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Radionuclides in the environment around the uranium mines in Guangxi, China
Applied Radiation and Isotopes 159 (2020) 109098
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Applied Radiation and Isotopes journal homepage: http://www.elsevier.com/locate/apradiso
Radionuclides in the environment around the uranium mines in Guangxi, China Ruirui Wang a, Jingyu Mai a, b, Yongjin Guan b, Zhiyong Liu a, * a
State Key Laboratory of Radiation Medicine and Protection, School for Radiological and Interdisciplinary Sciences (RAD-X) and Collaborative Innovation Centre of Radiation Medicine of Jiangsu Higher Education Institutions, Soochow University, Suzhou, 215123, China b Guangxi Key Laboratory for Relativistic Astrophysics, School of Physical Science and Technology, Guangxi University, Nanning, 530004, China
A R T I C L E I N F O
A B S T R A C T
Keywords: 238 U 239þ240 Pu activity 240 Pu/239Pu atom ratio Uranium mine Heavy mental
Uranium and plutonium are both poisonous radioactive elements, which are very harmful to human health and environment. Therefore, it is of great significance to study the distribution of 238U concentration and 239þ240Pu activity in the uranium mine surrounding soils. We have collected some surface soil sediments within 2 km of two uranium mines and a solid waste management center in Guangxi Province. The 238U concentration in these study areas is in the range of 1.44–83.91 mg/g, and the 238U concentration in the A uranium mine surrounding surface soils is higher than that in the B uranium mine and the solid waste management center. While the B uranium mine and the solid waster management center don’t pollute the surrounding soils because the 238U concentra tions in their surrounding soils are similar to the average 238U concentration in the soil. The 239þ240Pu activities in soil samples collected around the two uranium mines and the solid waste management center are close ranged from 0.06 mBq/g to 0.51 mBq/g. Moreover, the 240Pu/239Pu atom ratios in our study samples are ranged from 0.15 to 0.23, which indicate the Pu may come from the global fallout. In addition, we study heavy metals in our collected samples, only heavy metal Tl has weak positive correlations with 238U concentrations and 239þ240Pu activities. And there is a weak positive correlation between 238U concentrations and 239þ240Pu activities.
1. Introduction Uranium is a kind of energy and a very important strategic resource. Uranium provides sufficient guarantee for the sustainable development of nuclear energy, and also plays an important role in nuclear military industry and nuclear power industry (Donohue, 1998; Wallenius et al., 2007; Wang et al., 2011; Herring, 2018). As a highly sensitive strategic resource with dual-use military and civilian applications, uranium is widely used with the gradual maturity and perfection of nuclear power technology (Briner, 2010; Burns and Finch, 2018). It is used in national defense construction and national economy, so the demand for fuel is increasing. According to the calculation of Hou et al. (2007), the average uranium content in the Earth is (4.42–5.85) � 10 8 mg/g. Uranium in the world is very rich, but the overall distribution is uneven (Herring, 2013). In China, the uranium deposits are mainly of medium and low grade. Although the scale of a single deposit is small, the number of deposits is large (Liu et al., 1992; Zhang et al., 2008; Yan et al., 2011). Uranium in nature has a mixture of three isotopes: 238U (T1/2 ¼ 4.468 � 109 years), 239U(T1/2 ¼ 7.038 � 108 years), and 240U(T1/2 ¼
2.455 � 106 years). Among them, the content of 238U accounts for 99.28% of the total content. These three isotopes of uranium are found in many minerals containing uranium, of which pitchblende contains the highest amount of uranium. Meanwhile, the uranium tailing is a serious pollution challenge for the environment and very harmful to human health (Bednar et al., 2007; Yang et al., 2011), and there are many technologies to deal with uranium in the environment (Vanden hove et al., 2001; Rufyikiri et al., 2004; Li and Zhang, 2012; Li et al., 2013). Plutonium is another radionuclide closely related to the nuclear in dustry including weapons industry and energy industry. Pu enters the environment mainly through nuclear weapon testing (Sholkovitz, 1983), accidental release (Zheng et al., 2012) and discharges from nu clear fuel reprocessing sites and nuclear power plants (Dai et al., 2005). Anthropogenic radionuclides 239Pu (T1/2 ¼ 24100 yr) and 240Pu (T1/2 ¼ 6560 yr) have high radiological toxicity and long-term retention in the environment. 239Pu and 240Pu with relatively long half-life have been used as tracers to analyze a series processes of Pu in nature, including the sources of Pu, Pu inputs and distribution, rebuilding the Pu historical
* Corresponding author. E-mail address: [email protected] (Z. Liu). https://doi.org/10.1016/j.apradiso.2020.109098 Received 27 October 2019; Received in revised form 2 January 2020; Accepted 23 February 2020 Available online 27 February 2020 0969-8043/© 2020 Elsevier Ltd. All rights reserved.
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Fig. 1. Map of sampling sites near the uranium mines in Guangxi province.
events, migration of Pu in seas and land, and special impacts on the environment (Kelley et al., 1999; Muramatsu et al., 2001; Warneke et al., 2002; Ketterer et al., 2004; Chamizo et al., 2011; Zheng et al., 2013; Wu et al., 2014, 2018). In addition, The 240Pu/239Pu atom ratio can be effectively used to trace the sources of Pu. Because the 240Pu/239Pu atom ratio varies among nuclear weapon construction sites, weapon test fallout, reproc essing plants, reactor or satellite accidents, and weapon types and yields (Muramatsu et al., 2001; Ketterer et al., 2004). The 240Pu/239Pu atom ratio of weapon-grade material is 0.01–0.077, while that of nuclear reactor-grade material is 0.2–1.08. The average 240Pu/239Pu atom ratio for regional fallout from the Pacific Proving Ground (PPG) in the Marshall Islands close-in fallout is 0.33–0.369, which is distinctly different from the ratio of the global fallout (0.176 � 0.014) (Ketterer et al., 2004). Plutonium from the Chernobyl Nuclear Power Plant (CNPP) accident has 240Pu/239Pu atom ratio ranged from 0.186 to 0.348 (Ketterer et al., 2004), while plutonium from the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident has 240Pu/239Pu atom ratio of 0.330 � 0.032 (Zheng et al., 2013). In China, the activity of 239þ240Pu in surface soils has shown obvious latitude-dependent distribution, with maximum settlement occurring between 40� N and 50� N (Xing, 2015). The average atom ratio of 240Pu/239Pu is 0.19, indicating that the Pu source in surface soils in China is mainly from the global fallout (Liu et al., 2007; Li et al., 2007; Zhuang et al., 2019). In this study, we choose a production stoppage uranium mine and a decommissioned uranium mine and a solid waste management center to study the uranium and plutonium in their surrounding soils. The pur poses are: (1) to find out the distribution of 238U concentration, Pu ac tivity and provide the background value of 238U concentration and Pu activity in soils around the two uranium mines and the solid waste management center; (2) to determine the source of Pu; (3) to look for some correlations of 238U concentration and Pu activity with some heavy
metal contents. 2. Sampling and measurements 2.1. Study area The two uranium mines are located in Guangxi province (Fig. 1). The production stoppage uranium mine is the “A uranium mine”, which has a history of more than 30 years, located in Daxing, Guangxi province. The mining area includes open pit ruins, western waste rock field, eastern waste rock field, industrial site, mining road and living area. The uranium mine was formally put into production in the early 1980s, when only open-pit mining was carried out, not smelting. The mining project was shut down in 1994. In 2000, the residual ore recovery project was implemented, mainly for mining and recycling of the exposed ore. The decommissioned uranium mine is the “B uranium mine”, which is located in Xiangyangping, mountain area with deep landform and large slope. The mine is still in operation and now has stopped pro duction and begun the residual ore recovery project. In our study, there is also a solid waste management center, which is used to dispose the wast from uranium mines. Meanwhile, the radio nuclides in soils around it are also the focus of our study. 2.2. Sample collection A total 38 soil samples were collected around the two uranium mines and the solid waste management center in 2015, which are in the ura nium mine isolation zone. These samples were taken from fresh soil samples at the sampling site, taken by the recovery of box cores with top 3 cm sediments. In these soil samples, 17 soil samples were collected around the A uranium mine, 14 samples were collected around B 2
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uranium mine, and the remaining 7 samples were collected around the solid waste management center. First, all these samples were packed into clean polypropylene sample bags, and then kept in a 4 � C refrig erator until analysis. The detailed site information and sample proper ties are shown in Table A1. Since these mines are uranium mines, it is inconvenient for us to provide specific locations of uranium mines there.
loading onto the second resin column, AG MP-1 M (2.5 ml, 6.5–8.5 mm � 58 mm, i.d.). Pu is eluted from the AG MP-1 M column with 16 ml HBr. And the mixture is evaporated to near dryness, 1 ml of concentrated HNO3 is added and heated to remove any trace of HBr. When near dryness, the final residual is dissolved in 0.8 ml of 40% HNO3 in prep aration for the sector field inductively coupled plasma mass spectrom etry (SF-ICP-MS; Element 2) analysis. The SF-ICP-MS is conducted in the Key Laboratory of Radiation Medicine and Protection at Soochow Uni versity (Suzhou, China). An APEX-Q sample introduction system with a membrane desolvation unit (ACM) and a conical concentric nebulizer are used as sample introduction systems to improve the sensitivity of the SF-ICP-MS. The overall Pu recoveries range from 80%–95%, and the average recovery was 87% � 3%. A certified Pu isotopic standard so lution (NBS-947) is used for mass bias correction. All measurements are made in the self-aspirating mode to reduce the risk of contamination by the peristaltic pump tubing. Details on the instrument optimization and determination of Pu isotopes have been described previously (Zheng and Yamada, 2006).
2.3. U and Pu isotope analysis The samples are dried at 100 � C for 24 h and pass through 20-mesh sieves to remove rhizome and coarse stones and then transfer to crucible and dry at 800 � C for over 8 h to podzolize the samples. The details of uranium analytical procedures are described in Kikawada et al. (2009). Uranium in surface soil samples is determined by scintillation gamma-spectrometry. Measurements are carried out using a gamma-spectrometer with a low-background NaI (Tl) well scintillation detector with a volume of 200 � 200 mm (well dimensions of 70 � 150 mm). The determination of the equivalent uranium (U (Ra)) concen trations are carried out using the analytical gamma-line of the isotope 214 Bi 1764.5 keV, based on the assumption of a radioactive equilibrium between 238U and 226Ra. The lower limit of detection of U (Ra) is about 0.4 ppm. The relative error of U (Ra) concentrations determination is not worse than 15% (2σ). The accuracy and reproducibility of the analysis are controlled by parallel measurements of Russian national geological reference materials SG-1A, SG-3, SG-2, DVG, DVT, ZUK-1, BIL-1, ST-1A (Govindaraju, 1994), and we use the SG-2 to control the accuracy in this study. To measure the plutonium in surface soils, these surface soil sedi ments are dried at 80 � C and ground in an amber mortar in preparation for Pu isotopic analyses. A 2.5 g dried samples is weighted out. Then 1.14 pg 242Pu is added for each samples as a chemical yield tracer. We leach the Pu acid with 50 ml 8 M HNO3 by heating on a hot plate at 180–200 � C for at least 4 h using a tightened Teflon vessel (150 ml) (Savillex Corporation, Minnesota, USA). After filtration supernatant, a NaNO2 is added to the sample solution to a concentration of 0.2 M and heated at 40 � C for 30 min to take Pu to the tetravalent state prior to loading onto the first SG 1-X8 resin column (2.5 ml, 6.5–8.5 mm � 58 mm, i.d.). After removing U, Pb, and Fe from the column using 50 ml of 8 M HNO3 (Zheng and Yamada, 2006), the sample solution is ready for
Fig. 2. Relationship between
239þ240
Pu activity and
3. Results and discussion The analysis results of 238U concentrations, 239þ240Pu activities and Pu/239Pu atom ratios are presented in Table A1.
240
3.1. Levels of
238
U concentrations in soils
238 U concentrations in the two uranium mines and the solid waste management center surrounding soils are in the range of 1.44–83.91 mg/g. Among them, the 238U concentrations range from 1.83 mg/g to 83.91 mg/g in the A uranium mine surrounding soils, with the average of 19.67 mg/g. The 238U concentrations are in the range of 1.44–6.87 mg/g (with the average of 3.11 mg/g) in the B uranium mine sur rounding soils, and the 238U concentrations of 2.48–3.81mg/g (with the average of 3.03 mg/g) are in the solid waste management center sur rounding soils. The 238U concentrations in the B uranium mine and solid waste management center surrounding soils are similar to the order of magnitude in igneous and sedimentary racks (0.45–3.7 mg/g) and Chinese loess (2.72–4.08 mg/g) (Tayler et al., 1983; Ding et al., 2001). A stark contrast is that the 238U concentrations in the A uranium
240
Pu/239Pu atom ratio in surface sediments from river basins in southern China. 3
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Fig. 3. Content of 239þ240Pu activity (mBq/g), 238U concentration (mg/g) and heavy metals (mg/kg) in surface sediments of river basins in southern China. The activities of 239þ240Pu are on a scale of 10 times.
mine surrounding soils are relatively higher than that in the B uranium mine and solid waste management center surrounding soils, especially in samples 2, 4, 74, and the highest 238U concentration is 83.91 mg/g (Table A1). Since the 238U concentrations in the B uranium mine and solid waste management center surrounding soils are similar to the 238U concentrations in Chinese loess (2.72–4.08 mg/g) (Tayler et al., 1983), that the 238U concentrations in the B uranium mine and solid waste management center surrounding soils are the background values of 238U concentrations in soils in Guangxi province, which is 1.44–6.87 mg/g. According to the above results, it can be confirmed that the A uranium mine may contaminate the surrounding soils, and the soils around the B uranium mine and the solid waste management center do not be polluted. Thus, we should pay more attention to the environment around the A uranium mine. 3.2.
239þ240
Pu activity and
and the samples collected from the solid waster management center surrounding soils have the 239þ240Pu activities of 0.08–0.15 mBq/g (with the average of 0.10 � 0.009 mBq/g). Obviously, the 239þ240Pu activities in the A uranium mine surrounding soils vary widely, and have relatively higher values than the 239þ240Pu activities in the B uranium mine and the solid waste management center surrounding soils. Besides, the 239þ240Pu activities in the B uranium mine and the solid waste management center surrounding soils have similar values. On the whole, the soils around our study sites have relatively close 239þ240Pu activities. The 240Pu/239Pu atom ratios in our study samples are ranged from 0.15 to 0.23, with an average of 0.19 � 0.020. The 240Pu/239Pu atom ratios in the A uranium mine surrounding soils are 0.15–0.23, with an average of 0.19 � 0.017. The 240Pu/239Pu atom ratios in the B uranium mine surrounding soils are 0.16–0.23, with an average of 0.19 � 0.025. The samples collected from the solid waster management center sur rounding soils have the 240Pu/239Pu atom ratios of 0.20–0.23, with an average of 0.22 � 0.021. Even the 240Pu/239Pu atom ratios in the solid waste management center surrounding soils are relatively higher than that in the A and B uranium mines surrounding soils. All these 240 Pu/239Pu atom ratios are well with the global fallout value (0.178 � 0.019, 0–30� N) (Kelley et al., 1999). Besides, these results are also consistent with previous studies about the 240Pu/239Pu atom ratios in surface soils in southern China (Liu et al., 2007; Li et al., 2007; Zhuang et al., 2019). Moreover, the major source of Pu in these surface soils is mainly from the global fallout. In central and northern China, similar 240 Pu/239Pu atom ratios have also been reported (Zheng and Yamada, 2006; Wang et al., 2011; Zhuang et al., 2019). In Fig. 2, we can see clearly that most of the 240Pu/239Pu atom ratios of the A and B uranium mines surrounding soils are in the global fallout area. While the 240Pu/239Pu atom ratios of the solid waste management center surrounding soils are above the global fallout and below the Pacific Proving Grounds (PPG). Although these 240Pu/239Pu atom ratios in the solid waste management center surrounding soils differ in some extent, they are all in value range of the global fallout (0.178 � 0.019, 0–30� N).
240
Pu/239Pu atom ratio
The 239þ240Pu activities in these soil samples collected around the two uranium mines and the solid waste management center are ranged from 0.06 mBq/g to 0.51 mBq/g (mean: 0.18 � 0.016 mBq/g). The samples collected from the A uranium mine surrounding soils have the 239þ240 Pu activities of 0.09–0.5 mBq/g (with the average of 0.25 � 0.018 mBq/g), the 239þ240Pu activities in soil samples collected around the B uranium mine are 0.07–0.37 mBq/g (mean: 0.14 � 0.016 mBq/g),
3.3. Factors effecting
238
U content and
239þ240
Pu activity in soils
To find out some factors effected the 238U concentrations and the Pu activity distributions, we also do some heavy metal analysis on these soil samples, such as the contents of Cr, Co, Ni, Cu, Zn, Cd, Tl, Pb. The analysis results are presented in Table A2. Several studies have reported that uranium has a unique bound with Cr and only indirectly with Mn, Fe, Ni, Co in soil, and 238U concentration shows a strong
239þ240
Fig. 4. The correlation between U, Pu and other heavy metal. 4
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Table 1 The correlation between Cr Cr Co Ni Cu Zn Cd Tl Pb U Pu a b
1 0.497a 0.714a 0.633a 0.633a 0.041 0.287 0.767a 0.347b 0.381b
238
U concentrations, Co
239þ240
Pu activities and other heavy metals.
Ni a
0.497 1 0.879a 0.659a 0.574a 0.456a 0.383 0.466a 0.283 0.262
Cu a
0.714 0.879a 1 0.845a 0.727a 0.438 0.429a 0.693a 0.339b 0.321b
Zn a
0.633 0.659a 0.845a 1 0.753a 1.465 0.461a 0.742a 0.156 0.088
Cd a
0.633 0.574a 0.727a 0.753a 1 0.167 0.373b 0.776a 0.182 0.249
0.041 0.456a 0.438a 0.465a 0.167 1 0.052 0.091 0.076 0.097
Tl
1
0.287 0.383b 0.429a 0.461a 0.373b 0.052
0.386b 0.31 0.306
Pb
U a
0.767 0.466a 0.693a 0.742a 0.776a 0.091 0.386b 1 0.104 0.253
Pu b
0.347 0.283 0.339b 0.156 0.182 0.076 0.309 0.104 1 0.408b
0.381b 0.262 0.321b 0.088 0.249 0.097 0.306 0.253 0.408b 1
Correlation is significant at the 0.01 level (2-tailed). Correlation is significant at the 0.05 level (2-tailed).
positive correlation with the content of K, Pb, Be, Zn, Cr, Co, Fe, V, Ni, Cu, Al (Malikova et al., 2020). Wang and Gao (2014) have indicated that the content of heavy metals in soils has a gradual decreasing trend with increasing distance from the tailing. Meanwhile, the 238U concentrations in these samples are evidently correlated with the contents of Cr and Cu and no correlativity with the Cd content. However, the 239þ240Pu ac tivities only have positive correlation with the content of As in the samples from the river basins in southern China (Wang et al., 2019). The 238U concentrations in our study surface soils has weak bounds with the content of Tl (correlation coefficient is 0.309) and the 239þ240Pu activities (correlation coefficient is 0.408), while has negative correla tions with other heavy mental elements (Cr, Co, Ni, Cu, Zn, Cd, Pb). 239þ240 Pu activities, like 238U concentrations, show weak positive cor relations with the content of Tl (correlation coefficient is 0.306) and 238 U concentrations (correlation coefficient is 0.408), also have indi rectly correlations with other heavy mental contents (Fig. 3 and Fig. 4) (Table 1). Compared with previous studies, 238U concentrations have negative correlations with the content of Cr in our study samples. Be sides, the 239þ240Pu activities have a weak positive relationship with the content of Tl (Fig. 3). It is clearly to see the correlations between the 238U concentrations and heavy mental contents have different results in different study areas, and the same true for 239þ240Pu activities. The reason for this inconsistency is largely due to the complexity of the environment in different study areas. For example, there are other fac tors more important for the distributions of 238U concentration and 239þ240 Pu activity in soils, including the peculiarities of soil formation, the evaporation process, the degree of salinity, variations in the mineralogical composition (Malikova et al., 2020). Previous studies have reported that uranium is deposited in the process of the adsorption on clay minerals (Titaeva, 2000; Kabata- Pendias, 2000; Santos-Frances et al., 2018), and the content and prop erties of clay minerals affect the sorption of uranium (Kayzar et al., 2014; Filistovic et al., 2015). Lin et al. (2018) has reported the depth distribution of uranium is probably controlled by its leaching and deposition cycles. In addition, the forms of uranium are important fac tors for uranium content in soils (Reinoso-Maset and Ly, 2016). Mean while, more studies have indicated the mobility of uranium forms is determined not only by redox conditions and the PH of the water, but also by biogeochemical processes (Andersson et al., 2001; Frederickson et al., 2002; Sharp et al., 2011; Newsome et al., 2015; Strakhovenk and Gaskova, 2018). It is believed that the mobility of uranium in the environment depends on the form of organic matter (Meng et al., 2017). Combined with the above studies, our research results are inconsistent with the published results, which is more due to the uncontrollable environment, and there will be more influencing factors for uranium migration in the environment in future studies. In addition, the 238U concentrations have a weak positive correlation with the 239þ240Pu activities in our study. However, no conclusive conclusions have been made about their correlation, and we will further study their correlation and the factors influencing their distribution in the environment.
4. Conclusion The average of 238U concentrations in the A uranium mine sur rounding soils (19.67 mg/g) is higher than the 238U concentrations in the B uranium mine (mean: 3.11 mg/g) and the solid waste management center surrounding soils (mean: 3.03 mg/g). Moreover, the 239þ240Pu activities in these soil samples collected around the A, B uranium mines and the solid waste management center are close, which are 0.09-0.5 mBq/g (mean: 0.25�0.018 mBq/g), 0.07-0.37 mBq/g (mean: 0.14�0.016 mBq/g) and 0.08-0.15 mBq/g (mean: 0.10�0.009 mBq/g), respectively. In addition, the Pu in all collected samples is from the global fallout. And, for the contents of these heavy metals, only the Tl content has weak positive correlations with the 238U concentrations (correlation coefficient is 0.306) and the 239þ240Pu activities (correla tion coefficient is 0.306). Meanwhile, there is a weak positive correla tion between the 238U concentrations and the 239þ240Pu activities (0.408). Declaration of competing interest 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. Acknowledgement This work was supported jointly by grants from the National Science Foundation of China (41773004, 11665006), the Jiangsu Provincial Key Laboratory of Radiation Medicine and Protection, the Priority Academic Program Development of Jiangsu Higher Education Institutions. Appendix A. Supplementary data All of the test results about 239þ240Pu activities, 240Pu/239Pu atom ratios and 238U concentrations (Tab. A1), the content of heavy metals (mg/kg) in surface soil samples. (Tab. A2). This information is available free of charge via the Internet at https://www.elsevier.com/openacce sspricing Supplementary data to this article can be found online at https://doi. org/10.1016/j.apradiso.2020.109098. References Andersson, P.S., Porcelli, D., Gustafsson, O., Ingri, J., Wasserburg, G.J., 2001. The importance of colloids for the behavior of uranium isotopes in the low-salinity zone of a stable estuary. Geochem. Cosmochim. Acta 65, 13–25. Bednar, A.J., Medina, V.F., Ulmer-Scholle, D.S., Frey, B.A., Johnson, B.L., Brostoff, W.N., Larson, S.L., 2007. Effects of organic matter on the distribution of uranium in soil and plant matrices. Chemosphere 70 (2), 237–247. Briner, W., 2010. The toxicity of depleted uranium. Int. J. Environ. Res. Publ. Health 7 (1), 303–313. https://doi.org/10.3390/ijerph7010303. Burns, P.C., Finch, R.J., 2018. Uranium: Mineralogy, Geochemistry, and the Environment, vol. 38. Walter de Gruyter GmbH and Co KG.
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Radionuclides in the environment around the uranium mines in Guangxi, China Ruirui Wang a, Jingyu Mai a, b, Yongjin Guan b, Zhiyong Liu a, * a
State Key Laboratory of Radiation Medicine and Protection, School for Radiological and Interdisciplinary Sciences (RAD-X) and Collaborative Innovation Centre of Radiation Medicine of Jiangsu Higher Education Institutions, Soochow University, Suzhou, 215123, China b Guangxi Key Laboratory for Relativistic Astrophysics, School of Physical Science and Technology, Guangxi University, Nanning, 530004, China
A R T I C L E I N F O
A B S T R A C T
Keywords: 238 U 239þ240 Pu activity 240 Pu/239Pu atom ratio Uranium mine Heavy mental
Uranium and plutonium are both poisonous radioactive elements, which are very harmful to human health and environment. Therefore, it is of great significance to study the distribution of 238U concentration and 239þ240Pu activity in the uranium mine surrounding soils. We have collected some surface soil sediments within 2 km of two uranium mines and a solid waste management center in Guangxi Province. The 238U concentration in these study areas is in the range of 1.44–83.91 mg/g, and the 238U concentration in the A uranium mine surrounding surface soils is higher than that in the B uranium mine and the solid waste management center. While the B uranium mine and the solid waster management center don’t pollute the surrounding soils because the 238U concentra tions in their surrounding soils are similar to the average 238U concentration in the soil. The 239þ240Pu activities in soil samples collected around the two uranium mines and the solid waste management center are close ranged from 0.06 mBq/g to 0.51 mBq/g. Moreover, the 240Pu/239Pu atom ratios in our study samples are ranged from 0.15 to 0.23, which indicate the Pu may come from the global fallout. In addition, we study heavy metals in our collected samples, only heavy metal Tl has weak positive correlations with 238U concentrations and 239þ240Pu activities. And there is a weak positive correlation between 238U concentrations and 239þ240Pu activities.
1. Introduction Uranium is a kind of energy and a very important strategic resource. Uranium provides sufficient guarantee for the sustainable development of nuclear energy, and also plays an important role in nuclear military industry and nuclear power industry (Donohue, 1998; Wallenius et al., 2007; Wang et al., 2011; Herring, 2018). As a highly sensitive strategic resource with dual-use military and civilian applications, uranium is widely used with the gradual maturity and perfection of nuclear power technology (Briner, 2010; Burns and Finch, 2018). It is used in national defense construction and national economy, so the demand for fuel is increasing. According to the calculation of Hou et al. (2007), the average uranium content in the Earth is (4.42–5.85) � 10 8 mg/g. Uranium in the world is very rich, but the overall distribution is uneven (Herring, 2013). In China, the uranium deposits are mainly of medium and low grade. Although the scale of a single deposit is small, the number of deposits is large (Liu et al., 1992; Zhang et al., 2008; Yan et al., 2011). Uranium in nature has a mixture of three isotopes: 238U (T1/2 ¼ 4.468 � 109 years), 239U(T1/2 ¼ 7.038 � 108 years), and 240U(T1/2 ¼
2.455 � 106 years). Among them, the content of 238U accounts for 99.28% of the total content. These three isotopes of uranium are found in many minerals containing uranium, of which pitchblende contains the highest amount of uranium. Meanwhile, the uranium tailing is a serious pollution challenge for the environment and very harmful to human health (Bednar et al., 2007; Yang et al., 2011), and there are many technologies to deal with uranium in the environment (Vanden hove et al., 2001; Rufyikiri et al., 2004; Li and Zhang, 2012; Li et al., 2013). Plutonium is another radionuclide closely related to the nuclear in dustry including weapons industry and energy industry. Pu enters the environment mainly through nuclear weapon testing (Sholkovitz, 1983), accidental release (Zheng et al., 2012) and discharges from nu clear fuel reprocessing sites and nuclear power plants (Dai et al., 2005). Anthropogenic radionuclides 239Pu (T1/2 ¼ 24100 yr) and 240Pu (T1/2 ¼ 6560 yr) have high radiological toxicity and long-term retention in the environment. 239Pu and 240Pu with relatively long half-life have been used as tracers to analyze a series processes of Pu in nature, including the sources of Pu, Pu inputs and distribution, rebuilding the Pu historical
* Corresponding author. E-mail address: [email protected] (Z. Liu). https://doi.org/10.1016/j.apradiso.2020.109098 Received 27 October 2019; Received in revised form 2 January 2020; Accepted 23 February 2020 Available online 27 February 2020 0969-8043/© 2020 Elsevier Ltd. All rights reserved.
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Fig. 1. Map of sampling sites near the uranium mines in Guangxi province.
events, migration of Pu in seas and land, and special impacts on the environment (Kelley et al., 1999; Muramatsu et al., 2001; Warneke et al., 2002; Ketterer et al., 2004; Chamizo et al., 2011; Zheng et al., 2013; Wu et al., 2014, 2018). In addition, The 240Pu/239Pu atom ratio can be effectively used to trace the sources of Pu. Because the 240Pu/239Pu atom ratio varies among nuclear weapon construction sites, weapon test fallout, reproc essing plants, reactor or satellite accidents, and weapon types and yields (Muramatsu et al., 2001; Ketterer et al., 2004). The 240Pu/239Pu atom ratio of weapon-grade material is 0.01–0.077, while that of nuclear reactor-grade material is 0.2–1.08. The average 240Pu/239Pu atom ratio for regional fallout from the Pacific Proving Ground (PPG) in the Marshall Islands close-in fallout is 0.33–0.369, which is distinctly different from the ratio of the global fallout (0.176 � 0.014) (Ketterer et al., 2004). Plutonium from the Chernobyl Nuclear Power Plant (CNPP) accident has 240Pu/239Pu atom ratio ranged from 0.186 to 0.348 (Ketterer et al., 2004), while plutonium from the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident has 240Pu/239Pu atom ratio of 0.330 � 0.032 (Zheng et al., 2013). In China, the activity of 239þ240Pu in surface soils has shown obvious latitude-dependent distribution, with maximum settlement occurring between 40� N and 50� N (Xing, 2015). The average atom ratio of 240Pu/239Pu is 0.19, indicating that the Pu source in surface soils in China is mainly from the global fallout (Liu et al., 2007; Li et al., 2007; Zhuang et al., 2019). In this study, we choose a production stoppage uranium mine and a decommissioned uranium mine and a solid waste management center to study the uranium and plutonium in their surrounding soils. The pur poses are: (1) to find out the distribution of 238U concentration, Pu ac tivity and provide the background value of 238U concentration and Pu activity in soils around the two uranium mines and the solid waste management center; (2) to determine the source of Pu; (3) to look for some correlations of 238U concentration and Pu activity with some heavy
metal contents. 2. Sampling and measurements 2.1. Study area The two uranium mines are located in Guangxi province (Fig. 1). The production stoppage uranium mine is the “A uranium mine”, which has a history of more than 30 years, located in Daxing, Guangxi province. The mining area includes open pit ruins, western waste rock field, eastern waste rock field, industrial site, mining road and living area. The uranium mine was formally put into production in the early 1980s, when only open-pit mining was carried out, not smelting. The mining project was shut down in 1994. In 2000, the residual ore recovery project was implemented, mainly for mining and recycling of the exposed ore. The decommissioned uranium mine is the “B uranium mine”, which is located in Xiangyangping, mountain area with deep landform and large slope. The mine is still in operation and now has stopped pro duction and begun the residual ore recovery project. In our study, there is also a solid waste management center, which is used to dispose the wast from uranium mines. Meanwhile, the radio nuclides in soils around it are also the focus of our study. 2.2. Sample collection A total 38 soil samples were collected around the two uranium mines and the solid waste management center in 2015, which are in the ura nium mine isolation zone. These samples were taken from fresh soil samples at the sampling site, taken by the recovery of box cores with top 3 cm sediments. In these soil samples, 17 soil samples were collected around the A uranium mine, 14 samples were collected around B 2
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uranium mine, and the remaining 7 samples were collected around the solid waste management center. First, all these samples were packed into clean polypropylene sample bags, and then kept in a 4 � C refrig erator until analysis. The detailed site information and sample proper ties are shown in Table A1. Since these mines are uranium mines, it is inconvenient for us to provide specific locations of uranium mines there.
loading onto the second resin column, AG MP-1 M (2.5 ml, 6.5–8.5 mm � 58 mm, i.d.). Pu is eluted from the AG MP-1 M column with 16 ml HBr. And the mixture is evaporated to near dryness, 1 ml of concentrated HNO3 is added and heated to remove any trace of HBr. When near dryness, the final residual is dissolved in 0.8 ml of 40% HNO3 in prep aration for the sector field inductively coupled plasma mass spectrom etry (SF-ICP-MS; Element 2) analysis. The SF-ICP-MS is conducted in the Key Laboratory of Radiation Medicine and Protection at Soochow Uni versity (Suzhou, China). An APEX-Q sample introduction system with a membrane desolvation unit (ACM) and a conical concentric nebulizer are used as sample introduction systems to improve the sensitivity of the SF-ICP-MS. The overall Pu recoveries range from 80%–95%, and the average recovery was 87% � 3%. A certified Pu isotopic standard so lution (NBS-947) is used for mass bias correction. All measurements are made in the self-aspirating mode to reduce the risk of contamination by the peristaltic pump tubing. Details on the instrument optimization and determination of Pu isotopes have been described previously (Zheng and Yamada, 2006).
2.3. U and Pu isotope analysis The samples are dried at 100 � C for 24 h and pass through 20-mesh sieves to remove rhizome and coarse stones and then transfer to crucible and dry at 800 � C for over 8 h to podzolize the samples. The details of uranium analytical procedures are described in Kikawada et al. (2009). Uranium in surface soil samples is determined by scintillation gamma-spectrometry. Measurements are carried out using a gamma-spectrometer with a low-background NaI (Tl) well scintillation detector with a volume of 200 � 200 mm (well dimensions of 70 � 150 mm). The determination of the equivalent uranium (U (Ra)) concen trations are carried out using the analytical gamma-line of the isotope 214 Bi 1764.5 keV, based on the assumption of a radioactive equilibrium between 238U and 226Ra. The lower limit of detection of U (Ra) is about 0.4 ppm. The relative error of U (Ra) concentrations determination is not worse than 15% (2σ). The accuracy and reproducibility of the analysis are controlled by parallel measurements of Russian national geological reference materials SG-1A, SG-3, SG-2, DVG, DVT, ZUK-1, BIL-1, ST-1A (Govindaraju, 1994), and we use the SG-2 to control the accuracy in this study. To measure the plutonium in surface soils, these surface soil sedi ments are dried at 80 � C and ground in an amber mortar in preparation for Pu isotopic analyses. A 2.5 g dried samples is weighted out. Then 1.14 pg 242Pu is added for each samples as a chemical yield tracer. We leach the Pu acid with 50 ml 8 M HNO3 by heating on a hot plate at 180–200 � C for at least 4 h using a tightened Teflon vessel (150 ml) (Savillex Corporation, Minnesota, USA). After filtration supernatant, a NaNO2 is added to the sample solution to a concentration of 0.2 M and heated at 40 � C for 30 min to take Pu to the tetravalent state prior to loading onto the first SG 1-X8 resin column (2.5 ml, 6.5–8.5 mm � 58 mm, i.d.). After removing U, Pb, and Fe from the column using 50 ml of 8 M HNO3 (Zheng and Yamada, 2006), the sample solution is ready for
Fig. 2. Relationship between
239þ240
Pu activity and
3. Results and discussion The analysis results of 238U concentrations, 239þ240Pu activities and Pu/239Pu atom ratios are presented in Table A1.
240
3.1. Levels of
238
U concentrations in soils
238 U concentrations in the two uranium mines and the solid waste management center surrounding soils are in the range of 1.44–83.91 mg/g. Among them, the 238U concentrations range from 1.83 mg/g to 83.91 mg/g in the A uranium mine surrounding soils, with the average of 19.67 mg/g. The 238U concentrations are in the range of 1.44–6.87 mg/g (with the average of 3.11 mg/g) in the B uranium mine sur rounding soils, and the 238U concentrations of 2.48–3.81mg/g (with the average of 3.03 mg/g) are in the solid waste management center sur rounding soils. The 238U concentrations in the B uranium mine and solid waste management center surrounding soils are similar to the order of magnitude in igneous and sedimentary racks (0.45–3.7 mg/g) and Chinese loess (2.72–4.08 mg/g) (Tayler et al., 1983; Ding et al., 2001). A stark contrast is that the 238U concentrations in the A uranium
240
Pu/239Pu atom ratio in surface sediments from river basins in southern China. 3
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Fig. 3. Content of 239þ240Pu activity (mBq/g), 238U concentration (mg/g) and heavy metals (mg/kg) in surface sediments of river basins in southern China. The activities of 239þ240Pu are on a scale of 10 times.
mine surrounding soils are relatively higher than that in the B uranium mine and solid waste management center surrounding soils, especially in samples 2, 4, 74, and the highest 238U concentration is 83.91 mg/g (Table A1). Since the 238U concentrations in the B uranium mine and solid waste management center surrounding soils are similar to the 238U concentrations in Chinese loess (2.72–4.08 mg/g) (Tayler et al., 1983), that the 238U concentrations in the B uranium mine and solid waste management center surrounding soils are the background values of 238U concentrations in soils in Guangxi province, which is 1.44–6.87 mg/g. According to the above results, it can be confirmed that the A uranium mine may contaminate the surrounding soils, and the soils around the B uranium mine and the solid waste management center do not be polluted. Thus, we should pay more attention to the environment around the A uranium mine. 3.2.
239þ240
Pu activity and
and the samples collected from the solid waster management center surrounding soils have the 239þ240Pu activities of 0.08–0.15 mBq/g (with the average of 0.10 � 0.009 mBq/g). Obviously, the 239þ240Pu activities in the A uranium mine surrounding soils vary widely, and have relatively higher values than the 239þ240Pu activities in the B uranium mine and the solid waste management center surrounding soils. Besides, the 239þ240Pu activities in the B uranium mine and the solid waste management center surrounding soils have similar values. On the whole, the soils around our study sites have relatively close 239þ240Pu activities. The 240Pu/239Pu atom ratios in our study samples are ranged from 0.15 to 0.23, with an average of 0.19 � 0.020. The 240Pu/239Pu atom ratios in the A uranium mine surrounding soils are 0.15–0.23, with an average of 0.19 � 0.017. The 240Pu/239Pu atom ratios in the B uranium mine surrounding soils are 0.16–0.23, with an average of 0.19 � 0.025. The samples collected from the solid waster management center sur rounding soils have the 240Pu/239Pu atom ratios of 0.20–0.23, with an average of 0.22 � 0.021. Even the 240Pu/239Pu atom ratios in the solid waste management center surrounding soils are relatively higher than that in the A and B uranium mines surrounding soils. All these 240 Pu/239Pu atom ratios are well with the global fallout value (0.178 � 0.019, 0–30� N) (Kelley et al., 1999). Besides, these results are also consistent with previous studies about the 240Pu/239Pu atom ratios in surface soils in southern China (Liu et al., 2007; Li et al., 2007; Zhuang et al., 2019). Moreover, the major source of Pu in these surface soils is mainly from the global fallout. In central and northern China, similar 240 Pu/239Pu atom ratios have also been reported (Zheng and Yamada, 2006; Wang et al., 2011; Zhuang et al., 2019). In Fig. 2, we can see clearly that most of the 240Pu/239Pu atom ratios of the A and B uranium mines surrounding soils are in the global fallout area. While the 240Pu/239Pu atom ratios of the solid waste management center surrounding soils are above the global fallout and below the Pacific Proving Grounds (PPG). Although these 240Pu/239Pu atom ratios in the solid waste management center surrounding soils differ in some extent, they are all in value range of the global fallout (0.178 � 0.019, 0–30� N).
240
Pu/239Pu atom ratio
The 239þ240Pu activities in these soil samples collected around the two uranium mines and the solid waste management center are ranged from 0.06 mBq/g to 0.51 mBq/g (mean: 0.18 � 0.016 mBq/g). The samples collected from the A uranium mine surrounding soils have the 239þ240 Pu activities of 0.09–0.5 mBq/g (with the average of 0.25 � 0.018 mBq/g), the 239þ240Pu activities in soil samples collected around the B uranium mine are 0.07–0.37 mBq/g (mean: 0.14 � 0.016 mBq/g),
3.3. Factors effecting
238
U content and
239þ240
Pu activity in soils
To find out some factors effected the 238U concentrations and the Pu activity distributions, we also do some heavy metal analysis on these soil samples, such as the contents of Cr, Co, Ni, Cu, Zn, Cd, Tl, Pb. The analysis results are presented in Table A2. Several studies have reported that uranium has a unique bound with Cr and only indirectly with Mn, Fe, Ni, Co in soil, and 238U concentration shows a strong
239þ240
Fig. 4. The correlation between U, Pu and other heavy metal. 4
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Table 1 The correlation between Cr Cr Co Ni Cu Zn Cd Tl Pb U Pu a b
1 0.497a 0.714a 0.633a 0.633a 0.041 0.287 0.767a 0.347b 0.381b
238
U concentrations, Co
239þ240
Pu activities and other heavy metals.
Ni a
0.497 1 0.879a 0.659a 0.574a 0.456a 0.383 0.466a 0.283 0.262
Cu a
0.714 0.879a 1 0.845a 0.727a 0.438 0.429a 0.693a 0.339b 0.321b
Zn a
0.633 0.659a 0.845a 1 0.753a 1.465 0.461a 0.742a 0.156 0.088
Cd a
0.633 0.574a 0.727a 0.753a 1 0.167 0.373b 0.776a 0.182 0.249
0.041 0.456a 0.438a 0.465a 0.167 1 0.052 0.091 0.076 0.097
Tl
1
0.287 0.383b 0.429a 0.461a 0.373b 0.052
0.386b 0.31 0.306
Pb
U a
0.767 0.466a 0.693a 0.742a 0.776a 0.091 0.386b 1 0.104 0.253
Pu b
0.347 0.283 0.339b 0.156 0.182 0.076 0.309 0.104 1 0.408b
0.381b 0.262 0.321b 0.088 0.249 0.097 0.306 0.253 0.408b 1
Correlation is significant at the 0.01 level (2-tailed). Correlation is significant at the 0.05 level (2-tailed).
positive correlation with the content of K, Pb, Be, Zn, Cr, Co, Fe, V, Ni, Cu, Al (Malikova et al., 2020). Wang and Gao (2014) have indicated that the content of heavy metals in soils has a gradual decreasing trend with increasing distance from the tailing. Meanwhile, the 238U concentrations in these samples are evidently correlated with the contents of Cr and Cu and no correlativity with the Cd content. However, the 239þ240Pu ac tivities only have positive correlation with the content of As in the samples from the river basins in southern China (Wang et al., 2019). The 238U concentrations in our study surface soils has weak bounds with the content of Tl (correlation coefficient is 0.309) and the 239þ240Pu activities (correlation coefficient is 0.408), while has negative correla tions with other heavy mental elements (Cr, Co, Ni, Cu, Zn, Cd, Pb). 239þ240 Pu activities, like 238U concentrations, show weak positive cor relations with the content of Tl (correlation coefficient is 0.306) and 238 U concentrations (correlation coefficient is 0.408), also have indi rectly correlations with other heavy mental contents (Fig. 3 and Fig. 4) (Table 1). Compared with previous studies, 238U concentrations have negative correlations with the content of Cr in our study samples. Be sides, the 239þ240Pu activities have a weak positive relationship with the content of Tl (Fig. 3). It is clearly to see the correlations between the 238U concentrations and heavy mental contents have different results in different study areas, and the same true for 239þ240Pu activities. The reason for this inconsistency is largely due to the complexity of the environment in different study areas. For example, there are other fac tors more important for the distributions of 238U concentration and 239þ240 Pu activity in soils, including the peculiarities of soil formation, the evaporation process, the degree of salinity, variations in the mineralogical composition (Malikova et al., 2020). Previous studies have reported that uranium is deposited in the process of the adsorption on clay minerals (Titaeva, 2000; Kabata- Pendias, 2000; Santos-Frances et al., 2018), and the content and prop erties of clay minerals affect the sorption of uranium (Kayzar et al., 2014; Filistovic et al., 2015). Lin et al. (2018) has reported the depth distribution of uranium is probably controlled by its leaching and deposition cycles. In addition, the forms of uranium are important fac tors for uranium content in soils (Reinoso-Maset and Ly, 2016). Mean while, more studies have indicated the mobility of uranium forms is determined not only by redox conditions and the PH of the water, but also by biogeochemical processes (Andersson et al., 2001; Frederickson et al., 2002; Sharp et al., 2011; Newsome et al., 2015; Strakhovenk and Gaskova, 2018). It is believed that the mobility of uranium in the environment depends on the form of organic matter (Meng et al., 2017). Combined with the above studies, our research results are inconsistent with the published results, which is more due to the uncontrollable environment, and there will be more influencing factors for uranium migration in the environment in future studies. In addition, the 238U concentrations have a weak positive correlation with the 239þ240Pu activities in our study. However, no conclusive conclusions have been made about their correlation, and we will further study their correlation and the factors influencing their distribution in the environment.
4. Conclusion The average of 238U concentrations in the A uranium mine sur rounding soils (19.67 mg/g) is higher than the 238U concentrations in the B uranium mine (mean: 3.11 mg/g) and the solid waste management center surrounding soils (mean: 3.03 mg/g). Moreover, the 239þ240Pu activities in these soil samples collected around the A, B uranium mines and the solid waste management center are close, which are 0.09-0.5 mBq/g (mean: 0.25�0.018 mBq/g), 0.07-0.37 mBq/g (mean: 0.14�0.016 mBq/g) and 0.08-0.15 mBq/g (mean: 0.10�0.009 mBq/g), respectively. In addition, the Pu in all collected samples is from the global fallout. And, for the contents of these heavy metals, only the Tl content has weak positive correlations with the 238U concentrations (correlation coefficient is 0.306) and the 239þ240Pu activities (correla tion coefficient is 0.306). Meanwhile, there is a weak positive correla tion between the 238U concentrations and the 239þ240Pu activities (0.408). Declaration of competing interest 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. Acknowledgement This work was supported jointly by grants from the National Science Foundation of China (41773004, 11665006), the Jiangsu Provincial Key Laboratory of Radiation Medicine and Protection, the Priority Academic Program Development of Jiangsu Higher Education Institutions. Appendix A. Supplementary data All of the test results about 239þ240Pu activities, 240Pu/239Pu atom ratios and 238U concentrations (Tab. A1), the content of heavy metals (mg/kg) in surface soil samples. (Tab. A2). This information is available free of charge via the Internet at https://www.elsevier.com/openacce sspricing Supplementary data to this article can be found online at https://doi. org/10.1016/j.apradiso.2020.109098. References Andersson, P.S., Porcelli, D., Gustafsson, O., Ingri, J., Wasserburg, G.J., 2001. The importance of colloids for the behavior of uranium isotopes in the low-salinity zone of a stable estuary. Geochem. Cosmochim. Acta 65, 13–25. Bednar, A.J., Medina, V.F., Ulmer-Scholle, D.S., Frey, B.A., Johnson, B.L., Brostoff, W.N., Larson, S.L., 2007. Effects of organic matter on the distribution of uranium in soil and plant matrices. Chemosphere 70 (2), 237–247. Briner, W., 2010. The toxicity of depleted uranium. Int. J. Environ. Res. Publ. Health 7 (1), 303–313. https://doi.org/10.3390/ijerph7010303. Burns, P.C., Finch, R.J., 2018. Uranium: Mineralogy, Geochemistry, and the Environment, vol. 38. Walter de Gruyter GmbH and Co KG.
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