Fulvic acid (FA)
has important properties and is used widely in
agriculture, industry, medicine, and other fields. However, there
is a lack of environmentally friendly and efficient extraction methods
for coal-based FA and its molecular structural characterization. In
this study, FA was extracted cleanly and efficiently from low-rank
lignite via the innovative method of microwave combined with hydrogen
peroxide and glacial acetic acid, followed by purification by the
sulfuric acid–acetone method. The molecular structures of FA
were precisely characterized by UV–vis spectroscopy, infrared
(IR) spectroscopy, 1H nuclear magnetic resonance (NMR)
spectroscopy, and gas chromatography–mass spectrometry (GC–MS).
The results showed that the microwave combined with hydrogen peroxide–glacial
acetic acid method had stronger oxidative degradation ability compared
with the conventional method. Under optimized conditions, the yield
of FA reached 60.97%. During the oxidation process, the macromolecular
network structure of coal was destroyed, resulting in the production
of many oxygen-containing functional groups. According to the IR and
UV–vis spectra, there were abundant oxygen-containing functional
groups such as hydroxyl, carboxyl, carbonyl, and quinone groups in
the molecular structure of FA. Determination of the total acid group
content in the oxygen-containing functional groups of FA showed that
the content of carboxyl groups was much higher than that of phenolic
hydroxyl groups. The 1H NMR showed that there were hydrogen
atoms present as part of carboxyl, aromatic, phenolic hydroxyl, and
aliphatic groups in FA. The (GC–MS) results suggested that
FA is a mixture of dozens of complex compounds, including alkanes,
alcohols, esters, etc.