# Extraction of Humic Acid from Lignite by KOH-Hydrothermal Method
## 3 Results
### Physicochemical Characterization and Mechanism Analyses
However, the intensity of the -CH\({}_{2}\) absorption peak at 2853 cm\({}^{-1}\) was somewhat diminished, probably because the hydrothermal reaction broke the CH\({}_{2}\)-CH\({}_{2}\) bond of the long-chain aliphatic macromolecular compounds in the raw coal and increased the -CH\({}_{3}\) content. The peaks in the 1900 to 1000 cm\({}^{-1}\) range were mainly attributed to the oxygen-containing functional groups in HA. The peak at 1696 cm\({}^{-1}\) represented the sharp C=O stretching vibration peak of carboxylic acids, aldehydes, and ketones. The peak at 1604 cm\({}^{-1}\) may be attributed to C=O in amides (amide I band), quinones, or ketones. The peaks in the broad frequency range of 1550 to 1790 cm\({}^{-1}\) could be mainly attributed to protonated carboxylic acid (-COOH), carboxylate anion (-COO-), and ester carbonyl (-COOR) groups, as well as to some other functional groups. The peak at 1440 cm\({}^{-1}\) was assigned as the out-of-plane stretching vibrational absorption peak of C-H in aliphatic CH\({}_{3}\). The peak at 1371 cm\({}^{-1}\) was attributed to the deformation of the hydroxyl group, the C-O stretching vibration of a phenolic hydroxyl group, the C-H deformation of a methyl or methylene group, or the asymmetric stretching vibration of COO-. Aromatic structures had an absorption band within the range of 900 to 700 cm\({}^{-1}\), which may be a deformation vibration of the aromatic ring out of the CH plane, indicating that the groups around the aromatic ring are substituted. HA showed significantly higher absorption band at 1040 cm\({}^{-1}\) than YM and CY, indicating that HA had higher carbohydrate contents than YM and CY, since the absorption band at 1040 cm\({}^{-1}\) was found to represent the C-O asymmetric stretch vibration of carbohydrates. The element analytical result showed that O/C ratios of HA was slightly higher than those of YM and CY, this verified the conclusion that HA contain more carbohydrates than YM and CY [60]. The characteristic peaks at 1150 cm\({}^{-1}\), representing asymmetric stretching of the C-O-C bridge. The peak observed at 1253 cm\({}^{-1}\) can be assigned to the -COO (ester) group [61]. Figure 7: Fourier-transform infrared (FTIR) spectroscopy of YM, HA, and CY. UV-VIS is an important method for detecting the molecular structure of HA. The location and intensity of the peaks are closely related to the unsaturated bonds and substituent groups of the molecule. UV-VIS (TU-1810 SPC, Beijing Purkinje General Instrument Co., Ltd., China) analysis of the extracted HA solution was performed by recording the spectra from 200 to 900 nm and determining the absorbance variation of HA. Briefly, 20 mg of dry HA sample was added to 70 mL of a NaHCO\({}_{3}\) solution, and the pH was adjusted to 8.0 with 1% NaOH solution or 0.1 mol/L HCl solution. Another sample without HA was used as the blank sample. As shown in Figure 8, the absorption signal of HA decreased monotonically in the visible light region (400-700 nm). For the near-ultraviolet region (200-400 nm), there was a weak absorption plateau (aromatic structure) at approximately 300 nm. A higher coal rank corresponds to deeper aromatization and a more pronounced absorption plateau [62]. It was inferred that the aromatized structure of the raw coal may have been destroyed by the hydrothermal reaction; there was a large absorption peak at 240 nm, indicating the presence of conjugated carbonyl groups with two double bonds in the HA molecule. The absorption peaks at 240 nm represented the unsaturated thiol, aldehyde and hydrazine carbonyl groups [63]. This result was consistent with the results of the infrared spectrum analysis and indicated that HA contained more oxygen-containing functional groups than YM or CY. The mineralogical compositions of YM, HA, and CY were determined by XRD, and the XRD patterns were recorded with an X-ray diffractometer (D/max-rA) over scanning angles (20) with a step of 0.02\({}^{\circ}\), as shown in Figure 9.  
Figure 9 shows the XRD crystal phase analyses of YM, CY, and HA.