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

Guanqun Gong; Liangwei Xu; Yingjie Zhang; Weixin Liu; Ming Wang; Yufeng Zhao; Xin Yuan; Yajun Li

doi:10.1021/acsomega.0c03388

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

ACS Omega. 2020 Oct 22;5(43):27953-27961. doi: 10.1021/acsomega.0c03388. eCollection 2020 Nov 3.

Authors

Guanqun Gong^{1

2}, Liangwei Xu², Yingjie Zhang^{1

2}, Weixin Liu², Ming Wang², Yufeng Zhao², Xin Yuan², Yajun Li²

Affiliations

¹ Key Laboratory of Coal Processing and Efficient Utilization of Ministry of Education, Xuzhou 221116, China.
² School of Chemical Engineering & Technology, China University of Mining and Technology, Xuzhou 221116, China.

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 low-rank lignite by hydrogen peroxide (H₂O₂) in a microwave field, and the functional groups of FA were characterized. The optimal extraction process was determined, with the H₂O₂ 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 H₂O₂ oxidation of lignite, the content of -COOH increased, long-chain 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.