Structure and Characteristic Analysis of Humic Acids Extracted from Two Typical Lignites with Different Extractants

Properties of humic acids (HAs) depend on source of origin as well as method of extraction, and the physicochemical properties of HAs are directly affected by their structure, so it is of great significance to study HAs extracted with different extractants. HAs isolated from two lignite samples with three extractants (KOH, NaOH, Na4P2O7) respectively were characterized using spectral methods and ultimate analysis and compared with each other. Comparison of the HAs extracted from two lignites with different extractants shows that they are very similar to each other in chemical nature regarding the at least degree of aromaticity and humification, molecular weight, and the distribution of function groups. The experimental results indicate that the extraction of HAs from lignite with higher humification degree is less affected by the type of extractant. Additionally, the six HAs are rich in oxygen-containing groups, and both the aliphatic and aromatic carbons are main existing forms of carbon in HAs. The results of XRD patterns indicate that the content of aliphatic carbon is higher than the aromatic carbon in the six HAs. In this work, the capacity of lignites as parent sources of HAs for various applications is appreciated mainly on the base of extractability and content of oxygen-containing groups. In addition to high extraction efficiency, the extractant with little influence on HAs structure should be selected. So studies on the physicochemical properties of HAs can be helpful in predicting the behavior characteristics of HAs in practical application, and provide valuable reference for the future application of HAs.