Microbial mechanism of mine water quality evolution and sulfate removal in closed coal mine goaf areas

In recent years, the number of coal mine closures in our country has gradually increased, and abandoned coal mine goaf areas have become potential sources of groundwater pollution, characterized primarily by high sulfate mine water. Under certain conditions, native microbial communities within these goaf areas can drive the sulfur cycle, stimulating the potential for natural purification of mine water. To explore the evolutionary control mechanism of mine water quality in closed coal mine goaf areas and the microbial mechanism of sulfate removal, this paper used a typical coal mine in Northwest China as the research object. Through the design of five long-term indoor microcosm experiments under different conditions (experimental group, coal multi-group, culture medium group, culture medium and microbial group, and sterilization group), and utilizing water chemistry testing, 16S rDNA high-throughput sequencing, and other testing methods, the hydrochemical characteristics and microbial mechanisms of sulfate removal in mine water were systematically investigated. The hydrochemical characteristics study showed that the \mathrmSO_4^2- concentration in the experimental group and the coal multi-group system first increased and then decreased. The \mathrmSO_4^2- concentration decreased by 96% in the culture medium group, by 95% in the culture medium and microbial group, and by 57% in the sterilization group. These hydrochemical indicators suggest that, under anaerobic conditions and with available carbon sources, closing the coal mine goaf has the potential for low-cost biological removal of sulfate from mine water. Microbial sequencing results showed that exogenous microorganisms may disrupt the native microbial ecological metabolic network. Bacteria species that degrade micromolecular organic matter in coal exist in the goaf system, forming a synergistic system with sulfate-reducing bacteria. The relative abundance of sulfate-reducing bacteria in the three groups (culture medium group, culture medium and microbial group, and sterilization group) was significantly increased, reaching as high as 70.1% and 79%, respectively, indicating that the microbial reduction process is key to controlling sulfate removal. Based on the above research findings, the evolution mechanism of mine water quality in closed coal mine goaf areas was elucidated: Under conditions without exogenous small-molecule carbon sources, indigenous SRB (stomach-derived biofilm organisms) are in a low-activity or dormant state, and the evolution of mine water quality in goaf areas is dominated by hydrochemical processes such as pyrite oxidation, mineral dissolution, and clay mineral adsorption. However, with the addition of exogenous carbon sources, microbial activity becomes the main controlling factor. The type and concentration of the carbon source determine the direction of the mutual transformation and coupling between the hydrochemical field and the microbial field in the evolution of mine water quality in closed coal mine goaf areas. If indigenous bacterial communities with the ability to degrade coal organic matter can be domesticated or enriched, low-cost self-purification of mine water can be achieved.