| As the natural radioelements,uranium and thorium widely existed in the environment(soil,water,air)and the content in soil and water is ?g/g or ?g/L magnitude,respectively.The content of thorium is higher than that of uranium in environmental medium.At present,the mainly source of uranium and thorium in the air is the dust from crust or soil.However,the continuous human production activities(such as ore mining and smelting,coal-fired thermal power generation,etc.)also affect the concentration of radioactive uranium and thorium.The main ways to enter into human body is by inhaling air,drinking water or intake of food,while the inhalation is one of the most important and most dangerous way.Uranium and thorium are both radioactive poison and chemical poison.They mainly accumulate in the bones,liver,kidney and other human organs which can not only induce hematopoietic,nervous and immune system diseases,but also induce leukemia and other diseases.The radioactive materials are adsorbed by suspended particulate matter in the air and form the radioactive aerosols.The degree of harm to human body is closely related to the concentration of uranium and thorium in the atmosphere,the physiological state of the body and the aerodynamic diameter of the aerosol,etc.Foreign countries have carried out relevant monitoring and research work on the content of uranium and thorium in atmospheric particulate matter.The United Nations Scientific Committee on the Effects of Atomic Radiation(UNSCEAR)has formulated the limits of uranium and thorium in aerosols.In China,the investigation of uranium and thorium mainly focused on the mining area and uranium mine.The study on the monitoring of uranium and thorium in environmental aerosols had been carried out in China.There is no standard method for the determination of uranium and thorium in aerosols and also no concentration limit.Therefore,the analysis of uranium and thorium in atmospheric particles and monitoring of China aerosol(especially fine particulate)concentrations of uranium and thorium play an important role in the maintenance of public health and the prevention and control of atmospheric pollution.In this work,the analytical methods of ultra-trace uranium and thorium in multi-grade large-scale particulate matter were studied and the analytical conditions were optimized.The concentration of uranium and thorium in atmospheric particulate matter were analyzed and summarized.The source of pollutions and the estimation of dose were discussed as well.The main research contents are as follows:(1)Experimental study on analytical method for determination of ultra trace uranium and thorium in six grade atmospheric particulates.The effects of different filter membrane,acid digestion and microwave digestion on the content of uranium and thorium were analyzed.The cellulose membrane with lower content of uranium and thorium was used to collect air samples.Aqua regia with hydrogen peroxide was applied to digest samples in the pretreatment process.The parameters of the mass spectrometer were also optimized in experiment.A new analytical method for determination of ultra trace uranium and thorium in the atmospheric particulates by microwave digestion inductively coupled plasma mass spectrometry was established.The containing uranium and thorium standard membrane was used to confirm this method,and the results showed that the relative error of this method was below 10%,the precision of the method of uranium and thorium was 3%and 8%,respectively.The detection limit of uranium and thorium were 2×10-4 ng/m3 and 1.7×10-3 ng/m3,respectively.The extended uncertainty of this method was 8%(Uuranium)and 4%(Uthorium).Thus,the method was effective and reliable.(2)Study on the distribution characteristics of the concentration of uranium and thorium and it's correlation with meteorological factors.The actual aerosol samples which were collected continuously were analyzed using the established analytical method and the data were discussed.After the Spearman non parametric test,it was concluded that the concentration of ultra trace uranium and thorium in different particle size had a positive correlation with air quality index and humidity,and had a negative correlation with temperature and wind speed;The mass concentration of uranium and thorium tended to be lower in summer,higher in winter,lower in the daytime and higher at night;Uranium and thorium were distributed more in fine particles which were more affected by the meteorological conditions.(3)Concentration levels of ultra trace uranium and thorium in different particle sizes of atmospheric particulatesAerosol samples collected in Beijing city were measured and analyzed by the established method.The mass concentration range of uranium in PM2.1?PM10.2?TSP were 0.039-0.145 ng/m3,0.088?0.238 ng/m3,0.113?0.275 ng/m3;The mass concentration range of thorium in PM2.1?PM10.2?TSP were 0.070?0.227 ng/m3?0.166?0.461 ng/m3,0.222?0.546 ng/m3.The range of activity concentration of 238U was 0.291?5.007 ?Bq/m3,with an average value of 2.194 ?Bq/m3,while the activity concentration of 232Th was 0.334?3.712 ?Bq/m3,with an average value of 1.541?Bq/m3.(4)Estimation of internal exposure dose and discussion on traceability of uranium and thorium particlesThe effective dose was estimated by the kinetic model and the dose conversion coefficient(committed effective dose coefficient)provided by national standard GB18871-2002.It was concluded that the average annual effective dose for the inhalation of uranium and thorium was 0.345 Sv/a the maximum value was about 0.828 Sv/a,and the minimum value was about 0.073 Sv/a.These values were far less than 2.4 mSv/a,which was the annual effective dose of natural radiation sources given by the UNSCEAR report.Three high pollution processes of uranium and thorium in the air in Beijing were analyzed using HYSPLIT model and the airflow was traced for nearly 72 hours.The model revealed that foreign pollution mainly came from the northwest and north neighboring countries;Domestic pollution mainly originated from mineral-rich areas,densely populated areas or related provinces. |
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