Energy &Fuels 1993, 7, 219-221
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A Convenient Method for the Quantitative Determination of Elemental Sulfur in Coal by HPLC Analysis of Perchloroethylene Extracts David H. Buchanan,* Kenneth J. Coombs, and Patrick M. Murphy Chemistry Department, Eastern Illinois University, Charleston, Illinois 61920-3099
Chusak Chaven Illinois State Geological Survey, 615 East Peabody Drive, Champaign, Illinois 61820 Received September 14,1992 ~~
A convenient method for the quantitative determination of elemental sulfur in coal is described. Elemental sulfur is extracted from the coal with hot perchloroethylene (PCE) (tetrachloroethene, CzC14) and quantitatively determined by HPLC analysis on a CISreverse-phase column using UV detection. Calibration solutions were prepared from sublimed sulfur. Results of quantitative HPLC analyses agreed with those of a chemical/spectroscopic analysis. The HPLC method was found to to 2 X g/L. The lower detection limit was be linear over the concentration range of 6 X 4 X lo4 g/L, which for a coal sample of 20 g is equivalent to 0.0006% by weight of coal. Since elemental sulfur is known to react slowly with hydrocarbons at the temperature of boiling PCE, standard solutions of sulfur in PCE were heated with coals from the Argonne Premium Coal Sample program. Pseudo-first-order uptake of sulfur by the coals was observed over several weeks of heating. s-l, too small For the Illinois No. 6 premium coal, the rate constant for sulfur uptake was 9.7 X for retrograde reactions between solubilized sulfur and coal to cause a significant loss in elemental sulfur isolated during the analytical extraction. No elemental sulfur was produced when the following pure compounds were heated to reflux in PCE for up to 1week: benzyl sulfide, octyl sulfide, thiane, thiophene, benzothiophene, dibenzothiophene, sulfuric acid, or ferrous sulfate. A sluury of mineral pyrite in PCE contained elemental sulfur which increased in concentration with heating time.
Introduction Concern for the adverse environmental effects of largescale combustion of sulfur containing fuels has led to intense interest in the forms of sulfur found in fossil fuels, especially The ASTM Forms of Sulfur in Coal analysis recognizes three forms: sulfatic, usually found only in weathered samples; pyritic, FeS2; and organic, calculated by differen~e.~Elemental sulfur (SO) is not recognized as a separate form by this method and, if present, would be counted as "organic sulfur". The presence or absence of So is currently of interest because of reports that extraction of coal by hot perchloroethylene (PCE) (tetrachloroethylene,C2C14) removes large amounts of organic sulfur which is recovered as So.p6 The source of the So and the amount of authentic organic sulfur extracted in the process is still a matter of some contro~ersy.~-l~ The current status of analytical work on the presence of So in coal has been recently reviewed by (1)Orr, W. H., White, C. M., Eds. Geochemistry of Sulfur in Fossil Fuels; ACS Symposium Series 429;American Chemical Society: Washington, DC, 1990. (2) Markuszewski, R.;Wheelock, T. D.Processing and Utilization of High-Sulfur Coals, III; Elsevier: Amsterdam, 1990. (3)American Society for Testing and Materials. Annual Book of ASTM Standards, D-2492,Forms of Sulfur in Coal, 1991; p 289-293. (4)Lee, S.: Kesavan, S. K.: Lee, B. G.: Ghosh, A.: Kulik. C. J. Fuel Sci. Technol. Int. 1989, I, 443-468. (5)Lee, S.;Fullerton, K. L.; Kesavan, S. K.; Parameswaran, V. R.; Lee, B. G.; Nitirahardjo, S.,Proceedings: Fourteenth Annual EPRI Fuel Conference on Fuel Science; EPRI Report GP-6827,1990;pp 7.1-7.24. (6)Atwood, G.A.; Leehe, H. H. Coal Prep. 1991,11,77-86. (7)Narayan, R.;Kullerud, G.; Woods, K. V.,Prepr.Pap.-Am. Chem. SOC.,Diu. Fuel Chem. 1988,33(1), 193-197.
0887-0624/93/2507-0219$04.00/0
Stock and Wolny, who concluded that pristine (unweathered) coal contains little or no So." In the course of our investigation into the PCE extraction process, we developed a reliable and convenient method for the quantitative determination of Soin coala8Because of the simplicity of the method and the mild conditions employed, we wish to report the experimental details at this time. Since So is reactve, care must be taken in the quantitative determination of sulfur in the presence of organic material. Casagrande showed that So, enriched in 35S,was taken up by the humic acids isolated from peat in chloroform at reflux within 6-48 h.12 There was a nearly linear increase in the organic sulfur content with time. White has shown that So reacts slowly with simple hydrocarbon in sealed tubes at 118 "C over 6 months to give organosulfur compounds; e.g., dibenzothiophene was detected in mixtures of So and bi~heny1.l~ Given these observations, any analytical procedure employingelevated temperatures either during the removal or analytical phase may underestimate the amount of So originally present through conversion to organic sulfur compounds. The method described here employs rapid extraction at 120 (8) Buchanan, D.H.; Coombs, K.; Murphy, P. M.; Chaven, C.; Hackley, K. C.; Kruse, C. W., Proceedings: Fourteenth Annual Fuel Science Conference, EPRI Report GS-6827,1990; p. 8.1-8.13. (9)Buchanan, D. H.; Warfel, L. C. Prepr. Pap.-Am. Chem. SOC.,Diu.
Fuel Chem. 1990,35(2), 516-522. (10)Buchanan, D.H.; Coombs, K.; Chaven, C.; Kruse, C. W.; Hackley, K. C., Chapter 8 in ref 2, pp 79-87. (11)Stock, L. M.; Wolny, R. Chapter 14 in ref 1, pp 241-248. (12)Casagrande, D. J.; Ng, L. Nature 1979,282, 598-599. (13)White, C. M.; Douglas, L. J.; Schmidt, C. E.; Hackett, M. Energy Fuels, 1988,2,22Cb223.
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220 Energy d Fuels, Vol. 7, No. 2, 1993
"C with a solvent in which So is very soluble, followed by room temperature chromatographic analysis. Experimental Section C o d samples from the Illinois Basin Coal Sample Program (IBC coais, Illinois State Geological Survey) and the Argonne Premium Coal Sample Program (APCS coals) were dried to constant weight a t 13.3 Pa., 100 "C in an Abderhalden apparatus before use. Samples were supplied as either -60 or -100 mesh and were not further reduced in size. ASTM Forms of Sulfur analyses were performed a t the Illinois State Geological Survey Applied Laboratory following the ASTM D-2492 p r ~ c e d u r e . ~ High-purity PCE (Aldrich HPLC grade) was fractionally distilled before use to remove traces of UV-active material which could interfere with the analysis. HPLC grade methanol was used as received and deionized water was purified with a Milli-Q cartridge system. Elemental sulfur for preparation of analytical standards and reactions with coal was vacuum sublimed immediately before use. Florisil, 60-100 mesh (Aldrich),was activated at 550 "C for 2 h and stored in a 95 OC oven until used in the sample clean-up column. Soxhlet extraction thimbles were preextracted with PCE, washed with methanoliwater, and vacuum dried before use in analytical experiments. With care, they may be reused many times. Soxhlet extraction is convenient for quantities of coal larger than 20 g. For smaller amounts, batch extractions yield similar results for So content. A thermometer or thermocouple probe was arranged to monitor the temperature inside the extraction thimble in the upper one third of the coal mass, and an electric heating tape (Brisket) was warpped around the glass barrel of the Soxhlet apparatus holding the thimble to maintain the PCE inside at a slow boil (120 "C) during the extraction. Under these conditions, any constriction in the siphon tube of the apparatus interrupted smooth cycling of hot solvent. Solvent was added to the apparatus and brought to temperature a t which time the cod sample was added to the soivent in the thimble. Total solvent volume in milliliters was 10 times the sample weight in grams (10-fold ratio). After extraction for 3 h, the slurry in the thimble was vacuum filtered on a 0.45-pm P T F E membrane filter and the filtrate was cooled for analysis. In batch extraction experiments, a weighed sample of dry coal (0.2-15 g) was quickly added to a 10-fold volume of well-stirred PCE maintained at reflux in a round-bottom flask in an electrically heated oil bath. After 30 min, the slurry was filtered through a 0.45-pm P T F E membrane filter and the filtrate was cooled for analysis. T o prevent possible retrograde reactions between extracted So and organic material from coal, a heated filter holder was used to maintain the solution at 119 "C during filtration. The HPLC system consisted of a Beckman 112 solvent delivery module, Altex 210 injector with a 20-pL loop, 4.6 X 250 mm Cl* Alltech Econosphere 5p reverse-phase HPLC column protected by a Waters C18 pBondapak Guard-Pak Pre-Column, and a Schoeffel GM 770 Monochromator operated a t 290 nm. The mobile phase was 90-95% methanol/water a t 1.0-1.5 mL/min. Aliquots (5 mL) of the solvent extracts were analyzed for elemental sulfur by HPLC after passage through a dry-packed Florisil chromatographycolumn (11X 100mm)to remove organic compounds, some of which contain sulfur, which interfere with the analysis. The Florisil column flow was adjusted to 1dropis and the sample eluted with an additional 4 mL of pure PCE. The first 1mL eluted was discarded and the second 1 mL eluted from the column was collected for HPLC analysis. The method was calibrated with freshly prepared standard solutions of vacuum sublimed Sodissolved in distilled PCE. Calibration samples were collected from the Florisil clean-up column prior to HPLC injection in the same manner as for the unknowns. Independent analysis of the concentration of Soin PCE solution by a modified version of a standard chemical reductionspectroscopic detection method was used to check the accuracy
B u c h a n a n e t al. of the new HPLC analysis.'* Approximately 1g of granular zinc was amalgamated with 2 mL of saturated HgC12 solution in a three-neck, round-bottom reaction flask for 1min. Excess HgClz was removed and the amalgamated zinc was rinsed twice with distilled water. An aliquot of PCE extract (0.1-5 mL) was added to the flask which was quickly attached througharefluxcondenser to a gas collection system. The system was purged with flowing dinitrogen a t a rate of 100 mL/min for 5 min a t which time 10 mL of 6N HC1 was added via a pressure-equalizing addition funnel. The mixture was heated toreflux for 1hand the entrained gas was scrubbed free of 02 and NO, bypassage througha solution of 10% pyrogallol and NaH2P04.H20mixture. The H2S produced was absorbed in Zn(0Ac)z:NaOAc solution to form methylene blue upon the addition of PADA (acidified solution of N,Ndimethyl-p-phenylenediamine sulfate in the presence of Fe3+) whereupon the absorption of the solution was determined spectrophotometrically at 670 nm. The resonse was linear over the range 0-100 pg/mL (ppm).
Results and Discussion HPLC analytical methods for estimating elemental sulfur contents of soil samples15J6and cereal grains17have also been reported. In all cases including this work, choosingan extraction solvent to maximize sulfur removal while minimizing extraction of organic materials which interfere with the analysis is of primary importance. Chromatographic sample cleanup prior to HPLC analysis expands the range of suitable extraction solvents. PCE is a good solvent for elemental sulfur but a poor solvent for the organic material in coal. Narayan reports So solubility in PCE of 30 g/100 mL.7 We have observed that while freshly sublimed So will dissolve without a residue, sublimed So which has been stored at room temperature for several weeks will always leave a small amount of a nearly white insoluble residue which we believe is amorphous sulfur. Even after prlonged Soxhlet extractions of coal with PCE, less than 2 wt 5% of the coal was soluble in the PCE. GUMS analysis of the PCE extract of an Illinois No. 6 coal showed that, in addition to So, dibenzothiophene and at least 11 other organosulfur compounds were also present.ls Least-squares analysis of HPLC detector response versus Soconcentration in PCE for six calibration solutions was linear (regression coefficient 0.9997) over the range of 6 X lo4 to 2 X g/L. The detection limit was 4 X 10-4 g/L. On a routine basis, the lower limit of detection of So is about 0.0006 wt 5% of coal. Standard solutions and several coal extract samples were also analyzed by the modified Zn amalgam reduction method.14 The Zn reduction results agreed with the HPLC results within 4 % relative error, the HPLC values usually being slightly higher. We have determined the So content of several coal samples supplied by other investigators. Narayan supplied +lo0 and -100 mesh splits of a waste coal for which he had found an average of 1.54 wt 5% Sousing PCE extraction and mass spectrometric quantitation.7 Note that the origin of this coal was initially misidentified and subsequently corrected." ASTM Forms of Sulfuranalyses of the Narayan samples revealed a total sulfur content of (14) Aspiras, R. B.; Kenny, D. R.; Chesters, G. Anal. Lett. 1972, 5 , 425-432. (15)Lauren, D. R.; Watkinson, J. H. J. Chromat. 1985,348,317-320. (16) Watkinson, J. H.; Lee, A.; Lauren, D. R. Aust. J.Soil Res. 1987, 25. 167-178. (17) Gerstl, R.; Ranfft, K. Deutsch. Lebensmittel-Rundschau 1990, 86, 176-178. (18) Vorres, K. S. P r e p . Pap.-Am. Chem. SOC.,Diu. Fuel Chem. 1990, 35(2),523-529.
Energy & Fuels, Vol. 7, No. 2, 1993 221
Elemental Sulfur in Coal 0.1 5
0
-0.1 2
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-0.66 -0.93
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Figure 1. Plot of In [SJS,] vs time for the uptake of elemental sulfur from PCE at 120 OC by Argonne Premium Coal Samples. Solid lines are least-squares fits to the data points shown. Coals are identified in the text and described in detail in ref 20.
11.7% and sulfatic sulfur content of 5.96%. By our method,SOcontentsof 1.45(+100meshsample) and 1.64% (-100 mesh sample) (average 1.54%) were found. Stock and co-workers used a sulfur-selective GC detector to analyze solvent extracts of several coals for Soand, for the Argonne Premium coals, both Stock'sllJg and our analyses found no detectable So. In order to evaluate how much, if any, SO would be lost to reaction with the coal during extraction and workup, solutions of sublimed So in PCE were heated a t reflux in the presence and absence of coal for up to 900 h. In the absence of coal, less than 5 % of the original So was lost. All coals tested reacted slowly with So in hot PCE such that, after several weeks, no free SO remained in solution. Pseudo-first-order plots for the uptake of elemental sulfur by Argonne Premium Sample Coals 301 (Illinois No. 6), 501 (Pocahontas No. 31, and 801 (Buelah-Zap Lignite)20 are shown in Figure 1. The pseudo-first-order rate (19) Duran, J. E.; Mahassay, S. R.; Stock, L. M. Fuel 1986,65,11671168. (20) Vorres, K. S. Energy Fuels 1990,4, 420-426.
constant was found to be 9.7 X le7s-1 for the uptake of SO from hot PCE by the Illinois No. 6 coal.g Using this rate constant, the time for a 1% loss of So from solution by reaction with solid coal is estimated to be 2.9 h. Since the batch extractions for this analytical method are completed within 30 min, errors from this source are insignificant. In order to investigate possible sources of interference and screen model compounds as potential sources of So, a variety of pure organic and inorganic sulfur-containing compounds were heated at reflux in PCE for up to 1week and the solutions were analyzed for Soproduction. No So was produced from benzyl sulfide, octyl sulfide, thiane, thiophene, benzothiophene, dibenzothiophene, sulfuric acid, or ferrous sulfate. Traces of So were present in commercial benzyl trisulfide upon room temperature extraction, but the amount did not increase upon heating. SO was present on mineral pyrite at room temperature and the amount detected increased upon heating for 4 h in PCE. By measurement of stable sulfur isotope ratios, we have also shown that the Soextracted with PCE from IBC107 coal (Illinois No. 6) was produced by oxidation of pyrite.21 At this time, our results are consistent with pyrite oxidation as the source of Soin all the coals we have studied. Additional studies of the variation in elemental sulfur extracted from coal by PCE as a function of coal oxidation and extraction conditions are currently in progress.
Acknowledgment. We thank Linda Warfel for experimental assistance, Leon Stock for advice on use of the zinc amalgam, and Carl Kruse (Illinois State Geological Survey), Karl Vorres (Argonne National Laboratory), and Ramani Narayan (PurdueUniversity) for the coal samples used in this study. We gratefully acknowledgethe financial support of the Electric Power Research Institute and the Illinois Coal Development Board through the Center for Research on Sulfur in Coal. (21) Hackley, K. C.; Buchanan, D. H.; Coombs, K.; Chaven, C.; Kruse,
C.
W.Fuel Process. Technol. 1990,24, 431-436.
