Structural investigations of Australian coals—III. A 13C-NMR study on the effects of variation in rank on coal humic acids

A quantitative ¹³C-NMR technique was applied to humic acids, isolated from total solvent extracts obtained from coals, varying in rank from lignite to medium volatile bituminous. The results from elemental analysis and ¹³C-NMR structural ratios are interpreted in the form of three distinct structural phases in the maturation of coal humic acids. These phases reflect the rank of the parent coals. A decrease in polar functional group content and corresponding increase in aromaticity is observed with increasing maturation of humic acid chemical structure. Limits are postulated for the oxygen content and level of aromaticity for humic acids derived from higher rank coals.