Geochemical and mineralogical evaluations of coal, shale, and mine waste overburden from Makum coalfield of the Northeast India

The Cenozoic-age Makum coal from northeastern India offers numerous research opportunities because of its diverse geochemical and geological characteristics. Due to its high sulfur content, the coal has been found to be less useful for industrial purposes. It can, however, serve as a hub for ongoing research on coal-based derivative products. The aim of this research work is to investigate the mineralogical and geochemical compositions of the coal, mine overburden (MOB) and shale samples taken from the Makum coal field and also on establishing a mutual relationship between them. To characterize the geochemical controlling factors of the Makum coal field, the study employs coal petrography, FTIR, mineralogical, and geochemical analysis. According to X-ray diffraction analysis, the major minerals like quartz, kaolinite, haematite, illite, pyrite, and calcite are present in coal and MOB. Pyrite is observed by SEM–EDS analysis as cubic-shaped particles that are smaller than a few µm in size. The presence of sulfide minerals represents a phase of pyrite mineralization. The petrography study was used to better understand the environment that existed during the formation of the plant material, which aids us in determining the quantity of detrital mineral sediment contained in the coal. According to the ICP-MS analysis, the samples indicate significant levels of rare earth elements including yttrium. The present study reveals higher concentrations of potential hazardous elements in the coal samples, with V, Cr, Ni, Cu, and Zn content in coal being considerably enriched compared to world-average concentrations. The correlation analysis reveals that the potential hazardous elements like Co, Ni, As, and Cu are associated with pyrite as they have strong affinity towards pyrite. Thus, numerous minerals and rare earth elements (REEs) exist, opening up a fresh avenue for more research in the area. This study also assists researchers in understanding the significance of Makum coal and provides numerous ideas for coal characterization.


Introduction
Coal, a combustible sedimentary rock originating from ancient vegetative matter, has a diverse range of organic and inorganic components that contributed to its creation and post-depositional actions that account for its structure and metamorphism (Dai et al. 2020).Australia, Russia, China, India, and the United States have the largest coal deposits.Coal is utilised in the production of energy as well as in industrial applications such as cement, iron, steel, chemicals, and fuels (Oxford Analytica 2017).The bulk of coal is used to create electricity, which today accounts for 40% of worldwide energy consumption (IEA 2020(IEA , 2022)).
The discovery of large coal deposits in the northeastern region (NER) of India over the years has sparked interest in its potential for several sectors.Because of the significant quantity of sulfur present in these NER coals, in the power and industrial sectors, they are less prevalent.Sulfur compositional differences in coal are mostly influenced by sedimentary settings during peat deposition and post-depositional modifications (Chou 2012), particularly the sulfate in the saltwater that inundated the peat swamps.Sulfur and several trace elements in coal undergo major changes during coal combustion and lead to atmospheric pollution (Mukherjee and Srivastava 2005;Rabha et al. 2018;Islam et al. 2020).Northeastern India coals have a high yield of vitrinite and volatile matter, yielding double the amount of tar in comparison to other Indian coals (Baruah and Khare 2010).With the world becoming more environmentally conscious, coal-based industries are in jeopardy due to the harm that coal ash and acid mine drainage inflict in rivers and streams.In India, according to the Indian Bureau of Mines, 755,440 hectare (ha) (7554.4km 2 ) of land is under mining lease with 5% [approximately 37,772 ha (377.7 km 2 )] actively mined (Juwarkar and Jambhulkar 2008; Indian Bureau of Mines (IBM) 2001).The coal mining activity also generates a large volume of mine waste containing rock, shale, and different types of soil along with minerals and potentially hazardous elements.These coal and mine wastes, termed as mine overburden (MOB), mainly have high sulphur content (2%-12%, as received basis) in the Makum coal fields of northeast India.The coal mining activity leads to major changes in water bodies through generation of acid mine water, soil topography, soil erosion, retardation of plant growth, and environmental pollution (Islam et al. 2021;Dutta et al. 2017).Coal associated with some higher concentrations of potentially hazardous elements such as lead (Pb), zinc (Zn), cadmium (Cd), mercury (Hg), chromium (Cr), arsenic (As), Nickel (Ni), etc. and the mining activity or improper utilization of coal may cause severe health problems (Dutta et al. 2020).Dutta et al. (2020) reported that the presence of potentially hazardous elements in coal can cause cytotoxic to both human and animal during the exposure of higher concentration (300 µg/ml).The elemental and mineralogical studies in coal samples provide information about the paleoenvironment of coal formation, along with formational and regional history, mode of occurrence and their identification with potential mobility of particular trace elements.The mineralogical study also helps in evaluation of the behavior of different coals in the preparation and utilization processes (Saikia et al. 2015b).Therefore, the investigation of the distribution of elements and mineral matter present in any coals are of prime importance not only for its geochemical study but also for clean coal initiatives.In earlier studies, some of the physico-chemical characteristics of coal and mine waste over burden from Ledo and Tirap colliery of Makum coalfield were established (Islam et al. 2021;Dutta et al. 2017), however, comprehensive geochemical and mineralogical investigations have not been performed by using advanced analytical techniques.
The main objective of the present research work is to evaluate the mineralogy and geochemistry of coal, shale, and MOB collected from the Tirap, Tikak, and Tipong collieries of the Makum coalfield.This study evaluates the petrological analysis of coal samples that may lead us to the identification of coal depositional environment and their nexus among India's major coalfields.Due to the growing application of rare earth elements (REEs) in modern civilization, finding alternate sources of the REEs recovery is a topic of high interest.The major novelty of the present study is to determine the distribution of REEs and potentially hazardous elements (PHEs) in coal, MOB, and shale samples of Makum coalfield.This study also focuses on the relationship among the major trace and rare earth elements that may offer additional insight into the origin of these elements.

Geological characteristics of the study site
The Makum coalfield, which stretches from 27°15′ to 27°25′N latitude and 95°40′ to 95°50′E longitude along the western most side of the Patkai range, is one of the most important coalfields in NE India.Out of the five mining sites of the coal field, Tikak and Tirap have open-cast mines, whereas Baragolai, Ledo, and Tipong have underground mines.The Oligocene Tikak Parbat Formation, which consists of alternating sandstone, siltstone, mudstone, carbonaceous shales, and coal seams, is where all of the coal beds are found.A linear "Belt of Schuppen" overthrusts run through the whole region.The lithological composition of the Tikak Parbat formation (Stratigraphic succession is presented in Table 1) is characterized by the massive to well-bedded, fine to medium grained, yellowish to light grey, and greyish white sandstones which are entrapped with shale and coal seams.Two thick, commercially viable coal seams occur in the lower part of the segment.The base of the lowest seam, which goes by the name of the "18 m" seam, delineates the conformable boundary between the Baragolai and Tikak Parbat Formations (Evans 1932) The "6 m" seam is located at a higher elevation in the section.These seams are coal-rich units with occasional siliciclastic partings rather than continuous deposition of pure coal.According to Ahmed (1996), the coals are categorised as high volatile B bituminous, which denotes a level of maturation connected to the local tectonics.

Sampling program
Using Standard method (ASTM 2010), coal samples were collected from one of the largest coal mines in the Makum coalfield.The rock types were mainly sandstone, shale, and carbonaceous clay.Several coal, MOB, and shale samples were gathered from three distinct sites (shown in Fig. 1) of the area.Three main locations were used to gather the samples are Tirap, Tipong, and Tikak colliery.All of the coal samples from the Tirap colliery were labelled as NBTP-1, NBTP-2, NBTP-3, NBTP-4, and NBTP-5.Similarly, one coal sample named TPM (1) and one shale sample named TPCS from the Tipong colliery.From Tikak area TPC 20 was labelled as a coal sample, while TPOB20 and TPOB60 were designated as MOB.

Proximate and ultimate analysis
The proximate analysis and the forms of sulfur analysis were carried out using the appropriate ASTM standard procedures (ASTM 2011a, b, c).The proximate analysis of the coal sample was done in the 'Proximate Analyzer' (Model: TGA 701; Leco Corporation, USA) by following the ASTM method.The carbon and hydrogen were estimated by using 'Elemental Analyzer' (Model: Perkin-Elmer 2400).

SEM analysis
Scanning electron microscopic analyses were also performed on the raw coal and MOB samples.The samples were coated with platinum byAuto Fine Coater and examined in a scanning electron microscope of model No. ZEISS, SIGMA, Make: Carl ZEISS Microscopy, Germany.Observations and photographs with the SEM were made at 20 kV with a working distance of 11 mm and spot size 36.The SEM images were further developed by using "Image J" program (software version 1.47).

Determination of forms of sulfur
ASTM 2002 technique was used in the current study to determine the different forms of sulfur.Only two of sulfur's three forms, sulfate and pyritic, are directly measured in this process.Organic sulfur, calculated by difference, is the third form: The total sulfur content of the coal sample was measured using a Leco S-144 DR dual range sulfur analyzer.

Petrographic characterisation
For petrology, the coal was crushed to a top size of 20 mesh (840 µm); encased as epoxy-bound particulate pellets; prepared to a final 0.05-µm alumina polish; and examined with a 50×, oil-immersion, reflected-light objective.Description of the coal samples with nomenclature following the terminology recommended by the International Committee for Coal and Organic Petrology (ICCP) for vitrinite (ICCP 1998), inertinite (ICCP 2001), and liptinite (Pickel et al. 1994).The measurement of vitrinite reflectance was carried out using the appropriate ASTM standard.

Elemental analysis (ICP-MS) of coal
The trace and REEs concentrations in coal and MOB samples were quantified by using the High Resolution Inductively Coupled Plasma Mass Spectrometer (Element XR; Model: ThermoFisher Scientific).Collision/reaction cells (Q Cell technology) of ICP MS was also used to determine some selected ions present in coal and MOB samples.The recommended ASTM procedure was applied for sample digestion and further analytes determination (ASTM 2021).A microwave-induced combustion (MIC) procedure was applied for coal digestion for subsequent determination of As, Cd, and Pb by inductively coupled plasma mass spectrometry (ICP-MS) and Hg using cold vapor (CV) generation coupled to ICP-MS.Pellets of coal (500 mg) weighed and transferred together the filter paper to the quartz holder of MIC system.

FTIR analysis
The presence of functional groups in each coal and shale sample was detected by the Fourier transform infrared (FT-IR) spectroscopy using the potassium bromide (KBr) powder method.The coal samples were mixed with KBr in Fig. 1 Makum Coal Basin regional geological map, Assam, India, modified after Ahmed (1996) the ratio 1:100 and then converted them into pellets using a hydraulic press.The FTIR spectrum thus generated was then recorded from wavenumber; 4000-400 cm −1 in transmittance mode (T, %).The scanning was done using the FTIR spectrophotometer (Model: Spectrum Two, Make: Perkin Elmer).Attenuated Total Reflection Fourier Transform Infrared spectroscopic (ATR-FTIR) analysis of MOB was carried out in an FTIR spectrometer (Spectrum-Two; Perkin Elmer) in the range of 400-4000 cm −1 to determine the functional group present.

X-ray diffraction (XRD) analysis
The raw samples were analyzed using an X-ray diffractometer (Rigako, Ultima IV) in accordance with the technique described elsewhere (Islam et al. 2019).The materials' X-ray diffraction data were collected using a starting angle of 2.00° and a stopping angle of 90.00°, as well as a step angle of 0.02° and a scanning rate of 1° C/min, with a target Cu Kα (1.54056A).Mineral peaks were identified using the library database 'Rigaku PDXL 1.2.0.1' application.

X-ray photoelectron spectrometer (XPS) analysis
The X-ray photoelectron spectrometer (XPS) analysis was carried out to determine the surface structure and composition of coal, mine overburden and shale samples.
The analysis was performed with a Thermo-Scientific ESCALAB Xi + spectrometer using a monochromatic Al Kα X-ray source (1486.6 eV) and a spherical energy analyzer that operates in the constant analyser energy (CAE) mode using the mode of the electromagnetic lens.The CAE for high-resolution spectra is 50 eV and that for survey spectra is 100 eV.The five samples (TPC 20, TPM 1, TPOB 20, TPOB 60, and TPCS) were also examined for the distribution  3.The current samples have a total sulfur content ranging from 0.40% to 2.84% (wt%), of which 27%-73% is organic.There is pyritic sulfur present, ranging from 25% to 42.5% of the total sulfur.The distribution of pyritic sulfur does not follow any regular pattern in these coals.Sulfur distribution studies aid in the exploration and selective mining of coals, as well as providing information on deposition conditions.However, detailed basin analysis including paleoclimate, surface and ground water, and tectonic accommodation, in some cases indicate that high sulfur content in coal may not be solely a function of marine influence (Greb et al. 2002;Turner and Richardson 2004).

Physico-chemical analysis
In the absence of detail basin analysis and temporal factors, the influx of sulfur from other sources cannot be commented upon at present.

Petrology of the coal samples
Table 4 shows the maceral compositions of the seven coals.We collected coal samples from all three study sites, and the maceral makeup of the analyzed samples is essentially the same.On a mineral-free basis, all of the current coals contain more than 92% vitrinite.The vitrinite is dominated by the telovitrinite which, in turn, has abundant collotelinite.The inertinite groups consists of varying concentrations of fusinite, semifusinite, and funginite.Funginite occurs as sclerotia (Fig. 2d-f) and hyphae (Fig. 2c, f).Some sense of the complexity of the maceral assemblages can be seen in (Fig. 2a, d) where the funginite occurs in detrital matrices.Liptinite occurs as sporinite, cutinite (Fig. 3a, b), suberinite (Fig. 3c, d), exsudatinite (Fig. 3e), and resinite (Fig. 3a).Resinite occurs within cutinite (Fig. 2a, b) and as clusters or layers in association with vitrinite (Fig. 3).
Silicate, sulfide, carbonate, and other mineral phases are prevalent in the samples, with silicate having a maximum concentration of 5.5% in TPM (1) and a minimum concentration is observed in below detection limit for NBTP 04.Sulfides have been observed in lower concentrations, with a highest value of 1.6% in TPM 1 and lowest in NBTP04 with 0.4%.Because of their tiny size and close connection in most coals, determining sulfur in individual macerals is challenging.The total sulfur concentration of individual macerals isolated from two coals was determined to be the lowest in fusinite.The liptinite (mostly sporinite) has the most organic sulfur while inertinite contains the least, sulfur level in vitrinite macerals is moderate (Ward and Gurba 1998).

FTIR analysis of functional groups
The surface functional groups of raw coal, two MOBs, and one carbonaceous shale samples were examined using FTIR spectroscopy.Figure 5 depicts the FTIR analysis of the samples.The stretching vibration bands of S-S and 2− bonds are more prominent in the Tirap coal (TPM (1)).The peak at 2850 cm −1 , which appears as sharp peaks of medium intensity, may be assigned to aliphatic and alicyclic CH 3 , CH 2 , and CH groups.However, major contributions are anticipated to be due to CH groups (Fig. 5).

Observations from SEM-EDS analysis
Figures 6 and 7 show SEM pictures of the minerals and carbonaceous structure in samples of coal, MOB, and carbonaceous shale.The particle size and shape distributions are erratic according to the SEM micrographs of the samples.The surface morphology revealed a non-uniform topography with minerals embedded in a carbon matrix.According to the elemental analysis, there are significant amounts of silicon, iron, aluminium, carbon, oxygen, and traces of sulfur.Pyrite was detected by SEM-EDS analysis in the form of particles with cubic shapes less than a few µm.EDS reveals the presence of the Si-Al clay mineral kaolinite.Gypsum was also confirmed by the SEM images.Its origin can be regarded as the weathering product of calcite with SO 4 2− derived from pyrite oxidation (Saikia et al. 2015a).

Abundance of elements in coal, MOB and shale samples
The result of the major and trace element data from representative samples of the study area are listed in Table 5.
From the previous study (Saikia et al. 2015a), it has been reported that the major minerals associated with coals in the Northeast India include kaolinite, zircon, pyrite, goyazite, chlorite, xenotime, siderite, titanium oxide (anatase or rutile), iron-bearing gahnite, rhabdophane, carbonate, jarosite, illite, and mixed-layer illite/smectite.However, the coals are also found to be rich in F, Pb, Sr, Zr, and Ba.Earlier work confirmed the presence of element Cr, which is regarded as a marker or index element for the Oligocene coals of the Makum coalfield of Northeastern India (Mukherjee et al 1992).In our study,we have also observed a significant Cr concentration in the samples.Additionally, the coals have low concentrations of most other elements of environmental concern, such as Co, Mo, Mn, etc.Further it was also established that the Eocene and the Oligocene coals of Northeastern India originated under marine conditions.Sulfides, such as pyrite, in coal can be easily weathered; once the crystal lattice of pyrite is destroyed, trace elements with a sulfur affinity may be released into the environment.The origin of pyrite in coal has been attributed to the marine-influenced depositional environment with it's the influx of SO 4 2− ions (Spears 2000).), respectively (Fig. 8c, f, h).From the high-resolution S 2p XPS deconvoluted spectra, we can see that for TPCsample the intensity of binding energy for FeS 2 possess higher intensity than that of SO 4 2− , while for TPOB-20 and TPOB-60, SO 4 2− possesses higher intensity of binding energy than that of FeS 2 .

Correlation analysis of trace and rare earth elements (REY)
The possible source of trace elements and REEs in coal and MOB were qualitatively determined with the Pearson correlation coefficient techniques.This analysis also shows the relationship among the elements and REEs in both coal and MOB samples.A statistical tool for the data analysis (STHDA, R-software) method was used to determine the correlation coefficient (r) for the elements and REEs concentrations obtained from coal, MOB, and soil samples of Makum coalfield of India.In this study, Copper also shows strong relationship (r > 0.8) with Co, Ni, and As, and is also positively correlated with all other trace elements (Fig. 9).This correlation indicates that they (Co, Ni, As, and Cu) are associated with pyrite mineral as they have strong affinity towards pyrite.Copper is mainly associated with mineral matter as chalcopyrite which is a common sulphide mineral in the coal.Figure 10 shows  The key factors affecting the concentration of elements in coal relate to the coal forming environment as evidenced by the kind and provenance of the sediment that provided information on terrigenous deposits (Dai et al. 2005(Dai et al. , 2012)).

Conclusions
A thorough investigation is carried out by combining analytical methods that include sophisticated mineralogical techniques such as XRD, XPS, SEM, and ICP-MS  examination of coal, shale, and MOB samples.Clay minerals, quartz, pyrite, calcite, and haematite were discovered in Makum coal and MOB samples.Petrographic analysis is a potent investigative method for determining the amount of macerals assemblages in coal samples and evaluating their process circumstances.The carbon content of the shale sample under inquiry was discovered to be quite high, signifying that it is a carbonaceous shale that requires additional investigation.The existence of trace and significant REY is observed in all three categories of samples.The presence of aliphatic amino (NH) and aromatic C=C groups, as well as S-S and C-S, C-O-C, C-H, and SO 4 2− functional groups, is revealed by FTIR analysis.The petrology study revealed a high concentration of sulphide minerals, as well as a variety of maceral compositions, which may imply a time of pyrite mineralization supported by organic matter (macerals).This study suggest that the coal is high volatile B bituminous, which is typical of coal from Northeast India.The composition of the macerals is comparable to one of India's largest coal reserves.During the sulfur analysis, the investigation discovered the presence of a moderate amount of sulfur.This study reveals increased concentrations of potentially hazardous elements (in ppm) in the coal samples, with V, Cr, Ni, Cu, and Zn content in coal being considerably enriched compared to world-average concentrations.The overall mineralogical and geochemical investigation presented in this study will be effective in designing the mitigation processes for the environmental consequences like acid mine water generation in the coal mining area and also provides a new avenue for the further investigation on gainful and sustainable recovery of REEs from coal, MOB, and shale samples of Makum coalfield.

Fig. 3 a
Fig. 3 a and b White-light and blue-light images of resinite (r).c and d White-light and blue-light images of resinite (r).e and f White-light and blue-light images of resinite (R) with telovitrinite (tv).g and h White-light and blue-light images of cutinite and resinite (R)

Fig. 5
Fig. 5 FTIR analyses of raw coal, MOB, and shale samples showing the presence of functional groups

Fig. 6
Fig. 6 SEM micrographs of a TPOB-20, b TPOB-60 samples, c TPM(1), and d TPC samples; e and f shows EDS analysis of TPOB-20 and TPOB-60, respectively.Presence of Fe-sulfideare seen in the morphology of the micrographs (see EDS analysis) the significant and strong relationship of Nb is observed (r > 0.8) among Ti, Zr, Cr, and As.Niobium is also positively correlated with other trace elements.This strong relationship indicates similar source of origin of these elements.Irregularly scattered patches of clay minerals have been reported in the coal from Tikak Parbat formation, which may be responsible for the presence of Nb, Zr, Li, Ba, and V in coal and MOB samples.Clay minerals are expected to contribute to the emissions of trace elements of environmental importance, mainly Cr, V, and Zn to a lesser amount.Cobalt shows very strong relationship with Ni (r = 0.97), which is supposed to form Cobalt-organic complexes.The significant relationship (r = 0.71) of Cr and Ni indicates that Ni might be partly linked with clay minerals.

Fig. 7
Fig. 7 SEM micrographs of a, c NBTP 1, b NBTP 2 and d NBTP 3 samples.Micrograph b denotes cubic morphology in the matrix; a, c and d showslong prismatic to needle-shaped gypsum crystals, clay minerals, Fe-hydroxides (mostly weathering products)

Fig. 8
Fig.8The XPS survey spectra of the samples show the presence of C 1s , O 1s , S 2p , and Fe 2p (a, d, g, i); the Fe 2p XPS peak splits into two main peaks which are located at 710 eV (Fe 2+ ), and 725 eV (Fe 3+ ) for all the samples (b, e, j)

Fig. 9
Fig. 9 Pearson's correlation coefficient of trace elements coal, MOB, and shale samples

Fig. 10
Fig. 10 Pearson's correlation coefficient of REEs in coal, MOB, and shale samples

Table 1
Raja Rao 1981)cession of the coalfields of the foreland basin (modified afterRaja Rao 1981)Greyish to yellowish white sandstone, sandyshale,coal seams Greyish to bluish grey or yellowish red mudstone, shale,sandstone, cabonaceous shale and thin coal seam Compact,fine grained, dark grey sandstone with bands of splintery shale

Table 2
Physico-chemical properties of the coal MOB, and shale samples (as received wt%) Note: M moisture, VM volatile matter, FC fixed carbon, C carbon, H hydrogen various forms of sulfur.The coal samples exhibit relatively high volatile matter and moderate content characteristics.The forms of sulfur obtained are shown in the Table of

Table 5
Summary