Naturally Occurring Radioactive Materials in Uranium-Rich Coals and

Article Cite This: Environ. Sci. Technol. XXXX, XXX, XXX-XXX

pubs.acs.org/est

Naturally Occurring Radioactive Materials in Uranium-Rich Coals and Associated Coal Combustion Residues from China Nancy Lauer,# Avner Vengosh,# and Shifeng Dai*,†,‡ #

Division of Earth and Ocean Sciences, Nicholas School of the Environment, Duke University, Durham, North Carolina 27708, United States † State Key Laboratory of Coal Resources and Safe Mining, China University of Mining and Technology, China ‡ School of Resources and Geosciences, China University of Mining and Technology, Xuzhou 221116, China S Supporting Information *

ABSTRACT: Most coals in China have uranium concentrations up to 3 ppm, yet several coal deposits are known to be enriched in uranium. Naturally occurring radioactive materials (NORM) in these U-rich coals and associated coal combustion residues (CCRs) have not been well characterized. Here we measure NORM (Th, U, 228 Ra, 226Ra, and 210Pb) in coals from eight U-rich coal deposits in China and the associated CCRs from one of these deposits. We compared NORM in these U-rich coals and associated CCRs to CCRs collected from the Beijing area and natural loess sediments from northeastern China. We found elevated U concentrations (up to 476 ppm) that correspond to low 232Th/238U and 228Ra/226Ra activity ratios (≪1) in the coal samples. 226Ra and 228Ra activities correlate with 238U and 232Th activities, respectively, and 226Ra activities correlate well with 210Pb activities across all coal samples. We used measured NORM activities and ash yields in coals to model the activities of CCRs from all U-rich coals analyzed in this study. The activities of measured and modeled CCRs derived from U-rich coals exceed the standards for radiation in building materials, particularly for CCRs originating from coals with U > 10 ppm. Since beneficial use of high-U Chinese CCRs in building materials is not a suitable option, careful consideration needs to be taken to limit potential air and water contamination upon disposal of U- and Ra-rich CCRs.

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(222Rn and 220Rn), lead (210Pb), and Polonium (210Po) and collectively termed Naturally Occurring Radioactive Materials (NORM). While data on U content of some Chinese coals have been previously reported,2,3,5 as far as we are aware no data have been published thus far on the radionuclides of the U and Th decay chains in coals and CCRs in China. Previous studies have demonstrated that the combustion of coal and the resulting elimination of the coal’s organic matter leads to the enrichment of the radionuclides in the CCRs to activities that are proportional to radionuclide activities and ash yields of the original coals.9,10 Additionally, data from CCRs from the U.S. have shown further enrichment of 210Pb, particularly in fine CCR particles, reflecting Pb volatilization and subsequent reattachment to fly ash.10,11 Therefore, CCRs typically have significantly higher NORM than their respective parent coals. Accordingly, the enrichment of NORM in CCRs originating from high-U coals could pose potential human and environmental health concerns upon inhalation of CCR particles and mobilization to the ecosystem.12 The use of CCRs for cement and other building materials could also be

INTRODUCTION China is the largest coal producer and consumer in the world, contributing ∼50% of the global coal production and ∼50% of the global coal consumption in recent years, and accordingly, China is the largest CCR-producing country in the world (CCRs, coal combustion residues). In China, about ∼70% of CCRs are used every year in various ways including cement and building industry, brick manufacturing, road/dam construction, structural fill, backfill, and agriculture fertilizer, while ∼30% are disposed in coal ash ponds and landfills. Most coals worldwide, including the majority of coal deposits in China, are characterized by uranium (U) and thorium (Th) concentrations of 1−3 and 3 ppm, respectively.1−5 However, several coal deposits in northwestern (Xijiang Autonomous Region), southwestern (Sichuan, Guizhou, and Yunnan Provinces), and southern (Guangxi Province) China are enriched in U up to several hundred ppm.2,6−8 Given that many of the U-rich coals are also enriched in sulfur,2,4,7 typically these coals are not exclusively used in coal-fired power plants. Instead, they are often blended with low-sulfur and low-U coals. One exception is high-U Xiaolongtan coal, which is solely used in a 600megawatt coal-fired power plant in Xiaolongtan, Yunnan Province of southwestern China. 238 U and 232Th decay into a series of additional radioactive elements, notably including radium (226Ra and 228Ra), radon © XXXX American Chemical Society

Received: July 10, 2017 Revised: October 1, 2017 Accepted: October 30, 2017

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DOI: 10.1021/acs.est.7b03473 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Environmental Science & Technology

deposits in China with large variations in U concentrations. These include Lopingian coals (Heshan,18 Yishan,7 Guxu,19 Moxinpo,6 Guiding20), Middle Jurassic Bituminous coals (Muli21), Miocene coals (Lincang8), and Miocene lignites (Xiaolongtan). For more information on the location and geological setting of the different coal deposits see Figure 1 and Supporting Information Table 1 (Table S1). Radionuclide Analyses. Coal and CCR samples were powdered, homogenized, and packed in Petri-style dishes (6.5 cm in diameter, 2 cm high). The samples were then sealed with electrical tape and coated in wax to prevent the escape of 222Rn (t1/2 = 3.8 days). Following at least 27 days of incubation, the samples were counted for 1−3 days on a Canberra Broad Energy 5030 germanium (BEGe) gamma detector surrounded by 10 cm of lead shielding at the Laboratory for Environmental Analysis of RadioNuclides (LEARN) at Duke University. During the incubation time, 222Rn and its short-lived decay products, 214Bi (t1/2 = 19.7 min) and 214Pb (t1/2= 26.9 min), are expected to approach radioactive secular equilibrium with 226Ra (t1/2= 1600 years), and 228Ac (t1/2= 6.1 h) is also expected to approach equilibrium with 228Ra (t1/2= 5.8 years). The assumption that radioactive secular equilibrium is reached in these sections of the U and Th decay series allows for the analysis of 228Ra and 226Ra through their decay products when sufficient peaks are not available for direct measurement (e.g., the significant interference of 235U (54% yield) on the 186 keV peak of 226Ra). Accordingly, 226Ra was analyzed through the 214 Pb (351 keV) peak, 228Ra was analyzed through the 228Ac (911 keV) peak, and 210Pb was analyzed directly through its 47 keV peak. We corrected for self-adsorption of the relatively weak 210Pb gamma emissions using a 210Pb point source by methods described in Cutshall et al.22 Detector efficiencies were determined using a Canadian Certified Reference Materials Project (CCRMP) U−Th ore reference material (DL-1a) that contains U−Th series radionuclides in equilibrium that was packaged and incubated in the same geometry as the samples. Background counts and efficiency checks were performed routinely over the same time period as sample analyses. Statistical counting errors for all radionuclides were typically less than 10% (1σ). Th and U concentrations in coal and CCR samples were measured by inductively coupled plasma mass spectrometry at State Key Laboratory of Coal Resources and Safe Mining in China University of Mining and Technology (ThermoFisher, X series II ICP-MS). Coal samples were digested using an UltraClave Microwave High Pressure Reactor (Milestone). The

restricted due to the potential indoor radiation that could be induced by elevated NORM in the building materials.13−17 This study examines the occurrence and distribution of radionuclides from the U and Th decay series in some U-rich coals from China and models NORM activities in CCRs that would be generated from the combustion of these coals. The objectives of this study are (1) to characterize NORM in the Urich coals from different coal deposits in China, (2) to quantify the enrichment of NORM in CCRs originating from the combustion of U-rich coals, and (3) to evaluate implications for CCR management through disposal and use for building materials.

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MATERIALS AND METHODS Sample Collection from Coal-Fired Power Plants. Coal samples from different deposits in China (n = 57), coal combustion residues (n = 12), and, for a comparative investigation, loess sediments that represent surface loess deposits west of Beijing (n = 4) were analyzed in this study (Figure 1, Table 1). The coal samples represent different

Figure 1. A map of coalfields in China and the location of the coal deposits investigated in this study. The locations of the coal deposits are 1 - Xiaolongtan; 2 - Lincang; 3 - Guiding; 4 - Muli; 5 - Heshan; 6 -Moxinpo; 7 - Guxu; and 8 - Yishan. The rectangle in the lower right section shows the South China Sea Islands.

Table 1. Mean Values of Ash Yields (%; Dry Basis), Concentrations of U and Th (ppm; on Whole-Coal Dry Basis), and Radionuclides 226Ra, 228Ra, and 210Pb (Bq/kg) in Coals, CCRs, and Loess Sediments Investigated in This Study sample coal

fly ash loess

deposits/sources

n

ash yield

U

Th

226

Lincang Guiding Muli Heshan Moxinpo Yishan Guxu Xiaolongtan Xiaolongtan Beijing Area Northeastern China

12 6 7 5 4 8 12 3 6 6 4

22 23 17 37 42 18 21 11

69 206 1.9 39 376 82 16 13 68 8.0 1.5

3.1 3.1 6.6 14 12 6.3 8.1 1.1 12 31 10

757 2503 18 517 4601 1056 219 104 804 96 26

B

Ra

228

Ra

15 14 17 45 38 17 50 6 50 118 42

210

Pb

744 2545 21 542 4453 1086 231 120 771 N/A N/A


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Environmental Science & Technology digestion reagents for each 50-mg coal sample were 5-mL 65% HNO3 and 2-mL 40% HF. Inorganic Ventures standards, including CCS-1, CCS-4, CCS-5, and CCS-6, were used for calibration of trace element concentrations. Details of the ICPMS analysis technique for coal and coal-related materials are more fully described by Dai et al.23

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RESULTS AND DISCUSSION U- and Th-Decay Series Nuclides in the U-Rich Coals from China. U concentrations in Chinese coals in this study varied between 0.26 and 476 ppm (3 to 5875 Bq/kg 238U; Tables 1 and S1), up to a 160-fold higher U relative to typical U contents in coals (1−3 ppm1). Concentrations of Th in these Chinese coals vary from 0.16 to 24 ppm (0.6 to 96 Bq/kg 232 Th). This range also surpasses the average Th concentrations in Chinese (5.8 ppm),2,4,24 U.S. (3.2 ppm), and global (3.3 ppm) coals.1,2 Coals from the Muli deposit have relatively low U concentrations (0.26−6.5 ppm U) compared to coal samples from other deposits in this study and are more in the range of average concentrations for typical Chinese and global coals. Additionally, Muli coals had Th/U mass ratios of 3.4 ± 0.5 (activity ratio 1.1 ± 0.2). However, coals from other deposits analyzed in this study typically had higher concentrations of U compared to Th (Figure 2). The U enrichment in these Chinese coal are therefore independent of Th concentrations, as demonstrated by the decreases of the Th/U ratios with increasing U concentrations, resulting in a shift from a Th/U mass ratio of ∼3, which is typical for common coals and continental crust, to ratios below 1 in the U-rich coals (Figure 2). 226 Ra activities correlate linearly with 238U activities, and 210 Pb activities correlate linearly with 226Ra activities (slope ∼1; Figure 3). 228Ra activities also correlated with 232Th activities (Figure 4). Average 226Ra/238U and 228Ra/232Th activity ratios across all coal samples were found to be 1.1 ± 0.3 and 1.2 ± 1.1, respectively, and average 210Pb/226Ra activity ratios were found to be 1.1 ± 0.2. Average ratios close to unity indicate that U−Th series radionuclides in high-U Chinese coals fairly well approximate radioactive secular equilibrium (i.e., the activity of the decay product is equal to the activity of the parent nuclide), especially in the U-series radionuclides. While 226Ra/238U activity ratios deviate from unity in some samples, the majority of coal samples investigated in this study have 226Ra/238U activity ratios between 0.8 and 1.2 (i.e., within 20% of 1). Therefore, we can infer that there is a similar enrichment of all of the nuclides in the 238U decay series in U-rich coals. Likewise, 228Ra/226Ra ratios were also relatively consistent with 232 Th/238U activity ratios and were relatively low, reflecting the selective enrichment of U over Th, as presented in Figure S1 (Supporting Information Figure 1). For example, most U.S. coals are characterized by 232Th/238U activity ratios of 0.3 to 0.8, while the U-rich coals from China in the present study had distinctively lower 232Th/238U and 228Ra/226Ra activity ratios of 0.01 to 0.2 (Figure S1). We used loess samples from northeastern China as a reference for natural soil (Table 1). In contrast to the coals, the loess samples showed much lower U concentrations (∼1.5 ppm) but with similar Th (10 ppm) concentrations and thus a distinctively higher 232Th/238U mass ratio of ∼6.7 (232Th/238U activity ratio of 1.6). Radionuclide Enrichment in CCRs. Previous studies have shown that the 226Ra and 228Ra activities in CCRs are controlled by the original radionuclide activities and ash yields

Figure 2. Thorium and Th/U (mass ratio) versus U concentrations in the investigated coals from China. Uranium is enriched in investigated Chinese coals relative to common coals with typically 1−3 ppm U. Thorium concentrations in some coals were also higher than common coals (∼3 ppm) but were typically less than 10 ppm.

of the parent coals.10 This relationship is also consistent with previous studies that have shown similar NORM enrichment in CCRs9,10,25,26 and indicates that the enrichment simply due to the removal of the coal’s organic matter during combustion primarily causes the enrichment of radionuclide activities in CCRs. Therefore, with knowledge of the coal’s ash yield and original U and Th (or 226Ra and 228Ra, if secular equilibrium can be assumed), the 226Ra and 228Ra activities of CCRs derived from coals can be approximated. This estimation does not take into account the redistribution and enrichment that may occur due to the volatile nature of certain elements; volatization, however, has been found to have a relatively minor effect on U and Ra compared to more volatile elements like lead.9,10 The ash yield of the Chinese coals investigated in this study ranged from 6% to 50% (mean = 23%), suggesting ∼2- to 16fold enrichments in NORM in CCRs produced from these coals when the organic matter is fully eliminated during combustion. In Xiaolongtan, the local coal is exclusively used C


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Figure 4. Activities of 228Ra versus 232Th in coals from China. The dashed line represents a 1:1 ratio or radioactive secular equilibrium.

Figure 3. Activities of 226Ra versus 238U and 210Pb versus 226Ra in coals from China. The dashed line represents a 1:1 ratio or radioactive secular equilibrium. Coals investigated in this study fairly well approximate secular equilibrium in the U decay series.

Figure 5. 228Ra versus 226Ra activities in Xiaolongtan coals, Xiaolongtan fly ash, and hypothetical activities calculated from initial Ra activities and ash yields in Xiaolongtan coals. Hypothetical Ra activities in modeled CCRs are consistent with Ra activities measured in actual Xialongtan coal samples.

for combustion in a local coal plant. This sample set provides the opportunity to test whether Ra activities in CCRs estimated from U and Th and ash yields of feed coal are consistent with Ra activities measured in actual CCR samples. We found that the 226Ra and 228Ra activities in the actual CCRs from Xiaolongtan coals are indeed consistent with the theoretical Ra activities (Figure 5). Based on the Ra activities and ash yields of the coals, we calculated the hypothetical Ra activities in CCRs (Table S3). 226Ra activities in CCRs originating from the U-rich coals are expected to reach up to 11,250 Bq/kg (the average values in hypothetical CCRs from Guiding), which is 40- to 90-fold higher relative to 226Ra activities reported for CCRs from the U.S. (120 to 230 Bq/kg).10 The ability to estimate Ra activities of CCRs from U-rich coals is important as Ra is often used in calculations of radiation hazard indices. Implications for CCR Use and Disposal. To assess the potential radiation hazard of CCRs from U-rich Chinese coals, we employed several measures, including the radium equivalent

activity (Raeq), the external hazardous index (Hex), and the gamma index. These indices consider the external radiation dose from γ radiation from 226Ra, 232Th, and 40K in building materials that could induce exposure risk to indoor inhabitants. The Raeq is defined as Raeq = 226Ra + 1.43232Th + 0.07740K (where 226Ra, 228Ra, and 40K are the specific activities measured in building materials in Bq/kg). Based on United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR13), a Raeq value of 370 Bq/kg in building materials will produce an annual dose rate of about 1.5 mSv/y to the inhabitants. Consequently, external exposure in dwellings by γ rays emitted from nuclides of the uranium and thorium decay chains in building materials would result in potential health risks for indoor inhabitants. In order to protect public health, standards and guidelines have been developed for an upper level of radiation permitted in building materials.14,15,17,27,28 D


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Environmental Science & Technology The Raeq level of 370 Bq/kg has been recommended to be the threshold for homes or any other residential applications.13 Similarly, the Hex index is calculated as Hex = (226Ra/370) + (232Th/259) + (40K/4810). A Hex level above unity, which corresponds to a Raeq activity of 370 Bq/kg, is considered a potential radiation risk for indoor inhabitants. The European Commission has also developed the gamma index, defined as I = (226Ra/300) + (232Th/200) + (40K/3000).15 Gamma index values above unity correspond to an annual dose rate of approximately 1 mSv/y to the inhabitants. Since I is a more conservative hazard index, values that exceed the Raeq and Hex thresholds will also exceed an I of 1. Because 40K was not analyzed in this study, Raeq, Hex, and I values are calculated using solely 226Ra and 232Th, which are still quite representative due to the relatively minor contribution of 40K to these indices. Our data show that Raeq levels in modeled CCRs originating from the U-rich coals measured in the present study far exceed the Raeq threshold of 370 Bq/kg, as well as Hex and I values of 1, and can reach to levels of up to 16,000 Bq/kg. The 226Ra and 232 Th activities and thus the Raeq values in CCRs are associated with the U, Th, and ash yields of the original coals (Figure 6).

< 1. For example, the CCR samples collected in the Beijing area have relatively low Raeq (∼100 to 400 Bq/kg) and Hex values mostly below unity (0.3 to 1.1), which indicate that building materials composed of CCRs derived from typical coals will likely result in a radiation dose below the upper dose threshold (Figure 6). Figure 6 presents the Raeq and Hex values calculated for modeled CCRs versus the U contents of their original coals. We found that the combustion of coals with U contents great than 10 ppm (and ash yield of 20% to 40%) can potentially generate CCRs that exceed thresholds for safe building materials (Raeq > 500 Bq/kg; Hex > 1.3). At about 60 ppm U in coals, the Hex values in their CCRs potentially become an order of magnitude (i.e., > 10) higher than the safe radiation threshold for building materials. Since beneficial use of these high-U Chinese CCRs in building materials is not a suitable option, careful consideration should be given to the potential environmental and human health risks that may are arise if other disposal options are sought. Given the evidence of NORM migration and bioaccumulation in the ambient environment,12,31,32 the radiation of CCRs should be considered. Such risks include fugitive emission of fine CCR particles from dry storage sites such as landfills. Similar to radon exposure, inhalation of very fine and Ra-rich particulate matter could result in accumulation of radium and its decay-nuclides in the respiratory system. The 226 Ra activities of modeled CCRs originating from U-rich coals can potentially reach 200 times the 226Ra activities of Chinese loess sediments. Future studies should further evaluate these risks in areas of extensive CCR emission and population exposure. Overall, this study examined the occurrence and distribution of radionuclides from the U and Th decay series in U-rich coals and associated CCRs from several coal deposits in China, typical CCRs from Beijing area, and loess sediments in northeastern China. It was found that U is selectively enriched (up to 476 ppm) in the coals, resulting in low Th/U and 228 Ra/226Ra activity ratios (≪1), relative to typical coals and CCRs with lower U contents (typically 1 to 3 ppm). The enrichment of nuclides in the CCRs originating from the U-rich coals is proportional to the U and ash yields of the coals. The radiation of modeled CCRs originating from most of the U-rich coals exceeds the upper limit of radiation hazard indices for building materials. Empirical data from the Chinese coals and associated CCRs indicate that the threshold for radiation levels above the acceptable threshold values for building materials corresponds to U content of ∼10 ppm in the source coals. We therefore suggest that CCRs originating from other coal resources with U concentrations >10 ppm could pose radiation risks if used for building materials in residential building.33,34 Additionally, these CCRs could pose potential human health risks in areas located downwind from sites with fugitive CCR emissions. Future studies should examine the global occurrence of coals with U > 10 ppm and the management and disposal of CCRs originating from such U-rich coals.

Figure 6. Radium equivalent activity (Raeq, left y-axis) and external hazardous index (Hex, right y-axis) in hypothetical CCRs versus U concentrations of the parent coals analyzed in this study. CCRs originating from coals with U > 10 ppm would be associated with Raeq and Hex values above thresholds of 370 and 1, respectively, which represents the upper limit permitted for radiation in building materials.13,15

Thus, CCRs originating from relatively lower U content and lower ash yield (e.g., Guiding, U = 150−290 ppm; ash = 20%; Yishan, U = 47−123; ash = 20%) would have similar Raeq values compared to coals with relatively higher U and higher ash yield (e.g., Moxinpo, U = 295−476 ppm; ash = 40%; Figure 6). Actual CCR samples from Xiaolongtan had Raeq values of 830 to 970 Bq/kg and Hex values ranging from 2 to 4, which is higher than the safe radiation threshold for building materials. The Raeq values in the CCRs originating from the U-rich coals are several orders of magnitude higher than levels typically measured in common cements (Raeq ∼ 100 Bq/kg; Hex values typically 10 ppm for building materials, especially for housing, should be restricted. In contrast, typical coals with U content of 1−3 ppm would typically be expected to generate CCRs with Raeq < 370 and Hex

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.est.7b03473. Table S1, background information for investigated coals; Table S2, radionuclide data and 95% confidence E


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intervals; Table S3, hypothetical radionuclides activities in CCRs; Figure S1, 228Ra/226Ra vs Th/U (activity ratios) in investigated coals (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Avner Vengosh: 0000-0001-8928-0157 Shifeng Dai: 0000-0002-9770-1369 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the National Key Basic Research Program of China (no. 2014CB238902) and the National Natural Science Foundation of China (no. 41420104001). The authors would also like to thank Dr. James Kaste at the College of William and Mary for laboratory use. We also thank Dr. Huaming Guo from China University of Geosciences for providing the loess and fly ash samples from Beijing area.

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Naturally Occurring Radioactive Materials in Uranium-Rich Coals and

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