The Occurrence of Selected Radionuclides and Rare Earth Elements in Waste at the Mine Heap from the Polish Mining Group

Abstract

The paper presents the results of research on rare earth elements (REY) and selected radionuclides in barren rocks deposited on a heap at a mine belonging to the Polish Mining Group (the largest producer of hard coal in EU countries). The maximum concentration of REE_s determined in silstones was 261.6 mg/kg and in sandstones 221.2 mg/kg. The average uranium and thorium content in silstones was 6.8 mg/kg and 11.6 mg/kg, respectively. On the other hand, the samples of burnt coal shales contain on average 3.5 mg/kg of uranium and 9.7 mg/kg of thorium. In all coal waste samples, the REE values are higher than in hard coal (15.7 mg/kg). Carriers of REY, uranium, and thorium in coal waste are detritic minerals: monazite and xenotime, which are part of the grain skeleton of barren rocks. Coal waste samples are characterized by a variable distribution of REY concentrations as well as a variable content of radionuclides. The ²²⁶Ra, ²²⁸Ra, and ⁴⁰K measurements in the investigated samples were performed using the gamma spectrometry technique. The concentrations of the analyzed isotopes differed depending on the mineralogical composition of the investigated samples. The present study results may be important in determining the possibility of utilization of wastes of barren rocks stored in the mine heap and in assessing environmental and radiological hazards.

1. Introduction

Hard coal production is associated with the generation of significant amounts of mining waste, which amounts to about 34 million megagram (Mg) annually in Poland [1]. It is estimated that coal mining waste varies between 0.3 to 0.7 Mg for each Mg of coal mined in the Upper Silesian Coal Basin (GZW) [2,3]. The waste material is generated when mining the rock mass, winning coal, and its processing. The coal mining [4] waste stored in heaps mainly includes silstone (40–98%), mudstone (2–40%), sandstone (0–33%), coal shale (2–25%), and hard coal (3–10%). This waste mostly consists of mineral matter in variable amounts, with organic matter accounting for up to 15% [5]. The minerals mostly comprise clay minerals (50–70%), quartz (20–30%), and other minerals (10–20%), including chlorite, pyrite, siderite, ankerite, gypsum, and jarosite [5]. The composition of coal mine waste depends on the geological composition of the basin and the development of preparation technologies [6]. Coal mining wastes are treated to a small extent as a source of mineral raw materials used in the economy [2]. Mining waste was and still continues to be stored in the form of heaps, dumping grounds, or sedimentation ponds. Two hundred and twenty-six [6] such facilities have been identified in the Silesian Voivodeship (Figure 1). Coal waste heaps and dumping grounds excluded from social and economic use may negatively influence the environment in the area of their origin (pollution of soil, underground, and surface waters). Dumping grounds containing thermally active coal waste pose a threat to inhabitants due to high surface temperature and loosening of the embankment material. Oxidation of organic matter from waste can lead to self-heating and emission of pollutants, and can negatively affect air quality in the vicinity of the dumping ground [7,8].

Coal and barren sedimentary rocks contain varying concentrations of naturally radioactive material (NORM) including uranium (²³⁸U), thorium (²³²Th), and potassium (⁴⁰K) [9,10]. Carboniferous rocks erode easily when exposed to water and air. Radionuclides (²²⁶Ra, ²³²Th, ⁴⁰K) contained in coal mineral matter and coal mining waste may have a potential negative impact on the environment in the heap vicinity [11]. The element erosion processes in barren rock dumping sites may have an indirect impact on the environment. Uranium and thorium erode easily in an acidic environment and pose a potential threat to humans [12,13]. Rock weathering contributes to the geochemical mobilization of uranium, during which uranium oxidizes (from +IV valence state to +VI), and is easily leached in this form. REEs present in hard coal and coal mining waste may be of economic significance. Coal deposits enriched in REEs are researched by many authors [14,15,16,17,18,19]. The pyrite, present in mining waste, under hypergenic conditions, is subject to weathering and the weathering products can react with REE-containing minerals, e.g., phosphates (monazite, xenotime) or aluminophosphates of the crandallite group [20]. Under hypergenic conditions, minerals containing lanthanides are not subject to weathering. In contrast, feldspars and biotite present in mining waste degrade, releasing REEs into the environment (e.g., Ce³⁺ is oxidised to Ce⁴⁺ and yttrium can be transported long distances from the waste dumping site, lanthanum occurs as La³⁺ and readily forms compounds with carbonates, silicates, and oxides) [21].

The studied coal mining waste mainly includes waste material deposited in the floor and the roof of a coal bed together with mineral interlayers brought to the surface together with the mined and processed material. This research was conducted to determine the content of REE, U, and Th in coal mining waste obtained from the bottom dumping ground layer, which is partially thermally active. We have identified the mineral phases being carriers of these elements. We measured the concentrations of radionuclides in hard coal and coal mining waste to determine their potential impact on the environment and the health of the population.

2. Research Materials and Methods

The research covered coal mining waste from the surface layer of a heap originating from a hard coal mine belonging to the Polska Grupa Górnicza (Polish Mining Group). The heap of mining waste is partially thermally active in some of its parts. The rocks coexisting with coal beds, originating from roofs, floors, and interlayers, are genetically varied and characterized by variable mineral and chemical composition. The following samples were taken for analyses: silstone, mudstones (with admixtures of coal), sandstones, and burnt coal shales. Mudstones and sandstones are the most abundant in the coal mining waste deposited on the heap. Samples of hard coal deposited in the heap were also collected for analysis. A total of 28 samples were taken, including mining waste (20) and hard coal (8). The samples come from the heap surface and a depth of up to 0.7 m. The lower part of the heap is directly adjacent to residential houses and is not covered with vegetation. The duplicate method was used for sampling; it consists of taking a sample and its duplicate from the same test site and analyzing them twice (balanced strategy) [22]. Eight mining waste samples and two hard coal samples (a proper sample and a duplicate) were selected for further research. These samples were selected due to their low macroscopic variation, the remaining samples were rejected. The following labelling was used for the analyzed samples in Tables 1–4: 1, 1A-silstone; 2, 2A-sandstone; 3, 3A-mudstone; 4, 4A-burnt coal shale (red); 5, 5A-hard coal. Samples labelled 1–4 represent the surface of the heap, whereas samples 1A–4A were taken from a depth of approximately 0.7 m. Samples 5 and 5A represent hard coal.

The measurements of natural radioactivity in the analyzed samples were performed using a gamma spectrometer. This spectrometer is equipped with a high purity germanium detector (GC2018, Mirion Technologies (Canberra), Inc. (Meriden, CT, USA)), with a crystal diameter of 60.7 mm and an electrically powered cryostat Cryo-Pulse 5 from Mirion Technologies (Canberra), Inc. (Meriden, CT, USA). The detector is shielded with an 11 cm thick low-background lead shield (Ortec). The HPGe detector has a relative efficiency of 20%. The full width at half maximum (FWHM) for 1.332 MeV gamma line from ⁶⁰Co is equal to 1.71 keV and Peak-to-Compton (P/C) ratio of 53.8:1. Data analysis and acquisition was performed using Genie 2000 software (V3.4.1). The analyzed samples were dried, ground, mixed, placed in Marinelli containers, and left for a month to achieve a radioactive equilibrium between ²²⁶Ra, ²²⁸Ra and their derivatives. The activities of the analyzed radionuclides were calculated on the basis of a standard prepared from certificated materials obtained from the Central Laboratory for Radiological Protection (CLOR) in Poland. The concentrations of ²²⁸Ra and ²²⁶Ra isotopes in the analyzed samples were calculated as a weighted mean from the activities of ²²⁸Ac (338.3, 911.1 keV) and ²¹⁴Pb (295.2, 351.9 keV), ²¹⁴Bi (609.3, 1120.3 keV), respectively. The activity concentration of the ⁴⁰K isotope was assessed based on the 1460.8 keV gamma line. Radionuclide concentrations were measured once in the studied samples.

REE (La, Ce, Nd, Sm, Eu, Tb, Yb, Lu), U, and Th contents in the samples were determined by instrumental neutron activation analysis (INAA) at Activation Laboratories Ltd. (Ancaster, ON, Canada) using a 2MW Pool research reactor. Gamma radiation was measured using Ge ORTEC and CANBERRA detectors. Rare earth elements such as Pr, Gd, Dy, Ho, Er, and Tm + Y were determined by fusing the INAA, ICP-MS (Inductively Coupled Plasma Mass Spectrometry), and ICP-OES (Inductively Coupled Plasma Atomic Emission Spectroscopy) methods (Activation Laboratories Ltd., Ancaster, ON, Canada). REY, U, and Th concentrations were analyzed twice for each sample.

A Philips XL 30 ESEM/TMP microscope software (fully MS Windows NT compatible) with an EDS unit was used to determine the chemical composition of the mineral phases present in the coal waste samples by scanning electron microscopy. The accelerating voltage of the electron beam was 15–20 kV and the current was 20 nA. Tests were performed using preparations embedded in epoxy resin and polished.

3. Results and Discussion

3.1. Radionuclides

The ^226,228Ra and ⁴⁰K concentrations, U and Th content are presented in

Table 1. The activity concentrations in [Bq/kg] of ²²⁶Ra, ²²⁸Ra, ⁴⁰K, and content in [mg/kg] of U and Th in coal waste.

Sample Number		226Ra [Bq/kg]	228Ra(228Ac) [Bq/kg]	40K [Bq/kg]	U [mg/kg]	Th [mg/kg]
Silstone	1	49.5 ± 1.7	44.5 ± 1.0	749 ± 12	4.3	11.3
	1A	112.3 ± 4.1	54.2 ± 1.0	862 ± 13	9.2	11.9
Sandstone	2	9.3 ± 0.3	7.6 ± 0.2	148 ± 3	0.9	2.2
	2A	24.4 ± 0.9	12.5 ± 0.4	207 ± 4	2.1	3.1
Mudstone	3	39.0 ± 1.3	27.7 ± 0.6	761 ± 12	3.3	7.2
	3A	70.8 ± 2.6	57.6 ± 1.2	1008 ± 16	5.9	2.9
Burnt coal shale	4	69.9 ± 2.3	47.3 ± 0.9	769 ± 12	5.8	11.6
	4A	12.0 ± 0.5	10.6 ± 0.3	192 ± 4	1.1	7.8
Hard coal	5	20.3 ± 0.7	7.7 ± 0.3	64 ± 2	1.4	1.9
	5A	24.1 ± 0.9	15.1 ± 0.4	168 ± 4	2.1	3.9

. The concentrations of the analyzed radionuclides in the investigated samples are variable. Lower values of ^226,228Ra and ⁴⁰K isotopes were observed in hard coal and sandstone samples. High concentrations of the analyzed isotopes were observed in the silstone sample. Lower concentrations of radionuclides were observed for particular samples taken from the surface layer of the dump. The reverse trend was observed only in the case of the fired burnt shale samples.

The activity concentrations of ^226,228Ra and ⁴⁰K in hard coal are within the range of values found in literature data for GZW coals [23]. In coal mining rocks the concentrations of radionuclides vary greatly depending on the type of waste. The highest uranium concentrations were found in silstone and mudstone samples, whereas the lowest ones were found in sandstone samples (Table 1). The most variable concentration of U was found in the samples of silstone, mudstone and burnt coal shale. In the silstone samples, the Th concentration (11.3–11.9 mg/kg) is at a similar level. The largest variation in Th content was found in samples of mudstone (2.9–7.2 mg/kg) and burnt coal shale (7.8–11.6 mg/kg).

In coal samples from the heap, U and Th concentrations are within the range determined for GZW hard coals; i.e., for U < 0.1–8.5 mg/kg and for Th 0.1–14.9 mg/kg [24,25]. In European hard coals, the concentration ranges for U and Th are 5–29 mg/kg and 5–65 mg/kg, respectively [25]. Chinese coals normal exhibit background values for trace elements except, for example, Th (5.84 mg/kg) [26]. Some of these coals can be enriched in both U (54.5 to 100 mg/kg) and Th (up to 19.3 mg/kg) [27,28,29]. South African coals also contain U in a wide range of concentrations, varying from 2.9 to 199 mg/kg [30].

In order to assess the interdependence between the analyzed isotopes, the Pearson correlation analysis was performed (Figure 2). The ²³⁸U and ²³²Th concentrations were calculated based on U and Th content (Table 1). The correlation coefficient values range from +1 (perfect positive correlation) to −1 (perfect negative correlation). On the other hand, a correlation coefficient close to zero showed no linear correlation [31]. The values of this coefficient between −1 and 1 indicate the degree of linear dependence between the variables.

A full correlation between activity concentrations of the daughter ²²⁶Ra and parent ²³⁸U isotope was observed (r = 0.99). Very high correlation between pairs: (²²⁶Ra-²²⁸Ra: r = 0.91), (²³⁸U-⁴⁰K: r = 0.85) was noted. A moderate correlation between ²³⁸U and ²³²Th (r = 0.64) was observed. Previous studies [32] also showed a very high correlation between pairs: (²³⁸U-²²⁸Ra), (²³⁸U-⁴⁰K) for different feed hard coal samples (hard coal, culm, and silt), and for ²³⁸U and ²²⁶Ra isotopes, the correlation was high.

Waste deposited in landfills sometimes showed a high radioactive content and may have a negative impact on the environment. Therefore, the radiological risk from gamma radionuclides present in the landfills was investigated. The following risk indicators: radium equivalent activity (Ra_eq), external hazard index (H_ex) were assessed [33] on the basis of equations:

$${\mathrm{Ra}}_{\mathrm{eq}}={}_{ }{}^{226}\mathrm{R}\mathrm{a}+1.43·{}_{ }{}^{232}\mathrm{T}\mathrm{h}+0.077·{}_{ }{}^{40}\mathrm{K},$$

(1)

$${\mathrm{H}}_{\mathrm{ex}}=\frac{{}_{ }{}^{226}\mathrm{R}\mathrm{a}}{370}+\frac{{}_{ }{}^{232}\mathrm{T}\mathrm{h}}{259}+\frac{{}_{ }{}^{40}\mathrm{K}}{4810},$$

(2)

where: concentrations of ⁴⁰K, ²²⁶Ra, ²³²Th isotopes are expressed in Bq/kg.

Ra_eq should be lower than 370 Bq/kg [34] and H_ex value less than or equal to unity to limit the external gamma radiation dose below 1.5 mSv/y [33,35].

The absorbed dose rate in [nGy/h] from gamma radiation at a high of 1m above the ground level can be calculated as follows [34,36]:

$$\mathrm{D}\left[\frac{\mathrm{nGy}}{\mathrm{h}}\right]=0.462{}_{ }{}^{226}\mathrm{R}\mathrm{a}+0.604{}_{ }{}^{232}\mathrm{T}\mathrm{h}+0.0417{}_{ }{}^{40}\mathrm{K},$$

(3)

The annual effective dose rate [36] was calculated based on Equation:

$$\mathrm{ED} \left[\frac{\mathrm{mSv}}{\mathrm{y}}\right]=D\times 0.7\times 0.2\times 8760\times {10}^{-6},$$

(4)

In dose calculation, according to UNSCEAR [36] report, the conversion coefficient from the absorbed dose to the effective dose equal to 0.7 [Sv/Gy] and the outdoor occupancy factor equal to 0.2 were used.

The values of radium equivalent activity (R_aeq), external hazard index (H_ex) (

Table 2. The calculated values of Radium equivalent activity (R_eq), external hazard index H_ex air absorbed dose rates D [nGy/h], and annual effective dose ED [mSv/y].

Sample Number		Raeq [Bq/kg]	Hex	D [nGy/h]	ED [mSv/y]
Silstone	1	173	0.5	83.5	0.10
	1A	248	0.7	117.0	0.14
Sandstone	2	33	0.1	16.7	0.02
	2A	58	0.2	28.2	0.03
Mudstone	3	139	0.4	68.2	0.08
	3A	165	0.4	82.8	0.10
Burnt coal shale	4	197	0.5	93.6	0.11
	4A	72	0.2	33.4	0.04
Hard coal	5	36	0.1	15.3	0.02
	5A	60	0.2	28.6	0.04

), are lower than the limits.

The absorbed dose in the air reaches the value of 117.0 [nGy/h]. According to the report [36], the average absorbed dose rate in the air outdoors from terrestrial gamma radiation is equal to 60 [nGy/h]. The effective radiation dose reaches the value of 0.14 mSv/y, which is below the limit of 1 mSv/y [36].

3.2. REE

Table 3. The concentrations in [mg/kg] of REY in coal waste samples.

Element	Sample Number
	Silstone		Sandstone		Mudstone		Burnt Coal Shale		Hard Coal
	1	1A	2	2A	3	3A	4	4A	5	5A
La	50.3	52.2	36.4	39.1	31.6	36.8	37.7	25.2	1.9	2.1
Ce	111.0	114.0	87.2	91.2	76.5	99.2	88.2	67.3	5.5	5.3
Pr	12.5	13.8	10.3	11.8	8.5	9.9	10.2	7.7	0.8	0.8
Nd	42.5	49.8	42.4	49.5	30.8	34.6	26.7	21.8	3.6	3.5
Sm	8.2	9.8	7.8	8.4	5.1	5.5	7.2	6.1	0.8	0.7
Eu	1.3	1.6	1.6	1.6	1.1	1.3	0.7	0.3	0.8	0.6
Gd	7.3	7.8	6.1	7.6	3.8	4.5	3.1	2.5	0.7	0.7
Tb	0.9	0.9	1.1	1.1	0.6	0.8	0.4	0.2	0.1	0.1
Dy	4.6	5.8	4.2	5.1	3.3	3.6	2.8	2.1	0.6	0.7
Ho	1.2	1.1	0.6	0.9	0.7	0.9	0.1	0.1	0.1	0.1
Er	2.9	2.1	1.8	2.2	2.3	2.0	1.2	0.8	0.4	0.4
Tm	0.1	0.1	0.1	0.1	0.1	0.2	0.1	0.1	0.1	0.1
Yb	2.4	2.5	2.0	2.3	1.9	2.3	1.4	0.9	0.3	0.5
Lu	0.3	0.1	0.3	0.3	0.3	0.3	0.2	0.1	0	0.1
Y	29.1	31.3	23.6	27.8	30.3	34.2	21.5	18.6	4.4	4.6
∑REE	245.5	261.6	201.9	221.2	166.3	201.9	180.0	135.2	15.6	15.7
LREY	224.5	239.6	184.1	200.0	152.5	186.0	170.0	128.1	12.6	12.4
MREY	43.2	47.4	36.6	43.2	39.1	44.4	28.5	23.7	6.6	6.4
HREY	6.9	5.9	4.8	5.8	5.3	5.7	3.0	2.0	0.9	1.2

presents the results of REY determinations in coal waste samples.

In coal samples, REE concentrations were determined from 15.6 mg/kg to 15.7 mg/kg. The low REE content (15.7 mg/kg) in Polish hard coal is confirmed by studies by Całusz-Mołoszko and Białecka [37] and Żelazny et al. [38], who report REE contents in the range of 6–60 mg/kg. Coals generally contain REE in the range up to 69 mg/kg [39]. Anomalies of ∑REE + Y in hard coal have been found in Russia, Poland, Romania, the United States, and China [40]. Coal from China [29,41] and India (100–200 mg/kg) was found to have high concentrations of REE (172–232.2 mg/kg) [42]. Both hard coal and barren rocks can be enriched in REE [16].

Coal mining wastes are clearly enriched in lanthanides; these include clays, which contain REY ranging from 245.5 mg/kg to 261.1 mg/kg (Table 3).

Based on the geochemical classification of Seredin and Dai [16], the REY in hard coal and coal waste is classified into light (LREY-La, Ce, Pr, Nd, and Sm), medium (MREY-Eu, Gd, Tb, Dy, and Y), and heavy (HREY-Ho, Er, Tm, Yb, and Lu) groups (Table 3).

La, Ce, Nd, Eu, Sm, Yb, and Lu concentrations determined in the silstone samples are consistent with the results of the study by Kokowska-Pawlowska [43]. The lowest REE content (135.2–180 mg/kg) was determined in burnt coal shale (Table 3).

When studying mining waste Nowak and Kokowska-Pawłowska (2017) [44] found that the REE_s content varies depending on the degree of thermal transformation of the waste. In thermally unprocessed and moderately processed waste, the average REE_s content is similar, ranging from 184.0 mg/kg to 187.5 mg/kg. According to Nowak and Kokowska-Pawłowska (2017) [44], higher content of REE_s (225.6 mg/kg) is found in mining waste from the highly thermally processed zones. In these wastes, they found higher concentrations of La (40.1 mg/kg), Nd (32.9 mg/kg), Sm (5.8 mg/kg), and Y (55.6 mg/kg), compared with the waste from the thermally unprocessed and moderately processed zones.

The determined lanthanide concentration values for coal waste samples were normalized to NASC (North American Shale Composites) [45] (

Table 4. Anomaly coefficients of NASC-normalized concentrations of individual lanthanides.

Anomaly Coefficient	Sample Number
	Silstone		Sandstone		Mudstone		Burnt Coal Shale		Hard Coal
	1	1A	2	2A	3	3A	4	4A	5	5A
Ce/CeNASC	5.7	5.8	4.5	4.7	3.9	5.1	4.5	3.5	0.6	0.3
La/YbNASC	16.2	16.8	11.7	12.6	10.2	11.8	12.1	8.1	0.6	0.4
La/SmNASC	8.4	8.7	6.1	6.5	5.3	6.2	6.3	4.2	0.5	0.6
Sm/YbNASC	2.6	3.2	2.5	2.7	1.6	1.8	2.3	2.0	1.4	0.7
Eu/EuNASC	5.9	7.2	7.2	7.2	5.0	5.9	3.1	1.3	3.6	2.7
REE+Y [mg/kg] Concentrations in Shales (NASC) 1
Y	La	Ce	Pr	Nd	Sm	Eu	Gd	Tb	Dy	Ho
27	31.1	67.033	7.9	30.4	5.98	1.25333	5.5	0.85	5.75	1.2

¹ Migaszewski and Gałuszka, 2019 [45].

). Anomaly ratio values below 0.8 indicate a negative anomaly and those above 1.2 indicate a positive anomaly [45,46]. The values of REY concentrations normalized to NASC are indicative of the studied coal mining waste being enriched in LREY. The mean numerical values of the Ce/Ce_NASC (Ce/Ce_NASC = 0.5La_NASC + 0.5Pr_NASC) [47] (3.5–5.8) anomaly ratio are indicative of a positive anomaly in the studied coal waste samples (Table 4). The values of the Ce/Ce_NASC (1.3–3.6) anomaly ratio vary the most in the burnt shale samples. This is due to significant variation in concentrations of REY determined for samples 4 and 4A. The degree of shale thermal transformation is one of the factors that may influence its heterogeneity in chemical and mineral composition.

Coal waste is enriched in LREY and La/Yb_NASC (8.1–16.8) ratio is normalized, which is indicative of strong lanthanide fractionation.

Graphs showing the REY concentration values normalized to NASC in coal waste are presented in Figure 3.

Curves representing REY concentrations normalized to NASC indicate a large variation in the lanthanides in the investigated coal mining waste samples. The strong anomaly of Ce and Eu in the tested samples from the coal waste heap is probably due to the nature of these samples. This anomaly is most common in coal rocks, as confirmed by the studies of Serdin and Dai (2016) [16]. A strong positive Eu anomaly may be the result of the high temperature on the heap (even up to 200 °C) and the chemical composition of the coal mine waste (iron sulfides). The coals with positive Eu anomalies are characterized by a high pyrite content [47]. Heating inside a coal waste dump is typically highly variable with temperatures ranging from ambient to >500 °C [48]. Coal wastes deposited on the dump undergo a number of processes which can completely change their initial chemical or mineral composition [5]. The normalized Ho/Ho_NASC content in coal mine waste samples is from 1.0 (silstone) to 0.08 (burnt coal shale and hard coal). Sandstone (2, 2A) and mudstone (3, 3A) samples are the most stable as the concentration curves of REY normalized to NASC in these waste types are similar. The greatest variation in normalized REY concentration values was found in the burnt coal shale (4 and 4A).

Coal samples (5, 5A) normalized to NASC (Figure 4) are indicative of a marked increase in Eu compared to the other normalized lanthanide concentrations. The value of the normalized Eu/Eu_NASC (Eu/Eu_NASC = Eu_NASC/(Sm_NASC·Gd_NASC)^0.5) [49] anomaly ratio is 2.7–3.6.

The Y concentration (4.4–4.6 mg/kg) in the coal samples is low compared to the coal waste samples (31.3–18.6 mg/kg). The lowest Y content was found in the burned shale samples and the highest in the silstone samples (Table 3). The Y content in all coal mining waste samples was correlated with the LREY and HREY content. Strong correlations were found between Y concentration and HREY content (r = 0.91), and with LREY (r = 0.92) (Figure 5). All coal waste samples were analyzed for correlation. According to Lefticariu et al. [50], the Y/Ce ratio can be used to predict enrichment in REY. In the analyzed coal waste samples, the Y/Ce ratio is in the range of 0.3–0.5. Coal samples exhibited a higher value of the Y/Ce ratio (0.8–0.9). In the investigated coal waste samples, the relationship between Y concentration and Ce concentration is also moderate (r = 0.63).

3.3. Phase Composition

In the coal samples, rare earth phosphates, mainly monazite (Ce, La, Nd, Th, U, Pr)[PO₄], are also the sources of REY, U, and Th. Xenotime [YPO₄] was rarely found and therefore cannot be considered as a source of actinides in hard coal stored in heaps.

Monazite, which is a carrier of REY, U, and/or Th, was also identified in coal waste samples. The identified monazite grains can be divided into 3 types with varied morphology and varied content of identified rare earth oxides, uranium and thorium (Figure 6a–f). All types of monazite were found in the mudstone and silstone samples, including solid and compact grains, where cracks were rarely observed (Figure 6a,b). Monazite grains exhibit high concentrations of ThO₂ (>14%) and much lower concentrations of UO₂ (up to 1.72%). These grains are characterized by concentrations of Nd, Pr, and Sm oxides.

The second monazite type includes highly fractured, irregularly shaped grains with ThO₂ concentration not exceeding 8% (Figure 6c,d). Lower content of La₂O₃ and Ce₂O₃ was found in monazite compared to grains of the first type. No UO₂ was found in this monazite type, and the ThO₂ concentration does not exceed 7%.

The third monazite type is characterized by partially or strongly crushed grains, some of which exhibit traces of dissolution (Figure 6e). Monazite of this type contains either uranium or thorium. The UO₂ (up to 0.34%) and ThO₂ (up to 1%) oxide content is low compared to monazite of the first and second types. The monazite grains exhibit higher La₂O₃ and Ce₂O₃ concentrations, which is at a similar level as in monazite of the first type. Gd₂O₃ content was determined in these monazite grains and its concentration was found to be in the range of 0.76–1.51%.

In the sandstone samples, monazite grains of the first type were observed, which are weak in UO₂ and ThO₂ compared to the mudstone and silstone samples (Figure 6g,h).

In the burnt coal shale samples monazite grains of the first and third type were found (Figure 7). The chemical composition of monazite varies a lot, which results from the thermal transformation of the coal waste. Variable concentrations of UO₂ (0.36–1.38%) and ThO₂ (2.46–10.39%) were found in monazite. The concentrations of La₂O₃, Ce₂O₃, and Nd₂O₃ also vary a lot.

The chemical composition of the xenotime (Y) in REY exhibited variable contents of MREY and HREY, as well as UO₂ and/or ThO₂, were found (Figure 8). It was mainly observed in the silstone (Figure 8a) and mudstone samples (Figure 8b). Xenotime grains are rarely found in sandstone (Figure 8c) and burnt coal shale samples (Figure 8d).

The observed xenotime grains have Y₂O₃ content ranging from 49.61% to 56.08%. Among LREY and MREY, samarium and holmium occur sporadically with concentrations not exceeding 0.70%. The chemical composition of xenotime exhibited varied oxide contents: Gd₂O₃ (1.12–3.47%), Tb₂O₃ (2.39–8.52%, Dy₂O₃ (2.03–6.75%), and Yb₂O₃ (2.67–4.60%). Higher concentrations were recorded for UO₂ (0.68–1.38%; max. 6.83%) than ThO₂ (0.03–0.32%). The strong correlation between Y and HREY and LREY concentration (Figure 5) suggests that the rare earths are bound by xenotime and another substance, probably coal organic matter, which has often been found in coal mining waste (silstone and mudstone).

Numerous very fine agglomerations of rare earth phosphates (monazite and occasionally xenotime) have been found in hard coal. According to Seredin and Dai [16], and Ramakrishna [39], REE-rich minerals in hard coal mainly include fine-grained autogenous minerals (including REE-rich aluminum phosphates and sulphates, phosphates, and carbonates), as well as organic compounds. Dai et al. (2012) [51] found LREY associations in goyazite and gorceixite, MREY and HREY in boehmite, and some indications of MREY and HREY associations in accessory minerals. In addition to REE-rich minerals monazite and zircon, Hower et al. [52,53] found minerals containing rare earth elements on the nanoscale (monazite interlaid with kaolinite 10 nm to 1 µm). According to Pluta et al. [54], uranium is also found in secondary phosphates found on the cleavage surface of clay minerals in coal and is found in association with iron and zinc sulfides, and barite.

4. Conclusions

Preliminary results derived from a statistically small number of mining waste samples allow limited conclusions to be drawn, and further research in this area is needed.

The varying values of REY, U and Th concentrations in coal mining waste are due to the heterogeneity in chemical and mineral composition of the material deposited on the heap.

Monazite (Ce) in coal waste material on heaps is the main source of LREY. Quantitatively, xenotime (Y) is less important than monazite. Monazite cannot be the sole REY carrier like xenotime, which contains variable amounts of HREY (the positive correlation between Y concentration and HREY concentration is insufficient to determine the source of radioactivity). It might be assumed that rare earth phosphates in the studied coal mining waste take the form of very fine agglomerations in other minerals of Carboniferous rocks.

The analyzed barren rocks were characterized by variable strength of radioactive activity of the isotopes, with the highest values found in silstone and mudstone samples.

With the calculated radiological indices, the environmental impact of the coal waste heap and its influence on the health of the local population may be judged insignificant. Please note, however, the variability within the same type of waste (higher values in waste deposited in the heap profile down to the depth of 0.7 m), which requires further research.