slight decrease iln ric reaction rate with pulp density. Altthough the was 15%. The inclusion of yeast extract, pepexact cause is not known, particle aggllomeration tone, and 0.01 M FeCl3 into mineral salts mediof gaseous or reduction in solubility and transfer ( um adversely affected the rate and the extent of nutrients (mainly 02 and C02) are aimong the sulfur removal from coal. main factors causing reduced surface reaction ACKNOWLEDGMENTS rates at high pulp densities (Pd > ) The sulfur removal data obtained from high This study was supported in part by National Science sulfur content coal (i.e., coal refuse Mvith -11% Foundation grant CPE The summer internship given to J.M.R. by the Energy Research Center of Lehigh Universi- used to ty is acknowledged. total sulfur) in exponential phase wais determine the doubling time for sulfatte release into liquid medium. Microbial growtl h in expo- LITERATURE CITED nential phase is described by the follovwing equa- 1. American Society of Testing Materials Annual book tion: of ASTM standards, part 26, methods D and In X =,t + K (4) D American Society of Testing Materials, Philadelphia. where X is the biomass concentratiion in the 2. Brierley, C. L Molybdenite-leaching: use of a high reaction medium (grams per liter),,. is the temperature microbe. J. Less Common Met. 36: specific rate of microbial growth, and t is time. 3. Brierley, C. L Bacterial leaching. Crit. Rev. Micro- to be biol. 6: Sulfur removal from coal was assunned 4. Brierley, C. L., and J. A. Brierley A chemoauto- biomass trophic and thermophilic microorganism isolated from an growth associated. Since the increase iin concentration is proportional to ATP generated acid hot spring. Can. J. Microbiol. 19: from the oxidation of sulfur compounds and the 5. Brierley, C. L., and L. E. Murr Leaching: use of a thermophilic and chemoautotrophic microbe. Science ATP generated is proportional to electrons re- 179: moved from sulfur and reduced iiron com- 6. Brierley, J. A., and C. L. Brierley Microbial leach- ing of copper at ambient and elevated temperatures, p. pounds, then the increase in biomass econcentration can be assumed to be proportionali to sulfate In L. E. Murr, A. E. Torma, and J. A. Brierley released into the liquid medium. Therrefore, (ed.), Metallurgical applications of bacterial leaching and efore, related microbiological phenomena. Academic Press, X-Xo = Y [(5) - (S)O] (5) Inc., New York. 7. Brock, T. D Thermophilic microorganisms and life where X0 is the initial concentration o f biomass, at high temperatures, p Springer-Verlag New Y is the yield coefficient, [S] is sulfur iin the form York, Inc., New York. of soluble sulfate at time t (milligramvs of sulfur 8. Brock, T. D., K. M. Brock, R. T. Belly, and R. L. Weiss. per liter), and [SO] is initial sulfur Sulfolobus: a new genus of sulfur oxidizing bacteria living at low ph and high temperature. Arch. Microbiol. Assuming X0 << X and substitutinig equation 84: into equation 4 yields 9. Capes, C. E., A. E. McIlhlnney, A. F. Siianni, and I. E.

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