# Extraction of Humic Acid from Lignite by KOH-Hydrothermal Method
## 1 Introduction
Hydrothermal extraction is based on the theory that subcritical water can dissolve organic matter. The reaction is carried out in a special closed container with hot-compressed water above 100 \({}^{\circ}\)C [36], and undissolvable/insoluble substances are dissolved in aqueous medium [37]. Elemental analysis and spectroscopic methods including UV-VIS and FTIR spectroscopy are usually used for the characterization of HAs. [PERSON] et al. [38] examined the effects of water content and particle size on the yield and reactivity of lignite char. A thermogravimetric analyzer (TGA) was used for measuring the reactivity of the chars after pyrolysis and gasification. For the <75 um coal particles, the high water content decreased the char yield; for the 0.9-2.0 mm coal particles, the high water content contributed to the increase in char yield. [PERSON] et al. [39] studied the structural changes in lignite during hydrothermal treatment. The weak bonds in lignite were broken, and the oxygen functional groups decreased gradually during hydrothermal treatment below 240 \({}^{\circ}\)C. The covalent bonds began to break above 240 \({}^{\circ}\)C. [PERSON] et al. [40] and [PERSON] et al. [41] modified lignite by the hydrothermal method. The carboxyl and carbonyl content decreased significantly with an increase in the temperature after hydrothermal treatment. [PERSON] et al. [42] studied the effects of hydrothermal treatment on the swelling, extraction, and liquefaction performances of Shenhua coal. The results showed that hydrothermal treatment could hydrogenate, deash, and devolatilize lignite. The IR spectra showed that the forms of noncovalent bonds (hydrogen bonds) were changed by hydrothermal treatment, the weak covalent bonds (e.g., ether bonds and ester bonds) hydrolyzed, and the side-chains of the aromatic rings broke. The hydrothermal temperature had a strong influence on the reaction process. [PERSON] et al. [43] extracted fulvic acid from weathered coal using hydrothermal catalytic oxidation with nano-copper oxide as the catalyst and hydrogen peroxide as a green oxidant. The maximum total HA and fulvic acid acid contents were 73.23% and 8.98%, respectively. HA could be broken up into two soluble and two insoluble fractions by successive dissolution in deionized water at near-neutral pH [44]. The above analyses showed that hydrothermal pretreatment destroys the macromolecular structure of lignite and increases the content of oxygen-containing functional groups. In the present study, KOH solution was adopted to extract HA from lignite. The reaction equation is given by  
\[\mathrm{R-(COOH)_{n}+nKOH\to R-(COOK)_{n}+nH_{2}O} \tag{1}\]  
In this study, the extraction process of HA from lignite was optimized by evaluating the effects of different factors, i.e., alkali-to-carbon mass ratio, reaction time, reaction temperature, water-to-coal mass ratio, and coal size. Characterization of the extracted humic acid was also done to further evaluate its composition and structure through proximate analysis by UV-VIS, FTIR, XRD, etc. This study will further the economic value and application of lignite [45].