Analysis of sulfur transformation pathways in coal-carbonate superimposed strata: Considering carbonate buffering and microbial promotion.

Sulfur pollution is pervasive in coal-carbonate superimposed strata. The oxidation of sulfide minerals in coal produces high-sulfate, acidic wastewater, while sulfate-reducing bacteria (SRB) can metabolize sulfate to produce hydrogen sulfide, posing potential threats to mining safety and environmental quality. To analyze the environmental risks associated with sulfur, this study integrates field surveys, laboratory simulations, and theoretical analysis to elucidate the mechanisms. The results demonstrate that sulfate concentrations increase significantly in coal-bearing strata, whereas they decrease in the underlying carbonate strata, accompanied by a concurrent rise in dissolved sulfide. Specifically, experimental results reveal that carbonate rocks serve as a natural pH buffer in coal-bearing karst systems, neutralizing acidity from pyrite oxidation and thereby improving the habitat for SRB, enhancing sulfate reduction activity and sustainability. Temperature further regulates sulfur transformation pathways. At 30 °C, SRB are rapidly activated, accelerating sulfate reduction, whereas at 15 °C (lower temperature), sulfur oxidation is prolonged. Functional gene prediction indicates a temporal metabolic shift in the microbial community, shifting from sulfur absorption toward dissimilatory reduction. Ultimately, a conceptual mode of sulfur speciation and transformation was established, centered on the local lithology, specifically, the relationship between carbonate layers and coal seams, within the context of prevailing environmental conditions. This study provides a theoretical foundation for the prevention and management of sulfur pollution in groundwater systems coal-carbonate superimposed strata.