Exploring rare earth elements in coalmine overburden: nanoscale insights from FESEM, TEM and XPS analysis

Rare earth elements and yttrium (REY) are critical for various advanced technologies, particularly in electronics, and play a vital role in the economic growth of any country. Coal and its by-products could be potential precursors of these commodities and other natural resources. While coal and coal fly ash have been assessed for their REY content, the coal mine overburden (OB) or waste remains unexplored as a potential source of rare earth elements. The coal and coalmine OB samples of the Makum coalfield from the Northeastern region (NER) of India are examined and found to be promising sources of REY. This study presents the existence, distribution, and depositional conditions of REY in the coal and coalmine OB using various advanced analytical techniques, such as nanoscale morphology, Field Emission Scanning Electron Microscopy (FE-SEM), and High-Resolution Transmission Electron Microscopy (HR-TEM) to provide light on the geochemical behaviour and potential commercial viability of REY. The average value of REY in the study area is 167.66 mg kg^-1 on a whole sample basis, in which the mean light (LREY) to heavy (HREY) ratio is 37.67. The average values of the europium anomaly (δEu), cerium anomaly (δCe), and Gadolinium anomaly (δGd) are 3.20, 0.71, and 5.30, respectively. The coal-forming conditions are characterized by slightly oxidizing and highly reducing environments, dominated by M-type enrichment, which are favourable for the weathering process. These conditions, marked by the absence of anaerobic microbial activity, facilitated the preservation of organic matter. Thus, the OB and coal deposits in this region present an opportunity for further exploration and assessment regarding the potential future recovery of REY.