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Rare Earth Elements and Yttrium (REY) in coal mine drainage from the Illinois Basin, USA
Journal Pre-proof Rare Earth Elements and Yttrium (REY) in coal mine drainage from the Illinois Basin, USA
Liliana Lefticariu, Kyle L. Klitzing, Allan Kolker PII:
S0166-5162(19)30912-7
DOI:
https://doi.org/10.1016/j.coal.2019.103327
Reference:
COGEL 103327
To appear in:
International Journal of Coal Geology
Received date:
12 September 2019
Revised date:
18 October 2019
Accepted date:
29 October 2019
Please cite this article as: L. Lefticariu, K.L. Klitzing and A. Kolker, Rare Earth Elements and Yttrium (REY) in coal mine drainage from the Illinois Basin, USA, International Journal of Coal Geology(2019), https://doi.org/10.1016/j.coal.2019.103327
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© 2019 Published by Elsevier.
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Rare Earth Elements and Yttrium (REY) in coal mine drainage from the Illinois Basin, USA Liliana Lefticariu1*, Kyle L. Klitzing1, and Allan Kolker2 1
Department of Geology, Southern Illinois University, Carbondale, IL 62901, USA U.S. Geological Survey, 956 National Center, Reston, Virginia 20192 USA
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*Corresponding author: email [email protected], phone: 618 453 7373; fax: 618 453 7393
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Graphical abstract:
Keywords: Rare Earth Elements and Yttrium, Illinois Basin, coal mine drainage, hydrothermal activity. Highlights 1
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Illinois basin coal-mine drainages (CMD) have high contents of ΣREY as well as critical-ΣREY
CMD with high ΣREY contents have also high metal (i.e., Zn, Ni, Co) contents
CMD rich in trace metals and REY are mostly located along the west and northern part of the Illinois basin
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Y/Ce ratios can be used as a proxy for predicting matrixes with high critical-ΣREY
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Journal Pre-proof ABSTRACT The ever-growing demand in the USA and in the modern economy for rare earth elements, defined here as the lanthanide elements plus yttrium (REY), motivates the development of economically feasible and environmentally friendly approaches for domestic REY resources. Non-conventional sources, including coal mine drainage (CMD) hosting elevated concentrations of REY, have been explored as attractive secondary sources for metals recovery. Consequently, in this study we investigate CMD from abandoned coal mines in the Illinois Basin as a potential REY resource. We collected CMD from 35 abandoned coal mine sites in the Illinois Basin and investigated trends in
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REY enrichment and their association with CMD geochemical characteristics, including pH, the presence of other chemical elements of economic significance (i.e., Zn, Ni, Cr, Cu, V) as well as spatial patterns in
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relation to Permian magmatic intrusions of the Hicks Dome, a potential source of REY in the Illinois
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Basin. Additionally, the REY contents in possible source rocks for Illinois CMD were measured including coal from an underground mine and coal preparation plant in Southern Illinois, mafic igneous rocks
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associated with Hicks Dome, and hydrothermal fluorite from Illinois fluorspar district. The REY concentrations in samples of CMD (filtered fraction) and associated coal and non-coal solids
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were determined by ICP-MS. The total concentration of REY in CMD, ΣREYCMD varied between 1 and 9,879 µg L−1 with an average of 1,059 µg L−1 and critical-ΣREYCMD (Nd, Eu, Tb, Dy, and Y) varied between
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0.3 and 7,577.5 µg L−1 with an average of 637 µg L−1. Basin-wide, we found no statistical correlation between ΣREYCMD and (1) pH of the CMD, or (2) the proximity to the Permian igneous magmatic
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intrusion at Hicks Dome in Southern Illinois. The REYCMD values normalized to North American Shale Composite (NASC) REYCMD/REYNASC exhibit enrichments in middle-REY for all CMD, but also enrichments
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in light-REY in some CMD. Statistically, we observed that ΣREYCMD displays significant direct correlations with Al, Si, SO4, Zn, Ni, Co, and Cu and no-correlation with Fe, PO4, Ba, and V. Synthesis of the geochemical data suggests that coal mining sites with the highest ΣREYCMD and critical-ΣREY contents include non-coal bedrock dominated by silicates (high clay content) and coal mine wastes that were impacted by metal-rich hydrothermal solutions, which are predominantly located along the western and northern margins of the Illinois Basin. Particularly, sites with the highest hydrothermal input as approximated by Mississippi Valley-type mineralization (i.e., Zn contents) are prime candidates for REY, as well as other elements of potential economic value such as Al, Ni, Cr, V, and Co. Thus, CMD in the Illinois basin could become a potential alternative domestic source of extractable REY, especially critical-ΣREY, as well economically valuable metals.
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Journal Pre-proof 1. INTRODUCTION Rare earth elements (REE) and yttrium (hereafter REY) are a group of 16 elements comprising the lanthanides and yttrium that exhibit similar physicochemical characteristics and tend to co-occur in nature (McLennan, 1989; Wedepohl, 1995; IUPAC, 2005). Scandium is chemically similar to the lanthanides and yttrium, and is sometimes included in the REEs, but it does not occur in the same geologic settings (Van Gosen et al., 2014) and is not considered further in this study. The REE are generally sub-divided based on their atomic numbers into light-REE (LREE), also called the “cerium group,” including lanthanum (57) to europium (63); middle-REE (MREE), including Sm, Eu, Gd, Tb, and
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Dy, and heavy-REE (HREE) including gadolinium (64) to lutetium (71), also called the “yttrium group” (Henderson, 1984; Dołęgowska and Migaszewski, 2013). Yttrium (Y, Z=39) is not a lanthanide, however,
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is usually grouped with the HREY because of their geochemical similarities (Dai et al., 2016a). The above
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classification is often used to emphasize the enrichments of a specific group of REY in some environments, such as of MREY in acid mine drainage (Serano et al., 2000; Verplanck et al. 2004;
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Steward et al., 2017; Sun et al., 2012; Sahoo et al., 2012; Grawunder et al., 2014). Commercial uses of REY include incorporation in numerous emerging technologies (i.e., catalysts,
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fuel cells, permanent magnets used in wind turbines and electric/hybrid car motors, photovoltaics, and hybrid car batteries) and consumer products (i.e., phones, computers, appliances, and automotive
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sensors) (Alonso et al., 2012; Goodenough et al., 2018.). REY are relatively abundant in the Earth’s crust, with average crustal contents between 160 and 205 mg kg−1 (Wedepohl, 1995). Still, they are relatively
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immobile through most geological processes and thus rarely form their own minerals or concentrate in ore deposits that are technologically possible to mine and economically profitable (Wedepohl, 1995;
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DOE, 2011; Long et al., 2012; Weng et al., 2013; Van Gosen et al., 2017). In the USA, there are a limited number of REY reserves (DOE, 2011; Graedel et al., 2015; Van Gosen et al., 2017) and currently (2019) REY production is limited to one mine, at Mountain Pass, CA where bastnäsite ore is produced (Gambogi, 2019). The remaining production is from imports, primarily from China. Significantly, the national demand of REY is projected to increase rapidly within the next several decades and many REY with high criticality, so called critical-REY, namely Nd, Eu, Tb, Dy, and Y (cf. DOE, 2011; Goodenough et al., 2018; Roskill, 2019), are already in high demand (Alonso et al., 2012; Gambogi, 2019). Limited natural reserves and rapid growth in demand is also of global concern (McLellan et al., 2014; Migaszewski and Gałuszka, 2015) since restricted market availability of REY could lead to supply risk, vulnerability to supply restriction, and environmental implications (Graedel et al., 2015).
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Journal Pre-proof Securing potentially sustainable domestic sources of these commercially critical elements has become increasingly important, requiring the diversification of supplies to include recovery of REY from recycling (Binnemans et al., 2013) and alternative resources such as coal (Seredin and Dai, 2012; Hower et al. 1999; Hower et al. 2016; Honaker et al 2017; Dai and Finkelman, 2018; Finkelman et al., 2019), coal combustion byproducts (Hower et al., 2013; Taggart et al., 2016; Kolker et al., 2017; Smith et al., 2019), coal mine waste (Honaker et al., 2017, 2018; Zhang and Honaker, 2018.), and mine drainage (Ayora et al., 2016; Ziemkiewicz et al., 2016; Hedin et al., 2019). Geochemical studies of coal mine drainage (CMD) from the Appalachian Basin, USA (Ziemkiewicz et
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al., 2016; Stewart et al., 2017), China (Zhao et al., 2007; Sun et al., 2012), and India (Sahoo et al., 2012), as well as drainage from metal mining districts in Europe (Grawunder et al., 2014; Ayora et al., 2016),
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show that mine drainage is often enriched in total REY (ΣREY). The feasibility of economic recovery of
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REY from metal and coal mine drainage will depend on factors such as its ΣREY, the share of criticalΣREY, and the ability to co-extract additional elements of economic value as well as the contaminant
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contents, operational cost, and capital cost. CMD may become an important target for REY recovery because REY are present largely in a dissolved form and ΣREYCMD is often elevated, in the range of
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hundreds to thousands of µg/L (Ziemkiewicz et al., 2016; Hedin et al., 2019). Furthermore, many CMDs have been shown to contain elevated contents of other metals, including Zn, Cu, Al, Cr, and V (Cravotta,
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2008; Lefticariu et al., 2015; Ayora et al., 2016; Stewart et al., 2017), thus making CMD an attractive potential source for metals recovery. Notably, recovery of REY and valuable metals from CMD and coal
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mine waste products can occur as a co-benefit of environmental remediation of abandoned and/or active coal mining sites (Ayora et al., 2016; Hedin et al., 2019).
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Potential REY resources in the Illinois Basin were reported in association with both the hydrothermal fluorite and the Permian mafic igneous intrusions at the Hicks Dome in southern Illinois (Moorehead, 2013; Denny et al., 2015; Gambogi, 2019). Hydrothermal ore deposits in the Illinois Basin includes well-known mining districts (1) Illinois-Kentucky fluorite district (IKFD) in Southern Illinois and Western Kentucky (Richardson and Pinckney, 1984; Trace and Amos, 1984; Chesley et al., 1994; Plumlee et al., 1995; Kendrick et al., 2002; Denny et al., 2015) and (2) the Upper Mississippi Valley Zn-Pb district in northern Illinois (Hatch et al., 1976; Heyl and West, 1982). In the IKFD, fluorite in association with calcite with lesser amounts of sphalerite, galena, and barite were main ore components of the Mississippi Valley-type (MVT) mineralization (Richardson and Pinckney, 1984; Chesley et al., 1994; Denny et al., 2015). In northern Illinois, Cobb (1981) identified sphalerite in cleat and clastic dikes of Middle Pennsylvanian-age coal beds while the Upper Mississippi Valley Zn-Pb mineralization was 5
Journal Pre-proof dominated by sphalerite within a kaolinite-pyrite-sphalerite(pyrite)-paragenetic sequence (Cobb, 1981; Whelan et al., 1988; Kolker and Chou, 1994). Studies have indicated that even through the MVT mineralization was somehow different in southern and northern Illinois, the two districts could have been part of a single hydrothermal system, which probably originated at the Hicks Dome from where fluids migrated toward the edges of the basin where they crop out at different locations (Cobb, 1981; Kenderes and Appold, 2017). Alternatively, tectonically driven flow in the basin was responsible for the thermal anomalies in the Illinois Basin and the MVT deposits (Bethke, 1986; Rowan et al, 2002). The Permian mafic igneous intrusions at the Hicks Dome (Fig. 1) comprise a mineralized intrusive
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center of alkaline mafic dikes, sills, and diatreme breccias (Bradbury and Baxter, 1992; Moorehead, 2013; Denny et al., 2015). Numerous mafic igneous dikes, sills, and diatremes are also common features
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in coal mines in southern Illinois (Hower et al., 2002; Stewart et al., 2005; Schimmelmann et al., 2009;
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Quaderer et al., 2009; Rimmer et al., 2016) as well as at discrete locations in the Reelfoot Rift – Rough Creek Graben in southern Illinois (Maria et al., 2019) and northwestern Kentucky (Hower et al., 2000;
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2001). Exploration drilling at Hicks Dome has revealed possible resources of REE, F, Zn, Pb, and Ba mostly associated with fluorite ores that usually occur as veins, breccia infilling, and country rock
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replacement (Moorehead, 2013; Hower et al., 2000; Jackson and Christiansen, 1993). The mafic igneous rocks and diatremes are enriched in LREY, primarily Ce, and exhibit elevated ΣREY values up to 1,285
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mg/kg, whereas the fluorite and ore-stage calcite associated with hydrothermal mineralization showed enrichments in HREY, dominantly in Y, and overall lower ΣREY with concentrations <106 mg/kg (Denny
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et al., 2015). Therefore, we hypothesized that the magmatic-hydrothermal activity centered at Hicks
trace elements.
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Dome, postdating coal formation, affected the regional distribution of REY and possibly other major and
Coal and coal mining products contain variable amounts of REY (Dai et al., 2016a, b; Finkelman et al., 2019; Yan et al., 2019; Zhao et al., 2019), however, limited REY content data of coal and coalmine products from the Illinois Basin are currently available (Bryan et al., 2015). Recent studies reported that the coal ash from the Illinois Basin has generally lower ΣREY as compared to samples from the Appalachian Basin, but higher REY contents than coal ash derived from western coals (Taggart et al., 2016). In the present study, we investigated the geochemical characteristics of REY as well as of major and trace elements in CMD collected at 35 abandoned coal mine sites across Illinois. We also report REY data for solid matrices, which could represent parent-weathering materials for the generation of CMD, including: (1) samples of raw coal collected from an underground coal mine (raw coal), coal mining 6
Journal Pre-proof materials feeding preparation facilities (feed coal), refuse coal mining materials after preparation (coal refuse), and prepared coal output from a coal preparation facility in southern Illinois (prepared coal), (2) Permian mafic igneous rocks from southern Illinois, and (3) fluorite ore from the IKFD. We hypothesized that geochemical characteristics of the CMD and the proximity of a given CMD site to the mafic igneous intrusion at Hicks Dome are the two most important factors controlling the concentrations and patterns of dissolved REYCMD. Specifically, we tested whether there is: (1) a relationship between ΣREYCMD and pHCMD; and/or (2) a spatial relationship between the elevated values for ΣREYCMD and the proximity of CMD site to the Hicks Dome. Additionally, trends in ΣREYCMD and
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critical-ΣREYCMD (DOE, 2011; Seredin and Dai, 2012) were examined in relation to major and minor element contents in CMD as well as to spatial correlation of the geochemical trends relative to different
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structural and tectonic features in the Illinois Basin. These data were used to identify possible regional
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patterns for CMD with elevated contents of critical-ΣREYCMD and other economically valuable metals in
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CMD of the Illinois Basin.
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2. Materials and Methods
2.1. Study Area
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Coal mine drainage samples were collected at 35 sites of inactive coal mines across Illinois during in summer of 2013 and in spring and summer of 2017 (Fig. 1; Table S1). These sites represent
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abandoned coal mine operations with or without passive remediation systems. In many cases, the mining and remediation history of the abandoned coal mine sites is not fully known, however, the
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source of CMD predominantly included weathered Pennsylvanian coal and coalmine wastes. At some locations, such as the Tab Simco site (Behum et al., 2011), remediation systems were employed to address environmental issues such as low pH and metal discharge associated with CMD. At each site, we sampled a discrete outflow, most often from the main seep connected to a coal mine portal. The sampling sites were selected to represent a range of water chemistries (i.e., pH, TDS, sulfate, and metal contents) situated at various distances from the Hicks Dome mafic igneous complex. The CMD included acidic discharges (pH 1.9 to 4.5) as well as circumneutral and alkaline discharges with pH up to 7.5 (Fig. 2a). The wide range of discharge chemistry is representative of CMD on a basin-scale. Based on the proximity to the Hick’s Dome, we defined three distinct regions found on the southern, western, and northern margins (Fig. 1): (1) Region 1 (R1) comprised locations situated in southern and southeastern Illinois, in close proximity to the Hicks Dome, with most sampling sites situated along the 7
Journal Pre-proof Cottage Grove fault system. (2) Region 2 (R2) comprised locations situated in western Illinois along the Du Quine Monocline and included the Tab Simco site; (3) Region 3 (R3) comprised locations situated in northern Illinois, farthest from the Hicks Dome. Coal mining wastes at each CMD site contained heterogeneous materials, which, in addition to coal, include bedrock materials, coal refuse, and other coal mining-associated solid matrices. Geochemical information for these materials is critical to understanding and predicting the REY contents in Illinois CMD. In most cases, solids corresponding directly to CMD at a given site were not available for sampling. To circumvent this problem, we collected and analyzed three solid matrices, coalmine
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materials (CMM), mafic igneous rocks (MIR), and fluorite mineralization (FM) as analogues for solid
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materials which may be present at CMD sites in Illinois.
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2.2. Water sample collection
Aqueous samples were collected in 250-mL high-density linear-polyethylene (HDPE) sample
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bottles which were thoroughly pre-cleaned using trace hydrochloric acid and DI water procedures before being transported to the field. During each of the field sampling campaigns, measurements of
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the field parameters, including pH, temperature, specific conductance (SC), oxygen reduction potential (ORP), and dissolved oxygen (DO), were performed on unfiltered samples immediately following sample
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collection using a Hanna® multi-sensor probe. The pH electrode Hanna HI769828-1 field probe (pH/ORP) was calibrated with Orion pH buffers at 1.68, 4.01, and 7.00 and then checked against a pH 10 buffer.
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The oxidation-reduction potential (ORP) was measured using a factory-calibrated Hanna HI769828-1 Ag/AgCl probe and a single point calibration was performed using a Hamilton redox (ORP) buffer (475 ±
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5 mV). Dissolved oxygen (DO) was determined with a Hanna HI769828-2 amperometric probe calibrated to on-site atmospheric oxygen as 100% DO. The location of the CMD samples and the field parameters are presented in Table S1.
Upon collection, all samples were stored ~4 °C and transported directly to SIUC Geochemical Laboratory. In the laboratory, the CMD samples for the determination of anions, cations, and REY were immediately filtered through 0.45 µm cellulose acetate filter papers (Millpore® HAW). Laboratory standards and reagents were prepared using deoxygenated DI water from Milli-Q/Milli-Q Ultra Plus water system (>18 MΩ cm−1) with a UV photo-oxidation. For cation and REY analyses, one split of the filtered water sample was saved in a pre-cleaned 60-mL HDPE bottle and then immediately acidified to pH<2 using ultra-pure HNO3 (Fisher Scientific™). Alkalinity was measured on filtered, un-acidified water samples using a Hach™ digital titrator (Model AL-DT), a Hanna® multi-sensor probe, and 0.16-N H2SO4 8
Journal Pre-proof (for details see Lefticariu et al., 2015). Total alkalinity was measured in effluent samples that had pH values greater than 4.5 during field measurement (Table S1). A separate filtered, non-acidified aliquot was saved for anion analysis.
2.3. Solid matrix materials To understand the occurrence and distribution patterns of REY in CMD from Illinois, we collected three distinct types of solid matrix materials which we consider possible sources of REY to CMD that included: (1) coal mining and processing materials (CMM) from an active underground coal mine and a
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coal preparation plant in southern Illinois that was extracting and processing Herrin (No. 6) coal; (2) mafic igneous rocks (MIR) collected from an underground coal mine and locations in proximity to the
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Hicks Dome district in southern Illinois, and (3) fluorite mineralization (FM) from locations within the
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Illinois Kentucky Fluorite District (IKFD).
The Illinois Basin coals are commonly prepared prior to delivery to reduce mineral matter and
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improve heating value. During this process, two main CMM fractions are created, namely (1) the treated coal, which is sent to power plants for energy production, and (2) the coal refuse most likely to be
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stored at each coal mine site (Behum et al., 2018). For this study, we collected and analyzed samples of raw coal collected from an underground coal mine (raw coal), coal mining materials feeding the
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preparation facility (feed coal), refuse coal mining materials after preparation (coal refuse), and prepared coal output from a coal preparation facility in southern Illinois (prepared coal). At the time of
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sample collection, the mine was producing only Herrin (No. 6) coal, which was directly fed into the preparation plant, so all CMM fractions analyzed in this study are representative of one coal seam at
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one location. Additional geochemical information for Illinois Basin coals from operating mines is given by Kolker et al. (2019). From the same underground mine, we collected two samples of mafic igneous rock (MIR), namely a sample representing an igneous dike and a breccia sample. Additionally, in our analysis, we included previous REY data on Permian MIR from southern Illinois (Denny et al., 2015) and fluorite mineralization (FM) from the IKFD (Denny et al., 2015; Kipp et al., 2014). Information about sample type and REY contents of all solid matrices are presented in Table S4.
2.4 Analytical Methods All geochemical analysis of aqueous solutions, including anion, cation, and REY analyses, were performed on filtered samples. In this study, the dissolved split following filtration was exclusively included in geochemical analysis and the filtered colloids or solid materials were not measured. 9
Journal Pre-proof Major anions in filtered CMD were analyzed by ion chromatography (Dionex® ICS 2000) using an IonPac® AS18 anion-exchange column at Southern Illinois University. The standard deviation for duplicate sample analyses was consistently <5% for anions (i.e., SO42-, Cl-, F-). Samples for cation analysis, including REY were acidified to a 5 wt/v% trace-metal grade HNO3 (ACS certified) solution and analyzed on a contract basis by the ACME Laboratories, Inc. (Bureau Veritas Commodities), Vancouver, British Columbia, Canada (Table S2; S3). All samples were analyzed by ICP-MS, including two field replicates and one blank (18.2 MΩ.cm water) for all the CMD samples, in low-, medium-, and high-resolution modes, depending on individual element spectral values. A mixture of single-element standard solutions (STD
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TMDA-70 and STD TMDA-70.2) was used for instrument calibration. The analytical precision (±1σ) of standard concentration values was within 1%, whereas sample reproducibility was typically <±3%. All
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blanks were below detection. Resent research showed that accurate Eu concentration measurements by
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ICP-MS of coal and coal-related materials can be affected by high Ba concentrations if the Ba/Eu concentration ratios are > 1000 (Yan et al., 2018). However, most of the CMD samples have Ba/Eu ratios
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<20, with only one sample (S1) has a high Ba/Eu ratio of 1556, and therefore for the CMD samples considered in this study the reported Eu concentration values are accurate.
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Rare earth elements and yttrium (REY) for solid matrix materials were determined by ICP-MS at Activation Laboratories, Inc. (Actlabs), Ancaster, Ontario, Canada. Prior to analysis, 60-mesh splits of
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bulk samples were combusted by Actlabs at 525°C for 36 h to produce laboratory ash used in ICP-MS determinations. Compared to the ASTM D3174 ash yield determination at 750°C, coal combustion at
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525°C is thought to limit loss of moderately volatile trace elements, such as arsenic (As) and antimony (Sb), while retaining all but the most volatile elements such as mercury (Hg) and selenium (Se);
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(Golightly and Simon, 1989; Palmer, 1997). After laboratory combustion at 525°C, the resulting ash was digested using a sodium peroxide fusion digestion approach. Sodium peroxide fusion is superior to acid digestion for the decomposition of insoluble trace phases that carry REE (Meier, 1997). Detection limits by ICP-MS are 0.1 mg/kg (ppm) for the REE and Y with the exception of Ce (0.8 mg/kg), La and Nd (each 0.4 mg/kg), Dy (0.3 mg/kg) and Ho (0.2 mg/kg). The detection limit for major elements is typically 0.01%, but with addition of sodium peroxide, sodium cannot be determined as a major oxide. The REY contents of the raw coal, feed coal, and coal refuse sample collections (Table S4) are reported on whole sample basis.
3. Results
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Journal Pre-proof 3.1. Geochemistry of coal mine discharge (CMD)
3.1.1. Major and trace elements The geochemical results of all CMD, including field and laboratory data, are presented in Table S1, S2, and S3. Sulfate was the main anion present in CMD and in most cases representing >95% of the total anion load, with minor input from Cl and F (Table S2). Major cations measured included Fe followed by Al, Si, Ca, Mg, Mn, Na, and K. Overall, CMD samples from the Illinois Basin exhibit a fairly wide range of values for pH (1.9-7.5), TDS (532-6,525 mg/L), as well as dissolved ions, including SO4 (510-
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35,880 mg/L), Fe (1.8-3,633 mg/L), and Al (0.02-1,386 mg/L) (Table S2). It is noteworthy that one sample, SIU-41 of R3-CMD, exhibited unusually high values for SO4 (35,875 mg/L), Al (1,386 mg/L), Mg
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(6,890 mg/L), Mn (399.9 mg/L), and most of the metals (Table S2).
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Like major elements, trace elements detected in Illinois CMD display a wide range of concentration values, including for Zn (20.9-68,992 µg/L), Ni (0. 2-18,588 µg/L ), Co (2.7-8,925 µg/L), Li
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(72.8-37,804 µg/L), Cu (9.9-379.2 µg/L), V (0.2-275.0 µg/L), Cr (0.5-141.7 µg/L), U (0.1-160.8 µg/L), and Cd (0.1-688.3 µg/L) (Table S2). Among metals in CMD, Zn was the dominant element with values up to
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97% of the transitional metal load, followed by Ni (up to 32%), Co (up to 15%) and Cu (up to 4%). Additionally, Cd, V, Cr, U, and Pb have minor (<1%) contributions.
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Regionally, CMD geochemical parameters exhibited the following major spatial trends: (1) the average pH value was highest for R1-CMD at 4.3 followed by R3-CMD at 2.7 and R2-CMD at 2.6 (Fig. 2a);
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(2) SO4, Fe, and Mn have highest average concentrations in R2-CMD followed by R3-CMD (with the exception of sample SIU-41) and R1-CMD; (3) Al, Si, Mg, and Na have the highest average concentrations
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in R3-CMD followed by R2-CMD and R1-CMD; and (4) Ca and K have the highest average concentrations as well as the widest ranges of values in R1-CMD followed by R2-CMD and R3-CMD (Fig. S1). In terms of ΣREYCMD-metal relationships, similar regional patterns to those for metals were mapped, where the highest average concentrations were recorded in R3-CMD followed by R2-CMD and R1-CMD (Fig. S2). The site-specific geochemical composition of CMD was probably related to the inherent composition of coal mining wastes as well as the presence of materials added as treatment options for some abandoned mines (e.g., limestone).
3.1.2. Rare Earth Elements and Yttrium (REY) The ΣREYCMD concentrations varied between 0.69 µg/L and 9,879 µg/L with averages of 1,058 µg/L for all the samples as well as averages of 475 µg/L for R1-CMD, 1091 µg/L for R2-CMD, and 4,742 11
Journal Pre-proof for R3-CMD (Table S3). The unusually high average ΣREYCMD concentrations in R3-CMD was due to one sample SIU-41, which, in addition to high concentrations for major and minor elements (Table S3), had an exceptionally high ΣREYSIU41 value of 9,879 µg/L (Table S3). Overall, Y, Ce, and Nd exhibited the highest concentrations and the largest ranges of REYCMD values (Table S3), while in terms of percentage of individual element of ΣREYCMD, Y was the main REYCMD followed by Ce, Nd, Gd, Dy, Sm, and La (Fig. 2c). In Figs. 2d-f, REYCMD concentrations for R1, R2, and R3 are presented normalized to the North American Shale Composite (NASC; Gromet et al., 1984). The REYCMD/NASC patterns showed notable enrichment in Y and MREY (Fig. 2b). MREY enrichments similar to those shown in Fig. 3b were reported
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in drainage from coal mines in the Appalachian Basin, USA (Stewart et al., 2017), China (Zhao et al., 2007; Sun et al., 2012), India (Sahoo et al., 2012), and metal mining districts in Europe (Grawunder et al.,
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2014; Ayora et al., 2016). Regionally, while the overall REYCMD/NASC patterns for R1 (Fig. 2d), R2 (Fig. 2e),
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and R3 (Fig. 2f) are broadly similar with enrichment in MREY as the primary feature, key differences were also present, such as higher values for Gd and Dy for R1-CMD and predominantly Y and Gd for R2-
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CMD and R3-CMD. More specifically: (1) R1-CMD had the highest concentrations and largest range of values for Y, Ce, and Nd with highest percentage contribution to ΣREYCMD coming from Y, Ce, Nd, and La;
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(2) R2-CMD had the largest ranges of REY values for Y, Ce, and Nd, with highest percentage contribution to ΣREYCMD being from Y, Ce, and Nd, followed by La, Gd, and Dy; and (3) R3-CMD showed unique
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patterns with Y exhibiting high levels in terms of both values and percentage contribution to ΣREYCMD. It is noteworthy that sample SIU-41 had the ΣREY content (9,879 µg/L) approximately 10-times greater
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µg/L).
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than average value for ΣREYCMD (1,058 µg/L) as well as high contents for Gd (244.6 µg/L) and Y (264.7
3.1.3. Correlations of ΣREYCMD with major and trace elements Major and trace element data and correlations with pH and ΣREYCMD are presented in (Figs. 3, 4; Table S5). Similar to previous reports (Stewart et al., 2017), the pH had a major impact on ΣREYCMD contents, with significantly higher ΣREYCMD values in low-pH (< 4.5) CMD) compared to more alkaline CMD (Fig. 3a). All the CMD with alkaline pH were in R1, region closest to Hicks Dome (Fig. 2a). Generally, no statistical correlation was found between pH and ΣREYCMD (r = -0.25, p = 0.11; Table S5), with even poorer statistics if we consider only the CMD samples with pH <5 (r = -0.08, p = 0.67). Additionally, except for Si (r = -0.58, p < 0.005), none of the major or trace elements measured in Illinois CMD showed statistically significant correlations with pH. These results are surprising (Table S5) and probably reflect both the heterogeneous composition of the weathering coal mine wastes at different 12
Journal Pre-proof mine sites across the Illinois Basin as well as the prevailing geochemical conditions characterizing CMD in the Illinois Basin that were optimized to solubilize and mobilize these specific elements for a variety of original host phases. In contrast to pH, most of the major and trace elements displayed significant statistical correlation with ΣREYCMD (Table S5; Figs. 3b-i). Among the major elements, Al exhibits the strongest linear correlation with ΣREYCMD (r = 0.93, p < 0.005; Fig. 3b) for all CMD samples regardless of region. Similarly, good correlations with ΣREYCMD were measured for Si (r = 0.42, p = 0.006; Fig. 3c), Mg (r = 0.88, p < 0.005), Na (r = 0.59, p < 0.005), and Mn (r=0.91, p < 0.005; Fig. 3f). While SO4 showed a linear trend
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and a statistically significant correlation (r= 0.91, p < 0.005; Fig. 3d) with ΣREYCMD, Fe showed no correlation with ΣREYCMD (r= 0.13, p = 0.4; Fig. 3e) or with SO4 (r= 0.27, p = 0.08), suggesting that REY
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were not present in pyrite. Likewise, a wide range of values was measured for Ca (400-600 mg/L) and
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PO4 (0.03-11.12 mg/L) with no particular pattern for any region as well as no correlation with ΣREYCMD either for Ca (r= 0.24, p = 0.13; Fig. 3g), K (r= 0.20, p = -0.06; Fig. 3h), or for PO4 (r= 0.38, p = 0.01; Fig. 3i).
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The ΣREYCMD also displayed significant statistical correlation with many trace elements (Table S5; Figs. 4a-i), including for Zn (r= 0.37, p = 0.02; Fig. 4a), Ni (r= 0.90, p < 0.005; Fig. 4b), Co (r= 0.91, p <
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0.005; Fig. 4c), Cu (r= 0.71, p < 0.005; Fig. 4d), Li (r= 0.89, p < 0.005), Cr (r= 0.33, p = 0.03; Fig. 4g), U (r= 0.82, p < 0.005; Fig. 4h), and Cd (r= 0.87, p < 0.005; Fig. 4i). One key exception is V that did not show any
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statistical correlation with ΣREYCMD (r= 0.13, p = 0.43; Fig. 4f) and for which the highest concentration was recorded in R2-CMD, followed by R1-CMD and R3-CMD. Interestingly, Ba was the only trace
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element to show an inverse correlation with ΣREYCMD (r= -0.15, p = 0. 43; Fig. 4e).
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3.1.4. Geochemical patterns of critical-REYCMD In this study, the critical-REY include five REY, namely Nd, Eu, Tb, Dy, and Y, which were classified by US Department of Energy as “critical resources” due to their high economic demand for both the short and long term (DOE, 2011; USGS, 2018). Analogous to trends described above, the critical-ΣREYCMD values varied over a wide range from 0.26 µg/L to 7,212.51 µg/L with averages of 611.1 µg/L for all the samples as well as of 237.6 µg/L for R1-CMD, 577.5 µg/L for R2-CMD, and 3,313.6 µg/L for R3-CMD (Table S3). Similar to trends seen for ΣREYCMD, even though no statistical correlation was found between critical-ΣREY and pH, the acidic CMD (< 4.5) had significantly higher critical-ΣREY values compared to more alkaline CMD (Fig. 5a). Critical-ΣREYCMD exhibited a strong correlation with ΣREYCMD (r= 0.93, p < 0.005; Fig. 5b). In terms of individual REY, the highest concentrations and the widest ranges of values were measured for Y and Nd followed by Dy, Eu, and Tb (Fig. 5c). With respect to their 13
Journal Pre-proof contribution to critical-ΣREYCMD, the two main critical REY in Illinois CMD were Y (average 55.5%) and Nd (average 30.8%) (Fig. 5d). The strong statistical correlation between critical-ΣREYCMD and Y (r= 0.99, p < 0.005; Fig. 5e), is due most probably to the dominance of Y in Illinois CMD (Fig. 5e). The average value for Y contribution to critical-ΣREYCMD was 55.5% for all samples as well as 48.2% for R1-CMD, 56.5% for R2-CMD, and 66.2% for R3-CMD. These data are significant because none of the CMM samples analyzed in this study display high values for Y and, therefore, the high values of Y in CMD, most probably, did not exclusively reflect source enrichment. Significantly, critical-ΣREYCMD also showed a strong correlation with ΣHREYCMD/ΣLREYCMD ratio (r= 0.924, p < 0.005; Fig. 5f), such as the CMD samples with the highest
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3.2. Geochemical patterns of REY in solid matrixes
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ΣHREYCMD/ΣLREYCMD ratios also had the highest levels of critical-ΣREYCMD.
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Compared to CMD, REY in the solid matrixes are several orders of magnitude higher, with significant differences in the individual REY patterns for the three types of solid matrixes, namely CMM,
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MIR, and FM (Fig. 6a-l; Table S4). The CMM showed highly variable ΣREYCMM contents from 18.4 to 198.6 mg/kg with average values as follows: raw high-sulfur coal = 67.4 mg/kg (n=2), raw low-sulfur coal = 18.8
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mg/kg (n=2), preparation plant feed coal = 116.8 mg/kg (n=2), prepared coal = 29.5 mg/kg (n=4), and refuse coal 182.6 mg/kg (n=3) (Table S4; Fig. 6a). CMM display enrichments in LREY (i.e., Ce, La, and Nd)
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(Fig. 6d), have low ΣHREY/ ΣLREY ratios, and relatively flat ΣHREY/ΣLREY patterns, typical of crustal materials such as NASC (Fig. 6g), and a low fraction of critical-ΣREY dominated primarily by Nd and Y (Fig.
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6j).
The ΣREY values for MIR samples, ΣREYMIR, ranged from 300.4 to 414.2 mg/kg with an average of
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346.3 mg/kg (n=6) (Fig. 6b). Notably, two samples of an intruded mafic dike and an igneous breccia collected from the same coal mine as CMM samples had ΣREYMI values of 368.6 and 319.1 mg/kg, respectively (Table S1), significantly higher than CMM samples. Compared to CMM and FM, MIR display unique characteristics, namely it had the highest ΣREYMIR (Fig. 6b), the highest enrichments in LREY (i.e., Ce, La, and Nd) (Fig. 6e, h), the lowest ΣHREY/ΣLREY ratios, and the lowest fraction of critical-ΣREY dominated primarily by Nd (Fig. 6k). On a NASC normalized plot, MIR displayed substantial enrichments in LREY (Fig. 6h), with the high contents and range of values for Ce, La, Nd, followed by Y (Fig. 6d, e). The ΣREYFM, ranged from 23.5 to 65.4 mg/kg with an average of 38.5 mg/kg (n=7), which were the lowest values for solids compared to ΣREYMIR and ΣREYCMM (Fig. 6c). Significantly, Y was the predominant element in FM samples in terms of both highest concentration and the largest range of values (Fig. 6f) followed by Dy, Ce, Gd, Nd, Er, and La. Importantly, on ΣREYFM/ΣHREYNASC plot, FM 14
Journal Pre-proof exhibits evident enrichments in MREY (Fig. 6 i) and the highest ΣHREY/ ΣLREY ratios, with excessively high YFM levels with values 10-times higher than those of other solid matrixes (Fig. 6 l). Whereas fluorite is the main REY-carrier in the IKFD of southern Illinois, other minerals such sphalerite and calcite from IKFD have similar REY patterns with Y and MREY enrichments (data not shown), therefore, the REYFM patterns represent a good proxy for fingerprinting solid matrixes precipitated from hydrothermal fluids.
3.3. Comparing REY patterns of aqueous and solid samples To further decipher the factors controlling the REY patterns in CMD, especially those of critical-
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ΣREYCMD, geochemical criteria specific to Illinois CMD were employed to explain the REYCMD patterns in a broader context. By definition, there is almost perfect correlation between critical-ΣREY and ΣREY for all
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solid and aqueous matrixes considered in this study (Table S5). However, the fraction of critical-REY of
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the ΣREY varies broadly among different matrixes from 23.5% in MIR to 91.8% in FM (Fig. 7a). In a further analysis, we separated two end members: at one end, there are MIR and CMM, which display
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the lowest average critical-REY values of 25.9% and 34.6%, respectively and at the other end, the FM, which have exceptionally high fraction of critical-ΣREY with an average value of 80.6%. The Illinois CMD
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displayed values that range between these two end members, with an overall average value of 53.1% as well as 48.2.5% for R1-CMD, 56.5% for R2-CMD, and 66.2% for R3-CMD. Whereas the proportion of
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critical-REY in Illinois CMD is high, the concentrations of these elements in mine drainage is generally low. Methods for commercial recovery and separation of REY from coal-derived materials are in their
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infancy, but it is likely that further concentration of REY from AMD will be necessary before economic recovery is possible (Ayora et al., 2016; Hedin et al., 2019; Ziemkiewicz et al., 2016).
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Another remarkable feature of the solid and aqueous matrixes is the distinct positive Ce anomaly defined as Ce/Ce*=2 CeNASC/(LaNASC+PrNASC) (Bau and Dulski, 1996; Dai et al., 2016a). A caveat is that some La, Ce, and Pr contents, especially in the FM solids, were close to the detection limits and thus the range of Ce/Ce* ratios in FM will require further investigation. Most of the samples had Ce/Ce* > 2, with average values of 2.8 for CMM, 3.1 for MIR, and 4.7 for FM as well as of 4.6 for all CMD, 4.7 for R1-CMD, 5.6 for R2-CMD and 4.9 for R3-CMD (Fig. 7b). In contrast to critical-ΣREY patterns, within the geochemical pattern depicted by Ce/Ce* anomalies, no distinct endmembers for the solid matrixes could be separated. Whereas both CCM and MIR had lower Ce/Ce*values, the FM show a particularly wide range of values which mostly overlap those of CMD. This may indicate that hydrothermal solutions from which FM precipitated had elevated but variable amounts of LREY (La, Ce, and Pr), which resulted in variable Ce enrichments for FM. 15
Journal Pre-proof Yttrium is the main critical-ΣREY in the Illinois CMD (Fig. 5e), and, thus, to decipher the source of this enrichment we further explore Y relationships with other parameters, including critical-ΣREY, ΣREY, and ΣHREY/ΣLREY ratios. The fraction of Y of the critical-ΣREY varies widely among different solid and aqueous matrixes from 19.2 to 94.3% (Fig. 7c). Similar to the previously described approach, two end members were separated, namely at one end, the MIR and CMM matrixes have the lowest fractions of Y of the critical-ΣREY, with average values of 24.1% and 42.5%, respectively, and at the other end, the FM contain an extremely high fraction of Y of the critical-ΣREY, with an average value of 88.1%. The CMD samples have values between these two endmembers, with an overall average value of 55.5% for all
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CMD samples, 49.1% for R1-CMD, 60.6.4% for R2-CMD, and 65.9% for R3-CMD. The fraction of Y of the ΣREY showed large variations among different solid and aqueous matrixes
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with values from 3.8 to 56.3% (Fig. 7d). Interestingly, while the solid matrixes have average values of
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4.1% for MIR, 15.1% for CMM, and 26.5% for FM., the CMD showed a significantly wider range of values from 2.1 to 56.3% with averages of 30.8% for all CMM, 25.1% for R1-CMD, 34.8% for R2-CMD, and
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44.0% for R3-CMD. ΣHREY/ΣLREY ratios (Fig. 7e) displayed similar patterns with the fraction of criticalREY of the ΣREY (Fig. 7a), such as two end members can be separated, namely at one end, the MIR and
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CMM matrixes have low ΣHREY/ΣLREY ratios, with average values of 0.09 and 0.24, respectively, and at the other end, the FM contain extremely high ΣHREY/ΣLREY ratios with an average value of 3.6. The
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CMD samples have values spanning these two endmembers, with an overall average of 0.35 for all CMD samples and for 0.46 for R1-CMD, 0.71 for R2-CMD, and 1.12 for R3-CMD. Finally, we observed strong,
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statistically significant logarithmic relation (R2 = 0.96) between the fraction of critical-ΣREY and the Y/Ce
7f).
4. Discussion
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ratios, such as the samples with the highest share of critical-ΣREY also had the highest Y/Ce ratios (Fig.
In this study we tested two key factors which could influence the contents and distribution patterns of REY in CMD from the Illinois Basin, namely the pH and the spatial proximity of the CMD site to the Hicks Dome. In the following sections, both factors are evaluated.
4.1. Correlations of ΣREYCMD with pH and other geochemical parameters
4.1.1 Correlations of ΣREYCMD with pH
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Journal Pre-proof The impact of solution pH on the concentration of dissolved constituents in Illinois CMD is consistent with previous reports of natural and mine waters, which found that lower pH solutions usually carry a higher load of dissolved constituents (Gimero et al., 2000; Gammon et al., 2003; Cravotta, 2008; Nordstrom, 2011; Lefticariu et al., 2015; Noack et al., 2014; Stewart et al., 2017). In the present study, Illinois CMD also displayed the greatest enrichment in low-pH samples, but variation of REY with pH did not show a linear relationship (Fig. 3a) and, as such, there is not a significant statistical correlation (i.e., P-values < 0.05 indicate statistically significant non-zero correlations at the 95.0% confidence level) between pH and ΣREYCMD, including both all-CMD and low-pH CMD (Table S5), as well
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as with other REY-related parameters, namely ΣHREY/ΣLREY, Y/Ce, as well as critical-ΣREY. Similarly, at a
SO4, Al, Fe) and trace (i.e., metals) elements (Table S5).
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regional level, the values of pHCMD did not correlate with the concentration values for many major (i.e.,
Previous studies showed that, in low-pH, sulfate-rich waters, SO4 is the primary ligand for REY
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forming predominantly mono-sulphate complexes (LnSO4+; where Ln is a lanthanide element) and
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Ln(SO4)2− and free ionic (Ln3+) species contributing minor amounts to dissolved REY speciation (Gimeno et al., 2000; Verplanck et al., 2004; Zhao et al., 2007; Pérez-López et al., 2010). Furthermore, field
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experiments (i.e., Verplanck et al., 2004) document that under similar pH conditions as reported for Illinois CMD, the REE remain dissolved in sulfate-rich waters with solid-phase partitioning occurring
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exclusively in the waters with the highest pH (i.e., pH ≈ 6.6). Since dissolved SO4 was the prevailing ion in Illinois CMD regardless of the pHCMD and the SO4 concentrations were an order-of-magnitude higher
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than that of the other anions (Table S2), we conclude that REY-sulfate complexes were the dominant dissolved REY species in CMD analyzed in this study, with limited partitioning into the solid Fe-
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precipitates (Verplanck et al., 2004; Stewart et al., 2017). Collectively, these results suggest that in addition to pHCMD other key factors controlled the abundance and patterns of REY enrichment in Illinois CMD. Such factors could have included the bedrock geology, alteration mineralogy, biogeochemical processes at the CMD site, input of REY from outside systems (i.e., agricultural runoff), and coal mine drainage composition.
4.1.2. Correlations of ΣREYCMD with major elements Geochemical characteristics of CMD, mainly the identity, concentrations, and ratios among major elements in CMD could further decipher the identity of the heterogeneous, compositionally complex weathering solid matrixes at each CMD site. Aluminum and silicon were the two major elements that showed statistically strong correlations with ΣREYCMD (Table S5). None of these elements 17
Journal Pre-proof is redox sensitive and, thus, a limited number of processes control their behavior in natural environments, especially under low-pH conditions where both Al and Si are present as dissolved complexes in solution (Bigham and Nordstrom, 2000). Thus, the variation of Al and Si concentrations in CMD can be considered a direct reflection of coal mine weathering materials rather than that of the biogeochemical weathering stoichiometry. This information suggests that aluminosilicate sources and especially Al-rich clay minerals (i.e., kaolinite, illite) in coal mine wastes (i.e., Cobb, 1981; Finkelman et al., 2019) could have been an important source of REY in CMD. Previous research has demonstrated that clay minerals (i.e., kaolinite, halloysite, illite) are key repositories of REE in regolith-hosted REE deposits
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(Williams-Jones et al., 2012; Elliott et al., 2018; Cheshire et al., 2018) as well as in residual REE-clays and South China clay deposits (Bao and Zhao, 2008; Chi and Tian, 2008). In these cases, the REE were
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adsorbed at sites of negative charge on the layer surfaces and/or at the layer edges of clay minerals
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(Kynicki et al., 2012; Li and Yang, 2016).
Interestingly, we found that ΣREYCMD presents a strong statistical correlation with SO4 and no
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statistical correlation with Fe (Table S5). Both Fe and SO4 are redox sensitive elements and are extensively involved in redox cycling at CMD-impacted sites (Lefticariu et al., 2017a). However, under
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the pH-Eh conditions found at most CMD sites, Fe usually undergoes extensive redox transformations (Lefticariu et al., 2017b) while for SO4 this occurs to a lesser degree, mainly due to the restrictive
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environmental conditions necessary for bacterial sulfate reduction (Lefticariu et al., 2017a). Typically, Fe and SO4 concentrations in CMD reflect the alteration mineralogy, namely weathering of primarily pyrite
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(Lefticariu et al., 2006) but also other sulfides existing in coal mine wastes (i.e., sphalerite, galena, and chalcopyrite) (Cobb, 1981). The sulfides are not known as important repositories for REY, and, thus, the
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ΣREYCMD and SO4 statistical correlation may be due to interrelated factors, such as hydrothermal activity which could have enriched the host rocks in both sulfur-bearing species (i.e., pyrite) and REY (Dai et al., 2017).
At CMD sites, Fe often forms precipitates (Verplanck et al., 2004; Lefticariu et al., 2017b; Hedin et al., 2019). The biogeochemical processes associated with Fe redox cycling resulting in precipitation and/or dissolution of such Fe phases could also control the REY partitioning between the solids and the solution and thus affect the concentrations and geochemical patterns of REY in CMD (Verplanck et al., 2004; Hedin et al., 2019). Limited REY data of CMD solid phases at our sampling sites did not allow us to directly evaluate the role of Fe precipitates in controlling the REY contents and patterns in Illinois CMD. However, the lack of statistical correlation between ΣREYCMD and Fe concentrations for samples
18
Journal Pre-proof considered in this study suggest that CMD precipitates most probably had only minor control on ΣREYCMD and REYCMD patterns. The concentration of Ca, an element that does not undergo redox transformation, did not statistically correlate with ΣREYCMD (Table S3). Most probably, Ca levels reflect the mineralogy of coal mine wastes, with higher Ca levels reflecting the occurrence of carbonate rocks at the CMD site, either as part of the bedrock or added as a treatment option (i.e., Behum et al., 2011). These two sources can constitute a persistent supply of Ca to the CMD. However, limestone added at different CMD sites as a remediation option is prone to passivation due to coating by Fe-rich precipitates and as such, the Ca
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contribution from such sources often decreases over time to background levels (Lefticariu et al., 2015). Anthropogenic activities, especially the application of phosphate fertilizer (i.e., apatite, a REY-
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rich calcium phosphate mineral) could cause REY addition into the environment (Dinali et al., 2019).
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Overall, the PO4 concentrations in CMD were relatively low (Table S2) and the lack of statistically significant correlations with both ΣREYCMD and pH suggest that phosphate fertilizer generally had a
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minor, if any, contribution to the ΣREYCMD.
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4.1.3. Correlations of ΣREYCMD with trace elements
Remarkably, we found statistically significant correlations between ΣREYCMD and most of the
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metals detected in Illinois CMD, including Zn, Ni, Co, Cu, and Cd (Table S5). The metal enrichment is more likely derived from hydrothermal fluids that precipitated reduced sulfur and metals (i.e.,
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sphalerite, galena, and chalcopyrite) in Illinois coal seams. Such hydrothermally-derived enrichment for Hg and other metals was previously identified in pyrite in coal from southern Illinois (Lefticariu et al.,
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2011; Manceau et al., 2018) and coal seams from southern Webster and Union counties, Kentucky (Hower et al., 2000; 2001; 2002), yet any hydrothermal solution affecting the coal seams and associated bedrock would have contributed to increased contents of metals. Importantly, sites where CMD displays enrichment in metals, particularly Zn, Ni, Co, and Cu also exhibit elevated levels of ΣREYCMD. Thus, the significant correlation between metals and ΣREYCMD suggest that hydrothermal activity played a key role in REY enrichment found in Illinois CMD. Collectively, the major and trace element data together with pHCMD indicate that the heterogeneous composition of the coal mine wastes weathering at each site controlled the geochemical trends observed in Illinois CMD (Fig. 3, 4). This assumption is supported by the wide range of CMD chemical composition with respect to the major (Fig. 3) and trace (Fig. 4) constituents and no significant statistical correlation among elements which are usually associated with silicates (i.e., Al, Si), sulfides, 19
Journal Pre-proof such as pyrite (i.e., Fe, SO4), and carbonates (i.e., Ca, Mg) (Table S5). This indicates that mixed materials of bedrock, coal, coal refuse and added remediation materials, containing various proportion of silicates, sulfides, and carbonates have been weathering at each CMD site.
4.2. Proximity to the Hicks Dome and significance of REY regional patterns Perhaps one of the most important findings of this study is that distinct spatial patterns of CMD geochemical parameters were recorded from the three regions, namely R1, R2, and R3, including for major and minor elements as well as for REY (Figs. 2-4, 7; S1-S4).
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Whereas Ca (Fig. 3g; S1g) and pH (Fig. 3a) levels display variability throughout the basin, overall, more sites in R1 have higher Ca levels and alkaline pH suggesting greater amounts of carbonate were
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incorporated in the weathering coal mine waste, most probably from bedrock. Similarly, another
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noteworthy finding was Ba, which had the highest range of values and the highest average concentration for R1-CMD followed by R2-CMD and R3-CMD (Fig. 4g; S2e). These geochemical trends
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support previous reports that identified calcite and barite as the main ore components of the MVT mineralization in the IKFD (Richardson and Pinckney, 1984; Chesley et al., 1994; Denny et al., 2015). The
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CMD in R2 displayed the highest average concentrations for Fe (Fig. 3d, S1d), SO4, (Fig. 3e, S1e), and Mn (Fig. 3f, S1f) (anomalous sample SIU-41 in R3 not included), followed by R3-CMD and R1-CMD,
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suggesting significantly greater amounts of sulfides such as pyrite in the weathering coal mine wastes at R2-CMD sites. The CMD sites in R3 have higher Al (Fig. 3b; S1a) and Si (Fig. 3c; S1b) concentrations
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suggesting that higher amounts of silicates, particularly clay minerals (i.e., high Al concentrations), were present at R3-CMD sites. The regional trends in most metal concentrations of Illinois CMD, including Zn,
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Ni, Co, Cu, Li, Cr, U, and Cd, revealed similar patterns, with the highest metal concentrations measured in R3-CMD, followed by R2-CMD and R1-CMD (Fig. 4, S2). These results corroborate previous research (Cobb, 1981; Whelan et al., 1988) which describe kaolinite and sulfides as important components of the kaolinite-pyrite-sphalerite(pyrite)-paragenetic sequence which characterize the Upper Mississippi Valley Zn-Pb mineralization in the Illinois Basin. In the case of REY, contrary to our second hypothesis, R1-CMD did not display the highest average ΣREYCMD value (ΣREYR1-CMD, 475 µg/L) (Fig. S1), even though these CMD sites were spatially located closest to the Hicks Dome, a center of mafic igneous and hydrothermal activity in the Illinois Basin (Moorehead, 2013; Denny et al., 2015; Maria et al., 2019), and MIR were found to have the highest ΣREY of any of the samples investigated (Fig. 6). Instead, higher average ΣREYCMD values were recorded in R3CMD (4,742 µg/L) followed by R2-CMD (1,091 µg/L) that sampled sites along the northern and western 20
Journal Pre-proof edges of the Illinois Basin, further from Hicks Dome (Fig. 2b). Likewise, the highest average critical-ΣREY of 3,314 µg/L was recorded for R3-CMD, farthest from the Hicks Dome, followed by R2-CMD 578 µg/L and R1-CMD 238 µg/L (Fig. 7a). Moreover, other REY-related parameters including ΣHREY/ΣLREY and Y/Ce showed consistent regional patterns with the highest average value measured in R3-CMD followed by R2-CMD and the lowest one in R1-CMD (Fig. S2a, g). Many Illinois CMD samples display MREY enrichment on NASC-normalized REY plots (Fig. 2d-e) typical of low-pH environments (i.e., Gimeno Serrano et al., 2000; Johannesson and Zhou, 1999; Stewart et al., 2017). In other cases, even though the MREY enrichment was still present, the ΣLREY/ΣMREY
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ratios were similar or even larger than those of ΣHREY/ΣMREY (Figs. 2d-f). Regionally, this type of MREY pattern was found in many R1 and R2 sites (Figs. 2d-e). Local enrichments of coal mine wastes in LREY,
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particularly in Ce, Nd, and La in R1 (Fig. 2d) or in HREY, mainly in Y but also Dy in R3 (Fig. 2f) and R2 (Fig.
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2e) could explain the ΣREYCMD/ ΣREYNASC patterns (Figs. 2d-f) as well as the ΣHREY/ΣLREY, and Y/Ce,
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variability in Illinois CMD samples (Fig. S3).
4.3. Sources of REY
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Numerous studies investigating fluid−rock interaction during weathering, employing major, trace, and REY geochemical behavior have emphasized the extensive influence of the source rock
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geochemical signatures on the interacting solutions (Cravotta, 2008; Chi and Tian, 2008; Migaszewski, and Gałuszka, 2015; Zaharescu et al., 2017; Frings and Buss, 2019). Herein, we employ a similar
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approach by determining the inherent REY signatures for three solid matrixes considered as possible sources of REY in Illinois CMD, namely CMM, MIR, and FM. These geochemically distinct matrixes
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exhibit dissimilar ranges for ΣREY and critical- ΣREY values (Fig. 6a, b, c) as well as unique individual NASC-normalized REY patterns for CMM (Fig. 6g), MIR (Fig. 6h), and FM (Fig. 6i). In the case of CMM, different samples displayed wide ranges of ΣREYCMM and critical-ΣREYCMM values. The highest ΣREYCMM and critical-ΣREYCMM values were measured in the coal refuse samples (Table S4), the CMM fraction most likely to be stored at coal mine sites and become part of the coal mine waste (Behum et al., 2018). The treated coal, which is the CMM fraction sent to power plants for energy production, had the lowest ΣREYCMM and critical- ΣREYCMM contents (Table S4). On average, the coal refuse samples from the coal preparation facility had 4 to 6-times higher ΣREYCMM concentrations compared to those of treated coal prepared for commercial use. Previous studies have showed that in coal, REY are primarily controlled by detrital minerals such as apatite, monazite, xenotime, titanite, and, to a lesser extent, by zircon (Seredin and Dai, 2012; Hower et al., 2016; Dai and Finkelman, 2018; 21
Journal Pre-proof Finkelman et al., 2019). Additionally, REY can be associated with clay minerals (Elliott et al., 2018; Kynicki et al., 2012; Li and Yang, 2016), which are abundant in coal but also in coal refuse such as shale and mudstone (Hower et al., 1999; Seredin and Dai, 2012). In addition to the detrital component (Finkelman et al., 2019) hydrothermal input can also account for the distribution of REY among the studied coals (Dai et al., 2017). Despite the distinct ΣREYCMM ranges for different CMM fractions, their individual REYCMM/REYNASC patterns were very similar and show a crustal distribution pattern (Fig. 6g), thus, implying that the weathering of any combination of CMM fractions would most likely produce similar REYCMD/REYNASC patterns.
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In southern Illinois, coal seams are intruded by mafic igneous dikes, sills, and diatremes, which are a common feature of surface and underground coal mines (Stewart et al., 2005; Schimmelmann et al.,
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2009; Rimmer et al., 2016). However, at any coal mine location, MIR represent only as a minor
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component of the weathering coal mine wastes due to the overall trivial mined volume of MIR compared to that of CMM. Nonetheless, we used the REYMIR data as a reference for both the Hicks
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Dome composition and igneous intrusions in coal. MIR display distinct REY patterns, having the highest ΣREY values of all the solid matrixes considered (Fig. 6b), the greatest enrichment in LREY (i.e., Ce, La,
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and Nd) (Fig. 6e, h), a distinct Eu anomaly with respect to NASC, and the lowest fraction of critical-ΣREY, dominated primarily by Nd (Fig. 6k). In NASC-normalized patterns, MIR exhibit distinct LREY enrichment
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(Fig. 6h) due to the presence of REY-rich minerals, including monazite, apatite, zircon, and REY-rich fluorocarbonates, previously described in Hicks Dome MIR (Moorehead et al., 2013). The addition of
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MIR to weathering coal mine wastes will produce CMD enriched in Ce, La, and Nd, and overall, in LREY above the levels seen in weathering of CMM.
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The FM samples analyzed in this study had the lowest ΣREYFM values compared to ΣREYMIR and ΣREYCMM (Fig. 6 c). However, FM displayed distinct REY patterns with remarkable enrichment in MREY on ΣREYFM/ΣHREYNASC plots (Fig. 6 g) and the highest ΣHREY/ ΣLREY ratios (Table S3) among solid matrixes. Importantly, Y, the main REY in Illinois CMD (Fig. 5 b, c), was also the dominant REY in FM (Fig. 6 i, l) with YFM concentration values 10-times higher than those of the other REYFM (Fig. 6 l). While fluorite is the main REY-carrier in the IKFD of southern Illinois, other minerals from IKFD such calcite and sphalerite exhibit similar REY patterns, particularly Y and MREY enrichment (data not shown). While our data are limited, the results suggest that the described REYFM patterns might be used as proxies for materials formed from or interacted with hydrothermal fluids (Migdisov and Williams-Jones, 2014; Migdisov et al., 2016). Thus, we speculate that the interaction of REY-bearing MVT hydrothermal solutions with coal and
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Journal Pre-proof associated sedimentary rocks could enrich these solid matrixes generally in REY, and predominantly in Y, in addition to metals (i.g. Zn), producing unique geochemical signatures resembling those of FM.
4.4. The role of host rocks and hydrothermal activity
4.4.1. Host rock-hydrothermal interaction The combined geochemical data for both aqueous (i.e., CMD) and solid (i.e., CMM, MIR, and FM) matrixes (Fig. 7a-f) show evidence for two main factors controlling the REY concentrations and
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patterns in Illinois CMD, namely (1) the overall composition of the source materials, and (2) the degree of hydrothermal alteration of the weathering coal mine wastes. These two factors also had an important
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effect on the critical-ΣREY composition as well as the content of metals of economic value (i.e., Zn, Al,
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Ni, Cr, V, and Co) identified in Illinois CMD.
Overall, our combined critical-ΣREY data for solid and aqueous matrixes (Fig. 7a) are consistent with
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previously published results which showed that the mafic igneous rocks are depleted in critical-ΣREY (Denny et al., 2015) whereas acidic solutions (i.e., hydrothermal, mine drainages) tend to be enriched in
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critical-ΣREY (Stewart et al., 2017). Significantly, in the Illinois Basin, the critical-ΣREYCMD regional patterns (Fig. 7a) mirror those for metal concentrations (Fig. S2), in which a regional enrichment in the
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order of R3 CMD>R2 CMD>R1 CMD could be related to increased hydrothermal imprint on the CMD source materials from the Hicks Dome to the western and northern margins of the Illinois Basin. This
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observation likely implies that CMD originating from parent materials that interacted with hydrothermal solutions would be enriched in ΣREY and most importantly in critical-ΣREY. This hypothesis is also
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supported by critical-ΣREY values of solid matrixes, which showed that FM had the highest fraction of critical-ΣREY and by extension, that past interaction of sedimentary rocks with hydrothermal solutions was necessary to produce weathering coal mine wastes enriched in REY, critical-ΣREY and metals (Fig. 7a; S1). Significantly, the fraction of the critical-ΣREY values to ΣREY for aqueous CMD and solid matrixes, namely CCM, MIR, and FM, from the Illinois Basin are considerably higher than those reported in conventional REY ores, such as those dominated by bastnäsite (typically less than 15%) (Williams-Jones et al., 2012; Van Gosen et al., 2017), but at much lower overall ΣREY concentrations.
4.4.2 Cerium anomalies Illinois CMD are characterized by a narrow range of positive values for Ce/Ce* increasing in the order of R1CMD
Journal Pre-proof have two oxidation states within the Eh−pH regime of CMD, namely Ce(III) and Ce(IV) (Brookins, D.G., 1983). Upon oxidation of Ce(III) to Ce(IV), Ce(IV) exhibits strong affinity for partitioning into the solid phases, usually forming precipitates such as CeO2(s) , which leads to fractionation from its non-redox reactive lanthanide neighbors La(III) and Pr(III) (Brookins, D.G., 1989). Previous studies have showed that surface waters unaffected by mine drainage are more likely to have negative Ce/Ce* values, as expected for oxidative scavenging (Smedley, 1991; Leybourne and Johannesson, 2008), while Appalachian CMD have positive Ce/Ce* anomalies with values mostly lower than 1.3 (Cravotta, 2008; Cravotta and Brady, 2015; Stewart et al., 2017). The positive Ce/Ce* anomalies observed in CMD could be due to: (1)
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selective enrichment of Ce in the weathering coal mine wastes due to higher amounts of Ce-bearing minerals; (2) involvement of biogeochemical processes along the CMD flow path that selectively oxidize
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and mobilize Ce (Yoshida et al., 2004; Bau et al., 2013; Kraemer et al., 2015; Zaharescu et al., 2017);
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and/or (3) enrichment of Ce by adsorption on Fe oxyhydroxides with subsequent release during reductive dissolution (Bau, 1999; Ohta and Kawabe, 2001). The Ce/Ce* values for Illinois CMD were
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much higher than those reported for Appalachia CMD (Stewart et al., 2017), suggesting that source enrichment was probably a controlling factor of Ce/Ce* anomalies in Illinois CMD (i.e., Williams-Jones,
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2012). The source enrichment in Ce could be attributed to higher abundance of Ce-rich REY minerals, such as monazite, apatite, and Ce-rich fluorocarbonates (Moorehead, 2013; Denny et al., 2015).
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Alternatively, since FM had some of the highest Ce/Ce* values (Fig. 7b) this implies that hydrothermal solutions could become enriched in Ce and thus through subsequent interactions capable of producing
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solid matrixes with higher Ce/Ce* ratios (Williams-Jones, 2012; Migdisov et al., 2016). In the Illinois Basin, the CMD samples with elevated Ce/Ce* ratios also exhibit high metal contents (i.e., Zn) suggesting
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that Ce-rich hydrothermal solutions probably played a pivotal role in higher Ce/Ce* ratios in the CMD.
4.4.3 Preferential mobilization of yttrium Yttrium is the main critical-REY in the Illinois CMD (Fig. 5g), with values for the Y contribution to critical-ΣREYCMD up to 77% (Fig. 7c). Although Y shares many physico-chemical characteristics with REE (Bau and Dulski, 1995; Richens, 1997), it has also unique properties among REY (Thompson et al., 2015). Namely, trivalent Y ion, Y3+, has the lowest polarizability (ionization potential) and the lowest covalency relative to other trivalent ions of the REY3+ group (Misono et al., 1967). This is known as the Misono softness parameter (Misono et al., 1967) and for Y it has an anomalously low value compared to the other REY, resulting in anomalous behavior in the environment (Bau et al., 1999; Thompson et al., 2015), such as during water-rock interaction, Y3+ will preferentially remain in solution rather than partitioning 24
Journal Pre-proof into solid matrixes by co-precipitation or sorption processes. Thus, during weathering, Y3+ being more soluble compared to other ions of the REY3+ group, will be preferentially leached out of the solid matrix (Thompson et al., 2015). Indeed, the predominance of Y in REYCMD (Fig. 5b, c) reflects REY variation in Misono softness such as during weathering of coal mine wastes, the CMD preferentially become enriched Y compared to the other REY (Fig. 7c, d). Moreover, if the coal mine wastes contained Y-rich solid matrixes, subsequent to previous interactions with hydrothermal solutions, the weathering of such Y-bearing solid matrixes would lead to Y-rich CMD (Fig. 7d) with high Y share of critical-ΣREY (Fig. 7c) as well as elevated ΣHREY/ΣLREY ratios (Fig. 7e). In the case of solid matrixes, MIR, which were probably
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less affected by hydrothermal solutions had the lowest fraction of Y of critical-ΣREY (Fig. 7c) and the lowest ΣHREY/ ΣLREY ratios (Fig. 7e) while FM representing precipitates from hydrothermal solutions
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had the highest contribution of Y to critical-ΣREY (Fig. 7c) and the highest ΣHREY/ ΣLREY ratios (Fig. 7e).
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The CMD samples have values spanning between these two end members with increasing values probably reflecting increasing hydrothermal source enrichment in Y (Fig. 7c). The preferential
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mobilization of Y among REY in Illinois CMD was also evident when we examined the Y contribution to ΣREYCMD (Fig. 7d). Similar to trends described above, the share of Y contribution to ΣREYCMD for solid
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matrixes increases in the order MIR
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CMD,
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preferentially become enriched Y compared to the other REY (Fig. 7c, d).
4.4.4. Ce/Y as a proxy for fluid-rock interaction
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Cerium, in addition to Y, was also a main REY component in Illinois CMD (Fig. 2d, e). In terms of Misono softness, Y has an anomalously low value while Ce has the second highest value (Misono et al., 1967) and, thus, we expect that the Y/Ce ratios may be a reliable proxy for determining the extent of fluid-rock interactions. While both Ce (Fig. 6d, e) and Y (Fig. 6f) can be enriched in solid matrixes, Y3+ being more soluble and thus more mobile assures that during prolonged water-rock interaction the Y/Ce ratios in solution will progressively increase (Thompson et al., 2015; Williams-Jones et al., 2012). Overall, the average Y/Ce values for solid and aqueous matrixes were 0.2 for MIR, 0.5 for CMM, and 10.5 for FM as well as 2.1 for all CMD. Indeed, both MIR and CMM have the two lowest ratios among all the aqueous and solid matrixes, suggesting limited water-rock interaction, while FM, which are precipitates from hydrothermal solutions, have the highest Y/Ce values (Fig. 7c), while the CMD plot on a continuum between the two end members. 25
Journal Pre-proof Despite the wide range of REY-source materials and local processes that controlled the REYCMD patterns at different sites across Illinois, we found a remarkably strong exponential relationship (R2 = 0.96) between critical-ΣREY and Y/Ce for all the aqueous- and solid-matrix samples considered in this study (Fig. 7f). This strong relationship points towards coherent behavior of REY across various matrixes compositions and spatial scales, probably driven mostly by the anomalously low Misono softness of Y3+ relative to the other REY3+. The preferential mobilization of Y in solution and thus progressive Y enrichment relative to Ce with increased water-rock interaction results in increased Y/Ce ratios as well as increased critical-ΣREY values. This result may reinforce the idea that mine drainage and particularly
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acid drainage have increased capacity to mobilize Y and by extension critical-REY in solution. Therefore, in CMD from the Illinois Basin, Y/Ce ratio may be a key indicator for water-rock interaction, with the
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highest values fingerprinting both enriched sources due to hydrothermal activity and secondary
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enrichment during weathering of parent materials and transport within the CMD-impacted systems. Our small geochemical dataset for Illinois CMD shows regionally distinct signatures for metals
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and REY which mirror the composition of hydrothermal mineralization described for southern (i.e., enrichments in Ba) and northern (enrichments in Al and Zn) parts of the Illinois Basin. Moreover, results
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of this study indicate that in the Illinois Basin, the assessment for the potential of REY recovery from CMD would greatly depend on better understanding the hydrothermal processes and their role in REY
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and metal enrichments in coal and coal mine wastes at various CMD sites across the basin, particularly the CMD sites located along the western and northern margins. These sites could be further targeted for
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REY recovery since both the CMD and coal mine wastes could be enriched in REY (i.e, high ΣREY and critical-ΣREY) as well as other metals of economic value (i.e., Zn, Ni, Al, Cr, and V). Furthermore, mine
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drainage treatment technologies could be designed to target both optimal extraction of REY and valuable metals from CMD and coal mine wastes as well as environmentally friendly CMD remediation efforts.
5. Conclusions The CMD in the Illinois Basin exhibit larger heterogeneities of REY patterns than those described in other USA coal-mining districts, such as the Appalachian Basin (Stewart et al., 2017). Significantly, the Illinois CMD have higher values for ΣREYCMD and critical-ΣREYCMD, with Y and Nd being the main criticalREYCMD. Furthermore, CMD in the Illinois Basin were found to contain small amounts of economically valuable metals (i.e., Zn, Ni, Co, V) that could potentially be co-extracted with REY to enhance the economic values of CMD. 26
Journal Pre-proof Synthesis of geochemical data suggests that hydrothermal activity probably played a key role in producing CMD enriched in metals, ΣREY, and most importantly, in critical-ΣREY. Specifically, our data point towards two sources of REY enrichment in Illinois CMD, namely coal mine wastes containing solid matrixes (1) with high Al and Si contents (i.e., clay minerals and silicates) and (2) impact of hydrothermal solutions enriched in metals (i.e, Zn, Ni, Co) and REY. The sites with the highest ΣREYCMD and critical-ΣREY CMD
contents were predominantly located along the western and northern margins of the Illinois Basin.
Particularly, sites with the highest hydrothermal input as approximated by Mississippi-Valley-type mineralization are prime candidate for REY and other metals (i.e., Zn, Ni, Co, Cu), potentially making
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Illinois CMD an attractive alternative source for REY and metals recovery.
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Acknowledgments
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This paper is dedicated to the memory of Prof. John C. Crelling. The Illinois Department of Natural Resources (IDNR) assisted our team with various tasks during the duration of this study. Angie Mick and Gregory Pinto of IDNR, Office of Mines and Minerals assisted in selecting the sampling sites as well as with the collection of coal mine drainage samples. Partial funding for this study was provided by the Geology Department-SIUC Porter Jobling grant to Kyle Klitzing. Coal sample analysis was supported in part by USGS Cooperative Agreement G16AC00449 to Southern Illinois University. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government. ■ ASSOCIATED CONTENT The Supporting Information is available free of charge on the Elsevier Publications website at DOI: 1xxx
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■ AUTHOR INFORMATION Corresponding Author *Phone: (618) 453-7373. E-mail: [email protected]
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Figure 1: Map of the Illinois Basin showing the location of coal mine drainage (CMD) sites sampled. Based on the proximity to the Hick’s Dome, we designated three distinct regions, namely: (1) Region 1, R1, comprised sites situated in southern and southeastern Illinois, in close proximity to the Hicks Dome; (2) Region 2, R2, comprised sites situated in western Illinois; (3) Region 3, R3, comprised sites situated in northern Illinois. Also indicated is the extent of Pennsylvanian-age coal-bearing strata in the Illinois Basin.
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Figure 2: Composite figure depicting: (a) ranges of pH values for R1-CMD, R2-CMD, and R3-CMD; (b)
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Ranges of total REY concentration values, ΣREY, for R1-CMD, R2-CMD, and R3-CMD; (c ) share of individual REY of ΣREY for all CMD; REYCMD concentrations are presented normalized to the North
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American Shale Composite (NASC; Gromet et al., 1984) for R1-CMD (d), R1-CMD (e ), and R1-CMD (f).
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Figure 3: Comparison of ΣREYCMD and (a) pH values, as well as the concentrations of dissolved major
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elements (b) aluminum Al; (c) silicon Si; (d) total iron Fe; (e) sulfate SO4; (f) manganese Mn; (g) calcium
logarithmic scale.
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Ca; (h) potassium K; and (i) phosphate PO4 in the Illinois CMD. The concentration values are plotted on a
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Figure 4: Comparison of ΣREYCMD and the concentrations of trace elements: (a) zinc Zn (b) nickel Ni; (c)
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cobalt Co; (d) copper Cu; (e) barium Ba; (f) vanadium V; (g) chromium Cr; (h) uranium U; and (i) cadmium
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Cd in the Illinois CMD. The concentration values are plotted on a logarithmic scale.
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Figure 5: Composite figure depicting geochemical patterns of critical-REYCMD: (a) comparison of criticalΣREYCMD and pHCMD values; (b) comparison of critical-ΣREYCMD and ΣREYCMD; (c) box and whisker plots of the individual critical-REYCMD range of values; (d) ) box and whisker plots of the share of individual critical-REYCMD; (e) comparison of critical-ΣREYCMD and YCMD concentration values; (f) comparison of critical-ΣREYCMD and ΣHREYCMD / ΣLREYCMD ratios. The concentration values are plotted on a logarithmic scale.
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Figure 6: Composite figure depicting the REY patterns for solid matrixes: ranges of ΣREY values for (a) coal mining and processing materials (CMM) from an active underground coal mine and a coal preparation plant in Southern Illinois; (b) mafic igneous rocks (MIR) from IKFD; and (c) fluorite mineralization from IKFD; box and whisker plots of the individual REY range of values for (d) CMM; (e) MIR; and (f) FM; REY concentrations normalized to the North American Shale Composite values for (g) CMM; (h) MIR; and (i) FM; box and whisker plots of the individual critical-REY range of values for (j) CMM; (k) MIR; and (l) FM.
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Figure 7: Composite figure comparing the REY geochemical patterns for aqueous (R1-CMD, R2-CMD, and R3-CMD) and solid (CMM; MIR, FM) matrixes considered in this study: (a) box and whisker plots of the share of individual critical-REY; (b) Ce/Ce* ratios defined as CeNASC/(LaNASC+PrNASC) (Bau and Dulski, 1996); (c) box and whisker plots for the fraction of Y of the critical-ΣREY; (d) box and whisker plots for the fraction of Y of the ΣREY; (e) box and whisker plots for the ΣHREY/ ΣLREY ratios; (f) comparison of Y/Ce and fraction of critical-ΣREY values. 43
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Journal Pre-proof Declaration of interests
☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
☒The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
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Liliana Lefticariu
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Associate Professor Southern Illinois University
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Carbondale Illinois
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Journal Pre-proof Highlights
Illinois basin coal-mine drainages (CMD) have high contents of ΣREY as well as critical-ΣREY
CMD with high ΣREY contents have also high metal (i.e., Zn, Ni, Co) contents
CMD rich in trace metals and REY are mostly located along the west and northern part of the Illinois basin
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Y/Ce ratios can be used as a proxy for predicting matrixes with high critical-ΣREY
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Liliana Lefticariu, Kyle L. Klitzing, Allan Kolker PII:
S0166-5162(19)30912-7
DOI:
https://doi.org/10.1016/j.coal.2019.103327
Reference:
COGEL 103327
To appear in:
International Journal of Coal Geology
Received date:
12 September 2019
Revised date:
18 October 2019
Accepted date:
29 October 2019
Please cite this article as: L. Lefticariu, K.L. Klitzing and A. Kolker, Rare Earth Elements and Yttrium (REY) in coal mine drainage from the Illinois Basin, USA, International Journal of Coal Geology(2019), https://doi.org/10.1016/j.coal.2019.103327
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
© 2019 Published by Elsevier.
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Rare Earth Elements and Yttrium (REY) in coal mine drainage from the Illinois Basin, USA Liliana Lefticariu1*, Kyle L. Klitzing1, and Allan Kolker2 1
Department of Geology, Southern Illinois University, Carbondale, IL 62901, USA U.S. Geological Survey, 956 National Center, Reston, Virginia 20192 USA
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*Corresponding author: email [email protected], phone: 618 453 7373; fax: 618 453 7393
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Graphical abstract:
Keywords: Rare Earth Elements and Yttrium, Illinois Basin, coal mine drainage, hydrothermal activity. Highlights 1
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Illinois basin coal-mine drainages (CMD) have high contents of ΣREY as well as critical-ΣREY
CMD with high ΣREY contents have also high metal (i.e., Zn, Ni, Co) contents
CMD rich in trace metals and REY are mostly located along the west and northern part of the Illinois basin
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Y/Ce ratios can be used as a proxy for predicting matrixes with high critical-ΣREY
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Journal Pre-proof ABSTRACT The ever-growing demand in the USA and in the modern economy for rare earth elements, defined here as the lanthanide elements plus yttrium (REY), motivates the development of economically feasible and environmentally friendly approaches for domestic REY resources. Non-conventional sources, including coal mine drainage (CMD) hosting elevated concentrations of REY, have been explored as attractive secondary sources for metals recovery. Consequently, in this study we investigate CMD from abandoned coal mines in the Illinois Basin as a potential REY resource. We collected CMD from 35 abandoned coal mine sites in the Illinois Basin and investigated trends in
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REY enrichment and their association with CMD geochemical characteristics, including pH, the presence of other chemical elements of economic significance (i.e., Zn, Ni, Cr, Cu, V) as well as spatial patterns in
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relation to Permian magmatic intrusions of the Hicks Dome, a potential source of REY in the Illinois
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Basin. Additionally, the REY contents in possible source rocks for Illinois CMD were measured including coal from an underground mine and coal preparation plant in Southern Illinois, mafic igneous rocks
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associated with Hicks Dome, and hydrothermal fluorite from Illinois fluorspar district. The REY concentrations in samples of CMD (filtered fraction) and associated coal and non-coal solids
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were determined by ICP-MS. The total concentration of REY in CMD, ΣREYCMD varied between 1 and 9,879 µg L−1 with an average of 1,059 µg L−1 and critical-ΣREYCMD (Nd, Eu, Tb, Dy, and Y) varied between
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0.3 and 7,577.5 µg L−1 with an average of 637 µg L−1. Basin-wide, we found no statistical correlation between ΣREYCMD and (1) pH of the CMD, or (2) the proximity to the Permian igneous magmatic
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intrusion at Hicks Dome in Southern Illinois. The REYCMD values normalized to North American Shale Composite (NASC) REYCMD/REYNASC exhibit enrichments in middle-REY for all CMD, but also enrichments
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in light-REY in some CMD. Statistically, we observed that ΣREYCMD displays significant direct correlations with Al, Si, SO4, Zn, Ni, Co, and Cu and no-correlation with Fe, PO4, Ba, and V. Synthesis of the geochemical data suggests that coal mining sites with the highest ΣREYCMD and critical-ΣREY contents include non-coal bedrock dominated by silicates (high clay content) and coal mine wastes that were impacted by metal-rich hydrothermal solutions, which are predominantly located along the western and northern margins of the Illinois Basin. Particularly, sites with the highest hydrothermal input as approximated by Mississippi Valley-type mineralization (i.e., Zn contents) are prime candidates for REY, as well as other elements of potential economic value such as Al, Ni, Cr, V, and Co. Thus, CMD in the Illinois basin could become a potential alternative domestic source of extractable REY, especially critical-ΣREY, as well economically valuable metals.
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Journal Pre-proof 1. INTRODUCTION Rare earth elements (REE) and yttrium (hereafter REY) are a group of 16 elements comprising the lanthanides and yttrium that exhibit similar physicochemical characteristics and tend to co-occur in nature (McLennan, 1989; Wedepohl, 1995; IUPAC, 2005). Scandium is chemically similar to the lanthanides and yttrium, and is sometimes included in the REEs, but it does not occur in the same geologic settings (Van Gosen et al., 2014) and is not considered further in this study. The REE are generally sub-divided based on their atomic numbers into light-REE (LREE), also called the “cerium group,” including lanthanum (57) to europium (63); middle-REE (MREE), including Sm, Eu, Gd, Tb, and
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Dy, and heavy-REE (HREE) including gadolinium (64) to lutetium (71), also called the “yttrium group” (Henderson, 1984; Dołęgowska and Migaszewski, 2013). Yttrium (Y, Z=39) is not a lanthanide, however,
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is usually grouped with the HREY because of their geochemical similarities (Dai et al., 2016a). The above
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classification is often used to emphasize the enrichments of a specific group of REY in some environments, such as of MREY in acid mine drainage (Serano et al., 2000; Verplanck et al. 2004;
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Steward et al., 2017; Sun et al., 2012; Sahoo et al., 2012; Grawunder et al., 2014). Commercial uses of REY include incorporation in numerous emerging technologies (i.e., catalysts,
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fuel cells, permanent magnets used in wind turbines and electric/hybrid car motors, photovoltaics, and hybrid car batteries) and consumer products (i.e., phones, computers, appliances, and automotive
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sensors) (Alonso et al., 2012; Goodenough et al., 2018.). REY are relatively abundant in the Earth’s crust, with average crustal contents between 160 and 205 mg kg−1 (Wedepohl, 1995). Still, they are relatively
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immobile through most geological processes and thus rarely form their own minerals or concentrate in ore deposits that are technologically possible to mine and economically profitable (Wedepohl, 1995;
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DOE, 2011; Long et al., 2012; Weng et al., 2013; Van Gosen et al., 2017). In the USA, there are a limited number of REY reserves (DOE, 2011; Graedel et al., 2015; Van Gosen et al., 2017) and currently (2019) REY production is limited to one mine, at Mountain Pass, CA where bastnäsite ore is produced (Gambogi, 2019). The remaining production is from imports, primarily from China. Significantly, the national demand of REY is projected to increase rapidly within the next several decades and many REY with high criticality, so called critical-REY, namely Nd, Eu, Tb, Dy, and Y (cf. DOE, 2011; Goodenough et al., 2018; Roskill, 2019), are already in high demand (Alonso et al., 2012; Gambogi, 2019). Limited natural reserves and rapid growth in demand is also of global concern (McLellan et al., 2014; Migaszewski and Gałuszka, 2015) since restricted market availability of REY could lead to supply risk, vulnerability to supply restriction, and environmental implications (Graedel et al., 2015).
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Journal Pre-proof Securing potentially sustainable domestic sources of these commercially critical elements has become increasingly important, requiring the diversification of supplies to include recovery of REY from recycling (Binnemans et al., 2013) and alternative resources such as coal (Seredin and Dai, 2012; Hower et al. 1999; Hower et al. 2016; Honaker et al 2017; Dai and Finkelman, 2018; Finkelman et al., 2019), coal combustion byproducts (Hower et al., 2013; Taggart et al., 2016; Kolker et al., 2017; Smith et al., 2019), coal mine waste (Honaker et al., 2017, 2018; Zhang and Honaker, 2018.), and mine drainage (Ayora et al., 2016; Ziemkiewicz et al., 2016; Hedin et al., 2019). Geochemical studies of coal mine drainage (CMD) from the Appalachian Basin, USA (Ziemkiewicz et
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al., 2016; Stewart et al., 2017), China (Zhao et al., 2007; Sun et al., 2012), and India (Sahoo et al., 2012), as well as drainage from metal mining districts in Europe (Grawunder et al., 2014; Ayora et al., 2016),
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show that mine drainage is often enriched in total REY (ΣREY). The feasibility of economic recovery of
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REY from metal and coal mine drainage will depend on factors such as its ΣREY, the share of criticalΣREY, and the ability to co-extract additional elements of economic value as well as the contaminant
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contents, operational cost, and capital cost. CMD may become an important target for REY recovery because REY are present largely in a dissolved form and ΣREYCMD is often elevated, in the range of
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hundreds to thousands of µg/L (Ziemkiewicz et al., 2016; Hedin et al., 2019). Furthermore, many CMDs have been shown to contain elevated contents of other metals, including Zn, Cu, Al, Cr, and V (Cravotta,
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2008; Lefticariu et al., 2015; Ayora et al., 2016; Stewart et al., 2017), thus making CMD an attractive potential source for metals recovery. Notably, recovery of REY and valuable metals from CMD and coal
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mine waste products can occur as a co-benefit of environmental remediation of abandoned and/or active coal mining sites (Ayora et al., 2016; Hedin et al., 2019).
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Potential REY resources in the Illinois Basin were reported in association with both the hydrothermal fluorite and the Permian mafic igneous intrusions at the Hicks Dome in southern Illinois (Moorehead, 2013; Denny et al., 2015; Gambogi, 2019). Hydrothermal ore deposits in the Illinois Basin includes well-known mining districts (1) Illinois-Kentucky fluorite district (IKFD) in Southern Illinois and Western Kentucky (Richardson and Pinckney, 1984; Trace and Amos, 1984; Chesley et al., 1994; Plumlee et al., 1995; Kendrick et al., 2002; Denny et al., 2015) and (2) the Upper Mississippi Valley Zn-Pb district in northern Illinois (Hatch et al., 1976; Heyl and West, 1982). In the IKFD, fluorite in association with calcite with lesser amounts of sphalerite, galena, and barite were main ore components of the Mississippi Valley-type (MVT) mineralization (Richardson and Pinckney, 1984; Chesley et al., 1994; Denny et al., 2015). In northern Illinois, Cobb (1981) identified sphalerite in cleat and clastic dikes of Middle Pennsylvanian-age coal beds while the Upper Mississippi Valley Zn-Pb mineralization was 5
Journal Pre-proof dominated by sphalerite within a kaolinite-pyrite-sphalerite(pyrite)-paragenetic sequence (Cobb, 1981; Whelan et al., 1988; Kolker and Chou, 1994). Studies have indicated that even through the MVT mineralization was somehow different in southern and northern Illinois, the two districts could have been part of a single hydrothermal system, which probably originated at the Hicks Dome from where fluids migrated toward the edges of the basin where they crop out at different locations (Cobb, 1981; Kenderes and Appold, 2017). Alternatively, tectonically driven flow in the basin was responsible for the thermal anomalies in the Illinois Basin and the MVT deposits (Bethke, 1986; Rowan et al, 2002). The Permian mafic igneous intrusions at the Hicks Dome (Fig. 1) comprise a mineralized intrusive
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center of alkaline mafic dikes, sills, and diatreme breccias (Bradbury and Baxter, 1992; Moorehead, 2013; Denny et al., 2015). Numerous mafic igneous dikes, sills, and diatremes are also common features
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in coal mines in southern Illinois (Hower et al., 2002; Stewart et al., 2005; Schimmelmann et al., 2009;
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Quaderer et al., 2009; Rimmer et al., 2016) as well as at discrete locations in the Reelfoot Rift – Rough Creek Graben in southern Illinois (Maria et al., 2019) and northwestern Kentucky (Hower et al., 2000;
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2001). Exploration drilling at Hicks Dome has revealed possible resources of REE, F, Zn, Pb, and Ba mostly associated with fluorite ores that usually occur as veins, breccia infilling, and country rock
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replacement (Moorehead, 2013; Hower et al., 2000; Jackson and Christiansen, 1993). The mafic igneous rocks and diatremes are enriched in LREY, primarily Ce, and exhibit elevated ΣREY values up to 1,285
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mg/kg, whereas the fluorite and ore-stage calcite associated with hydrothermal mineralization showed enrichments in HREY, dominantly in Y, and overall lower ΣREY with concentrations <106 mg/kg (Denny
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et al., 2015). Therefore, we hypothesized that the magmatic-hydrothermal activity centered at Hicks
trace elements.
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Dome, postdating coal formation, affected the regional distribution of REY and possibly other major and
Coal and coal mining products contain variable amounts of REY (Dai et al., 2016a, b; Finkelman et al., 2019; Yan et al., 2019; Zhao et al., 2019), however, limited REY content data of coal and coalmine products from the Illinois Basin are currently available (Bryan et al., 2015). Recent studies reported that the coal ash from the Illinois Basin has generally lower ΣREY as compared to samples from the Appalachian Basin, but higher REY contents than coal ash derived from western coals (Taggart et al., 2016). In the present study, we investigated the geochemical characteristics of REY as well as of major and trace elements in CMD collected at 35 abandoned coal mine sites across Illinois. We also report REY data for solid matrices, which could represent parent-weathering materials for the generation of CMD, including: (1) samples of raw coal collected from an underground coal mine (raw coal), coal mining 6
Journal Pre-proof materials feeding preparation facilities (feed coal), refuse coal mining materials after preparation (coal refuse), and prepared coal output from a coal preparation facility in southern Illinois (prepared coal), (2) Permian mafic igneous rocks from southern Illinois, and (3) fluorite ore from the IKFD. We hypothesized that geochemical characteristics of the CMD and the proximity of a given CMD site to the mafic igneous intrusion at Hicks Dome are the two most important factors controlling the concentrations and patterns of dissolved REYCMD. Specifically, we tested whether there is: (1) a relationship between ΣREYCMD and pHCMD; and/or (2) a spatial relationship between the elevated values for ΣREYCMD and the proximity of CMD site to the Hicks Dome. Additionally, trends in ΣREYCMD and
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critical-ΣREYCMD (DOE, 2011; Seredin and Dai, 2012) were examined in relation to major and minor element contents in CMD as well as to spatial correlation of the geochemical trends relative to different
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structural and tectonic features in the Illinois Basin. These data were used to identify possible regional
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patterns for CMD with elevated contents of critical-ΣREYCMD and other economically valuable metals in
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CMD of the Illinois Basin.
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2. Materials and Methods
2.1. Study Area
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Coal mine drainage samples were collected at 35 sites of inactive coal mines across Illinois during in summer of 2013 and in spring and summer of 2017 (Fig. 1; Table S1). These sites represent
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abandoned coal mine operations with or without passive remediation systems. In many cases, the mining and remediation history of the abandoned coal mine sites is not fully known, however, the
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source of CMD predominantly included weathered Pennsylvanian coal and coalmine wastes. At some locations, such as the Tab Simco site (Behum et al., 2011), remediation systems were employed to address environmental issues such as low pH and metal discharge associated with CMD. At each site, we sampled a discrete outflow, most often from the main seep connected to a coal mine portal. The sampling sites were selected to represent a range of water chemistries (i.e., pH, TDS, sulfate, and metal contents) situated at various distances from the Hicks Dome mafic igneous complex. The CMD included acidic discharges (pH 1.9 to 4.5) as well as circumneutral and alkaline discharges with pH up to 7.5 (Fig. 2a). The wide range of discharge chemistry is representative of CMD on a basin-scale. Based on the proximity to the Hick’s Dome, we defined three distinct regions found on the southern, western, and northern margins (Fig. 1): (1) Region 1 (R1) comprised locations situated in southern and southeastern Illinois, in close proximity to the Hicks Dome, with most sampling sites situated along the 7
Journal Pre-proof Cottage Grove fault system. (2) Region 2 (R2) comprised locations situated in western Illinois along the Du Quine Monocline and included the Tab Simco site; (3) Region 3 (R3) comprised locations situated in northern Illinois, farthest from the Hicks Dome. Coal mining wastes at each CMD site contained heterogeneous materials, which, in addition to coal, include bedrock materials, coal refuse, and other coal mining-associated solid matrices. Geochemical information for these materials is critical to understanding and predicting the REY contents in Illinois CMD. In most cases, solids corresponding directly to CMD at a given site were not available for sampling. To circumvent this problem, we collected and analyzed three solid matrices, coalmine
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materials (CMM), mafic igneous rocks (MIR), and fluorite mineralization (FM) as analogues for solid
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materials which may be present at CMD sites in Illinois.
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2.2. Water sample collection
Aqueous samples were collected in 250-mL high-density linear-polyethylene (HDPE) sample
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bottles which were thoroughly pre-cleaned using trace hydrochloric acid and DI water procedures before being transported to the field. During each of the field sampling campaigns, measurements of
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the field parameters, including pH, temperature, specific conductance (SC), oxygen reduction potential (ORP), and dissolved oxygen (DO), were performed on unfiltered samples immediately following sample
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collection using a Hanna® multi-sensor probe. The pH electrode Hanna HI769828-1 field probe (pH/ORP) was calibrated with Orion pH buffers at 1.68, 4.01, and 7.00 and then checked against a pH 10 buffer.
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The oxidation-reduction potential (ORP) was measured using a factory-calibrated Hanna HI769828-1 Ag/AgCl probe and a single point calibration was performed using a Hamilton redox (ORP) buffer (475 ±
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5 mV). Dissolved oxygen (DO) was determined with a Hanna HI769828-2 amperometric probe calibrated to on-site atmospheric oxygen as 100% DO. The location of the CMD samples and the field parameters are presented in Table S1.
Upon collection, all samples were stored ~4 °C and transported directly to SIUC Geochemical Laboratory. In the laboratory, the CMD samples for the determination of anions, cations, and REY were immediately filtered through 0.45 µm cellulose acetate filter papers (Millpore® HAW). Laboratory standards and reagents were prepared using deoxygenated DI water from Milli-Q/Milli-Q Ultra Plus water system (>18 MΩ cm−1) with a UV photo-oxidation. For cation and REY analyses, one split of the filtered water sample was saved in a pre-cleaned 60-mL HDPE bottle and then immediately acidified to pH<2 using ultra-pure HNO3 (Fisher Scientific™). Alkalinity was measured on filtered, un-acidified water samples using a Hach™ digital titrator (Model AL-DT), a Hanna® multi-sensor probe, and 0.16-N H2SO4 8
Journal Pre-proof (for details see Lefticariu et al., 2015). Total alkalinity was measured in effluent samples that had pH values greater than 4.5 during field measurement (Table S1). A separate filtered, non-acidified aliquot was saved for anion analysis.
2.3. Solid matrix materials To understand the occurrence and distribution patterns of REY in CMD from Illinois, we collected three distinct types of solid matrix materials which we consider possible sources of REY to CMD that included: (1) coal mining and processing materials (CMM) from an active underground coal mine and a
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coal preparation plant in southern Illinois that was extracting and processing Herrin (No. 6) coal; (2) mafic igneous rocks (MIR) collected from an underground coal mine and locations in proximity to the
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Hicks Dome district in southern Illinois, and (3) fluorite mineralization (FM) from locations within the
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Illinois Kentucky Fluorite District (IKFD).
The Illinois Basin coals are commonly prepared prior to delivery to reduce mineral matter and
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improve heating value. During this process, two main CMM fractions are created, namely (1) the treated coal, which is sent to power plants for energy production, and (2) the coal refuse most likely to be
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stored at each coal mine site (Behum et al., 2018). For this study, we collected and analyzed samples of raw coal collected from an underground coal mine (raw coal), coal mining materials feeding the
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preparation facility (feed coal), refuse coal mining materials after preparation (coal refuse), and prepared coal output from a coal preparation facility in southern Illinois (prepared coal). At the time of
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sample collection, the mine was producing only Herrin (No. 6) coal, which was directly fed into the preparation plant, so all CMM fractions analyzed in this study are representative of one coal seam at
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one location. Additional geochemical information for Illinois Basin coals from operating mines is given by Kolker et al. (2019). From the same underground mine, we collected two samples of mafic igneous rock (MIR), namely a sample representing an igneous dike and a breccia sample. Additionally, in our analysis, we included previous REY data on Permian MIR from southern Illinois (Denny et al., 2015) and fluorite mineralization (FM) from the IKFD (Denny et al., 2015; Kipp et al., 2014). Information about sample type and REY contents of all solid matrices are presented in Table S4.
2.4 Analytical Methods All geochemical analysis of aqueous solutions, including anion, cation, and REY analyses, were performed on filtered samples. In this study, the dissolved split following filtration was exclusively included in geochemical analysis and the filtered colloids or solid materials were not measured. 9
Journal Pre-proof Major anions in filtered CMD were analyzed by ion chromatography (Dionex® ICS 2000) using an IonPac® AS18 anion-exchange column at Southern Illinois University. The standard deviation for duplicate sample analyses was consistently <5% for anions (i.e., SO42-, Cl-, F-). Samples for cation analysis, including REY were acidified to a 5 wt/v% trace-metal grade HNO3 (ACS certified) solution and analyzed on a contract basis by the ACME Laboratories, Inc. (Bureau Veritas Commodities), Vancouver, British Columbia, Canada (Table S2; S3). All samples were analyzed by ICP-MS, including two field replicates and one blank (18.2 MΩ.cm water) for all the CMD samples, in low-, medium-, and high-resolution modes, depending on individual element spectral values. A mixture of single-element standard solutions (STD
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TMDA-70 and STD TMDA-70.2) was used for instrument calibration. The analytical precision (±1σ) of standard concentration values was within 1%, whereas sample reproducibility was typically <±3%. All
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blanks were below detection. Resent research showed that accurate Eu concentration measurements by
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ICP-MS of coal and coal-related materials can be affected by high Ba concentrations if the Ba/Eu concentration ratios are > 1000 (Yan et al., 2018). However, most of the CMD samples have Ba/Eu ratios
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<20, with only one sample (S1) has a high Ba/Eu ratio of 1556, and therefore for the CMD samples considered in this study the reported Eu concentration values are accurate.
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Rare earth elements and yttrium (REY) for solid matrix materials were determined by ICP-MS at Activation Laboratories, Inc. (Actlabs), Ancaster, Ontario, Canada. Prior to analysis, 60-mesh splits of
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bulk samples were combusted by Actlabs at 525°C for 36 h to produce laboratory ash used in ICP-MS determinations. Compared to the ASTM D3174 ash yield determination at 750°C, coal combustion at
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525°C is thought to limit loss of moderately volatile trace elements, such as arsenic (As) and antimony (Sb), while retaining all but the most volatile elements such as mercury (Hg) and selenium (Se);
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(Golightly and Simon, 1989; Palmer, 1997). After laboratory combustion at 525°C, the resulting ash was digested using a sodium peroxide fusion digestion approach. Sodium peroxide fusion is superior to acid digestion for the decomposition of insoluble trace phases that carry REE (Meier, 1997). Detection limits by ICP-MS are 0.1 mg/kg (ppm) for the REE and Y with the exception of Ce (0.8 mg/kg), La and Nd (each 0.4 mg/kg), Dy (0.3 mg/kg) and Ho (0.2 mg/kg). The detection limit for major elements is typically 0.01%, but with addition of sodium peroxide, sodium cannot be determined as a major oxide. The REY contents of the raw coal, feed coal, and coal refuse sample collections (Table S4) are reported on whole sample basis.
3. Results
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Journal Pre-proof 3.1. Geochemistry of coal mine discharge (CMD)
3.1.1. Major and trace elements The geochemical results of all CMD, including field and laboratory data, are presented in Table S1, S2, and S3. Sulfate was the main anion present in CMD and in most cases representing >95% of the total anion load, with minor input from Cl and F (Table S2). Major cations measured included Fe followed by Al, Si, Ca, Mg, Mn, Na, and K. Overall, CMD samples from the Illinois Basin exhibit a fairly wide range of values for pH (1.9-7.5), TDS (532-6,525 mg/L), as well as dissolved ions, including SO4 (510-
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35,880 mg/L), Fe (1.8-3,633 mg/L), and Al (0.02-1,386 mg/L) (Table S2). It is noteworthy that one sample, SIU-41 of R3-CMD, exhibited unusually high values for SO4 (35,875 mg/L), Al (1,386 mg/L), Mg
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(6,890 mg/L), Mn (399.9 mg/L), and most of the metals (Table S2).
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Like major elements, trace elements detected in Illinois CMD display a wide range of concentration values, including for Zn (20.9-68,992 µg/L), Ni (0. 2-18,588 µg/L ), Co (2.7-8,925 µg/L), Li
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(72.8-37,804 µg/L), Cu (9.9-379.2 µg/L), V (0.2-275.0 µg/L), Cr (0.5-141.7 µg/L), U (0.1-160.8 µg/L), and Cd (0.1-688.3 µg/L) (Table S2). Among metals in CMD, Zn was the dominant element with values up to
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97% of the transitional metal load, followed by Ni (up to 32%), Co (up to 15%) and Cu (up to 4%). Additionally, Cd, V, Cr, U, and Pb have minor (<1%) contributions.
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Regionally, CMD geochemical parameters exhibited the following major spatial trends: (1) the average pH value was highest for R1-CMD at 4.3 followed by R3-CMD at 2.7 and R2-CMD at 2.6 (Fig. 2a);
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(2) SO4, Fe, and Mn have highest average concentrations in R2-CMD followed by R3-CMD (with the exception of sample SIU-41) and R1-CMD; (3) Al, Si, Mg, and Na have the highest average concentrations
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in R3-CMD followed by R2-CMD and R1-CMD; and (4) Ca and K have the highest average concentrations as well as the widest ranges of values in R1-CMD followed by R2-CMD and R3-CMD (Fig. S1). In terms of ΣREYCMD-metal relationships, similar regional patterns to those for metals were mapped, where the highest average concentrations were recorded in R3-CMD followed by R2-CMD and R1-CMD (Fig. S2). The site-specific geochemical composition of CMD was probably related to the inherent composition of coal mining wastes as well as the presence of materials added as treatment options for some abandoned mines (e.g., limestone).
3.1.2. Rare Earth Elements and Yttrium (REY) The ΣREYCMD concentrations varied between 0.69 µg/L and 9,879 µg/L with averages of 1,058 µg/L for all the samples as well as averages of 475 µg/L for R1-CMD, 1091 µg/L for R2-CMD, and 4,742 11
Journal Pre-proof for R3-CMD (Table S3). The unusually high average ΣREYCMD concentrations in R3-CMD was due to one sample SIU-41, which, in addition to high concentrations for major and minor elements (Table S3), had an exceptionally high ΣREYSIU41 value of 9,879 µg/L (Table S3). Overall, Y, Ce, and Nd exhibited the highest concentrations and the largest ranges of REYCMD values (Table S3), while in terms of percentage of individual element of ΣREYCMD, Y was the main REYCMD followed by Ce, Nd, Gd, Dy, Sm, and La (Fig. 2c). In Figs. 2d-f, REYCMD concentrations for R1, R2, and R3 are presented normalized to the North American Shale Composite (NASC; Gromet et al., 1984). The REYCMD/NASC patterns showed notable enrichment in Y and MREY (Fig. 2b). MREY enrichments similar to those shown in Fig. 3b were reported
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in drainage from coal mines in the Appalachian Basin, USA (Stewart et al., 2017), China (Zhao et al., 2007; Sun et al., 2012), India (Sahoo et al., 2012), and metal mining districts in Europe (Grawunder et al.,
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2014; Ayora et al., 2016). Regionally, while the overall REYCMD/NASC patterns for R1 (Fig. 2d), R2 (Fig. 2e),
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and R3 (Fig. 2f) are broadly similar with enrichment in MREY as the primary feature, key differences were also present, such as higher values for Gd and Dy for R1-CMD and predominantly Y and Gd for R2-
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CMD and R3-CMD. More specifically: (1) R1-CMD had the highest concentrations and largest range of values for Y, Ce, and Nd with highest percentage contribution to ΣREYCMD coming from Y, Ce, Nd, and La;
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(2) R2-CMD had the largest ranges of REY values for Y, Ce, and Nd, with highest percentage contribution to ΣREYCMD being from Y, Ce, and Nd, followed by La, Gd, and Dy; and (3) R3-CMD showed unique
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patterns with Y exhibiting high levels in terms of both values and percentage contribution to ΣREYCMD. It is noteworthy that sample SIU-41 had the ΣREY content (9,879 µg/L) approximately 10-times greater
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µg/L).
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than average value for ΣREYCMD (1,058 µg/L) as well as high contents for Gd (244.6 µg/L) and Y (264.7
3.1.3. Correlations of ΣREYCMD with major and trace elements Major and trace element data and correlations with pH and ΣREYCMD are presented in (Figs. 3, 4; Table S5). Similar to previous reports (Stewart et al., 2017), the pH had a major impact on ΣREYCMD contents, with significantly higher ΣREYCMD values in low-pH (< 4.5) CMD) compared to more alkaline CMD (Fig. 3a). All the CMD with alkaline pH were in R1, region closest to Hicks Dome (Fig. 2a). Generally, no statistical correlation was found between pH and ΣREYCMD (r = -0.25, p = 0.11; Table S5), with even poorer statistics if we consider only the CMD samples with pH <5 (r = -0.08, p = 0.67). Additionally, except for Si (r = -0.58, p < 0.005), none of the major or trace elements measured in Illinois CMD showed statistically significant correlations with pH. These results are surprising (Table S5) and probably reflect both the heterogeneous composition of the weathering coal mine wastes at different 12
Journal Pre-proof mine sites across the Illinois Basin as well as the prevailing geochemical conditions characterizing CMD in the Illinois Basin that were optimized to solubilize and mobilize these specific elements for a variety of original host phases. In contrast to pH, most of the major and trace elements displayed significant statistical correlation with ΣREYCMD (Table S5; Figs. 3b-i). Among the major elements, Al exhibits the strongest linear correlation with ΣREYCMD (r = 0.93, p < 0.005; Fig. 3b) for all CMD samples regardless of region. Similarly, good correlations with ΣREYCMD were measured for Si (r = 0.42, p = 0.006; Fig. 3c), Mg (r = 0.88, p < 0.005), Na (r = 0.59, p < 0.005), and Mn (r=0.91, p < 0.005; Fig. 3f). While SO4 showed a linear trend
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and a statistically significant correlation (r= 0.91, p < 0.005; Fig. 3d) with ΣREYCMD, Fe showed no correlation with ΣREYCMD (r= 0.13, p = 0.4; Fig. 3e) or with SO4 (r= 0.27, p = 0.08), suggesting that REY
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were not present in pyrite. Likewise, a wide range of values was measured for Ca (400-600 mg/L) and
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PO4 (0.03-11.12 mg/L) with no particular pattern for any region as well as no correlation with ΣREYCMD either for Ca (r= 0.24, p = 0.13; Fig. 3g), K (r= 0.20, p = -0.06; Fig. 3h), or for PO4 (r= 0.38, p = 0.01; Fig. 3i).
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The ΣREYCMD also displayed significant statistical correlation with many trace elements (Table S5; Figs. 4a-i), including for Zn (r= 0.37, p = 0.02; Fig. 4a), Ni (r= 0.90, p < 0.005; Fig. 4b), Co (r= 0.91, p <
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0.005; Fig. 4c), Cu (r= 0.71, p < 0.005; Fig. 4d), Li (r= 0.89, p < 0.005), Cr (r= 0.33, p = 0.03; Fig. 4g), U (r= 0.82, p < 0.005; Fig. 4h), and Cd (r= 0.87, p < 0.005; Fig. 4i). One key exception is V that did not show any
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statistical correlation with ΣREYCMD (r= 0.13, p = 0.43; Fig. 4f) and for which the highest concentration was recorded in R2-CMD, followed by R1-CMD and R3-CMD. Interestingly, Ba was the only trace
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element to show an inverse correlation with ΣREYCMD (r= -0.15, p = 0. 43; Fig. 4e).
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3.1.4. Geochemical patterns of critical-REYCMD In this study, the critical-REY include five REY, namely Nd, Eu, Tb, Dy, and Y, which were classified by US Department of Energy as “critical resources” due to their high economic demand for both the short and long term (DOE, 2011; USGS, 2018). Analogous to trends described above, the critical-ΣREYCMD values varied over a wide range from 0.26 µg/L to 7,212.51 µg/L with averages of 611.1 µg/L for all the samples as well as of 237.6 µg/L for R1-CMD, 577.5 µg/L for R2-CMD, and 3,313.6 µg/L for R3-CMD (Table S3). Similar to trends seen for ΣREYCMD, even though no statistical correlation was found between critical-ΣREY and pH, the acidic CMD (< 4.5) had significantly higher critical-ΣREY values compared to more alkaline CMD (Fig. 5a). Critical-ΣREYCMD exhibited a strong correlation with ΣREYCMD (r= 0.93, p < 0.005; Fig. 5b). In terms of individual REY, the highest concentrations and the widest ranges of values were measured for Y and Nd followed by Dy, Eu, and Tb (Fig. 5c). With respect to their 13
Journal Pre-proof contribution to critical-ΣREYCMD, the two main critical REY in Illinois CMD were Y (average 55.5%) and Nd (average 30.8%) (Fig. 5d). The strong statistical correlation between critical-ΣREYCMD and Y (r= 0.99, p < 0.005; Fig. 5e), is due most probably to the dominance of Y in Illinois CMD (Fig. 5e). The average value for Y contribution to critical-ΣREYCMD was 55.5% for all samples as well as 48.2% for R1-CMD, 56.5% for R2-CMD, and 66.2% for R3-CMD. These data are significant because none of the CMM samples analyzed in this study display high values for Y and, therefore, the high values of Y in CMD, most probably, did not exclusively reflect source enrichment. Significantly, critical-ΣREYCMD also showed a strong correlation with ΣHREYCMD/ΣLREYCMD ratio (r= 0.924, p < 0.005; Fig. 5f), such as the CMD samples with the highest
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3.2. Geochemical patterns of REY in solid matrixes
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ΣHREYCMD/ΣLREYCMD ratios also had the highest levels of critical-ΣREYCMD.
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Compared to CMD, REY in the solid matrixes are several orders of magnitude higher, with significant differences in the individual REY patterns for the three types of solid matrixes, namely CMM,
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MIR, and FM (Fig. 6a-l; Table S4). The CMM showed highly variable ΣREYCMM contents from 18.4 to 198.6 mg/kg with average values as follows: raw high-sulfur coal = 67.4 mg/kg (n=2), raw low-sulfur coal = 18.8
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mg/kg (n=2), preparation plant feed coal = 116.8 mg/kg (n=2), prepared coal = 29.5 mg/kg (n=4), and refuse coal 182.6 mg/kg (n=3) (Table S4; Fig. 6a). CMM display enrichments in LREY (i.e., Ce, La, and Nd)
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(Fig. 6d), have low ΣHREY/ ΣLREY ratios, and relatively flat ΣHREY/ΣLREY patterns, typical of crustal materials such as NASC (Fig. 6g), and a low fraction of critical-ΣREY dominated primarily by Nd and Y (Fig.
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6j).
The ΣREY values for MIR samples, ΣREYMIR, ranged from 300.4 to 414.2 mg/kg with an average of
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346.3 mg/kg (n=6) (Fig. 6b). Notably, two samples of an intruded mafic dike and an igneous breccia collected from the same coal mine as CMM samples had ΣREYMI values of 368.6 and 319.1 mg/kg, respectively (Table S1), significantly higher than CMM samples. Compared to CMM and FM, MIR display unique characteristics, namely it had the highest ΣREYMIR (Fig. 6b), the highest enrichments in LREY (i.e., Ce, La, and Nd) (Fig. 6e, h), the lowest ΣHREY/ΣLREY ratios, and the lowest fraction of critical-ΣREY dominated primarily by Nd (Fig. 6k). On a NASC normalized plot, MIR displayed substantial enrichments in LREY (Fig. 6h), with the high contents and range of values for Ce, La, Nd, followed by Y (Fig. 6d, e). The ΣREYFM, ranged from 23.5 to 65.4 mg/kg with an average of 38.5 mg/kg (n=7), which were the lowest values for solids compared to ΣREYMIR and ΣREYCMM (Fig. 6c). Significantly, Y was the predominant element in FM samples in terms of both highest concentration and the largest range of values (Fig. 6f) followed by Dy, Ce, Gd, Nd, Er, and La. Importantly, on ΣREYFM/ΣHREYNASC plot, FM 14
Journal Pre-proof exhibits evident enrichments in MREY (Fig. 6 i) and the highest ΣHREY/ ΣLREY ratios, with excessively high YFM levels with values 10-times higher than those of other solid matrixes (Fig. 6 l). Whereas fluorite is the main REY-carrier in the IKFD of southern Illinois, other minerals such sphalerite and calcite from IKFD have similar REY patterns with Y and MREY enrichments (data not shown), therefore, the REYFM patterns represent a good proxy for fingerprinting solid matrixes precipitated from hydrothermal fluids.
3.3. Comparing REY patterns of aqueous and solid samples To further decipher the factors controlling the REY patterns in CMD, especially those of critical-
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ΣREYCMD, geochemical criteria specific to Illinois CMD were employed to explain the REYCMD patterns in a broader context. By definition, there is almost perfect correlation between critical-ΣREY and ΣREY for all
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solid and aqueous matrixes considered in this study (Table S5). However, the fraction of critical-REY of
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the ΣREY varies broadly among different matrixes from 23.5% in MIR to 91.8% in FM (Fig. 7a). In a further analysis, we separated two end members: at one end, there are MIR and CMM, which display
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the lowest average critical-REY values of 25.9% and 34.6%, respectively and at the other end, the FM, which have exceptionally high fraction of critical-ΣREY with an average value of 80.6%. The Illinois CMD
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displayed values that range between these two end members, with an overall average value of 53.1% as well as 48.2.5% for R1-CMD, 56.5% for R2-CMD, and 66.2% for R3-CMD. Whereas the proportion of
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critical-REY in Illinois CMD is high, the concentrations of these elements in mine drainage is generally low. Methods for commercial recovery and separation of REY from coal-derived materials are in their
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infancy, but it is likely that further concentration of REY from AMD will be necessary before economic recovery is possible (Ayora et al., 2016; Hedin et al., 2019; Ziemkiewicz et al., 2016).
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Another remarkable feature of the solid and aqueous matrixes is the distinct positive Ce anomaly defined as Ce/Ce*=2 CeNASC/(LaNASC+PrNASC) (Bau and Dulski, 1996; Dai et al., 2016a). A caveat is that some La, Ce, and Pr contents, especially in the FM solids, were close to the detection limits and thus the range of Ce/Ce* ratios in FM will require further investigation. Most of the samples had Ce/Ce* > 2, with average values of 2.8 for CMM, 3.1 for MIR, and 4.7 for FM as well as of 4.6 for all CMD, 4.7 for R1-CMD, 5.6 for R2-CMD and 4.9 for R3-CMD (Fig. 7b). In contrast to critical-ΣREY patterns, within the geochemical pattern depicted by Ce/Ce* anomalies, no distinct endmembers for the solid matrixes could be separated. Whereas both CCM and MIR had lower Ce/Ce*values, the FM show a particularly wide range of values which mostly overlap those of CMD. This may indicate that hydrothermal solutions from which FM precipitated had elevated but variable amounts of LREY (La, Ce, and Pr), which resulted in variable Ce enrichments for FM. 15
Journal Pre-proof Yttrium is the main critical-ΣREY in the Illinois CMD (Fig. 5e), and, thus, to decipher the source of this enrichment we further explore Y relationships with other parameters, including critical-ΣREY, ΣREY, and ΣHREY/ΣLREY ratios. The fraction of Y of the critical-ΣREY varies widely among different solid and aqueous matrixes from 19.2 to 94.3% (Fig. 7c). Similar to the previously described approach, two end members were separated, namely at one end, the MIR and CMM matrixes have the lowest fractions of Y of the critical-ΣREY, with average values of 24.1% and 42.5%, respectively, and at the other end, the FM contain an extremely high fraction of Y of the critical-ΣREY, with an average value of 88.1%. The CMD samples have values between these two endmembers, with an overall average value of 55.5% for all
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CMD samples, 49.1% for R1-CMD, 60.6.4% for R2-CMD, and 65.9% for R3-CMD. The fraction of Y of the ΣREY showed large variations among different solid and aqueous matrixes
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with values from 3.8 to 56.3% (Fig. 7d). Interestingly, while the solid matrixes have average values of
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4.1% for MIR, 15.1% for CMM, and 26.5% for FM., the CMD showed a significantly wider range of values from 2.1 to 56.3% with averages of 30.8% for all CMM, 25.1% for R1-CMD, 34.8% for R2-CMD, and
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44.0% for R3-CMD. ΣHREY/ΣLREY ratios (Fig. 7e) displayed similar patterns with the fraction of criticalREY of the ΣREY (Fig. 7a), such as two end members can be separated, namely at one end, the MIR and
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CMM matrixes have low ΣHREY/ΣLREY ratios, with average values of 0.09 and 0.24, respectively, and at the other end, the FM contain extremely high ΣHREY/ΣLREY ratios with an average value of 3.6. The
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CMD samples have values spanning these two endmembers, with an overall average of 0.35 for all CMD samples and for 0.46 for R1-CMD, 0.71 for R2-CMD, and 1.12 for R3-CMD. Finally, we observed strong,
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statistically significant logarithmic relation (R2 = 0.96) between the fraction of critical-ΣREY and the Y/Ce
7f).
4. Discussion
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ratios, such as the samples with the highest share of critical-ΣREY also had the highest Y/Ce ratios (Fig.
In this study we tested two key factors which could influence the contents and distribution patterns of REY in CMD from the Illinois Basin, namely the pH and the spatial proximity of the CMD site to the Hicks Dome. In the following sections, both factors are evaluated.
4.1. Correlations of ΣREYCMD with pH and other geochemical parameters
4.1.1 Correlations of ΣREYCMD with pH
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Journal Pre-proof The impact of solution pH on the concentration of dissolved constituents in Illinois CMD is consistent with previous reports of natural and mine waters, which found that lower pH solutions usually carry a higher load of dissolved constituents (Gimero et al., 2000; Gammon et al., 2003; Cravotta, 2008; Nordstrom, 2011; Lefticariu et al., 2015; Noack et al., 2014; Stewart et al., 2017). In the present study, Illinois CMD also displayed the greatest enrichment in low-pH samples, but variation of REY with pH did not show a linear relationship (Fig. 3a) and, as such, there is not a significant statistical correlation (i.e., P-values < 0.05 indicate statistically significant non-zero correlations at the 95.0% confidence level) between pH and ΣREYCMD, including both all-CMD and low-pH CMD (Table S5), as well
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as with other REY-related parameters, namely ΣHREY/ΣLREY, Y/Ce, as well as critical-ΣREY. Similarly, at a
SO4, Al, Fe) and trace (i.e., metals) elements (Table S5).
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regional level, the values of pHCMD did not correlate with the concentration values for many major (i.e.,
Previous studies showed that, in low-pH, sulfate-rich waters, SO4 is the primary ligand for REY
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forming predominantly mono-sulphate complexes (LnSO4+; where Ln is a lanthanide element) and
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Ln(SO4)2− and free ionic (Ln3+) species contributing minor amounts to dissolved REY speciation (Gimeno et al., 2000; Verplanck et al., 2004; Zhao et al., 2007; Pérez-López et al., 2010). Furthermore, field
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experiments (i.e., Verplanck et al., 2004) document that under similar pH conditions as reported for Illinois CMD, the REE remain dissolved in sulfate-rich waters with solid-phase partitioning occurring
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exclusively in the waters with the highest pH (i.e., pH ≈ 6.6). Since dissolved SO4 was the prevailing ion in Illinois CMD regardless of the pHCMD and the SO4 concentrations were an order-of-magnitude higher
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than that of the other anions (Table S2), we conclude that REY-sulfate complexes were the dominant dissolved REY species in CMD analyzed in this study, with limited partitioning into the solid Fe-
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precipitates (Verplanck et al., 2004; Stewart et al., 2017). Collectively, these results suggest that in addition to pHCMD other key factors controlled the abundance and patterns of REY enrichment in Illinois CMD. Such factors could have included the bedrock geology, alteration mineralogy, biogeochemical processes at the CMD site, input of REY from outside systems (i.e., agricultural runoff), and coal mine drainage composition.
4.1.2. Correlations of ΣREYCMD with major elements Geochemical characteristics of CMD, mainly the identity, concentrations, and ratios among major elements in CMD could further decipher the identity of the heterogeneous, compositionally complex weathering solid matrixes at each CMD site. Aluminum and silicon were the two major elements that showed statistically strong correlations with ΣREYCMD (Table S5). None of these elements 17
Journal Pre-proof is redox sensitive and, thus, a limited number of processes control their behavior in natural environments, especially under low-pH conditions where both Al and Si are present as dissolved complexes in solution (Bigham and Nordstrom, 2000). Thus, the variation of Al and Si concentrations in CMD can be considered a direct reflection of coal mine weathering materials rather than that of the biogeochemical weathering stoichiometry. This information suggests that aluminosilicate sources and especially Al-rich clay minerals (i.e., kaolinite, illite) in coal mine wastes (i.e., Cobb, 1981; Finkelman et al., 2019) could have been an important source of REY in CMD. Previous research has demonstrated that clay minerals (i.e., kaolinite, halloysite, illite) are key repositories of REE in regolith-hosted REE deposits
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(Williams-Jones et al., 2012; Elliott et al., 2018; Cheshire et al., 2018) as well as in residual REE-clays and South China clay deposits (Bao and Zhao, 2008; Chi and Tian, 2008). In these cases, the REE were
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adsorbed at sites of negative charge on the layer surfaces and/or at the layer edges of clay minerals
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(Kynicki et al., 2012; Li and Yang, 2016).
Interestingly, we found that ΣREYCMD presents a strong statistical correlation with SO4 and no
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statistical correlation with Fe (Table S5). Both Fe and SO4 are redox sensitive elements and are extensively involved in redox cycling at CMD-impacted sites (Lefticariu et al., 2017a). However, under
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the pH-Eh conditions found at most CMD sites, Fe usually undergoes extensive redox transformations (Lefticariu et al., 2017b) while for SO4 this occurs to a lesser degree, mainly due to the restrictive
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environmental conditions necessary for bacterial sulfate reduction (Lefticariu et al., 2017a). Typically, Fe and SO4 concentrations in CMD reflect the alteration mineralogy, namely weathering of primarily pyrite
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(Lefticariu et al., 2006) but also other sulfides existing in coal mine wastes (i.e., sphalerite, galena, and chalcopyrite) (Cobb, 1981). The sulfides are not known as important repositories for REY, and, thus, the
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ΣREYCMD and SO4 statistical correlation may be due to interrelated factors, such as hydrothermal activity which could have enriched the host rocks in both sulfur-bearing species (i.e., pyrite) and REY (Dai et al., 2017).
At CMD sites, Fe often forms precipitates (Verplanck et al., 2004; Lefticariu et al., 2017b; Hedin et al., 2019). The biogeochemical processes associated with Fe redox cycling resulting in precipitation and/or dissolution of such Fe phases could also control the REY partitioning between the solids and the solution and thus affect the concentrations and geochemical patterns of REY in CMD (Verplanck et al., 2004; Hedin et al., 2019). Limited REY data of CMD solid phases at our sampling sites did not allow us to directly evaluate the role of Fe precipitates in controlling the REY contents and patterns in Illinois CMD. However, the lack of statistical correlation between ΣREYCMD and Fe concentrations for samples
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Journal Pre-proof considered in this study suggest that CMD precipitates most probably had only minor control on ΣREYCMD and REYCMD patterns. The concentration of Ca, an element that does not undergo redox transformation, did not statistically correlate with ΣREYCMD (Table S3). Most probably, Ca levels reflect the mineralogy of coal mine wastes, with higher Ca levels reflecting the occurrence of carbonate rocks at the CMD site, either as part of the bedrock or added as a treatment option (i.e., Behum et al., 2011). These two sources can constitute a persistent supply of Ca to the CMD. However, limestone added at different CMD sites as a remediation option is prone to passivation due to coating by Fe-rich precipitates and as such, the Ca
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contribution from such sources often decreases over time to background levels (Lefticariu et al., 2015). Anthropogenic activities, especially the application of phosphate fertilizer (i.e., apatite, a REY-
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rich calcium phosphate mineral) could cause REY addition into the environment (Dinali et al., 2019).
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Overall, the PO4 concentrations in CMD were relatively low (Table S2) and the lack of statistically significant correlations with both ΣREYCMD and pH suggest that phosphate fertilizer generally had a
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minor, if any, contribution to the ΣREYCMD.
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4.1.3. Correlations of ΣREYCMD with trace elements
Remarkably, we found statistically significant correlations between ΣREYCMD and most of the
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metals detected in Illinois CMD, including Zn, Ni, Co, Cu, and Cd (Table S5). The metal enrichment is more likely derived from hydrothermal fluids that precipitated reduced sulfur and metals (i.e.,
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sphalerite, galena, and chalcopyrite) in Illinois coal seams. Such hydrothermally-derived enrichment for Hg and other metals was previously identified in pyrite in coal from southern Illinois (Lefticariu et al.,
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2011; Manceau et al., 2018) and coal seams from southern Webster and Union counties, Kentucky (Hower et al., 2000; 2001; 2002), yet any hydrothermal solution affecting the coal seams and associated bedrock would have contributed to increased contents of metals. Importantly, sites where CMD displays enrichment in metals, particularly Zn, Ni, Co, and Cu also exhibit elevated levels of ΣREYCMD. Thus, the significant correlation between metals and ΣREYCMD suggest that hydrothermal activity played a key role in REY enrichment found in Illinois CMD. Collectively, the major and trace element data together with pHCMD indicate that the heterogeneous composition of the coal mine wastes weathering at each site controlled the geochemical trends observed in Illinois CMD (Fig. 3, 4). This assumption is supported by the wide range of CMD chemical composition with respect to the major (Fig. 3) and trace (Fig. 4) constituents and no significant statistical correlation among elements which are usually associated with silicates (i.e., Al, Si), sulfides, 19
Journal Pre-proof such as pyrite (i.e., Fe, SO4), and carbonates (i.e., Ca, Mg) (Table S5). This indicates that mixed materials of bedrock, coal, coal refuse and added remediation materials, containing various proportion of silicates, sulfides, and carbonates have been weathering at each CMD site.
4.2. Proximity to the Hicks Dome and significance of REY regional patterns Perhaps one of the most important findings of this study is that distinct spatial patterns of CMD geochemical parameters were recorded from the three regions, namely R1, R2, and R3, including for major and minor elements as well as for REY (Figs. 2-4, 7; S1-S4).
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Whereas Ca (Fig. 3g; S1g) and pH (Fig. 3a) levels display variability throughout the basin, overall, more sites in R1 have higher Ca levels and alkaline pH suggesting greater amounts of carbonate were
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incorporated in the weathering coal mine waste, most probably from bedrock. Similarly, another
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noteworthy finding was Ba, which had the highest range of values and the highest average concentration for R1-CMD followed by R2-CMD and R3-CMD (Fig. 4g; S2e). These geochemical trends
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support previous reports that identified calcite and barite as the main ore components of the MVT mineralization in the IKFD (Richardson and Pinckney, 1984; Chesley et al., 1994; Denny et al., 2015). The
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CMD in R2 displayed the highest average concentrations for Fe (Fig. 3d, S1d), SO4, (Fig. 3e, S1e), and Mn (Fig. 3f, S1f) (anomalous sample SIU-41 in R3 not included), followed by R3-CMD and R1-CMD,
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suggesting significantly greater amounts of sulfides such as pyrite in the weathering coal mine wastes at R2-CMD sites. The CMD sites in R3 have higher Al (Fig. 3b; S1a) and Si (Fig. 3c; S1b) concentrations
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suggesting that higher amounts of silicates, particularly clay minerals (i.e., high Al concentrations), were present at R3-CMD sites. The regional trends in most metal concentrations of Illinois CMD, including Zn,
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Ni, Co, Cu, Li, Cr, U, and Cd, revealed similar patterns, with the highest metal concentrations measured in R3-CMD, followed by R2-CMD and R1-CMD (Fig. 4, S2). These results corroborate previous research (Cobb, 1981; Whelan et al., 1988) which describe kaolinite and sulfides as important components of the kaolinite-pyrite-sphalerite(pyrite)-paragenetic sequence which characterize the Upper Mississippi Valley Zn-Pb mineralization in the Illinois Basin. In the case of REY, contrary to our second hypothesis, R1-CMD did not display the highest average ΣREYCMD value (ΣREYR1-CMD, 475 µg/L) (Fig. S1), even though these CMD sites were spatially located closest to the Hicks Dome, a center of mafic igneous and hydrothermal activity in the Illinois Basin (Moorehead, 2013; Denny et al., 2015; Maria et al., 2019), and MIR were found to have the highest ΣREY of any of the samples investigated (Fig. 6). Instead, higher average ΣREYCMD values were recorded in R3CMD (4,742 µg/L) followed by R2-CMD (1,091 µg/L) that sampled sites along the northern and western 20
Journal Pre-proof edges of the Illinois Basin, further from Hicks Dome (Fig. 2b). Likewise, the highest average critical-ΣREY of 3,314 µg/L was recorded for R3-CMD, farthest from the Hicks Dome, followed by R2-CMD 578 µg/L and R1-CMD 238 µg/L (Fig. 7a). Moreover, other REY-related parameters including ΣHREY/ΣLREY and Y/Ce showed consistent regional patterns with the highest average value measured in R3-CMD followed by R2-CMD and the lowest one in R1-CMD (Fig. S2a, g). Many Illinois CMD samples display MREY enrichment on NASC-normalized REY plots (Fig. 2d-e) typical of low-pH environments (i.e., Gimeno Serrano et al., 2000; Johannesson and Zhou, 1999; Stewart et al., 2017). In other cases, even though the MREY enrichment was still present, the ΣLREY/ΣMREY
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ratios were similar or even larger than those of ΣHREY/ΣMREY (Figs. 2d-f). Regionally, this type of MREY pattern was found in many R1 and R2 sites (Figs. 2d-e). Local enrichments of coal mine wastes in LREY,
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particularly in Ce, Nd, and La in R1 (Fig. 2d) or in HREY, mainly in Y but also Dy in R3 (Fig. 2f) and R2 (Fig.
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2e) could explain the ΣREYCMD/ ΣREYNASC patterns (Figs. 2d-f) as well as the ΣHREY/ΣLREY, and Y/Ce,
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variability in Illinois CMD samples (Fig. S3).
4.3. Sources of REY
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Numerous studies investigating fluid−rock interaction during weathering, employing major, trace, and REY geochemical behavior have emphasized the extensive influence of the source rock
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geochemical signatures on the interacting solutions (Cravotta, 2008; Chi and Tian, 2008; Migaszewski, and Gałuszka, 2015; Zaharescu et al., 2017; Frings and Buss, 2019). Herein, we employ a similar
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approach by determining the inherent REY signatures for three solid matrixes considered as possible sources of REY in Illinois CMD, namely CMM, MIR, and FM. These geochemically distinct matrixes
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exhibit dissimilar ranges for ΣREY and critical- ΣREY values (Fig. 6a, b, c) as well as unique individual NASC-normalized REY patterns for CMM (Fig. 6g), MIR (Fig. 6h), and FM (Fig. 6i). In the case of CMM, different samples displayed wide ranges of ΣREYCMM and critical-ΣREYCMM values. The highest ΣREYCMM and critical-ΣREYCMM values were measured in the coal refuse samples (Table S4), the CMM fraction most likely to be stored at coal mine sites and become part of the coal mine waste (Behum et al., 2018). The treated coal, which is the CMM fraction sent to power plants for energy production, had the lowest ΣREYCMM and critical- ΣREYCMM contents (Table S4). On average, the coal refuse samples from the coal preparation facility had 4 to 6-times higher ΣREYCMM concentrations compared to those of treated coal prepared for commercial use. Previous studies have showed that in coal, REY are primarily controlled by detrital minerals such as apatite, monazite, xenotime, titanite, and, to a lesser extent, by zircon (Seredin and Dai, 2012; Hower et al., 2016; Dai and Finkelman, 2018; 21
Journal Pre-proof Finkelman et al., 2019). Additionally, REY can be associated with clay minerals (Elliott et al., 2018; Kynicki et al., 2012; Li and Yang, 2016), which are abundant in coal but also in coal refuse such as shale and mudstone (Hower et al., 1999; Seredin and Dai, 2012). In addition to the detrital component (Finkelman et al., 2019) hydrothermal input can also account for the distribution of REY among the studied coals (Dai et al., 2017). Despite the distinct ΣREYCMM ranges for different CMM fractions, their individual REYCMM/REYNASC patterns were very similar and show a crustal distribution pattern (Fig. 6g), thus, implying that the weathering of any combination of CMM fractions would most likely produce similar REYCMD/REYNASC patterns.
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In southern Illinois, coal seams are intruded by mafic igneous dikes, sills, and diatremes, which are a common feature of surface and underground coal mines (Stewart et al., 2005; Schimmelmann et al.,
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2009; Rimmer et al., 2016). However, at any coal mine location, MIR represent only as a minor
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component of the weathering coal mine wastes due to the overall trivial mined volume of MIR compared to that of CMM. Nonetheless, we used the REYMIR data as a reference for both the Hicks
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Dome composition and igneous intrusions in coal. MIR display distinct REY patterns, having the highest ΣREY values of all the solid matrixes considered (Fig. 6b), the greatest enrichment in LREY (i.e., Ce, La,
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and Nd) (Fig. 6e, h), a distinct Eu anomaly with respect to NASC, and the lowest fraction of critical-ΣREY, dominated primarily by Nd (Fig. 6k). In NASC-normalized patterns, MIR exhibit distinct LREY enrichment
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(Fig. 6h) due to the presence of REY-rich minerals, including monazite, apatite, zircon, and REY-rich fluorocarbonates, previously described in Hicks Dome MIR (Moorehead et al., 2013). The addition of
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MIR to weathering coal mine wastes will produce CMD enriched in Ce, La, and Nd, and overall, in LREY above the levels seen in weathering of CMM.
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The FM samples analyzed in this study had the lowest ΣREYFM values compared to ΣREYMIR and ΣREYCMM (Fig. 6 c). However, FM displayed distinct REY patterns with remarkable enrichment in MREY on ΣREYFM/ΣHREYNASC plots (Fig. 6 g) and the highest ΣHREY/ ΣLREY ratios (Table S3) among solid matrixes. Importantly, Y, the main REY in Illinois CMD (Fig. 5 b, c), was also the dominant REY in FM (Fig. 6 i, l) with YFM concentration values 10-times higher than those of the other REYFM (Fig. 6 l). While fluorite is the main REY-carrier in the IKFD of southern Illinois, other minerals from IKFD such calcite and sphalerite exhibit similar REY patterns, particularly Y and MREY enrichment (data not shown). While our data are limited, the results suggest that the described REYFM patterns might be used as proxies for materials formed from or interacted with hydrothermal fluids (Migdisov and Williams-Jones, 2014; Migdisov et al., 2016). Thus, we speculate that the interaction of REY-bearing MVT hydrothermal solutions with coal and
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Journal Pre-proof associated sedimentary rocks could enrich these solid matrixes generally in REY, and predominantly in Y, in addition to metals (i.g. Zn), producing unique geochemical signatures resembling those of FM.
4.4. The role of host rocks and hydrothermal activity
4.4.1. Host rock-hydrothermal interaction The combined geochemical data for both aqueous (i.e., CMD) and solid (i.e., CMM, MIR, and FM) matrixes (Fig. 7a-f) show evidence for two main factors controlling the REY concentrations and
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patterns in Illinois CMD, namely (1) the overall composition of the source materials, and (2) the degree of hydrothermal alteration of the weathering coal mine wastes. These two factors also had an important
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effect on the critical-ΣREY composition as well as the content of metals of economic value (i.e., Zn, Al,
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Ni, Cr, V, and Co) identified in Illinois CMD.
Overall, our combined critical-ΣREY data for solid and aqueous matrixes (Fig. 7a) are consistent with
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previously published results which showed that the mafic igneous rocks are depleted in critical-ΣREY (Denny et al., 2015) whereas acidic solutions (i.e., hydrothermal, mine drainages) tend to be enriched in
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critical-ΣREY (Stewart et al., 2017). Significantly, in the Illinois Basin, the critical-ΣREYCMD regional patterns (Fig. 7a) mirror those for metal concentrations (Fig. S2), in which a regional enrichment in the
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order of R3 CMD>R2 CMD>R1 CMD could be related to increased hydrothermal imprint on the CMD source materials from the Hicks Dome to the western and northern margins of the Illinois Basin. This
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observation likely implies that CMD originating from parent materials that interacted with hydrothermal solutions would be enriched in ΣREY and most importantly in critical-ΣREY. This hypothesis is also
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supported by critical-ΣREY values of solid matrixes, which showed that FM had the highest fraction of critical-ΣREY and by extension, that past interaction of sedimentary rocks with hydrothermal solutions was necessary to produce weathering coal mine wastes enriched in REY, critical-ΣREY and metals (Fig. 7a; S1). Significantly, the fraction of the critical-ΣREY values to ΣREY for aqueous CMD and solid matrixes, namely CCM, MIR, and FM, from the Illinois Basin are considerably higher than those reported in conventional REY ores, such as those dominated by bastnäsite (typically less than 15%) (Williams-Jones et al., 2012; Van Gosen et al., 2017), but at much lower overall ΣREY concentrations.
4.4.2 Cerium anomalies Illinois CMD are characterized by a narrow range of positive values for Ce/Ce* increasing in the order of R1CMD
Journal Pre-proof have two oxidation states within the Eh−pH regime of CMD, namely Ce(III) and Ce(IV) (Brookins, D.G., 1983). Upon oxidation of Ce(III) to Ce(IV), Ce(IV) exhibits strong affinity for partitioning into the solid phases, usually forming precipitates such as CeO2(s) , which leads to fractionation from its non-redox reactive lanthanide neighbors La(III) and Pr(III) (Brookins, D.G., 1989). Previous studies have showed that surface waters unaffected by mine drainage are more likely to have negative Ce/Ce* values, as expected for oxidative scavenging (Smedley, 1991; Leybourne and Johannesson, 2008), while Appalachian CMD have positive Ce/Ce* anomalies with values mostly lower than 1.3 (Cravotta, 2008; Cravotta and Brady, 2015; Stewart et al., 2017). The positive Ce/Ce* anomalies observed in CMD could be due to: (1)
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selective enrichment of Ce in the weathering coal mine wastes due to higher amounts of Ce-bearing minerals; (2) involvement of biogeochemical processes along the CMD flow path that selectively oxidize
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and mobilize Ce (Yoshida et al., 2004; Bau et al., 2013; Kraemer et al., 2015; Zaharescu et al., 2017);
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and/or (3) enrichment of Ce by adsorption on Fe oxyhydroxides with subsequent release during reductive dissolution (Bau, 1999; Ohta and Kawabe, 2001). The Ce/Ce* values for Illinois CMD were
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much higher than those reported for Appalachia CMD (Stewart et al., 2017), suggesting that source enrichment was probably a controlling factor of Ce/Ce* anomalies in Illinois CMD (i.e., Williams-Jones,
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2012). The source enrichment in Ce could be attributed to higher abundance of Ce-rich REY minerals, such as monazite, apatite, and Ce-rich fluorocarbonates (Moorehead, 2013; Denny et al., 2015).
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Alternatively, since FM had some of the highest Ce/Ce* values (Fig. 7b) this implies that hydrothermal solutions could become enriched in Ce and thus through subsequent interactions capable of producing
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solid matrixes with higher Ce/Ce* ratios (Williams-Jones, 2012; Migdisov et al., 2016). In the Illinois Basin, the CMD samples with elevated Ce/Ce* ratios also exhibit high metal contents (i.e., Zn) suggesting
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that Ce-rich hydrothermal solutions probably played a pivotal role in higher Ce/Ce* ratios in the CMD.
4.4.3 Preferential mobilization of yttrium Yttrium is the main critical-REY in the Illinois CMD (Fig. 5g), with values for the Y contribution to critical-ΣREYCMD up to 77% (Fig. 7c). Although Y shares many physico-chemical characteristics with REE (Bau and Dulski, 1995; Richens, 1997), it has also unique properties among REY (Thompson et al., 2015). Namely, trivalent Y ion, Y3+, has the lowest polarizability (ionization potential) and the lowest covalency relative to other trivalent ions of the REY3+ group (Misono et al., 1967). This is known as the Misono softness parameter (Misono et al., 1967) and for Y it has an anomalously low value compared to the other REY, resulting in anomalous behavior in the environment (Bau et al., 1999; Thompson et al., 2015), such as during water-rock interaction, Y3+ will preferentially remain in solution rather than partitioning 24
Journal Pre-proof into solid matrixes by co-precipitation or sorption processes. Thus, during weathering, Y3+ being more soluble compared to other ions of the REY3+ group, will be preferentially leached out of the solid matrix (Thompson et al., 2015). Indeed, the predominance of Y in REYCMD (Fig. 5b, c) reflects REY variation in Misono softness such as during weathering of coal mine wastes, the CMD preferentially become enriched Y compared to the other REY (Fig. 7c, d). Moreover, if the coal mine wastes contained Y-rich solid matrixes, subsequent to previous interactions with hydrothermal solutions, the weathering of such Y-bearing solid matrixes would lead to Y-rich CMD (Fig. 7d) with high Y share of critical-ΣREY (Fig. 7c) as well as elevated ΣHREY/ΣLREY ratios (Fig. 7e). In the case of solid matrixes, MIR, which were probably
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less affected by hydrothermal solutions had the lowest fraction of Y of critical-ΣREY (Fig. 7c) and the lowest ΣHREY/ ΣLREY ratios (Fig. 7e) while FM representing precipitates from hydrothermal solutions
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had the highest contribution of Y to critical-ΣREY (Fig. 7c) and the highest ΣHREY/ ΣLREY ratios (Fig. 7e).
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The CMD samples have values spanning between these two end members with increasing values probably reflecting increasing hydrothermal source enrichment in Y (Fig. 7c). The preferential
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mobilization of Y among REY in Illinois CMD was also evident when we examined the Y contribution to ΣREYCMD (Fig. 7d). Similar to trends described above, the share of Y contribution to ΣREYCMD for solid
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matrixes increases in the order MIR
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CMD,
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preferentially become enriched Y compared to the other REY (Fig. 7c, d).
4.4.4. Ce/Y as a proxy for fluid-rock interaction
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Cerium, in addition to Y, was also a main REY component in Illinois CMD (Fig. 2d, e). In terms of Misono softness, Y has an anomalously low value while Ce has the second highest value (Misono et al., 1967) and, thus, we expect that the Y/Ce ratios may be a reliable proxy for determining the extent of fluid-rock interactions. While both Ce (Fig. 6d, e) and Y (Fig. 6f) can be enriched in solid matrixes, Y3+ being more soluble and thus more mobile assures that during prolonged water-rock interaction the Y/Ce ratios in solution will progressively increase (Thompson et al., 2015; Williams-Jones et al., 2012). Overall, the average Y/Ce values for solid and aqueous matrixes were 0.2 for MIR, 0.5 for CMM, and 10.5 for FM as well as 2.1 for all CMD. Indeed, both MIR and CMM have the two lowest ratios among all the aqueous and solid matrixes, suggesting limited water-rock interaction, while FM, which are precipitates from hydrothermal solutions, have the highest Y/Ce values (Fig. 7c), while the CMD plot on a continuum between the two end members. 25
Journal Pre-proof Despite the wide range of REY-source materials and local processes that controlled the REYCMD patterns at different sites across Illinois, we found a remarkably strong exponential relationship (R2 = 0.96) between critical-ΣREY and Y/Ce for all the aqueous- and solid-matrix samples considered in this study (Fig. 7f). This strong relationship points towards coherent behavior of REY across various matrixes compositions and spatial scales, probably driven mostly by the anomalously low Misono softness of Y3+ relative to the other REY3+. The preferential mobilization of Y in solution and thus progressive Y enrichment relative to Ce with increased water-rock interaction results in increased Y/Ce ratios as well as increased critical-ΣREY values. This result may reinforce the idea that mine drainage and particularly
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acid drainage have increased capacity to mobilize Y and by extension critical-REY in solution. Therefore, in CMD from the Illinois Basin, Y/Ce ratio may be a key indicator for water-rock interaction, with the
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highest values fingerprinting both enriched sources due to hydrothermal activity and secondary
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enrichment during weathering of parent materials and transport within the CMD-impacted systems. Our small geochemical dataset for Illinois CMD shows regionally distinct signatures for metals
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and REY which mirror the composition of hydrothermal mineralization described for southern (i.e., enrichments in Ba) and northern (enrichments in Al and Zn) parts of the Illinois Basin. Moreover, results
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of this study indicate that in the Illinois Basin, the assessment for the potential of REY recovery from CMD would greatly depend on better understanding the hydrothermal processes and their role in REY
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and metal enrichments in coal and coal mine wastes at various CMD sites across the basin, particularly the CMD sites located along the western and northern margins. These sites could be further targeted for
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REY recovery since both the CMD and coal mine wastes could be enriched in REY (i.e, high ΣREY and critical-ΣREY) as well as other metals of economic value (i.e., Zn, Ni, Al, Cr, and V). Furthermore, mine
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drainage treatment technologies could be designed to target both optimal extraction of REY and valuable metals from CMD and coal mine wastes as well as environmentally friendly CMD remediation efforts.
5. Conclusions The CMD in the Illinois Basin exhibit larger heterogeneities of REY patterns than those described in other USA coal-mining districts, such as the Appalachian Basin (Stewart et al., 2017). Significantly, the Illinois CMD have higher values for ΣREYCMD and critical-ΣREYCMD, with Y and Nd being the main criticalREYCMD. Furthermore, CMD in the Illinois Basin were found to contain small amounts of economically valuable metals (i.e., Zn, Ni, Co, V) that could potentially be co-extracted with REY to enhance the economic values of CMD. 26
Journal Pre-proof Synthesis of geochemical data suggests that hydrothermal activity probably played a key role in producing CMD enriched in metals, ΣREY, and most importantly, in critical-ΣREY. Specifically, our data point towards two sources of REY enrichment in Illinois CMD, namely coal mine wastes containing solid matrixes (1) with high Al and Si contents (i.e., clay minerals and silicates) and (2) impact of hydrothermal solutions enriched in metals (i.e, Zn, Ni, Co) and REY. The sites with the highest ΣREYCMD and critical-ΣREY CMD
contents were predominantly located along the western and northern margins of the Illinois Basin.
Particularly, sites with the highest hydrothermal input as approximated by Mississippi-Valley-type mineralization are prime candidate for REY and other metals (i.e., Zn, Ni, Co, Cu), potentially making
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Illinois CMD an attractive alternative source for REY and metals recovery.
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Acknowledgments
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This paper is dedicated to the memory of Prof. John C. Crelling. The Illinois Department of Natural Resources (IDNR) assisted our team with various tasks during the duration of this study. Angie Mick and Gregory Pinto of IDNR, Office of Mines and Minerals assisted in selecting the sampling sites as well as with the collection of coal mine drainage samples. Partial funding for this study was provided by the Geology Department-SIUC Porter Jobling grant to Kyle Klitzing. Coal sample analysis was supported in part by USGS Cooperative Agreement G16AC00449 to Southern Illinois University. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government. ■ ASSOCIATED CONTENT The Supporting Information is available free of charge on the Elsevier Publications website at DOI: 1xxx
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Figure 1: Map of the Illinois Basin showing the location of coal mine drainage (CMD) sites sampled. Based on the proximity to the Hick’s Dome, we designated three distinct regions, namely: (1) Region 1, R1, comprised sites situated in southern and southeastern Illinois, in close proximity to the Hicks Dome; (2) Region 2, R2, comprised sites situated in western Illinois; (3) Region 3, R3, comprised sites situated in northern Illinois. Also indicated is the extent of Pennsylvanian-age coal-bearing strata in the Illinois Basin.
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Figure 2: Composite figure depicting: (a) ranges of pH values for R1-CMD, R2-CMD, and R3-CMD; (b)
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Ranges of total REY concentration values, ΣREY, for R1-CMD, R2-CMD, and R3-CMD; (c ) share of individual REY of ΣREY for all CMD; REYCMD concentrations are presented normalized to the North
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American Shale Composite (NASC; Gromet et al., 1984) for R1-CMD (d), R1-CMD (e ), and R1-CMD (f).
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Figure 3: Comparison of ΣREYCMD and (a) pH values, as well as the concentrations of dissolved major
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elements (b) aluminum Al; (c) silicon Si; (d) total iron Fe; (e) sulfate SO4; (f) manganese Mn; (g) calcium
logarithmic scale.
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Ca; (h) potassium K; and (i) phosphate PO4 in the Illinois CMD. The concentration values are plotted on a
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Figure 4: Comparison of ΣREYCMD and the concentrations of trace elements: (a) zinc Zn (b) nickel Ni; (c)
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cobalt Co; (d) copper Cu; (e) barium Ba; (f) vanadium V; (g) chromium Cr; (h) uranium U; and (i) cadmium
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Cd in the Illinois CMD. The concentration values are plotted on a logarithmic scale.
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Figure 5: Composite figure depicting geochemical patterns of critical-REYCMD: (a) comparison of criticalΣREYCMD and pHCMD values; (b) comparison of critical-ΣREYCMD and ΣREYCMD; (c) box and whisker plots of the individual critical-REYCMD range of values; (d) ) box and whisker plots of the share of individual critical-REYCMD; (e) comparison of critical-ΣREYCMD and YCMD concentration values; (f) comparison of critical-ΣREYCMD and ΣHREYCMD / ΣLREYCMD ratios. The concentration values are plotted on a logarithmic scale.
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Figure 6: Composite figure depicting the REY patterns for solid matrixes: ranges of ΣREY values for (a) coal mining and processing materials (CMM) from an active underground coal mine and a coal preparation plant in Southern Illinois; (b) mafic igneous rocks (MIR) from IKFD; and (c) fluorite mineralization from IKFD; box and whisker plots of the individual REY range of values for (d) CMM; (e) MIR; and (f) FM; REY concentrations normalized to the North American Shale Composite values for (g) CMM; (h) MIR; and (i) FM; box and whisker plots of the individual critical-REY range of values for (j) CMM; (k) MIR; and (l) FM.
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Figure 7: Composite figure comparing the REY geochemical patterns for aqueous (R1-CMD, R2-CMD, and R3-CMD) and solid (CMM; MIR, FM) matrixes considered in this study: (a) box and whisker plots of the share of individual critical-REY; (b) Ce/Ce* ratios defined as CeNASC/(LaNASC+PrNASC) (Bau and Dulski, 1996); (c) box and whisker plots for the fraction of Y of the critical-ΣREY; (d) box and whisker plots for the fraction of Y of the ΣREY; (e) box and whisker plots for the ΣHREY/ ΣLREY ratios; (f) comparison of Y/Ce and fraction of critical-ΣREY values. 43
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Journal Pre-proof Declaration of interests
☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
☒The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
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Liliana Lefticariu
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Associate Professor Southern Illinois University
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Carbondale Illinois
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Journal Pre-proof Highlights
Illinois basin coal-mine drainages (CMD) have high contents of ΣREY as well as critical-ΣREY
CMD with high ΣREY contents have also high metal (i.e., Zn, Ni, Co) contents
CMD rich in trace metals and REY are mostly located along the west and northern part of the Illinois basin
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Y/Ce ratios can be used as a proxy for predicting matrixes with high critical-ΣREY
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