Nanopore Structure Evolution in Acid- and Alkali-Treated Coal Under Stress: Insights from SAXS Analysis

Research on the effects of acidic and alkaline solutions and stress on coal’s pore structure has traditionally focused on larger scales, leaving a gap in understanding nanoscale impacts. This study utilized a self-developed small-angle X-ray scattering (SAXS) miniature loading system and in situ synchrotron SAXS to investigate nanopore evolution under varying pH conditions and external stress. By analyzing the scattering data obtained, we investigated the changes in the internal nanopore structures of coal soaked in solutions with different pH values and subjected to external loading. The results showed that all coal samples exhibited negative Porod deviations. The degree of negative Porod deviation decreased after the coal samples were soaked in acidic solutions, while it increased after soaking in alkaline solutions. Negative Porod deviations increased notably under destructive loading. There are significant differences in the changes of internal nanopore structures in coal samples treated with chemical solutions of different pH values. The porosity and specific surface area of coal samples decreased significantly after soaking in acidic solutions, while coal samples treated with alkaline solutions showed substantial increases in both parameters. During subsequent loading, the samples soaked in acidic solutions exhibited minimal changes, whereas those treated with alkaline solutions experienced notable alterations. Chemically treated coal samples also showed increased sensitivity to external stress, especially in smaller nanopores. The study identifies three stages of nanopore evolution under stress: minor damage, compression, and rupture.