Water-induced molecular changes of hard coals and lignites

Abstract Coal deposits comprise a huge proportion of carbon sequestered within the Earth's crust. Many deposits are anthropogenically altered by mining operations. The changes in local hydrogeology include, first - water removal from the seam to enable excavations, second – restoration of natural water condition by cessation of water pumping. In each of these cases, there is a substantial change in the living conditions of microorganisms and coal sorption capacity towards gases. In the presented study, an attempt was made to connect results depicting structural changes (FT-IR), microbial community composition (NGS) and activity (GC) of four materials representing various stages of coalification upon hydratation. The issue is important as coal transformation often leads to generation of gases with high global warming potentials (CO2 and CH4). In all samples, hydratation led to coal dearomatization, decarboxylation and decompaction. The main gaseous product upon hydratation was carbon dioxide which was linked with both changes in coal sorption capacity upon hydratation and activity of the autochthonic microbiota. CO2 release was higher in lignites than in the hard coals. Within the experimental timeframes of 450 days, methane production was rather marginal. Furthermore, microbial communities composition suggests intrinsic turnover of that gas by the methanotrophic bacteria.