Effect of kaolin on biogenic coalbed methane production and the response of microbial community

Kaolin is a common inorganic mineral in nature, which can promote the growth and metabolism of microorganisms. There is a large amount of kaolin in coal, however, there are few reports about its bio-gas production of coal. In order to investigate the effect of kaolin on biogas production, the coal obtained from Yulin, Shaanxi Province was taken as the research object, domesticated microorganisms were taken as the research flora, and different qualities of kaolin were added into the culture media to make biological simulation of gas production. The changes of CH₄ content, volatile fatty acids (VFAs) concentration, coenzyme F₄₂₀ content, organic functional groups of coal and microbial community structure before and after gas production were studied by gas chromatograph, ELISA microplate analyzer, Fourier transform infrared spectroscopy and Illumina high throughout sequencing. The results show that kaolin affected the biological gas production process at different levels, and 0-8.0% kaolin supplementation could be divided into two ranges, 0-1.0% and 2.0%-8.0%. Methane production amount, cumulative methane production amount and concentration of F₄₂₀ show a trend of increasing first and then decreasing in these two ranges, while acetic acid and VFAs concentration show an opposite trend. After 50 days of simulated biological gas production, the cumulative methane production of the experimental group (0.5%) was up to 216 μmol/g coal, which was 55.4% higher than that of the blank group. Kaolin could increase the content of coenzyme F₄₂₀ up to 48.93 ng/L. The addition of kaolin contributes to the utilization of -OH, phenol-OH, -NH- and -NH₂ in coal by microorganisms. The effect of kaolin on the bacterial population structure in the coal biogas production system was not obvious, and only Firmicutes increased first and then decreased in 0-1.0% and 2.0%-8.0% addition ranges. In the contrast, the addition of kaolin has an obvious influence on the variation of Archaea diversity. Archaea were mainly assigned to Euryarchaeota, while Methanosarcina and Methanobacterium were the most abundant species. Euryarchaeota and Methanosarcina show a consistent change trend with cumulative methane production and F₄₂₀ in the interval of 0-1.0% and 2.0%-8.0%, while Methanobacterium and VFAs showed a consistent change trend. It presents that the supplement of kaolin affects the bio-simulated gas production of Yulin coal. Its methane production VFAs, F₄₂₀ enzyme activity, microbial community structure and organic functional group composition in coal have all changed. The present work provides a reference for studying the role of inorganic minerals in biogenic coalbed methane production in future.