Spectral Analysis of Humic and Fulvic Acids from Unoxidized and Oxidized Coal

Abstract

Spectral analysis is employed to compare oxidized and unoxidized coal samples and the humic acids and fulvic acids derived from them. Differences between naturally oxidized and unoxidized coal samples include increased yield of volatiles and humic materials; higher content of analytical moisture, ash. and moisture, and higher total content of oxygen, nitrogen, and sulfur (O + N + S); but lower carbon content. The greatest yield of humic materials is observed for oxidized 1B and 2B lignite (70.5%, consisting of 60.9% humic acids and 9.6% fulvic acids). The yield of humic materials is also high for oxidized long-flame D coal (42.4%, consisting of 34.8% humic acids and 7.6% fulvic acids). The yield of humic materials from G coal is minimal: no more than 0.7%. According to Fourier transform infrared (FTIR) spectra and CP/MAS ¹³C nuclear magnetic resonance (NMR) spectra, the structural group composition of the humic acids is largely determined by the composition of the initial coal. The content of aromatic structure f_ar and the hydrophilic/hydrophobic balance f_h/h calculated from the ¹³C NMR spectra for the coal samples and humic acids are directly related (R² = 0.71 and R² = 0.81, respectively). The structural group composition of the fulvic acids is largely unaffected by the composition of the initial coal and is more uniform. Overall, f_h/h is higher for oxidized coal than for the corresponding unoxidized coal. The humic acids have a larger content of carbon and aromatic fragments. The fulvic acids have a larger content of oxygen-bearing aliphatic groups. The elemental composition and structural group composition of the humic and fulvic acids from oxidized G and D coal samples are comparable with those for the lignite samples.