# Extraction of Humic Acid from Lignite by KOH-Hydrothermal Method
## 1 Introduction  
Humic acid (HA) is a complex mixture in which there can be small, large, and polydisperse molecules formed and accumulated through the decomposition and transformation of microorganisms and a series of geochemical reactions [1; 2]. [PERSON] and [PERSON] [3] proposed a structure for HA formed by alkyl benzene moieties attached through covalent bonds. [PERSON] [4] proposed that it is a self-assembled superstructure with relatively small heterogeneous molecules held together primarily by hydrophobic dispersive forces and hydrogen bonds. Many authors thought that HA had no precise structures, which depend on the source that generated them and the specific extraction conditions [5]. HA contains a high number of active groups such as carboxyl groups, phenolic hydroxyl groups, carbonyl groups, sulfonic acid groups, and methoxy groups, which have a crucial influence on the acidity, ion exchange properties, colloidal properties, and complexation properties [6]. HA is widely used in agriculture, medicine, health, wastewater treatment [7; 8], and other applications. The existence of carboxylic and phenolic groups results in HA carrying a predominantly negative charge in aqueous solutions under normal environmental conditions [9; 10]. Humic acid can efficiently purify metal ionsand improve water quality [11; 12]. In addition, HA can also be used as surfactants, flocculants, ceramic additives, battery cathode expansion agent, boiler anticrustator, water-soluble fertilizer, compound fertilizer, soil remediation agent, etc. [13; 14]. High functionality of the surface ensures that HA is a good adsorbent with an excellent capacity for treating the pollution caused by waste gases [15; 16]. HA widely exists in lakes, marsh, soils [17], peat, lignite, oxidized bituminous coal, weathered coal, shale, and flora/fauna residues [18; 19; 20]. Lignite has attracted considerable attention, as its reserves account for nearly 45% of global coal reserves [21; 22; 23]. However, its abundant oxygen functional moieties, low calorific value, and high moisture content have critically restricted its direct and wide use [24; 25; 26; 27]. Compared to HAs originating from soil and peat, lignite HAs are characterized by higher carbon content and biochemical activity, lower oxygen and nitrogen content, more aromatic moieties, and fewer carboxylic groups. Lignite HA is characterized by the presence of methylene and ethylene bridges between the aromatic rings. In contrast to soil humic acids, lignite humic acids were shown to contain saturated long chain alkanoic acids with a strong predominance of even numbered homologues [28]. Humic acids are insoluble under acidic conditions, but become soluble and extractable in alkaline solutions. Multiple approaches can be used to extract HA from lignite, including physical, chemical (alkaline [29] and acid), and biological methods [30]. [PERSON] et al. [31] extracted HA from Bulgarian lignite using a NaOH dissolution and HCl precipitation, obtaining 83% HA content. [PERSON] et al. [32] first dissolved South Moravian lignite with a NaOH (0.5 mol/L) and Na\({}_{4}\)P\({}_{2}\)O\({}_{7}\) (0.1 mol/L) solution, followed by pickling with a 0.5% HCl-HF solution, and HA was finally obtained by H\({}_{2}\)O\({}_{2}\)/HNO\({}_{3}\) oxidation treatment. [PERSON] et al. [33] prepared a water-soluble humic acid-like polycondensate (HALP) that mimicked the fundamental physicochemical and spectroscopic properties of natural HA. The polymerization procedures were as follows; a gallic acid and protocatechuric acid (mass ratio is 1:1) suspension liquid (0.5 L) was adjusted to pH 1 using HCl and continuously stirred for 12 h at 25 \({}^{\circ}\)C in a closed glass jar, and the pH was then adjusted to 10.5 with NaOH. [PERSON] et al. [34] isolated a water-soluble HA and melanin-like polymer complex from olive mill waste waters (OMWW) by using ammonium sulphate as a natural additive. Compared to the above methods, the hydrothermal method [35] is an effective approach for obtaining higher HA yields.