Extraction of Humic Acid from Lignite by KOH-Hydrothermal Method

Gan Cheng,Zeyu Niu,Xiaoming Zhang,Xusheng Li,Chuanxiang Zhang

doi:10.3390/app9071356

Abstract

Humic acid (HA) was extracted by a hydrothermal method from Huolinhe lignite from Inner Mongolia. The effects of the alkali-to-carbon mass ratio, water-to-coal mass ratio, reaction temperature, and reaction time on the HA yield were investigated. The physicochemical characterization of the products was performed, and the reaction mechanism was explored. Raw coal, HA, and residual coal were characterized using Fourier-transform infrared spectroscopy (FTIR), ultraviolet–visible spectroscopy (UV–VIS), elemental composition, and X-ray diffraction (XRD) analyses and compared to each other. The maximum HA yield (90.2%) was obtained from the 0.250–0.180 mm size fraction of the coal sample at a reaction temperature and time of 190 °C and 7 h. Proximate analysis proved that the ash and sulfur of lignite can be removed by hydrothermal treatment. Elemental analysis showed that the O/C and H/C ratios were highest for HA, followed by those for residual coal and raw coal, indicating an increase in the oxygen and hydrogen content of HA. FTIR and UV–VIS analyses showed that hydrothermal extraction destroyed the macromolecular structure of lignite. Moreover, the organics were degraded and hydrolyzed during the reaction process.

Highlights

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]
Many authors thought that HA had no precise structures, which depend on the source that generated them and the specific extraction conditions [5]
The results showed that hydrothermal treatment could hydrogenate, deash, and devolatilize lignite

Summary

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]. Schulten and Schnitzer [3] proposed a structure for HA formed by alkyl benzene moieties attached through covalent bonds. Piccolo [4] proposed that it is a self-assembled superstructure with relatively small heterogeneous molecules held together primarily by hydrophobic dispersive forces and hydrogen bonds. 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].

Methods

Results

Conclusion