Metamorphic transformations of nitrogen functionalities: Stabilization of organic nitrogen in anthracite and its effect on δ15N parameter

Marine and Petroleum Geology 112 (2020) 104090

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Research paper

Metamorphic transformations of nitrogen functionalities: Stabilization of organic nitrogen in anthracite and its effect on δ15N parameter

T

Anwitaa, Santanu Ghosha, Atul Kumar Varmaa,∗, Supriyo Kumar Dasb, Debayan Pala, Gaurav Solankia a

Coal Geology and Organic Petrology Laboratory, Department of Applied Geology, Indian Institute of Technology (Indian School of Mines) Dhanbad-826004, Jharkhand, India b Department of Geology, Presidency University, 86/1, College Street, Kolkata-700073, India

A R T I C LE I N FO

A B S T R A C T

Keywords: Stable nitrogen isotope Nitrogen functionalities TOC/TN ratio Metamorphic transformations Cyclazine structures

The coal metamorphism, as it evolves the microstructure of the coals, would also have significant influences on the structural transformations and isotopic discriminations of the primary chemical moieties. The present study aims to illustrate the nitrogen functionalities present within the chemical framework of the bituminous coal samples from the Raniganj and the Jharia Basins and anthracite A samples of the fold-thrust belts of the Sikkim Himalayas as well as to document the metamorphic transformations of these nitrogenous moieties with the advent in the degree of the coalification. The stable carbon (−24.08 to −21.67‰ for bituminous coal samples and −23.91 to −21.39‰ for anthracite A samples) and nitrogen isotopes (1.64–2.81‰ for the bituminous coal samples; 1.07–3.44‰ for the anthracite A samples) do not show any significant variation with the vitrinite reflectance and, therefore, correspond to the terrestrial higher plant sources of the organic matter. Further, the weak shifts of stable nitrogen isotope values in the anthracite A samples may be attributed to the preservation of the organic nitrogen in the thermally stable aromatic and heterocyclic structures. The total organic carbon to total nitrogen (TOC/TN) ratio may represent the vascular organic matter input into the mire in the case of bituminous coal samples (33.49–43.69), while for the anthracite A samples, this ratio may be an indication of the preservation of the organic nitrogen in the stable chemical framework within the microstructure. The primary alteration of the labile nitrogen-containing groups including the pyrrolic structures would have been initiated by the aqueous fluid entering into the reactive chemical framework during hydrothermal metamorphism. During these processes, most of the nitrogen atoms would have been quaternarized, which is evidenced by strong correlation (r = 0.96) between the relative area ratio of the graphitic to pyrrolic nitrogen and the mean random vitrinite reflectance. Moreover, the intensity and relative area ratio of the pyridinic to pyrrolic nitrogen are strongly correlated with the mean random vitrinite reflectance (r = 0.92 and 0.89, respectively) suggesting entanglement of the nitrogen atoms within the pyridinic forms as well with increasing metamorphic temperature. These structural rearrangements would have intensified the cyclazine structures and preserved the pyridinic forms in the anthracite A samples. Due to the increase in aromaticity and consequent increase in surface hydrophobicity and decrease in the interlayer spacing, the fluid would have lost its mobile phase and could not invade the relatively inert aromatic clusters. Thus, the organic nitrogen atoms were preserved within the thermochemically stable functionalities without showing any substantial isotopic variation. The scientific contribution of this present investigation, hence, lies in depicting the transformation of organic nitrogen among the pyrrolic, pyridinic and the cyclazine moieties with insignificant shifts in stable nitrogen isotope during coalification.

1. Introduction Nitrogen in the coal is primarily contributed from the chlorophyll, porphyrin groups, amino acids, proteins, etc. during accumulation of

∗

peat and this nitrogen is associated with the organic matter (OM) as well as fixed in inorganic form as the ammonium in the minerals (Zheng et al., 2015). Diagenetic alterations and burial transformations affect the OM preservation, and metamorphic processes enhance the

Corresponding author. E-mail address: [email protected] (A.K. Varma).

https://doi.org/10.1016/j.marpetgeo.2019.104090 Received 21 May 2019; Received in revised form 13 October 2019; Accepted 14 October 2019 Available online 19 October 2019 0264-8172/ © 2019 Elsevier Ltd. All rights reserved.

Fig. 1. Map of the Raniganj Basin, India (following Gee, 1932; Ghosh et al., 2018a, Hazra et al., 2015). Explanations:

= Sampling locations in the Raniganj Basin.

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Table 1 Stratigraphic successions of the Raniganj Basin (following GSI, 2003 and Mukhopadhyay et al., 2010). Age

Formation

Recent and Quaternery Tertiary

Cretaceous

Igneous intrusives

Cretaceous

Rajmaha Formation

Upper Triassic

Supra-Panchet Formation

Lower Triassic

Panchet Formation

Upper Permian

Raniganj Formation

Barren Measures Formation

Lower Permian

Barakar Formation with Kaharbari Formation (?) at base Talchir Formation

Precambrian

Chotanagpur Gneissic Complex

Lithology

Thickness (m)

Alluvial and residual soils: lateritic capping Unconformity Light grey mudstone and siltstone with bands of marlstone; white, soft fine grained clayey sandstone, mottled clay and loose sand with pebbles of vein quartz: occasionally lignite at the basal part Unconformity Basic (dolerite) dykes: Ultrabasic (mica-peridotite. mica-lamprophyre, lamprophyre) sills and dykes Greenish grey to black, line to medium grained vesicular porphyritic basalt and volcanic breccia: weathered aphanitic basalt at places: one to five inter-trappeans consisting of grey shale, fine grained sandstone and carbonaceous shale. Massive, very coarse to coarse quartzose sandstone, conglomeratic at places; bands of dark red silly shale Unconformity Coarse grained greenish yellow and greenish grey soil, micaceous, cross-bedded sandstone with slump structures: khaki green fissile silly shale: alternate bands of yellow coarse grained immature sandstone and bright reddish brown claystone with calcareous concretions: conglomeratic at the base Unconformity Grey to light grey fine and medium grained micaceous felspathic sandstone with calcareous clayey matrix in the upper part: siltstones and shales, often interlaminated with fine grained sandstone: carbonaceous shales and coal beds. Dark grey to black micaceous or carbonaceous, fissile shlaes with ferrugineous laminae and thin bands of dense, hard, cryptocrystalline day ironstones; rarely interbanded with fine-grained sandstone. Very coarse to medium grained arkosie sandstones, often cross-bedded: grey and carbonaceous shales, at times interbanded with fine grained sandstone: fire clay lenses and coal beds: pebbly and carbonaceous in lower part. Tillite or diamictite with sandy or clayey matrix at the base: medium to fine grained khaki or yellowish green feldspathic sandstone: siltstone, silty shale, needle shale and rhythmite with dropstones. Granite gneiss with migmatitic gneiss, hornblende schist, hornblende gneiss, mctabasic rocks, pegmatite and quartz veins etc..

90 300

––– 120

300

600

1150

600

750

500

–––

factor of ~1.2 after adding denzo-[c]-acridine (C17H11N) to the blend of coal. Tsubouchi et al. (2014), further, had reported that the increase in nitrogen content or net amount of either ammonia or hydrogen cyanide positively influenced the Gieseler maximum fluidity index. Moreover, now a day, N-doped graphene is widely used as oxygen reduction reaction catalysts (ORRC) (Bai et al., 2013; Liu et al., 2013; Kong et al., 2015; Zhang et al., 2014) that can augment the cathode reactions in the air batteries and the fuel cells (Cheng and Chen, 2012; Miao et al., 2017; Neburchilov et al., 2010). The Pt-based ORCCs, that are traditionally employed, are poorly stable and of high cost (Ahmed and Jeon, 2014; Chen et al., 2008; Miao et al., 2017). To overcome these situations and development of alternate affordable ORRCs, many efforts were made (Cheng and Chen, 2012; Chen et al., 2008; Kumar et al., 2017; Qi et al., 2011; Yuan et al., 2014) and the N-doped graphene came out as a useful material for this particular issue (Lu et al., 2017). The N-doped graphene electrodes are stable and can limit the current density much better than the traditional cathodes (Qu et al., 2010). The variation in the proportions of different nitrogen-containing functionalities (pyrrolic, pyridinic, graphitic) in N-doped graphene may diverse the catalytic activities. The electrolytic properties of these graphenes may depend on the concentration of the graphitic-N that limits the current density, whereas, the pyridinic nitrogen may influence the commencement of oxygen reduction reactions (Miao et al., 2017). Further, Unni et al. (2012) had reported that pyrrolic nitrogen and the mesoporous structures stimulated the catalytic activities of graphene. Further, graphene containing planar pyrrolic and pyridinic nitrogen exhibited significant electrolyte activities and good stability (Ding et al., 2013). Therefore, to characterize the nitrogen functionalities for enhancing the fluidity and caking property of the bituminous coal samples from the Raniganj and the Jharia Basins is a unique approach and has been understudied hitherto. Moreover, besides explaining the novelty of this study, the investigations of the nitrogen functionalities within the

aromatization and condensation of the OM that progressively produces high molecular weight insoluble macromolecular network (Zheng et al., 2015). Despite its low concentration in coal, nitrogen plays a crucial role in geochemical transformations and in bonding with the organic carbon atoms (Boudou et al., 2008). The present study investigates the bituminous coal samples from the Raniganj and the Jharia Basins and the anthracite A samples collected from the Himalayan fold-thrust belts of Sikkim, India to document the OM source through stable carbon and nitrogen (δ13C and δ15N) isotopic composition and metamorphic transformations of the nitrogen functionalities with increasing degree of coalification. The ratio of total organic carbon to total nitrogen content (TOC/TN) has been applied for documenting the OM sources in the bituminous coal. However, this ratio represents the inclusion of nitrogen atoms (N) in the heterocycles and aromatic systems during metamorphic reactions. The X-ray photoelectron spectroscopy (XPS) can monitor the transformations of the nitrogen-bearing moieties during metamorphism. The XPS signal assesses the ionization potentials of the orbitals through the measurement of the electron energy spectrum ejected from nitrogen atoms after bombarding them with X-rays (Boudou et al., 2008). The electrostatic interactions between the core and the valence electrons correspond to the chemical shifts (Boudou et al., 2008). When the X-ray irradiates the surface of the nitrogen atom, it loses one electron from the ‘K’ shell and more precisely, ‘1s’ subshell and that's why the recorded spectra have been termed as N 1s (Boudou et al., 2008). The XPS, here, has also been used to notice any shift of δ15N during the metamorphic course. Hence, the study of the nitrogen functionalities and their structural modifications during the metamorphism of coals from bituminous to anthracite defines the objective of this study. This organic nitrogen may affect the caking behavior (Tsubouchi et al., 2014) through influencing the fluidity of the coals (Clark et al., 1984; Mochizuki et al., 2013; Tsubouchi et al., 2014). Sakimoto et al. (2011) reported that the tensile strength of coke was enhanced by a 3

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anthracite A samples will help to utilize them for the production of Ndoped graphene so that those can be applied as ORRC in industrial scale.

2. Geological setting

Fig. 2. Map of the Jharia Basin, India (following Mishra and Cook, 1992). Explanations:

= Sampling locations in the Jharia Basin.

The Raniganj Basin within the Damodar Valley (Fig. 1) extends between 23°22′ N – 23°52′ N; 86°36′ E − 87°30′ E and covers 1900 km2 area. Initiating as a sag Basin, it took a homoclinal shape with the deposition of the Talchir and the Barakar sedimentary detritus (Gee, 1932; Ghosh et al., 2018a; Hazra et al., 2015). The organic detritus was accommodated within the mires through the meandering rivers at the north side of this Basin, while in the eastern part the peat mires were formed with fanglomeratic cones with intermittent alternations (Ghosh, 2002; Ghosh et al., 2018a). Meanwhile, the rivers flowing westerly to this Basin delivered the organic detritus in association with clastic sediments to deposit peat layers in the western part of this Basin (Ghosh, 2002; Ghosh et al., 2018a). Table 1 presents the general stratigraphy of the Raniganj Basin. Jharia Basin (Fig. 2) contains significant storage of bituminous coals with excellent caking abilities and those can be used for serving the purposes of the metallurgical industries (Mishra and Cook, 1992). This Basin extending from 23° 37′ - 23° 52′ N to 86° 06′ - 86° 30′ E has a total area of 450 km2 (Mishra and Cook, 1992) and forms a half-graben structure (Basu and Shrivastava, 1981; Mishra and Cook, 1992). The basinal axis plunges towards the west with an east-west trend, and the southern boundary fault deformed the southern flank of this Basin with a throw of ~1500 m (GSI, 1977; Mishra and Cook, 1992; Verma, 1983). The stratigraphic successions of this Basin are depicted in Table 2. Sikkim State (27° 05′ N to 28° 08′ N; 88°10′ E to 88° 55′ E) surrounded by the southern ranges of the Eastern Himalayan mountain belts has the areal extent over 7096 km2 (Ghosh et al., 2018b; GSI, 2012). The compressional forces had deformed the earlier thrust systems and developed the Himalayan fold-thrust belts (Bhattacharyya and Mitra, 2009; Gansser, 1964; Ray, 1995; Valdiya, 1980). The Teesta and Rangit rivers eroded this area and shaped the Teesta half window (Schwan, 1980). The ‘Lesser Himalayan Sequence’ exposes the rocks that had experienced greenschist facies metamorphism at this half window (Bhattacharyya and Mitra, 2011). Buxa and Daling rocks along with the Gondwana lithology were repeated multiple times in this sequence, a portion of which is exposed as Rangit duplex at the Rangit window (Fig. 3) lying within that half window (Bhattacharyya et al., 2006). The intense tectonism had enhanced the rank of the coal to anthracite A and influenced the microstructural characteristics, significantly (Ghosh et al., 2018b). The general stratigraphic layout within the Sikkim Himalayan folded thrust belts is shown in Table 3.

3. Material and methods 3.1. Field work Table 4 depicts the sampling sites of the Raniganj and the Jharia Basin coals, which are also marked in Figs. 1 and 2. The details of the anthracite sample collection from the fold-thrust belts of Sikkim Himalayan ranges and lithological descriptions had been discussed by Ghosh et al. (2018b). A total of seven (7) coal samples were collected from the Himalayan fold-thrust belts of Sikkim from Jorethang-NamchiKamling-Sikkip-Reshi towns of the west and south Sikkim. CG3300, CG3301, and CG3302 samples were collected from the Jorethang area and CG3303 was sampled near Namchi at South Sikkim (Fig. 3). Sample CG3304 was collected from the north of Kamling (Fig. 3). Furthermore, CG3305 and CG3306 were sampled near the Reshi Khola in Reshi town (Ghosh et al., 2018b) (Fig. 3).

4

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Table 2 Stratigraphic successions of the Jharia Basin (following Chandra, 1992; Fox, 1930; GSI, 1977; Murthy et al., 2015; Patra et al., 2018). Period Jurassic or Tertiary Lower Jurassic Upper Permian Middle Permian Lower Permian

Epoch

Lopingian (260 - 252.2 Ma) Guadalupian (271 - 260 Ma) Cisularian (299 - 271 Ma)

Formation

Raniganj Barren Measures Barakar Talchir

Prceambrian

Lithology Thickness Dolerite dykes Mica lamprophyre dykes and sills Fine- grained feldspathic sandstones, shales with coal seams Buff colored sandstones, shales and carbonaceous shales Buff colored coarse and medium grained feldspathic sandstones, grits, shales, carbonaceous shales and coal seams Greenish shale and line grained sandstones Meiamorphic rocks

800 m 730 m 1250 m 245 m

δ13CV-PDB = −10.43‰). IAEA-CH-6 is an inter-laboratory comparison standard distributed by the International Atomic Energy Agency (IAEA), Vienna. Further, the reference material used for nitrogen isotope analysis was IA-R001 (wheat flour, δ15NAIR = 2.55‰). For quality control purposes check samples of IA-R001, IA-R045 (ammonium sulfate, δ15NAIR = −4.71‰), IA-R046 (ammonium sulfate, δ15NAIR = 22.04‰) and IA-R069 (Tuna protein formula, δ15NAIR = 11.60‰) were analyzed during the batch analysis of the samples. These IA-R001, IA-R045, IAR046 and IA-R069 are, again, calibrated against and traceable to IAEAN-1 (ammonium sulfate, δ15NAIR = 0.4‰). IAEA-N-1 is, also, an interlaboratory comparison standard distributed by the IAEA, Vienna. In between every 8 samples, blanks, as well as internal standards, were run. The precision of these analyses (10 replicated standard samples) was 0.3% for C and 0.06‰ for δ13C along with 0.03% for N and 0.18‰ for δ15N. Twenty percent (20%) of the samples were, further, analyzed as duplicate. The duplicate analyses had reproducibility of 0.1‰.

3.2. Stable carbon and nitrogen (δ13C and δ15N) isotope analyses A Euro EA 3024 elemental analyzer was employed to determine the total organic carbon (TOC) and total nitrogen (TN) amounts within the studied bituminous and anthracite samples. The stable carbon and nitrogen (δ13C and δ15N, respectively) isotopes were analyzed in Elemental Analyzer Isotope Ratio Mass Spectrometry (EA-IRMS) at the Iso-Analytical in the United Kingdom. All the samples were mixed with hydrochloric acid (HCl) for removal of the carbonates before the TOC and δ13C determination. The TOC and TN data were expressed in weight percentage (wt%), while the δ13C and δ15N values were reported in per-mile (‰). The stable isotopes of carbon and nitrogen were calculated following eq. (1):

δ(%0) = [(R sample/R standard) − 1]× 103

Thickness

(1)

where Rsample for C was the 13C/12C values of the samples, Rstandard was the 13C/12C value of Vienna Pee Dee Belemnite (V-PDB), and for N, Rsample represented the 15N/14N values of the samples and Rstandard was the 15N/14N value of atmospheric N2. For carbon isotope analysis, the reference material was IA-R001 (wheat flour, δ13CV-PDB = of −26.43‰). Check samples of IA-R001, IAR005 (beet sugar, δ13CV-PDB = −26.03‰) and IA-R006 (cane sugar, δ13CV-PDB = −11.64‰) were analyzed for quality control purposes during the batch analysis of the samples. IA-R001, IA-R005 and IAR006 were calibrated against and traceable to the IAEA-CH-6 (sucrose,

3.3. X-ray photoelectron spectroscopic analysis (XPS) The X-ray photoelectron spectroscopic technique (XPS) of the samples was carried out at ‘Surface Characterization Lab’ in Indian Institute of Technology, Kanpur, India housing an “X-ray PhotoElectron Spectroscopy (XPS) with Auger Electron Spectroscopy (AES) module and C60 sputter gun” supplied by PHI 5000Versa Prob II, FEI,

Fig. 3. Map of the Rangit window, Sikkim Himalayan fold-thrust belts (following Bhattacharyya and Mitra, 2009). Explanations: RT = Ramgarh Thrust; TT = Tatapani Thrust; SKT = Sikkip Thrust; DT = Dong Thrust; JT = Jorethang Thrust; ST = Sorok Thrust; KT = Kitam Thrust; NH = Namchi Horse; RR = Rangit River. = Sampling locations; J = Jorethang; N = Namchi; K = Kamling; R = Reshi. 5

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Table 3 Generalized stratigraphic succession of Sikkim Himalaya (following GSI, 2012).

Table 4 Sampling sites of coals from the Raniganj and the Jharia Basins. Sample No.

Formation

Basin

CG1671 CG1672 CG1673 CG1674 CG1675 CG1676 CG1677 CG1679

Raniganj Barakar Barakar Raniganj Barakar Barakar Barakar Barakar

Raniganj Raniganj Jharia Jharia Jharia Jharia Jharia Jharia

Table 5 The mean random vitrinite reflectance (MVRO), stable isotopes of carbon (δ13C) and nitrogen (δ15N) along with TOC/TN ratio of the studied samples.

INC. The samples were irradiated by the X-rays to record the XPS signal. The number of electrons and the kinetic energy that escaped from the top surface to 7.5 nm of the samples was considered for recording the signal. A 100 W X-ray beam with 100-μm diameter was used to scan 1.4 mm in the non-dispersive direction of the X-ray monochromator at high velocity providing a rectangular large area having high sensitivity and resolution. Versaprobe chemical state maps were constructed through scanning the X-ray beam in each pixel over a defined areal extent. The spatial resolution of the spectroscopy was less than 10 μm. The binding energy signal of N1s spectra was calibrated using the C1s binding energy at the 285 eV (maxima of the principal C1s spectra). The raw N1s spectra were deconvoluted using the Gaussian function in Fityk 1.3.1. The intensities and the full width at the half maxima of the peaks were not restricted for obtaining the best fit of the N1s signal.

Sample No.

MVRO (%)

δ13C (‰)

δ15N (‰)

TOC/TN

Ranka

CG1671 CG1672 CG1373 CG1674 CG1675 CG1676 CG1677 CG1679 CG3300 CG3301 CG3302 CG3303 CG3304 CG3305 CG3306

0.65 0.58 0.95 1.09 0.90 0.76 1.00 1.00 5.12 4.94 5.15 5.36 4.11 4.15 4.73

−23.13 −24.17 −22.93 −21.67 −22.85 −24.08 −22.28 −23.24 −23.91 −21.39 −21.96 −22.57 −22.58 −22.96 −22.68

2.01 2.03 2.52 2.35 2.65 2.47 2.81 1.64 1.07 1.63 2.52 2.64 3.44 2.86 2.34

35.35 39.11 40.63 43.05 39.89 33.49 36.87 43.69 206.32 76.92 34.74 58.32 40.02 74.21 234.01

Bituminous C Bituminous D Bituminous C Bituminous B Bituminous C Bituminous C Bituminous B Bituminous B Anthracite Ab Anthracite Ab Anthracite Ab Anthracite Ab Anthracite Ab Anthracite Ab Anthracite Ab

Explanations: a Rank is mentioned following ISO (2005). b data reported from Ghosh et al. (2018b).

from 33.49 to 43.69, while in the anthracite A samples, this parameter is placed within 34.74 and 234.01 (Table 5). The δ13C parameter of the bituminous coal samples (CG1671–CG1679) varies from −24.08 to −21.67‰, while in the anthracite A samples (CG300–CG3306), it ranges between −23.91 and −21.39‰ (Table 5). The bituminous coal samples, on the other hand, exhibit shifts of δ15N in between 1.96 and 2.56‰, whereas the anthracite A samples reveal the δ15N variation from 0.36 to 2.47‰ (Table 5).

4. Results 4.1. δ13C, δ15N and TOC/TN ratio The TOC/TN ratio in the bituminous sample set shows the variation 6

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sources (Lamb et al., 2006). The TOC/TN ratio (33–44; Table 5) in the bituminous coal samples, hence, may indicate the terrestrial higher plant sources of the OM substantiating the discussion of δ13C and δ15N parameters (subsection 5.2). This indicates a significant contribution from the vascular plant tissues from conifers, ferns and other macrophytes that flourished during the development of the peat mire in the Permian period. The TOC/TN ratio is also used as a tool to trace the diagenetic alterations of peat OM (Kuhry and Vitt, 1996). Aerobic decomposition in acrotelm owing to the fluctuating water table may cause preferential loss of nitrogen and could enhance the TOC/TN ratio in the bulk peat OM. On the contrary, anaerobic microbial decomposition, especially methanogenesis, may cause a decline in the TOC/TN ratio (Kuhry and Vitt, 1996). This process occurs in catotelm at relatively stable groundwater level conditions. Further, a strong correlation between the δ13C and TOC/TN ratio may suggest a considerable influence of microbial degradation on stable isotope source signatures (Herczeg, 1988; Jones et al., 2010; Meyers, 1997; Sharma et al., 2005). The weak correlation between the δ13C and TOC/TN ratio in the studied samples suggests an insignificant influence of microbial decomposition on the δ13C. However, physicochemical changes of peat OM during the coalification process could overwrite the imprints of the microbial decomposition in the high-rank coals, and the relationship between stable isotopes (δ13C and δ15N) and elemental (TOC/TN) ratio in bituminous and anthracite may reflect the influence of coalification on the stable isotope signature although insignificant changes in the δ13C are linked to the limited expulsion of methane during coalification (Sackett, 1978; Whiticar, 1996). Weak relationships between the δ13C and TOC/TN, δ15N and TOC/TN and between δ13C and δ15N in the bituminous (R2 = 0.19, 0.11 and 0.04, respectively) and anthracite A samples (R2 = 0.30, 0.33 and 0.13, respectively) (Fig. 6 a,b,c) may suggest preservation of the original source signature of the OM and non-linear response of the stable isotopes to coalification and post-coalification metamorphic alterations. Moreover, weak variation in the δ15N may reflect weak isotopic discrimination during hydrothermal alterations (discussed in subsection 5.6).

Table 6 Subpeak assignments of the deconvoluted XPS spectra. Subpeak

Binding energy (eV)

Structure

N-6 N-5 N-Q1 N-Q2 N-X

398.8 400.6 401.6 402.8 403.6

Pyridinic nitrogen Pyrrolic/pyridone nitrogen [3.3.3] cyclazine-type (N–C3) nitrogen Pyridinic nitrogen linked with oxygen atoms R = N–O–R, where nitrogen atom is bonded to carbon (R) and to one oxygen atom

4.2. XPS spectra The N1s peak in the X-Ray photoelectron spectra reveals five subpeaks after spectral deconvolution. These subpeaks and related assignments (following Boudou et al., 2008; Casanovas et al., 1996; Chambrion et al., 1997; Kelemen et al., 2002; Moroeng et al., 2018; Pels et al., 1995; Phiri et al., 2017, 2018; Valentim et al., 2016; Xiao et al., 2005) are presented in Table 6. The exemplary structures of these nitrogen moieties are portraited in Fig. 4. Further, the intensity along with the areal percentage of each subpeak within the N1s peak is depicted in Table 7. The representative deconvoluted XPS spectra of bituminous coal (CG1675) and the anthracite A (CG3306) samples are presented in Fig. 5 a and b. The intensity and the areal percentage of the pyridinic nitrogen species show little increment towards the anthracite A samples (CG3300, CG3303, CG3305, CG3306) from the coal samples of the Raniganj and the Jharia Basins (CG1671, CG1673, CG1675, CG1679). Meanwhile, the subpeak intensity and the percentage of pyrrolic nitrogen decline, sharply, in the anthracite A samples. On the other hand, the [3.3.3] cyclazine type nitrogen (N-Q1) subpeak intensifies, steeply, in the anthracite A samples but the relative area does not exhibit substantial changes throughout the samples. The pyridinic nitrogen that is linked to oxygen atoms reveals declination in the peak intensities and the relative area towards the anthracite A samples. Finally, although, the oxidized nitrogen peak (N-X) shows an increase in the intensity towards the anthracite A samples (except CG3306), the relative area of this subpeak declines in these samples.

5.2. Atypical range of TOC/TN ratio in anthracite A samples Nitrogen, being a labile element and located at the edges of the heterocyclic compounds, is selectively and gradually removed through thermochemical reactions with an increasing degree of coalification. The depletion of N affects the TOC/TN ratio in high-rank coals, especially in the anthracite, as reflected in the studied anthracite samples (CG3300 and CG3306; Fig. 7). Interestingly, the TOC/TN values are not significantly higher in anthracite samples than that of bituminous coal samples. This may be caused by the incorporation of N within the heterocyclic and aromatic structures. In addition, amides and amines transform into pyridinic–N in association with heterocyclic pyrrolic–N during coalification (Kelemen et al., 1994; 2006; Geng et al., 2009; Valentim et al., 2011). During the courses of rank advancement, the protonation of this pyridinic–N forms the stable quaternary-N structures (Kelemen et al., 1994; Valentim et al., 2011). This, along with the

5. Discussions 5.1. TOC/TN ratio The TOC/TN parameter is applied extensively to interpret the OM sources (Das et al., 2008; Meyers, 1994; Meyers and Ishiwatari, 1993; Lamb et al., 2006). The TOC/TN ratio ranges from 6 to 9 in OM derived from plankton or algal biomass, 4–10 for aquatic nonvascular plants and > 15 for terrestrial vascular plants (Meyers, 1994; Meyers and Ishiwatari, 1993; Sampei and Matsumoto, 2001; Perdue and Koprivnjak, 2007). Organic matter derived from the terrestrial plants contains a substantial amount of carbon-rich lignocellulosic compounds and hence, is poor in nitrogen compared to the algal or planktonic

Fig. 4. Major nitrogen (N) functionalities detected in N1s XPS spectra of the samples (a) pyridinic -N, (b) pyrrolic-N, (c) graphitic-N. 7

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Table 7 Intensity and areal percentages of the subpeaks within the N1s spectra. Sample no.

CG1671 CG1673 CG1675 CG1679 CG3300 CG3303 CG3305 CG3306

Intensity

Areal percentage of subpeak within the N1s peak (%)

N-6

N-5

N-Q1

N-Q2

N-X

N-6

N-5

N-Q1

N-Q2

N-X

314.99 319.67 339.07 355.33 398.66 359.61 439.33 362.66

164.75 148.94 183.84 163.96 104.45 103.15 103.12 105.20

243.06 263.66 274.49 263.14 349.66 304.33 360.02 330.66

130.23 161.84 182.74 166.13 78.97 77.28 79.72 76.71

241.82 274.66 306.66 291.72 330.05 297.24 326.38 290.24

40.87 41.86 38.94 42.79 48.98 50.81 56.25 49.22

11.66 9.96 12.23 10.36 6.326 6.54 5.69 6.89

20.58 19.74 21.64 19.89 23.18 21.64 21.16 24.01

5.79 6.89 7.06 6.77 3.72 3.51 3.07 3.16

21.01 21.54 20.12 20.17 17.77 17.48 13.8 16.70

lighter δ15N signal (0–2‰) in comparison to the planktonic sources (δ15N = +8‰). The δ15N in the studied samples does not vary significantly with increasing coal rank (Fig. 8b). Instead, the δ15N for bituminous (1.64‰−2.81‰) and anthracite A samples (1.07 ‰–3.44‰; Table 2) suggest the terrestrial source of OM (Zheng et al., 2015). The δ15N in the terrestrial higher plants, further, declines with increasing average annual rainfall (Aranibar et al., 2004; Houlton et al., 2007). Changes in water use efficiency and increased precipitation during the long-wet season may lead to higher fractionation of the lighter 14N and result in low δ15N values (Craine et al., 2009; Houlton et al., 2007; Peri et al., 2012). Therefore, the δ15N values in the studied samples suggest semiarid to arid climatic spell with occasional rainfall during the formation of the peat mire. The plausible reasons behind the non-substantial shifts of this isotopic parameter in the anthracite A samples have further been discussed in subsection 5.6.

conversion of pyrrolic–N to graphitic–N may enhance the intensity of aromatic cyclazine structures in the anthracite samples. Further, the expansion of pyrrolic rings to pyridines (Pels et al., 1995) may conserve the nitrogen atoms within the coal microstructure. The preservation of the nitrogen in these partially condensed aromatic structures may lead to unusual TOC/TN values, which are almost similar to that of the bituminous coal samples, as discussed above. The loss of nitrogen may further cause a substantial increase in the ratio in CG3300 and CG3306 (Table 5) during the oxidation of N-containing structures at high temperature during anthracitization. 5.3. Stable carbon (δ13C) and nitrogen (δ15N) isotopic parameters The δ13C remains mostly unaffected by the coalification process even up to the meta-anthracite rank due to the presence of a large amount of carbon in coal and insignificant loss of the lighter carbon isotope through methane escape (Sackett, 1978; Whiticar, 1996). Moreover, the coalification driven variation of δ13C is restricted within ~5‰ shift from −22 to −27‰. Hence, the OM source signatures of δ13C remain unaltered (Compston, 1960; Whiticar, 1996). The δ13C of the OM derived from the C3 terrestrial vascular plants ranges between −22‰ and −33‰, and for C3 aquatic plants, it varies from - 13‰ to −27‰ (Whiticar, 1996). Compared to the C3 plants, the C4 plants show a heavier δ13C signature (−8‰ to −18‰) due to the incorporation of heavier carbon isotope during photosynthesis. The OM derived from vegetation exhibiting Crassulacean Acid Metabolism (CAM) in dry and warm climate conditions depicts the intermediate carbon isotopic signature of the C3 and the C4 plants (Whiticar, 1996). In the present study, the vast range and negligible shift of the δ13C values (Table 5; Fig. 8a) imply that this parameter has not been significantly affected by the thermal maturity, and retains the original OM source signature. The δ13C values of both the bituminous and anthracite A samples may, therefore, suggest terrestrial C3 vegetation source of the OM. The OM derived from the terrestrial plants would like to depict a

5.4. Nitrogen functionalities identified in the present study 5.4.1. N–6 (Pyridinic–N) subpeak The pyridinic nitrogen (Fig. 4a) corresponding to the N–6 subpeak within the N1s peak of the XPS spectra is a heterocyclic compound, where one methyne group (=CH–) is substituted by the N atom. This N atom is attached to two C atoms in the aromatic π-electron system (Boudou et al., 2008; Pels et al., 1995). Pyridine is characterized by a conjugated ring system with 6 delocalized π-electrons and an uneven distribution of electron density over the ring. In this type of structure, the N atom donates one unhybridized p-orbital to the aromatic πelectron system and projects a lone pair of electrons of sp2 hybridized orbital in the plane of the aromatic ring (Pels et al., 1995). 5.4.2. N–5 (Pyrrolic–N) subpeak The pyrrolic nitrogen (Fig. 4b) attributed to the N–5 subpeak, is, again, a heterocyclic compound, where an N atom replaces one C atom and is linked to one H and a couple of C atoms (Boudou et al., 2008).

Fig. 5. Fitted XPS spectra of (a) CG1675 (representative of the bituminous coal samples) and (b) CG3306 (representative of the anthracite A samples). 8

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Fig. 6. Cross-plots between (a) TOC/TN vs. δ13C, (b) TOC/TN vs. δ15N and (c) δ13C and δ15N parameters.

Fig. 7. TOC/TN ratio of the studied bituminous and anthracite A samples (modified after Boudou et al., 2008).

Although Mitra-Kirtley et al. (1993) reported the occurrences of pyridon-N in the coals, Kelemen et al. (1994) observed the peak of this pyridon-N at ~400.6 eV. The six-membered aromatic ring system shows two tautomers amongst which the molecular structure containing the carbonyl group is the most abundant (Pels et al., 1995). The N atom, in this case, is involved in bonding with the aromatic π-electron system through contributing two unhybridized p-electrons and it is also linked with one H atom in the plane of the aromatic ring. Often, the pyridinic rings consist of N atoms and oxygenated groups (Boudou et al., 2008; Schnadt et al., 2003; Vairavamurthy and Wang, 2002; Zhu et al., 1997). These oxygenated functional groups may offer potentially active sites for chemical reactions to take place, which in turn may lead to the removal of organic N from the bulk OM (Katritzky et al., 1997; Siskin and Katritzky, 2000). Due to the similarity in the chemical environment of the pyrrolic and pyridone-N, these two compounds, often, cannot be differentiated from the XPS spectra (Boudou et al., 2008; Pels et al., 1995; Zhu et al., 1997). The pyrrolic or the pyridone-N compounds are found to be present at the edges of the condensed and partially aromatized rings (Boudou et al., 2008). The intensity of this peak remains almost unaltered during low-grade metamorphism, while it undergoes sharp declination during anthracitization and graphitization (Boudou et al., 2008). 5.4.3. N–Q1 (quaternary–N) subpeak The N–Q1 subpeak corresponds to the N–C3 ([3.3.3] cyclazine type N), in which a single N atom replaces one C atom within a partially condensed aromatic ring system (‘centre’-N) (Fig. 4c). Here, the N atom either possesses positive charge or it may be neutral and it is linked with three C atoms (Boudou et al., 2008; Pels et al., 1995). The electron charge transfer from carbon to nitrogen may contribute, in part, to the larger binding energy of the N–C3. The rise in the relative intensity of the N-Q1 subpeak with escalation of metamorphic temperature may suggest that the N–Q1 is, primarily, associated with the N–C3 in the metamorphosed samples that would have experienced the removal of carboxyl and hydroxyl groups (Blom et al., 1957; Boudou et al., 2008; van Krevelen, 1961). At the low-grade metamorphic stage, this quaternary nitrogen may represent a type of pyridinic nitrogen with adjacent –OH or –COOH groups protonated by the formation of hydrogen bridges (Boudou et al., 2008; Kelemen et al., 1999). 5.4.4. N–Q2 and N–X subpeaks The assignment of the N–Q2 and the N–X subpeaks had been interpreted to be uncertain (Boudou et al., 2008). The former one may correspond to fixed ammonium ions within the clays (Buckley et al., 1995, 1996; Gong et al., 1999). The kerogen, usually, lacks any silicate mineral that has the possibility of hosting ammonium and thus, this 9

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Fig. 8. (a) Stable carbon (δ13C) and (b) nitrogen (δ15N) isotopic variations in the studied samples. Explanations: Vertical axis of both the diagram represents the sample number (1–15).

peak may not have arisen from the NH4+ fixed in clays rather it may be attributed to the pyridinic nitrogen linked with oxygen atoms (Boudou et al., 2008). Further, the N–X subpeak may be assigned to the oxidized nitrogen (Pels et al., 1995).

to hydroxyl and carboxyl groups in the bituminous coal samples (low metamorphic grade) (Kelemen et al., 1999), with increasing thermal maturity (anchizonal metamorphism) and consequent dehydroxylation and decarboxylation, the N atom would have been emplaced within the partially condensed ring system, preferably as the ‘centre’-N forming the [3.3.3] cyclazine structure. This may explain the increase in the intensity and relative area of the N-Q1 subpeak in the anthracite A samples. Further, the ratio of the N-Q1 intensity to the sum of the intensities corresponding to N-5 and N6 ([IN1/(IN-5+IN-6)]) shows a strong positive correlation (r = 0.95) with the MVRO (Fig. 10a), which, further, confirms the transition from the low metamorphic grade to anchizonal metamorphism, where the cyclazine structure becomes more dominant, possibly because of thermal decomposition of pyrrolic-N and protonation of the pyridinicN. Additionally, the ratio of relative area of the N-Q1within the N1s peak to that of the N-5 subpeak is strongly correlated (r = 0.96) with the MVRO (Fig. 10b), which may imply that pyrrolic-N would have been quaternarized to cyclazine type N with the advent of coal rank or increase in metamorphic grade owing to thermochemical degradation. In complementary, the intensity and the relative area ratio of pyridinic to pyrrolic nitrogen are strongly correlated (r = 0.92 and 0.89, respectively) with the MVRO (Fig. 10 c and d, respectively). The ring expansion of the pyrrolic-N due to rank advancement might have contributed to the pyridinic-N enrichment (Pels et al., 1995; Valentim et al., 2011). Hence, the pyrrolic-N is interpreted to contribute to the formation of both pyridinic and cyclazine-N, significantly, in the anthracite A samples. Interestingly, the intensity of the N-Q2 sharply decreases (r = −0.93) with the increase in thermal maturity (Fig. 11a). Further, the ratio of the relative area of the N-Q1 in the N1s peak exhibits increment over that of N-Q2 in the anthracite A samples (r = 0.93) from the bituminous coal samples (Fig. 11b). This may indicate that due to thermochemical reactions at high temperature, the pyridines linked

5.5. Metamorphic transformations of the nitrogen functionalities The peak intensity of the pyridinic nitrogen (N-6) shows a positive trend (r = 0.68) with the MVRO. The relative abundance of this heterocyclic group within the N1s peak exhibits, surprisingly, a sharp increment (r = 0.86) from the bituminous coal samples towards the anthracite A samples (Fig. 9 a and b). This is just the opposite situation that was observed by Boudou et al. (2008) in their study. Miao et al. (2017) suggested that the pyridinic-N content of N-doped graphene increased with the increment of the annealing temperature below 900 °C associated with a declination of pyrrolic-N (N-5) content. Further, the pyridinic nitrogen content may increase with the increasing maturation due to oxygen atom detachment from pyridones (Kelemen et al., 1994, 2006; Geng et al., 2009; Valentim et al., 2011). Here, with the advent the coal rank, the intensity of the pyrrolic nitrogen reveals a steep drop (r = −0.95), which is compounded by the sharp decline (r = −0.94) in the relative area of the pyrrolic-N within the N1s peak (Fig. 9 a and b). The pyrrolic-N possesses π-electron excessive heterocyclic structure and it behaves as nucleophiles. Therefore, it is extremely reactive and prone to attack the electrophile elements. Also, this structure becomes unstable at a high temperature and converts to other N containing compounds (Miao et al., 2017). These may, hence, explain the strong inverse relation of the pyrrolic-N intensity and relative area with the MVRO (%). Further, the peak intensity of the N-Q1 subpeak, as well as its relative area within the N1s spectra, are positively correlated (r = 0.86 and r = 0.73, respectively) with the MVRO (Fig. 9 a and b), which is in line with the work of Boudou et al. (2008) among others. Although existing as a form of pyridinic nitrogen linked

Fig. 9. (a) Correlation between the intensity of the pyridinic, pyrrolic and graphitic nitrogen subpeaks with mean random vitrinite reflectance (MVRO); (b) Relation between the relative area of the subpeaks with the mean random vitrinite reflectance (MVRO). 10

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Fig. 10. Correlation between mean random vitrinite reflectance (MVRO) and (a) Intensity of N-Q1 subpeak to the total intensities of N-6 and N-5 subpeaks; (b) relative area ratio of N-1 to N-5; (c) relative area ratio of N-6 to N-5; (d) intensity ratio of N-6 to N-5 subpeak.

spectroscopy (XPS) in Table 7 is independent of the TOC/TN ratio in Table 5. Hence, this may explain why the large TOC/TN values in CG3300 and CG3306 had not influenced the distributions of the nitrogen-bearing moieties in these two samples.

with O atoms might have converted to the quaternary-N with accelerated aromaticity and condensation of the aromatic rings in the anthracite A samples. Moreover, the N-X subpeak intensity is observed to reduce in the anthracite A samples from that in the bituminous coal samples. These may point towards the transformation of reactive oxidized nitrogen compounds into the quaternary cyclazine structures in the anthracite A samples at the relatively high temperature. Hence, with an increase in the degree of coalification (bituminous to anthracite), the unstable N-containing chemical functionalities within the chemical framework of the coal samples are modified to thermally stable inert partially condensed aromatic structures and the preservation of the organic nitrogen in those structures would have led to the unusual TOC/TN ratio in the anthracite A samples. The intensities and areal percentages of the pyridinic-N, pyrrolic-N, cyclazine-N and pyridines linked with oxygen atom and oxidized nitrogen moieties under the N1s spectra are independent from the total nitrogen concentration. Table 7 represents the distribution of nitrogen bearing moieties and their transformations throughout the samples irrespective of the total nitrogen content. Hence, deconvolution and quantification of the N1s spectra from the X-ray photoelectron

5.6. Effect of hydrothermal activity in geochemical signature of anthracite A samples Daniels et al. (1990) have reported hydrothermal alteration in anthracite from eastern Pennsylvania. Hower et al. (2019) found devolatilized megasporinites, and structureless vitrinites with devolatilization pores filled up with pyrolytic carbon in the Pennsylvania anthracites and linked it to the local scale hydrothermal alterations. Ghosh et al. (2018b) observed microstructural defects in the anthracite samples from Sikkim (same samples used in the present study) by using vitrinite reflectance anisotropy and Raman spectroscopy. The authors suggested tectonic episodes in Himalaya and consequent hydrothermal activities during the Greenschist facies metamorphism as the causes for the development of the microstructural defects. Interestingly, the deformation in the anthracite A samples could facilitate hydrothermal

Fig. 11. Relation between mean random vitrinite reflectance (MVRO) and (a) intensity of N-Q2 subpeak; (b) relative ratio of N-Q1 to N-Q2. 11

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fluid movements through the secondary fractures to reach to the labile reactive framework composed of aliphatic and heteroalipahtic compounds during regional metamorphic events. The thermochemical reactions introduced by the hydrothermal fluids could affect the labile organic N-containing moieties and generate NH3, N2 and oxides as indicated by the sharp peaks of N-X in the N1s XPS spectra of the anthracite A samples (Fig. 5b). As explained in subsection 5.5, the pyrrolic-N structures possess strong nucleophile characteristics and are prone to react with the electrophiles in the hydrothermal fluids. These reactions would have altered the labile pyrrolic structures and converted them into relatively more stable chemical moieties like pyridines and cyclazine. The thermochemical degradation of the pyrrolic-N would have enhanced the concentrations of the stable graphitic-N moieties in the anthracite rank. Moreover, some of the pyridinic-N would also have converted to stable cyclazine structures by protonation. Nitrogen atoms liberated from the labile structures during these reactions were encapsulated within the partially condensed aromatic rings of the quaternary moieties. These reactions, therefore, would have intensified the cyclazine structures as well as enhanced their relative area ratio with an increase in coal rank (Fig. 10 a, b). Further, the conversion of pyrrolic-N to pyridinic-N through ring expansion process might also result in an increase in the intensity and relative area of the pyridinic-N in the anthracite rank (Fig. 10 c, d). In complementary, the intensity of the pyridinic-nitrogen linked with oxygen atom is also observed to get lowered in the anthracite rank due to the plausible conversion of this relatively labile structure into a more stable cyclazine compound during hydrothermal metamorphism (Fig. 11 a, b). Intense graphitic-N (N-C3) peak in the anthracite A samples, compared to the bituminous coal samples, may reflect increasing thermochemical stability with increasing metamorphic degree (Fig. 10 a–d). The plausible scenario of this phenomenon would be like these: the organic nitrogen atoms became increasingly encapsulated and were conserved in the thermally stable cyclazine structures and partially in pyridinic forms. The invasion capability and reaction intensity of the hydrothermal fluids slowed down with increasing hydrophobic surface characteristics of the relatively inert aromatic stackings (Boudou et al., 2008). The greater stability of the aromatized moieties within these aromatic stackings may prevent the removal of the lighter nitrogen fractions leaving the stable isotope signature of the nitrogen unaffected by the hydrothermal alterations. Hence, the conserved nitrogen in the stable chemical structures may represent the bulk δ15N values of the studied anthracite A samples. These observations are in good agreement with the findings reported in Westphalian anthracites from the Western Middle field of Pennsylvania (U.S.A.) and the Bramsche Massif (Germany) (Ader et al., 1998), different regions of highly metamorphosed anthracitic, meta-anthracitic and graphitic terrains in U.S. A., China, Korea, Germany (Ader et al., 2006), coals from the Mahakam delta (Indonesia) (Boudou et al., 1984 a,b) and in type III kerogens (Whiticar, 1996).

would indicate preservation of the organic nitrogen within the thermochemically stable partially condensed aromatic and heterocyclic structures. Moreover, organic nitrogen was preserved within the anthracite microstructure with unaltered isotopic signature (δ15N) during the hydrothermal metamorphism. Acknowledgments The authors are indebted to Dr. Quanyou Liu, Associate Editor, Journal of Marine and Petroleum Geology and the Learned Reviewers for their valuable suggestions to upgrade the quality of the manuscript. The authors are thankful to the ‘Surface Characterization Lab’ in Indian Institute of Technology-Kanpur, India for XPS analysis of the samples. They would, further, like to extend their gratitude to Prof. Joan Esterle and Dr Sandra Rodrigues, School of Earth and Environmental Sciences, The University of Queensland, Australia for their guidance in vitrinite reflectance analysis of the anthracite A samples as well as to Dr. B.D. Singh, Scientist, Birbal Sahni Institute of Palaeosciences, Lucknow, India for vitrinite reflectance measurements of the bituminous coal samples. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.marpetgeo.2019.104090. References Ader, M., Boudou, J.-P., Javoy, M., Goffé, B., Daniels, E., 1998. Isotope study on organic nitrogen of Westphalian anthracites from the Western Middle field of Pennsylvania (U.S.A.) and from the Bramsche Massif (Germany). Org. Geochem. 29, 315–323. Ader, M., Cartigny, P., Boudou, J.-P., Petit, E., Oh, J.H., Javoy, M., 2006. Nitrogen isotopic evolution of carbonaceous matter during metamorphism: methodology and preliminary results. Chem. Geol. 232, 152–169. Ahmed, M.S., Jeon, S., 2014. Highly active graphene-supported NixP d100-x binary alloyed catalysts for electro-oxidation of ethanol in an alkaline media. Am. Chem. Soc. Catalysis 4, 1830–1837. Aranibar, J.N., Otter, L., Macko, S.A., Feral, C.J.W., Epstein, H.E., Dowty, P.R., Eckardt, F., Shugart, H.H., Swap, R.J., 2004. Nitrogen cycling in the soil-plant system along a precipitation gradient in the Kalahari sands. Glob. Chang. Biol. 10, 359–373. Bai, J., Zhu, Q., Lv, Z., Dong, H., Yu, J., Dong, L., 2013. Nitrogen-doped graphene as catalysts and catalyst supports for oxygen reduction in both acidic and alkaline solutions. Int. J. Hydrogen Energy 38, 1413–1418. Basu, T.N., Shrivastava, B.B.P., 1981. Structure and tectonics of Gondwana Basins of India. In: Cresswell, M.M., Vella, P. (Eds.), Int. Gondwana Symp. 5th (Wellington, NewZealand). Balkema, Rotterdam, pp. 172–182. Bhattacharyya, K., Mitra, G., 2009. A new kinematic evolutionary model for the growth of a duplex: an example from the Rangit duplex, Sikkim Himalaya, India. Gondwana Res. 16, 697–715. Bhattacharyya, K., Mitra, G., 2011. Strain softening along the MCT zone from the Sikkim Himalaya: relative roles of Quartz and Micas. J. Struct. Geol. 33, 1105–1121. Bhattacharyya, K., Mitra, G., Mukul, M., 2006. The geometry and implications of a foreland dipping duplex, the Rangit Duplex, Darjeeling–Sikkim Himalayas, India. Geol. Soc. Am. Abstract Progr. 38, 413. Blom, L., Edelhausen, L., van Krevelen, D.W., 1957. Chemical structure and properties of coal. XVIII. Oxygen groups in coal and related products. Fuel 36, 135–153. Boudou, J.-P., Mariotti, A., Oudin, J.L., 1984a. Unexpected enrichment of nitrogen during the diagenetic evolution of sedimentary organic matter. Fuel 63, 1508–1510. Boudou, J.-P., Pelet, R., Letolle, R., 1984b. A model of the diagenetic evolution of coaly sedimentary organic matter. Geochem. Cosmochim. Acta 48, 1357–1362. Boudou, J.-P., Schimmelmann, A., Ader, M., Mastalerz, M., Sebilo, M., Gengembre, L., 2008. Organic nitrogen chemistry during low-grade metamorphism. Geochem. Cosmochim. Acta 72, 1199–1221. Buckley, A.N., Kelly, M.D., Nelson, P.F., Riley, K.W., 1995. Inorganic nitrogen in Australian semi-anthracites; implications for determining organic nitrogen functionality in bituminous coals by X-ray photoelectron spectroscopy. Fuel Process. Technol. 43, 47–60. Buckley, A.N., Riley, K.W., Wilson, M.A., 1996. Heteroatom functionality in a high-sulfur Chinese bituminous coal. Org. Geochem. 24, 389–392. Casanovas, J., Ricart, J.M., Rubio, J., Illas, F., Jimenez-Mateos, J.M., 1996. Origin of the large N 1s binding energy in X-ray photoelectron spectra of calcined carbonaceous materials. J. Am. Chem. Soc. 118, 8071–8076. Chambrion, P., Suzuki, T., Zhang, Z.-G., Kyotani, T., Tomita, A., 1997. XPS of nitrogencontaining functional groups formed during the C-NO reaction. Energy Fuels 11, 681–685. Chandra, D., 1992. Introduction. Mineral resources of India 5. Jharia Coalfields. Geological Society of India, Gavipuram, Bangalore, 560019, pp. 4–5.

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