Experimental study of hydrochemical and microbial community characteristics during CO₂-water-mineral interactions

Water resources are the foundation for human survival and development. Identifying the mechanisms of water resource evolution is a prerequisite for achieving water resource regulation and promoting regional sustainable development. The vadose zone is an important site for the water cycle and plays a significant role in the transformation of atmospheric precipitation into groundwater. As an important participant in water-rock interactions, CO₂ also has a significant impact on alteration of hydrochemical composition and ecosystem evolution. This study focuses on the CO₂-water-mineral interactions. Calcite, potash feldspar, albite and hornblende were selected as the minerals for dissolution experiments under different CO₂ partial pressures (40 Pa, 200 Pa, 500 Pa, 1000 Pa, 1500 Pa, and 2000 Pa). Water samples were collected during the experiment to analyze the hydrochemical components and microbial communities, mineral samples were collected for scanning electron microscopy (SEM) analysis, and the interrelationships between the microbial communities and CO₂-water-mineral interactions were investigated. The results show that CO₂ can promote the dissolution of minerals by lowering the pH value of aqueous solutions, with the most pronounced effect observed on calcite. During the experiment, hornblende exhibited highest microbial community richness and diversity. With increasing CO₂ partial pressure, the overall microbial community richness and diversity tend to decrease during the dissolution of other minerals, except for a slight increase in microbial community diversity associated with potash feldspar. The dominant microbial phylum during the experiments was Proteobacteria, and its relative abundance increased with higher CO₂ partial pressure. Significant changes in the relative abundance of specific microbial phyla can also indicate the further dissolution of a particular mineral. Proteobacteria were affected by HCO₃^-, TDS, Ca²⁺, fCO₂, CO₂, and Mg²⁺, Bacteroidetes and Actinobacteria were primarily affected by pH and K⁺, and Firmicutes and Dependentiae were mainly affected by Na⁺. The CO₂-water-mineral interactions can affect microbial community characteristics, with variations in factors affecting microbial community characteristics during the dissolution of different minerals. Conversely, changes in microbial communities can also either promote or inhibit CO₂-water-mineral interactions. The research results are beneficial for studies of hydrochemical evolution and ecological environment protection.