Effects of oxidative weathering on the composition of organic matter in coal and sedimentary rock

The effect of oxidative weathering on the composition of sedimentary organic matter (SOM) of lignite, subbituminous coal, quartzose arenite, siliceous mudstone, siliceous shale and diatomaceous shale was investigated in Hokkaido, Japan, in a subarctic humid climate. For 15 samples in total, a humic substance extraction method was applied to divide SOM into lipid-C, humin-C and alkali soluble-C. The alkali soluble-C was further divided into fulvic acid (FA)-C, humic acid (HA)-C and a smaller molecular size fraction (SMSF)-C, which comprised organic molecules able to pass through a dialysis membrane (< 1000 Da) during the HA and FA purification processes. The alkali soluble-C (8–750 g/kg) increased with the degree of sample weathering in compensation for lowering of humin-C. For the alkali soluble-C, HA-C (undetectable to 698 g/kg) became dominant with sample weathering, in contrast to FA-C and SMSF-C. These observations imply that oxidative weathering converts humin-C into alkali soluble-C (mainly HA-C). The 13C nuclear magnetic resonance (NMR) analysis (precision ± 2.4%) revealed higher aromatic-C content of HA (average 40%) as compared to bulk humin (average 33%), suggesting that the re-generated HA was derived from condensed products (aromatic-C rich) in the humin, rather than from recalcitrant biomolecules (alkyl-C rich) in that fraction. The longer emission wavelength of the HA and FA from weathered samples in the excitation–emission matrix (EEM) spectra (precision ± 1.0%) is ascribed to the abundance of oxidized aromatic moieties, with the higher O/C atomic ratio of HA and FA from the weathered samples.