# Extraction of Humic Acid from Lignite by KOH-Hydrothermal Method
## 3 Results
### Physicochemical Characterization and Mechanism Analyses  
The residual coal and HA obtained under the optimal conditions were characterized. Here, raw coal and residual coal are denoted by YM and CY, respectively. The proximate and ultimate analyses of raw lignite before and after the hydrothermal reaction are summarized in Table 1.  
As shown in Table 1, compared with YM, fixed carbon (FC) of HA increased, while the ash (A\({}_{\text{d}}\)), volatiles (V\({}_{\text{d}\text{d}}\)), and moisture (M\({}_{\text{ad}}\)) of HA decreased to different extents. These results proved that the hydrothermal treatment could death and devolatilize lignite, and are in good agreement with previous reports in the literature. The content of the elements N, H, and O was higher in HA than in YM and CY. The O/C and H/C atomic ratios were considered as an index for the measurement of aromaticity, with higher O/C and H/C atomic ratios corresponding to lower aromaticity [53]. Thus, the aromaticity of HA was lower than those of YM and CY. The N/C atomic ratio reflected the amount of nitrogen in the organic material. The N/C ratio of HA was higher than that of YM and CY because nitrogen is mainly present in coal as organic pyridine or pyrrole [54]. The macromolecular structures of coal were destroyed, and the amount of oxygen-containing functional groups increased after the hydrothermal treatment. The variations in the organic functional groups present in the YM, HA and CY samples were evaluated by FTIR analysis [55; 56]. FTIR spectra were recorded in the range of 4500-400 cm\({}^{-1}\) (2 cm\({}^{-1}\) resolution) using an infrared spectrometer (VERTEX 70, Germany). The results are shown in Figure 7. \begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Products**} & \multicolumn{4}{c}{**Proximate Analysis/\%**} & \multicolumn{4}{c}{**Ultimate Analysis/\%**} & \multicolumn{4}{c}{**Atomic Ratios**} \\ \cline{2-11}  & **M\({}_{\text{ad}}\)** & **A\({}_{\text{d}}\)** & **V\({}_{\text{d}\text{d}}\)** & **FC\({}_{\text{d}\text{d}}\)** & **N\({}_{\text{d}}\)** & **C\({}_{\text{d}}\)** & **H\({}_{\text{d}}\)** & **O\({}_{\text{d}}\)** & **O/C** & **N/C** & **H/C** \\ \hline YM & 8.50 & 7.20 & 51.20 & 48.8 & 0.74 & 65.48 & 4.83 & 19.76 & 0.226 & 0.010 & 0.885 \\ HA & 0.96 & 4.92 & 40.09 & 59.91 & 1.68 & 55.44 & 6.02 & 21.28 & 0.288 & 0.026 & 1.303 \\ CY & 2.78 & 12.86 & 47.38 & 52.62 & 0.13 & 51.9 & 4.86 & 15.78 & 0.228 & 0.002 & 1.124 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Proximate and ultimate analyses of raw coal, residual coal, and HA. Figure 6: Effect of coal size on the HA yield. In the region of \(\sim\)3500 cm\({}^{-1}\), the hydrogen bonding interactions from free hydroxyl groups and between the hydroxyl groups and \(\pi\) bonds were weakened, indicating that hydrothermal treatment could change the hydrogen bond distribution of the raw coal [57]. In the 3600 to 3300 cm\({}^{-1}\) range, the HA molecules had strong and broad absorption peaks. Because lignite contains almost no alcohols [58], these peaks should be attributed to the hydrogen bond-associated -OH stretching or -NH stretching vibration absorption peaks in phenolic and carboxylic acid structures [59]. This result showed that the hydrothermal reaction led to the hydrolysis of oxygen-containing functional groups such as ether bonds and ester bonds in raw coal and increased the content of carboxyl groups and phenolic hydroxyl groups in HA molecules. The stretching vibration absorption bands of aliphatic C-H bonds were in the 3000 to 2700 cm\({}^{-1}\) range. Among these wavenumbers, 2923 cm\({}^{-1}\) and 2853 cm\({}^{-1}\) were the symmetrical and asymmetric stretching vibrational peaks of the C-H bonds in the saturated hydrocarbon groups -CH\({}_{3}\) and -CH\({}_{2}\), respectively. Within this range, the intensity of the absorption peak of CH\({}_{3}\) at 2923 cm\({}^{-1}\) was higher for HA than for the residual coal.