The evolution of sulfur during coalification in marine land interaction environments based on the sequential extraction of different sulfur species

Abstract

The geochemical study of sulfur in coal can not only reveal the source and occurrence of sulfur in coal and provide information about the depositional environment and coal formation process, but also provide necessary theoretical guidelines for coal desulfurization and acid mine wastewater treatment. This study established an extraction method for different forms of sulfur in coal and systematically analyzed the content, distribution and isotopic composition characteristics of various forms of sulfur in Andi-anticline coal in the Qianbei coal field, Guizhou Province, southwest China. The results showed that the main form of sulfur in coal in the research area is pyrite (S_py, averaging 50.3 ± 22.5%), with humic sulfur (HS, averaging 14.3 ± 10.2%) being relatively secondary. Elemental sulfur (S⁰, averaging 1.3 ± 0.5%) is a key intermediate, while ES (sulfur in water-soluble and exchangeable state, averaging 24.5 ± 11.6%) is the product of the oxidation of S_py and HS in the later stage of coalification. The sulfur isotope composition of the total sulfur in the coals was low (−34.1‰ ∼ +13.9‰, with a mean of −3.6‰). The δ³⁴S values of S_py and HS are notably lower than those of seawater sulfate (+10‰) from the Permian coal-forming period. The abundance of sulfate and the extent of sulfate reduction reactions control the evolution of total sulfur content and sulfur isotope composition in different layers. The sulfur isotope offset values Δ³⁴S_SO4-Spy of all coal samples are between +5‰ and +47‰, indicating that the bacterial sulfate reduction is the main controlling factor affecting the sulfur isotope offset in sulfate and sulfide in the entire coal formation system. The light sulfur isotope composition of S⁰ (−2.0 ± 7.3‰) indicate that some of the S⁰ comes from the thermal evolution of HS. In the future, it is necessary to further investigate the specific conversion mechanism between S⁰, organic sulfur and pyrite in the coal formation processes.