Preliminary Results of Spatial Distribution of Uranium and Thorium in Soil Profi Les
Total Page：16
File Type：pdf, Size：1020Kb
NUKLEONIKA 2016;61(3):367371 doi: 10.1515/nuka-2016-0061 ORIGINAL PAPER Preliminary results of spatial distribution Jin Wang, Juan Liu, of uranium and thorium in soil profi les Yongheng Chen, Gang Song, near a uranium industrial site, Diyun Chen, Tangfu Xiao, Guangdong province, China Huosheng Li, Chunlin Wang, Feng Jiang Abstract. Four soil profi les were collected from locations with different distances (5, 50, 250 and 1000 m) from a uranium mill tailings dam, Guangdong province, China, to investigate the pollution status of the soil in mining/ milling-related areas based on the contents of uranium (U) and thorium (Th), thus to understand the impacts of uranium industrial activities to the surroundings. The U and Th concentrations were determined by inductively coupled plasma mass spectrometry (ICP-MS) after conventional HF-HNO3-HClO4 digestion procedures. The results indicate that the soils within 50 m from uranium tailing were severely contaminated; those in 250 and 1000 m soil samples were observed to be with local background level even though these in 250 m had slightly higher U/Th ratios. Uranium concentrations varied from 5.50 ± 0.27 to 160.55 ± 8.03 mg/kg, with maximum values recorded in an intermediate layer of the 5-m distance soil profi le. In comparison, the concentration of Th ranged from 6.02 ± 0.30 to 84.71 ± 4.24 mg/kg, with maximum values observed in the top layer of the 1000-m distance soil profi le. The U/Th ratio varied from 0.15 to 11.99 compared with 0.20, 0.22 and 0.26 of the aver- age for Guangdong province, national China and the world, respectively. The mean U/Th of four soil profi les showed a reduction with distance from the uranium mill tailing dam, suggesting the relatively large magnitude of uranium elevation in soils within limited distances. Key words: spatial distribution • thorium • uranium • U/Th ratio Introduction J. Wang, G. Song, D. Y. Chen, F. Jiang School of Environmental Science and Engineering, The environmental issues concerning uranium min- Guangzhou University ing and milling have long been paid considerable and Guangdong Provincial Key Laboratory attention in wide aspects in the world [1–4]. It is of Radionuclides Pollution Control and Resources, well known that the industrial activities of mining 230 Waihuan West Str., 510006 Guangzhou, China and milling of uranium ores can lead to release of J. Liu, Y. H. Chen, H. S. Li a tremendous amount of radioactive pollutants Innovation Center and Key Laboratory of Waters Safety (wastewater and tailings, as representative carrier) & Protection in the Pearl River Delta, that consist typically of U and Th compounds. Also, Ministry of Education, the unintended stacking of uranium ores and waste Guangzhou University, rocks in surface environment would result in leach- 230 Waihuan West Str., 510006 Guangzhou, China, ing of U and Th under weathering and meteorologi- Tel.: +86 20 3936 6937, Fax: +86 20 3936 6946, cal conditions. Once being improperly treated, these E-mail: [email protected] radioactive pollutants can directly transfer to the environmental media, posing a signifi cant threat T. F. Xiao to the ecosystem and public health in the light of State Key Laboratory of Environmental Geochemistry, their radioactivity, toxicity, accumulation in the food Institute of Geochemistry, Chinese Academy of Sciences, chain and long-term persistence in nature [5–7]. 99 Lincheng West Str., 550081 Guiyang, China In the context of increased demand for uranium C. L. Wang resulting from skyrocketing development of nuclear Guangdong Provincial Academy of Environmental power plants in China, it is encouraged to carry out Science, investigation on uranium-induced pollution and 335 Middle Dongfeng Str., 510045 Guangzhou, China remediation strategy. In the present study, a large granitic-type uranium deposit in Guangdong, China, Received: 5 December 2015 was selected, whose mining and milling activities Accepted: 1 April 2016 commenced from 1960s. After 50 years of operation, - 10.1515/nuka-2016-0061 Downloaded from De Gruyter Online at 09/29/2016 03:06:43AM via free access 368 J. Wang et al. the distribution and level of radioactive elements in was accurately weighed and carefully transferred in a surrounding soil environment is of great concern. It 50-mL Tefl on beaker, then followed with HF-HNO3- is natural to wonder whether the local soil is safe -HClO4 digestion method on an electrothermal plate. for agriculture use. Therefore, the main aim of this Nitric acid (HNO3), hydrofl uoric acid (HF), and work is to investigate the pollution status and infl u- perchloric acid (HClO4) were all of super-pure grade. encing scale of this uranium industrial site and the The measurements of U and Th were performed related mining/milling activities to the ambient soil with inductively coupled plasma mass spectrometry environment with the focus on U and Th. Herein, (ICP-MS, PerkinElmer, Elan 6000) at the Guang- four soil profi les with different distances from mill zhou Institute of Geochemistry, Chinese Academy of tailings dam were studied to preliminarily illustrate Sciences. A certifi ed reference material was digested the spatial distribution of U and Th in the studied along with the samples to assure the precision and area and give insight into the fate and evolution of accuracy of analysis. The coeffi cients of variation these two elements in the soil medium under the of triplicate analysis for the studied elements were circumstance of anthropogenic infl uence of uranium below 10%, and the accuracy was confi rmed to be mining/milling-related activities. within ±5%. For the determination of soil pH, the method described in Ref. [8] was adopted. Air dried soil Experimental samples were mixed with ultra-pure water with a ratio of 1:2.5 (soil:water) and the pH of the clear Sample collection supernatant was measured using a pH meter (Sar- torius pH-2C, Germany). As schematically shown in Fig. 1, four soil pro- fi les were selected and sampled in the vicinity of a uranium mill tailings area and downstream area Results and discussion during a pilot survey in December 2011. With the purpose of investigating the pollution status and It is visible from Fig. 2 that the concentration of U infl uencing scale, the selection criteria of sampling in profi le A is very low, which seems to be affected points is based on mixed wind direction and water by little anthropogenic factors. However, U and Th fl ow direction; more sampling points were selected contents in the profi le are not seemingly low, that close to the most polluted site and less ones far is, 5.50 ± 0.27 to 19.37 ± 0.97 mg/kg, averaged away from pollution source. To prevent the sampling 11.83 ± 0.59 mg/kg, and 17.57 ± 0.88 to 84.71 ± contamination, the four soil profi les were collected 4.24 mg/kg, averaged 58.09 ± 2.90 mg/kg, respec- beginning from the most remote to the industrial tively, both twice the values of Guangdong province sites, marked as A, B, C and D, which were with (the average U and Th concentrations of 5.51 and different linear distances of about 1000, 250, 50 and 27.92 mg/kg, respectively) [9] and greatly exceeding 5 m, respectively, away from the uranium mill tail- the soil background value of 2.79 mg/kg in national ings dam. The total collection of 75 samples were China and the world of 2.0 mg/kg for U [10, 11]. from different depths of 1, 2, 3, 4, 5, 10, 15, 20, 25, Furthermore, both the maximum values emerged at 30, 35, 40, 45, 50, 55, 65, 75, 85 and 100 cm for each the surface layers. soil profi le. Each individual soil sample was carefully Many researchers use concentration ratios of excavated, mixed homogeneously and immediately U/Th to illustrate the U-related enrichment or placed in plastic bags. contamination [12, 13]. In particular, the U/Th ratio may distinguish U resulting from the artifi cial sources such as manufacture of nuclear fuels, coal- Sample preparation and analytical methods -combustion or from natural sources and indicate the relative level of U elevation [12]. In the present Before laboratory analysis, all glassware and Tefl on study, as shown in Fig. 3, the ratios in profi le A beakers were soaked in a 0.2 mol/L HNO3 solu- ranged from 0.15 to 0.31, averaged 0.21, which is tion for 24 h and then rinsed with ultra-pure water (18.25 M/cm) from a MilliQ-system (Millipore, Milford Corp., USA). Soil samples were air dried at 25°C and were ground to pass through a 0.149-mm nylon sieve. Hundred milligram of each soil sample Fig. 2. Spatial distribution of U and Th in profi les of A, B, C and D. The measurement uncertainty of concentrations Fig. 1. Schematic display of soil profi les location. for the two elements is about 5%. - 10.1515/nuka-2016-0061 Downloaded from De Gruyter Online at 09/29/2016 03:06:43AM via free access Preliminary results of spatial distribution of uranium and thorium in soil profi les... 369 and B. It indicates the process in which a great deal of U has largely migrated vertically down the profi le, other than Th, as a result of quite different chemical properties. The special geographical location of this soil profi le may help understand the high occurrence of U. Around profi le C, there was a polluted stream (stream 2 in Fig. 1), where were located with the outlets of processed wastewater piped from mill- ing area, also a wastewater pond full of pipelined effluent (pH ~3–4) with 2.091 mg/L of U and 0.030 mg/L of Th in the close vicinity found in our earlier investigation [6].