Characterization of humic acids extracted from a lignite and interpretation for the mass spectra

Abstract

Humic acids obtained from a Chinese lignite via alkali treatment were analyzed using Fourier transform infrared spectroscopy and Orbitrap mass spectrometry coupled with an electrospray ion source (ESI-Orbitrap-MS). Raw coal and the corresponding residue were characterized via scanning electron microscopy and energy dispersive spectrometry. Over 4700 heteroatom-containing compounds with wide distributions of molecular mass and unsaturation degree were detected via the ESI-Orbitrap-MS, and around 60 percent of the detected species were found to be oxygen-containing compounds. In addition, van Krevelen diagram and double-bond equivalent (DBE) plot were introduced to provide more structural details of the compounds. For the species only containing C, H, and O (HA_CHO), condensed aromatic compounds with a DBE value over 20 only contained 1 or 2 oxygen atoms. Carboxyl- and hydroxyl-containing aliphatic compounds (CHCACs) were predominant in HA_CHO with 5 or 6 oxygen atoms. Both the CHCACs and aromatic carboxylic acids or phenols were grouped into clusters in the van Krevelen diagram to be recognized. The introduction of a nitrogen atom to the HA_CHO species was based on the structures of the HA_CHO species, which is also indicated by the van Krevelen diagram.