Coal sulfur isotopes

Mass spectrometric investigations of isotopes in coal and coal minerals have also been very limited in scope. Rafter (25) published sulfur isotope data on 27 New Zealand coal samples but did not draw any conclusions from these data. Smith and Batts (26) determined the isotopic composition of sulfur in a number of Australian coals and concluded that, from this type of data, one might deduce the origin of the organically... 

Sulfur Isotope Composition and the Sources of Sulfur in Coal. Sulfur has four stable isotopes of atomic masses 32, 33, 34, and 36. The sulfur isotopic composition of a sample is generally characterized by its 34S/32S ratio, expressed in terms of 834S. It is the permil (%o) deviation in the 34S/32S ratio of a sample from a standard (troilite in the Canon Diablo meteorite) (102). 

Sulfur isotopic measurements can shed light on the origin of sulfur in coal. The 34S/32S ratio depends on the source of sulfur and the geologic processes involved during coal formation. For example, isotopic compositions are different for the two principal sources of sulfur in coal 1) the sulfur preserved from the precursor plant material, and 2) the sulfur derived from the bacterial reduction of dissolved sulfate in ambient waters. Plant assimilation of sulfur results in a slight depletion of 34S (4-4.5%c) relative to the 834S in the dissolved sulfate source (102.103). In contrast, the dissimilatory bacterial reduction of sulfate results in a large isotopic fractionation sulfide sulfur can be depleted as much as 60%o in the heavy isotope (89.104-106). 

High-sulfur coals of Pennsylvanian age in the Canadian Maritimes Basin contain 5-8% sulfur. However, this sulfur is associated predominantly with freshwater geologic settings. Recent sulfur isotopic results support the hypothesis that the sulfur in these coals is derived from a bedrock evaporite source (122). 

In order to determine the origin of sulfur in coals, Hackley and Anderson (18) studied the sulfur isotopic composition of various coals from the western United States. Coals from the Powder River Basin, Wyoming were found to have anomalously low values (-18.7 to 3.9 parts per mil). The unusual values of these coals suggest that original plant material and isotopically light secondary sulfur from bacterial activity were the major contributors to the sulfur content of these coals. 

The Hanna Basin coals from Wyoming had higher 63ife values than coals from the Powder River Basin. The pattern of sulfur isotopic composition found in coals from the Hanna Basin revealed that coals near the top of the coal seam contained more of the heavy sulfur isotope than coals stratigraphically lower in the coal seam. The sulfur isotope pattern observed in the Hanna Basin coals suggests that sulfur was introduced in sulfate-rich water from the top of the coal seam. 

Sulfur isotope abundance studies have elucidated a number of problems in the evolution, migration, and alteration of petroleum (Krouse, 1977b). In contrast, only recently have sufficient isotopic data been obtained with coal... 

Zhou, C.G., Yang, Q. and Kang X.D. et al. 2000. Sulfur-Isotope Evidence of Pyrite Generation in Coal of Late Palaeozoic in Northern China. Coal Geology of China 12(1) 19-22. 

Price FT, Shieh YN (1979) The distribution and isotopic composition of sulfur in coals from the Illinois Basin. Econ Geol 74 1445-1461... 

Worldwide, the amount of energy available from coal is estimated to be about ten times greater than the amount available from all petroleum and natural gas reserves combined. Coal is also the filthiest fossil fuel because it contains large amounts of such impurities as sulfur, toxic heavy metals, and radioactive isotopes. Burning coal is therefore one of the quickest ways to introduce a variety of pollutants into the air. More than half of the sulfur dioxide and about 30 percent of the nitrogen oxides released into the atmosphere by humans come from the combustion of coal. As with other fossil fuels, the combustion of coal also produces large amounts of carbon dioxide. 

This chapter reviews research on the abundance of sulfur in major coal basins in the U.S., the forms of sulfur in coals, the distribution of sulfur in coal lithotypes and macerals, and the nature of sulfur-containing organic compounds in coal. Next, the origin of sulfur in coal is reviewed based on the evidence from the distribution and speciation of sulfur in peat, and from stratigraphic, isotopic, and trace element data. Finally, the origin of sulfur in coal is explained by a geochemical model. 

In a coal desulfurization study, Narayan et al. (50) were able to extract an appreciable amount of elemental sulfur (36% of total sulfur) with perchloroethylene at 120 °C from weathered coal, but not from fresh coal. Hackley et al. (51) determined the isotopic composition of elemental sulfur extracted by perchloroethylene and obtained results consistent with the interpretation that the elemental sulfur originates from the oxidation of pyrite. 

Earlier work on the isotopic compositions of pyrite, elemental sulfur, and organic sulfur in coals from Japan, Australia, and Germany (107-110) was summarized by Nielsen (102). Smith and Batts (110) showed that organic sulfur in Australian coals ranging in age from Permian to Tertiary has a large isotopic variation (+2.9 to +24%c) in coals with more than % sulfur, whereas organic sulfur in low-sulfur coals (less than 1% sulfur) has a narrow isotopic composition between +4.6 and +7.3%o. The relatively uniform isotopic... 

Figure 4. Comparison of isotopic composition of pyritic and organic sulfur in low- and high-sulfur coals in the Illinois Basin. Data are taken form Westgate and Anderson (113). Samples were collected from the top, middle, and base of the Herrin Coal at each mine.

Basinal Brines as a Source of Sulfur in High-Sulfur Coals. Sulfide minerals, such as pyrite and sphalerite, in coal seams may be deposited from basinal hydrothermal fluids. The occurrence of epigenetic sphalerite in Illinois Basin coals has been described by Hatch et al. (119) and Cobb (120). Whelan et al. (121) studied the isotopic composition of pyrite and sphalerite in coal beds from the Illinois Basin and the Forest City Basin, and suggested that some of the coals were affected by Mississippi Valley-type hydrothermal solutions. 

Previous studies (4-6) have illustrated the difficulties in relating the isotopic composition of the organic and pyritic forms of sulfur in coals. This is particularly so where large additions of secondary pyrite further distort the frail relationship, if... 

Table I. Concentrations and Isotopic Compositions of the Forms of Sulfur in Coal...

Figure 2. Isotopic composition of organic sulfur in Permian Australian coals containing <1% total sulfur.

In a radically different analytical situation, Kelly et al. used thermal ionization and isotope dilution to determine the amount of sulfur in fossil fuels [31]. These materials (oil and coal) were to be used as reference materials for analyses by other analytical techniques the superior accuracy and precision of isotope di-... 

Smith, J.W. and Batts, B.D., 1974. The distribution of isotopic composition of sulfur in coal. Geochim. Cosmochim. Acta, 38 121—133. 

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