Characterization of organonitrogen species in Xianfeng lignite by sequential extraction and ruthenium ion-catalyzed oxidation

A three-step degradation, including sequential ultrasonic extraction (UE), sequential thermal extraction (TE), and ruthenium ion-catalyzed oxidation (RICO), of Xianfeng lignite (XL) was performed to characterize the organonitrogen species (ONSs) in XL. More than 87.3% of organic matter in XL was converted into soluble portions through the degradation. The analysis with X-ray photoelectron spectrometer shows that pyrrolic, amino, and quaternary nitrogen species are the main nitrogen forms both in XL and its residue from UE, while nitroaromatics, chemisorbed N-oxides, and pyrrolic nitrogen are predominant in the residue from TE. A series of ONSs, including pyridines, quinolines, benzo[d]imidazoles, and arylamines, were identified in the extracts from TE of the UE residue according to GC/MS analysis. Among the ONSs, pyridines and quinolines are the most abundant. The ONSs could be released by thermally destroying noncovalent bonds, such as hydrogen bonds and aromatic π - π interactions, during TE of the UE residue. Most of ONSs released from RICO of the TE residue could be generated from the degradation of nitrogen-containing macromolecular aromatics in XL matrix (XLM). Nitrobenzenecarboxylic acids are the most abundant ONSs released from RICO of the TE residue and should be released by the degradation of macromolecular nitroaromatics in XLM.