Rare earth element sources and modification in the Lower Kittanning coal bed, Pennsylvania: implications for the origin of coal mineral matter and rare earth element exposure in underground mines

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Abstract

In this study, we examine the variations in rare earth elements (REE) from the Lower Kittanning coal bed of eastern Ohio and western Pennsylvania, USA, in an attempt to understand the factors that control mineral matter deposition and modification in coal, and to evaluate possible REE mixed exposure hazards facing underground mine workers. The results of this study suggest that the Lower Kittanning coal mineral matter is derived primarily from a clastic source similar to that of the shale overburden. While highly charged cations like silicon, aluminum, and titanium remained relatively immobile within the coal mineral matter, iron (primarily as pyrite) was added from nonclastic sources, either during deposition of the coal mire vegetation or subsequent to burial. Other mobile cations (e.g., alkali and alkaline earth elements) appear to have been added to and/or leached from the originally deposited clastic mineral matter. Most of the sulfur in the Lower Kittanning coal bed is bound as FeS2 in the mineral matter, but a majority of samples contain a small excess of S that is most likely organically bound.

In general, the total rare earth element content (TREE) in coal ash is greater than that in the shale overburden. If the primary source of mineral matter is the same as that for the overlying shale, then REE must have been enriched in the coal mineral matter subsequent to deposition. The total rare earth element content of Lower Kittanning coals correlates strongly with Si concentration ([TREE]≈0.0024 [Si]), which provides a threshold for evaluating possible mixed exposure health effects. Chondrite-normalized REE patterns reveal a shale-like light rare earth element (LREE) enrichment for the coal, similar to that of the shale overburden, again suggesting a primarily clastic REE source. However, when normalized to the shale overburden, most of the coal ash samples display a small but distinct heavy rare earth element (HREE) enrichment. We surmise that the HREE were added and/or preferentially retained during epigenesis, possibly associated with groundwater flow through the coal unit, but not necessarily in close association with the addition of iron. At least some of the “excess” HREE could be organically bound within the Lower Kittanning coal.

Introduction

The amount and type of noncombustible mineral matter in coal can have profound effects on the economic viability of a coal bed, and on potential health effects from mining and burning of the coal. The origin of coal mineral matter is poorly understood, although it is thought that depositional (syngenetic) processes (e.g., input of clastic sediments) and postdepositional (epigenetic) processes (e.g., groundwater transport of elements through the peat/coal bed) are important factors Cecil et al., 1978, Finkelman, 1982, Stach et al., 1982. The use of rare earth elements (REE) in studies of clastic sediment provenance is well established Nance and Taylor, 1976, McLennan, 1989, Bock et al., 1994, and REE have been used as tracers for seawater, groundwater, and fluid flow processes during diagenesis Elderfield and Greaves, 1982, Elderfield et al., 1990, Byrne and Kim, 1990, Bau and Möller, 1993, Sholkovitz, 1993, Nath et al., 1997, Johannesson et al., 1999. Therefore, the potential exists for using the REE as tracers to understand both the source and the epigenetic modification of coal mineral matter. Here we examine the variations in REE from a single unit, the Lower Kittanning coal bed of eastern Ohio and western Pennsylvania, USA, in an attempt to understand the factors that control mineral matter deposition and modification in coal. In particular, we seek to address the following questions: (1) What can the REE tell us about the primary source(s) of mineral matter in coal? (2) What is the relationship of coal mineral matter to overlying and underlying sedimentary units? (3) To what extent is the REE content of a coal unit constant, both along strike and at different stratigraphic positions? (4) How do the REE covary with other geochemical parameters in coal, and what does this tell us about the origin and modification of coal mineral matter?

An additional factor motivating this investigation is concern over the possible negative occupational health aspects of rare earth elements, particularly to coal mine workers. While medical data suggest potential health problems related to worker exposures in high-REE environments, there is a lack of understanding regarding how REE exposures affect the human body, and few exposure data indicating concentration levels of REE in mining and related industrial environments Sulotto et al., 1986, McDonald et al., 1995, Pairon et al., 1995, Hirano and Suzuki, 1996. In underground mines, limitations in airflow can lead to relatively high concentrations of dust. The possibility exists for mixed exposure of REE with other known hazardous particles to worsen mining dust-related illnesses (e.g., coal workers pneumoconiosis, silicosis). In this study, we seek to understand (1) to what extent total rare earth element content can vary within a coal seam; (2) correlations of REE content with other potential sources of health problems; and (3) factors that can be used to predict the occurrence of rare earth element exposure during coal mining operations.

Section snippets

Rare earth elements in coal

The rare earth elements (REE), defined as elements with atomic number 57 (La) through 71 (Lu), are present at the parts per million (ppm) level in most rocks, and have long been used in petrogenetic studies of igneous rocks (see Hanson, 1980, for a review). All of the rare earth elements readily form 3+ ions under earth surface conditions; in addition, Ce4+ can be stable in an oxidizing, low-temperature environment, and Eu2+ is stable under certain reducing conditions at high (magmatic)

Study sites

The study sites are active Pennsylvania mines located in or near the cities of Clearfield, Enon Valley, Homer City, Ogle, and West Freedom (Fig. 2). The sites were chosen to represent coals with overburdens interpreted to be of various depositional environments, from fresh water to marine (Rimmer and Davis, 1986). Rooted (paleosol) underclays were present at the Enon Valley and Homer City sites.

The Clearfield site is an active surface coal mine in which only the Lower Kittanning coal bed is

Lower Kittanning coal bed lithotype analysis

Fig. 3 shows the Lower Kittanning coal lithotypes described for each site. The coal lithotype composition at all sites is dominated by clarain and vitrain. The clarain compositions are highly variable ranging from about 15% to about 85% bright banding. At all sites, the amount of vitrain and bright clarain increases towards the lower portion of the seam. Most of the sites show a boney, mineral-rich zone frequently enriched in durain towards the top of the seam. The exception is the Clearfield

Summary and conclusions

We carried out a petrographic, major element and rare earth element study of the Lower Kittanning coal bed mineral matter collected from sites with a variety of overburden depositional environments. The coal unit is thought to be compositionally homogeneous, although our petrographic study confirms a rank change from high volatile B to low volatile bituminous across the basin. The results of this study suggest that the Lower Kittanning coal mineral matter is derived primarily from a clastic

Acknowledgements

This research was carried out as a part of a PhD dissertation at the University of Pittsburgh by the first author. Basic Research project funding for this effort was provided by NIOSH. We would like to thank the following people for their valuable contributions to this research: Vik Skema, Willam Bragonier, Tim Miller, and Joe Ferrara for their assistance in selecting and obtaining field sites for the research; George Persetic for assistance at all field sites and in sample processing; Brian

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