Geochemical characterization of coal and waste rocks from a high sulfur bearing coalfield, India: Implication for acid and metal generation

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Highlights

  • Waste rocks in Jaintia Hills coalfield contain high amounts of pyrite.

  • Blocky pyrite and efflorescent salts have the highest affinity in generating AMD.

  • Efflorescent salts temporarily control AMD.

  • Mn is highly bioavailable in waste rocks.

  • Cadmium is highly enriched in coals with respect to crustal average.

Abstract

An integrated study on coal and mine wastes from the Jaintia Hills coalfield of Meghalaya, India involving mineralogy, acid base accounting (ABA), and net acid generation (NAG) potential and sequential leaching was undertaken to examine their potential in controlling the acid mine drainage (AMD). Mineralogical study revealed that pyrite is the major sulfide mineral in coal and mine waste, being more abundant in sandstone and carbonaceous shale; while, dolomite and calcite are abundant in a few shale and siltstones, and Fe and Al copiapites are enriched in sulfate salts. During the ABA test, all coals and > 50% of mine waste showed paste pH < 4, implying their acid generating nature. Further, the relations between net neutralization potential (NNP) and acid producing ratio (APR) revealed that blocky pyrite, pyritiferrous sandstone and efflorescent salts contribute acid much higher than coal, siltstone and carbonaceous shale. This inference is consistent with the NAG test. Partitioning of metals in mine waste indicates high proportions of them in the blocky pyrite being bioavailable than from other rocks. While Mn is highly bioavailable, major portions of Pb and Zn are in the reducible and oxidizable fractions. In efflorescent sulfate salts, > 80% of metals are available in water soluble fraction; thus these minerals can be considered as the highest polluting residue in the mining environment. These minerals easily precipitate from AMD solution in dry periods and can re-dissolve under rain events because of their high solubility; therefore, they can play an important role in controlling the chemistry of mine drainage in regard to local climate change. In the case of coal, only small fractions of Mn, Ni, Zn, Cd, and Pb are released to the environment, though high proportions of them can become bioavailable under oxidizing conditions besides their other bioavailable forms.

Introduction

Out of an endless list of environmental hazards confronting the mining industry worldwide, acid mine drainage (AMD) is one of the most serious ones, mainly associated with sulfide rich waste rocks and tailings (Kim and Chon, 2001, Nordstrom and Alpers, 1999, Sahoo et al., 2012a). Upon exposure to air and water these materials undergo aqueous and atmospheric oxidation and tend to acidify waters that contaminate the surface and sub-surface waters (Equeenuddin et al., 2010, Nordstrom and Alpers, 1999, Sahoo et al., 2012a). Control of AMD can be potentially achieved by inhibiting acid generation from mine materials. In coal deposits, acid-generating sulfide minerals, such as pyrite and marcasite, are either intergrown with or occur in close proximity to a variety of carbonate and aluminosilicate minerals. Thus, assessment of acid generating potential of each lithologic unit during and before mining is one of the key points required for effective management of AMD as well as mine wastes, especially if containing substantial amounts of pyrite (Jambor et al., 2003). The production of AMD is a function of the balance between the rates of acid production by sulfide mineral oxidation and buffering potential of the host rock (Jambor et al., 2002, Moon et al., 2008, White et al., 1999). This can be achieved by using the acid base accounting (ABA) test (Sobek et al., 1978) and/or in combination with the net acid generation (NAG) test (Miller, 1998, Schafer, 2000, Stewart et al., 2003). Nevertheless, understanding of the waste rock mineralogy and morphology of sulfide minerals is very much important to predict AMD generation because they strongly influence sulfide oxidation (Parbhakar-Fox et al., 2013, Parbhakar-Fox et al., 2014, Weisener and Weber, 2010).

Acid mine drainage also results in considerable mobility of metals present in trace quantities in the ore and waste materials, some of which are toxic. However, total concentration of elements provides only partial information about this impact, because their mobility and bioavailability are strongly influenced by the chemical speciation of metals in the mine materials (Concas et al., 2006, Pérez-López et al., 2008). Nevertheless, oxidation of pyrite can release a set of potentially toxic metals originally contained not only within the sulfides but also within the coal and host-rock minerals (Carbone et al., 2012). Therefore, the knowledge of the metal occurrence and their chemical association in mine materials and the ability to release into the aqueous environment are of great importance to understand the potential AMD impact of mine materials as well as the complex geochemical mobility of elements on the mine environment (Dold, 2003). Sequential extraction of metals is commonly carried out to understand the chemical association of metals and their mobility in mine materials. However, a number of investigators have highlighted the criticisms about the use of sequential extraction scheme (Nirel and Morel, 1990, Tessier and Campbell, 1988). The most serious criticisms are: the lack of adequate testing using standardized materials representative of field testing, the assumption that metal masses liberating using harsh inorganic reagents are representative of those that can be released by biological mechanisms under natural conditions (Ribeta et al., 1995), the poor selectivity of reagent used (Qiang et al., 1994), and the lack of quality control and possible readsorption of dissolved ions onto the solid material during the extraction (Xiao-Quan and Bin, 1993). Nevertheless, despite this limitation, sequential extraction schemes are widely used and are considered as an essential tool in establishing element fractionations from solid matrixes, which may be helpful for the assessment of contamination risk (Abollino et al., 2005, Palmer et al., 1998, Pérez-López et al., 2008). Furthermore, the mode of occurrence of elements and mineral matters in coal provide significant information about organic and inorganic associations of elements and the depositional conditions of the coal (Finkelman, 1995, Wang et al., 2008), which are important to understand the formation of AMD (Pinetown et al., 2007).

Meanwhile, evaporation of AMD in surficial environment produces efflorescent salts, consisting of a variety of metal-enriched hydrosulfates such as melanterite, rozenite, epsomite, halotrichite, and copiapite groups (Atanassova and Kerestedjian, 2009, Nordstrom, 2011, Romero et al., 2006). These minerals are commonly associated with surface waste rock deposits, tailings and coal storage areas (Carbone et al., 2013, Nordstrom, 2011, Nordstrom and Alpers, 1999, Romero et al., 2006). Because of the high solubility of efflorescent minerals, climate plays an important role on its mineral chemistry (Hammarstrom et al., 2005). Under arid climate or during prolonged dry period in humid conditions, thick crust of sulfate salts can be formed by oxidation, dehydration and neutralization processes (Jambor et al. 2000). While dissolution of the highly soluble sulfate salts occured during rainstorm that can have a catastrophic effect on aquatic ecosystems by releasing significant amounts of Fe, SO42  and potentially hazardous elements such as As, Cd, Cu, Ni and Pb as well as acidity into the leachate (Alpers et al., 1994, Alpers et al., 2003, Carbone et al., 2013, Hammarstrom et al., 2005, Jamieson et al., 2005, Nordstrom and Alpers, 1999, Romero et al., 2006). Thus, environmental significance of these sulfate minerals in areas of mining has spawned a great interest to understand their geochemical control on mine water chemistry, and the potential impact on water quality following rain events (Atanassova and Kerestedjian, 2009).

Tertiary coal deposits in many parts of the world are rich in Fe sulfide minerals occurring in both the coal seams and intercalated sedimentary strata, which pose a significant threat to the environment by generating AMD (Campbell et al., 2001, Kim and Chon, 2001, MacCauland and McTammany, 2007, Pinetown et al., 2007). In India, coalfields in the states of Assam and Meghalaya are severely affected by AMD (Equeenuddin et al., 2010, Equeenuddin et al., 2013, Sahoo et al., 2010, Sahoo et al., 2012a). Jaintia Hills coalfield of Meghalaya, the case in point, is one of the worst AMD affected zones in India, where the pH of drainage water is as low as 2, rich in sulfate, iron, and soluble heavy metals that severely contaminate the water around the surrounding (Sahoo et al., 2012a). Most of the earlier work in this area have mainly focused on the environmental impact of AMD on surface and groundwater, soil, and vegetation (Sahoo et al., 2010, Sahoo et al., 2012a, Sarma, 2005, Swer and Singh, 2003); role of Fe-oxyhydroxide precipitates in metal mobility (Sahoo et al., 2012b); inhibition of mine drainage (Sahoo et al., 2013); and geology, petrology and utilization potential of coal (Mishra and Ghosh, 1996, Singh and Singh, 2000). Although, few studies were carried out on the distribution of metals and their mode of occurrences in coal and overburden (Baruah and Khare, 2010), their role in the production of AMD along with the detailed mineralogy is largely unknown.

In this scenario, an attempt has been made to characterize the coal and mine wastes, including efflorescent salts at Jaintia Hills coalfield in terms of mineralogy, mode of occurrence of metals, acid–base accounting, and fractionation behavior to evaluate their potential role in controlling AMD generation, that is crucial for both prediction and designing the remediation strategies of this problem.

Section snippets

Study area and geology

The study area is located in Jaintia Hills district of Meghalaya, India (Fig. 1), which is one of the major coal producing zones in the north-east India with a total reserve of nearly 46 million tons. The district is bound by the state of Assam at the north and east, the East Khasi Hills at the west and Bangladesh in the south. The district is a continuous part of the Meghalaya plateau that represents a remnant of the plateau of the Precambrian Indian peninsular shield. The area is composed of a

Sampling and analytical technique

Representative rock samples were collected from various lithological units such as sandstone, siltstone and shale around the working coal mines, while efflorescent salts were sampled from the overburden dump nearby the mine sites (Fig. 1). In addition, twelve coal samples were collected from the working faces of the different collieries (Fig. 1). All samples were stored in tight sealed plastic bags to avoid contamination, and then brought to laboratory.

Mineralogy of mine waste and coal was

Mineralogy

Quartz is the most dominant mineral in waste rocks in addition to trace amounts of kaolinite and siderite (Table 1). Further, the presence of significant amounts of pyrite was observed associated with sandstone and laminated carbonaceous shale while appreciable amounts of dolomite and calcite were associated with shale and siltstone. In the coal, quartz is the major mineral though considerable amounts of kaolinite and minor amounts of pyrite (in most), calcite and dolomite (in a few) are also

Conclusions

Mineralogical study revealed that pyrite is the only sulfide mineral associated with coal and sandstone and carbonaceous shale bearing mine waste; while, dolomite and calcite are abundant in a few mine wastes bearing shale and siltstone. From the ABA and NAG tests, it is concluded that mine waste containing blocky pyrite, pyritiferous sandstone and secondary sulfate contribute more towards acid generation than coal and the rest of the mine waste, though containing pyrite. Consistency in results

Acknowledgments

The data presented in this paper is a part of the Ph.D. work of the first author submitted to the Indian Institute of Technology Kharagpur. Instrument support for SEM analysis was availed from Central Research Facility of IIT Kharagpur. The XRF and AAS analyses were carried out in the Department of Geology and Geophysics, IIT Kharagpur.

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