Transformation of organic to inorganic nitrogen in NH₄⁺-illite-bearing and Ga-Al-REE-rich bituminous coals: Evidence from nitrogen isotopes and functionalities

Nitrogen is one of the most important elements and mainly occurs as organic forms in coal, and in a few cases, in an inorganic form. Organic nitrogen in coal is mainly derived from the proteins of plants and micro-organisms but sources of inorganic nitrogen remain controversial. One hypothesis for the source of inorganic nitrogen is the organic matter of coal, but this lacks strong evidence. In the Daqingshan Coalfield of Inner Mongolia, northern China, an igneous intrusion in the east of the coalfield led to a coal rank increase from the Hailiushu through the Datanhao mines to the Adaohai mine from the west to east, with NH₄⁺-illite being detected in the coals from the Datanhao and Adaohai mines. These coals are highly enriched in Ga, Al, and rare earth elements (REE). This study aims to explore the transformation of organic to inorganic nitrogen in these coals by means of nitrogen isotopes, element distribution in NH₄⁺-illite, and nitrogen functional groups. The analysis of NH₄⁺-illite by Electron Probe Microanalysis (EPMA) indicates partial substitution of nitrogen during the transition from kaolinite to NH₄⁺-illite. Thermal metamorphism has resulted in the organic nitrogen isotope (δ¹⁵N_o) values in coals from the Adaohai Mine being more positive than those in the Datanhao and Hailiushu mines. ¹⁵N is more enriched in NH₄⁺-illite than in the associated organic matter. The distribution of nitrogen functional forms showed a decrease in pyrrolic nitrogen (N-5) and an increase in pyridinic nitrogen (N-6) from coals in the Hailiushu to Datanhao and Adaohai mines. Ammonium quaternary nitrogen (N-Q) was detected in the NH₄⁺-illite-bearing samples. This study provides significant evidence for the conversion of organic nitrogen to inorganic nitrogen in coals as a result of thermal metamorphism.