Extraction of Fulvic Acid from Lignite and Characterization of Its Functional Groups

Abstract

Fulvic acid (FA) is a complex organic mixture composed of small molecules. The structure and composition of FA vary greatly because of the different raw materials used for preparing FA. In this work, FA was extracted from shallow lowrank lignite by hydrogen peroxide (H 2 O 2 ) in a microwave field, and the functional groups of FA were characterized. The optimal extraction process was determined, with the H 2 O 2 concentration being the key factor affecting the yield of FA. Thermogravimetric analysis showed that FA was mainly composed of low molecular weight and readily pyrolyzed compounds. As shown by Fourier transform infrared spectroscopy, in the process of FA extraction by H 2 O 2 oxidation of lignite, the content of −COOH increased, longchain aliphatic compounds decreased, stretching vibrations of aromatic ring skeletons disappeared, and aromatic ring substitution became mainly tri-or disubstitution. Fluorescence spectroscopy indicated that FA had a low degree of aromaticity. X-ray photoelectron spectroscopy qualitatively and quantitatively revealed that the main modes of carbon−oxygen bonding in FA were C−O−, COO−, and CO. Thus, this study not only lays a foundation for studying the composition and structure of coal-based FA but also opens a new avenue for a clean and efficient utilization of lignite.

Fulvic acid – a small but powerful natural substance

“…In general, all methods for FA extraction are in one package with the HA employing the alkaline solution as extracting agent, pH adjustment, and centrifugation. Gong et al (2020a) as shown in Table 3, have outlined different extraction methods of FA. Based on techno-economical aspects, some methods were considered inefficient because of the time required.…”

“…Besides, as this process employs high amounts of sodium (Na + ) or potassium (K + ) ions in the process, it is hard to get FA with high purity by using this technique and the use of a large volume of alkali and acid, it is considered as a nonenvironmentally friendly method. (dimodifikasi dari Bannach-Sczott & Debska, 2008) Lately, Gong et al (2020a) described a novel method for FA extraction from low-quality lignite by using hydrogen peroxide (H2O2) in a microwave apparatus that was later modified by . By using oxidizing of the material approach, the researchers have been able to develop a fast and relatively simple extraction procedure to obtain less-cation-contaminated FA.…”

“…Hence, this method could be used as an effective technique for utilizing low-quality coal and provides a strong basic for an in-depth understanding of the molecule of FA extracted from coal. Gong and other co-workers (Gong et al, 2020b) also reported the molecular structure of FA obtained from the extraction of Halunbuir lignite in Inner Mongolia by using H2O2 and microwave-assisted technique (Gong et al, 2020a). In contrast, employed a similar technique in low-rank lignite extraction but with the addition of glacial acetic acid.…”

“…Schematic illustration of a microwave reactor (redrawn & modified from Gong et al, 2020a) Gambar 2. Gambaran skematis dari sebuah reaktor gelombang mikro (Digambar-ulang dari Gong et al, 2020a) presented in this patent document includes providing an extraction procedure to separate a liquid containing HA and FA from a mixture of compost with the crop materials that have been heated previously and added with water. In practice, the organic compost material could include straw and manure, which have been fermented a few days or a week aerobically.…”

“…Most include the elemental analysis, functional group identification, and micromorphological evidence of the material. As an illustration, the latest report by Gong and his coworkers (Gong et al, 2020a) provides the common analyses performed to evaluate the quality of FA obtained from a certain extraction procedure, i.e., H2O2 oxidation under microwave field (Figure 2), using lignite as extracted material. In short, these researchers performed elemental analysis by using a series of analyses employing fluorescence spectrophotometer, photoelectron spectroscopy, synchronous thermal analyzer, and Fourier transform infrared (FTIR) spectrometer.…”